results for au:Wang_J in:quant-ph

- Mar 14 2018 quant-ph physics.optics arXiv:1803.04449v1The ability to control multidimensional quantum systems is key for the investigation of fundamental science and for the development of advanced quantum technologies. Here we demonstrate a multidimensional integrated quantum photonic platform able to robustly generate, control and analyze high-dimensional entanglement. We realize a programmable bipartite entangled system with dimension up to $15 \times 15$ on a large-scale silicon-photonics quantum circuit. The device integrates more than 550 photonic components on a single chip, including 16 identical photon-pair sources. We verify the high precision, generality and controllability of our multidimensional technology, and further exploit these abilities to demonstrate key quantum applications experimentally unexplored before, such as quantum randomness expansion and self-testing on multidimensional states. Our work provides a prominent experimental platform for the development of multidimensional quantum technologies.
- Mar 06 2018 quant-ph arXiv:1803.01304v1In this paper, we present a detailed study on discrete-time Dirac quantum walks (DQWs) on triangular and honeycomb lattices. At the continuous limit, these DQWs coincide with the Dirac equation. Their differences in the discrete regime are analyzed through the dispersion relations, with special emphasis on Zitterbewegung. An extension which couples these walks to arbitrary discrete electromagnetic field is also proposed and the resulting Bloch oscillations are discussed.
- Feb 27 2018 quant-ph arXiv:1802.08990v1The quantum evolution can be accelerated in non-Markovian environment. Previous results showed that the formation of system-environment bound state governs the quantum speedup. Although a stronger bound state in the system-environment spectrum may seem like it should cause greater speed of evolution, this seemingly intuitive thinking may not always be correct. We illustrate this by investigating a qubit driven by a classical field and coupled to a photonic crystal waveguide in the presence of a mirror. The perfect mirror can force part of the emitted light to return back to the qubit, and thus induce non-Markovian dynamics. Within the considered model, we show how the evolution speed is influenced by the memory time and the classical driving strength. In particular, we find that the formation of bound state is not the essential reason for the acceleration of evolution. The quantum speedup is attributed to the flow of information, regardless of the direction in which the information flows. Our conclusion can also be used to other non-Markovian environments.
- Jan 24 2018 physics.optics quant-ph arXiv:1801.07341v1As a special experimental technique, weak measurement extracts very little information about the measured system and will not cause the measured state collapse. When coupling the orbital angular momentum (OAM) state with a well-defined pre-selected and post-selected system of a weak measurement process, there is an indirect coupling between position and topological charge (TC) of OAM state. Based on these ideas, we propose an experimental scheme that experimentally measure the TC of OAM beams from -14 to 14 through weak measurement.
- We study the dynamics of steering between two correlated Unruh-Dewitt detectors when one of them locally interacts with external scalar field via different quantifiers. We find that the quantum steering, either measured by the entropic steering inequality or the Cavalcanti-Jones-Wiseman-Reid inequality, is fragile under the influence of Unruh thermal noise. The quantum steering is found always asymmetric and the asymmetry is extremely sensitive to the initial state parameter. In addition, the steering-type quantum correlations experience "sudden death" for some accelerations, which are quite different from the behaviors of other quantum correlations in the same system. It is worth noting that the domination value of the tight quantum steering exists a transformation point with increasing acceleration. We also find that the robustness of quantum steerability under the Unruh thermal noise can be realized by choosing the smallest energy gap in the detectors.
- Jan 16 2018 quant-ph arXiv:1801.04418v1We perform decoy-state quantum key distribution between a low-Earth-orbit satellite and multiple ground stations located in Xinglong, Nanshan, and Graz, which establish satellite-to-ground secure keys with ~kHz rate per passage of the satellite Micius over a ground station. The satellite thus establishes a secure key between itself and, say, Xinglong, and another key between itself and, say, Graz. Then, upon request from the ground command, Micius acts as a trusted relay. It performs bitwise exclusive OR operations between the two keys and relays the result to one of the ground stations. That way, a secret key is created between China and Europe at locations separated by 7600 km on Earth. These keys are then used for intercontinental quantum-secured communication. This was on the one hand the transmission of images in a one-time pad configuration from China to Austria as well as from Austria to China. Also, a videoconference was performed between the Austrian Academy of Sciences and the Chinese Academy of Sciences, which also included a 280 km optical ground connection between Xinglong and Beijing. Our work points towards an efficient solution for an ultralong-distance global quantum network, laying the groundwork for a future quantum internet.
- We perform a protocol for multipartite quantum clock synchronization in a relativistic setting. The dynamics of the multipartite quantum system consisting of Unruh-DeWitt detectors when one of the detectors under accelerated motion is obtained. To estimate the time difference between the clocks, we calculate the time probability and analyze how the probability is influenced by the Unruh thermal noise and other factors. It is shown that both relativistic motion and the interaction between atom and the external scalar field affect the choice of optimal number of excited atoms in the initial state, which leads to a higher clock adjustment accuracy. The time probabilities for different types of initial states approach to the same value in the limit of infinite acceleration, while they tend to different minimums with increasing number of atoms. In addition, the accuracy of clock synchronization using bipartite initial state is found always higher than multipartite systems, while the $Z$-type multipartite initial states always perform better than the $W$-type states.
- High Fidelity Single-qubit Gates of a Single Neutral Atom in the Magic-Intensity Optical Dipole TrapDec 19 2017 quant-ph arXiv:1712.06306v1We demonstrate high fidelity single-qubit gate operation in a trapped single neutral atom. The atom is trapped in the recently invented magic-intensity optical dipole trap (MI-ODT) with more stable magnetic field. The MI-ODT efficiently mitigates the detrimental effects of light shifts thus sufficiently improves the performance of single qubit-gates. The gates are driven with microwave, and the fidelity of gate operation is characterized by using the randomized benchmarking method. We obtain an average error per Clifford gate of $3.0(7)\times10^{-5}$ which is much below the error threshold ($10^{-4}$) for fault-tolerance. This error is found to be dominated by qubit dephasing, and the corresponding coherence time relevant to the Clifford gates is also measured experimentally. This work is an essential step toward the construction of a scalable quantum computer with neutral atoms trapped in an MI-ODT array.
- Dec 19 2017 quant-ph arXiv:1712.06456v1It is challenging task to detect and measure genuine multipartite entanglement. We investigate the problem by considering the average based positive partial transposition(PPT) criterion and the realignment criterion. Sufficient conditions for detecting genuine tripartite entanglement are presented. We also derive lower bounds for the genuine tripartite entanglement concurrence with respect to the conditions. While the PPT criterion and the realignment criterion are powerful for detecting bipartite entanglement and for providing lower bounds of bipartite concurrences, our results give an effective operational way to detect and measure the genuine tripartite entanglement.
- Dec 11 2017 quant-ph arXiv:1712.03213v1We propose a machine-learning-based quantum state tomography scheme for pure states, along with a built-in fidelity estimation approach to access the reliability of the tomographic state. We prove the validity of the scheme theoretically and perform computer-simulated experiments on several representative target quantum states such as the W, cluster and dimer states. We found that the required number of measurements to meet the convergence criterion does not grow exponentially in the number of qubits, thus the scheme achieves high efficiency that is crucial for the tomography of large-scale quantum states realized in the laboratory.
- We report the first experimental two-dimensional infrared (2D IR) spectra of novel molecular photonic excitations - vibrational-polaritons. The application of advanced 2D IR spectroscopy onto novel vibrational-polariton challenges and advances our understanding in both fields. From spectroscopy aspect, 2D IR spectra of polaritons differ drastically from free uncoupled molecules; from vibrational-polariton aspects, 2D IR uniquely resolves hybrid light-matter polariton excitations and unexpected dark states in a state-selective manner and revealed hidden interactions between them. Moreover, 2D IR signals highlight the role of vibrational anharmonicities in generating non-linear signals. To further advance our knowledge on 2D IR of vibrational polaritons, we develop a new quantum-mechanical model incorporating the effects of both nuclear and electrical anharmonicities on vibrational-polaritons and their 2D IR signals. This work reveals polariton physics that is difficult or impossible to probe with traditional linear spectroscopy and lays the foundation for investigating new non-linear optics and chemistry of molecular vibrational-polaritons.
- Superconducting nanowire single photon detectors (SNSPDs) have advanced various frontier scientific and technological fields such as quantum key distribution and deep space communications. However, limited by available cooling technology, all past experimental demonstrations have had ground-based applications. In this work we demonstrate a SNSPD system using a hybrid cryocooler compatible with space applications. With a minimum operational temperature of 2.8 K, this SNSPD system presents a maximum system detection efficiency of over 50% and a timing jitter of 48 ps, which paves the way for various space applications.
- Nov 16 2017 quant-ph arXiv:1711.05041v1It is a computationally hard task to certify genuine multipartite entanglement (GME). We investigate the relation between the norms of the correlation vectors and the detection of GME for tripartite quantum systems. A sufficient condition for GME and an effective lower bound for the GME concurrence are derived. Several examples are considered to show the effectiveness of the criterion and the lower bound of GME concurrence.
- Oct 24 2017 quant-ph physics.atom-ph arXiv:1710.08088v2We give a derivation for the indirect interaction between two magnetic dipoles induced by the quantized electromagnetic field. It turns out that the interaction between permanent dipoles directly returns to the classical form; the interaction between transition dipoles does not directly return to the classical result, yet returns in the short-distance limit. In a finite volume, the field modes are highly discrete, and both the permanent and transition dipole-dipole interactions are changed. For transition dipoles, the changing mechanism is similar with the Purcell effect, since only a few number of nearly resonant modes take effect in the interaction mediation; for permanent dipoles, the correction comes from the boundary effect: if the dipoles are placed close to the boundary, the influence is strong, otherwise, their interaction does not change too much from the free space case.
- We study the behavior of monogamy deficit and monogamy asymmetry for Einstein-Podolsky-Rosen steering of Gaussian states under the influence of the Hawking effect. We demonstrate that the monogamy of quantum steering shows an extreme scenario in the curved spacetime: the first part of a tripartite system cannot individually steer two other parties, but it can steer the collectivity of the remaining two parties. We also find that the monogamy deficit of Gaussian steering, a quantifier of genuine tripartite steering, are generated due to the influence of the Hawking thermal bath. Our results elucidate the structure of quantum steering in tripartite quantum systems in curved spacetime.
- Sep 29 2017 cond-mat.quant-gas quant-ph arXiv:1709.09944v2We study the interaction-modulated tunneling dynamics of a Bose-Einstein condensate (BEC) in a deep double-well potential, where the tunneling between the two wells is modulated by another BEC trapped in a harmonic potential symmetrically positioned at the center of the double-well potential. The inter-species interactions couple the dynamics of the two BECs, which give rise to interesting features in the tunneling oscillations. Adopting a two-mode approximation for the BEC in the double-well potential and coupling it with the Gross-Pitaevskii equation of the harmonically trapped BEC, we numerically investigate the coupled dynamics of the BEC mixture, and map out the phase diagram of the tunneling dynamics. We show that the dynamical back action of the BEC in the harmonic trap leads to strong non-linearity in the oscillations of the BEC in the double-well potential, which enriches the system dynamics, and enhances macroscopic self trapping. The transition between the Josephson oscillation and the self-trapping dynamics can be identified by monitoring the oscillation frequency of the double-well BEC. Our results suggest the possibility of tuning the tunneling dynamics of BECs in double-well potentials.
- Sep 19 2017 quant-ph arXiv:1709.05445v2We investigate the decay properties of the quantumness including quantum entanglement, quantum discord and quantum coherence for two photonic qubits, which are partially entangled in their orbital angular momenta, through Kolmogorov turbulent atmosphere. It is found that the decay of quantum coherence and quantum discord may be qualitatively different from that of quantum entanglement when the initial state of two photons is not maximally entangled. We also derive two universal decay laws for quantum coherence and quantum discord, respectively, and show that the decay of quantum coherence is more robust than nonclassical correlations.
- Sep 12 2017 quant-ph arXiv:1709.03088v2We explore the quantum metrology in an optical molecular system coupled to two environments with different temperatures, using a quantum master equation beyond secular approximation. We discover that the steady-state coherence originating from and sustained by the nonequilibrium condition can enhance quantum metrology. We also study the quantitative measures of the nonequilibrium condition in terms of the curl flux, heat current and entropy production at the steady state. They are found to grow with temperature difference. However, an apparent paradox arises considering the contrary behaviors of the steady-state coherence and the nonequilibrium measures in relation to the inter-cavity coupling strength. This paradox is resolved by decomposing the heat current into a population part and a coherence part. Only the latter, coherence heat current, is tightly connected to the steady-state coherence and behaves similarly with respect to the inter-cavity coupling strength. Interestingly, the coherence heat current flows from the low-temperature reservoir to the high-temperature reservoir, opposite to the direction of the population heat current. Our work offers a viable way to enhance quantum metrology for open quantum systems through steady-state coherence sustained by the nonequilibrium condition, which can be controlled and manipulated to maximize its utility. The potential applications go beyond quantum metrology and extend to areas such as device designing, quantum computation and quantum technology in general.
- Generative modeling, which learns joint probability distribution from training data and generates samples according to it, is an important task in machine learning and artificial intelligence. Inspired by probabilistic interpretation of quantum physics, we propose a generative model using matrix product states, which is a tensor network originally proposed for describing (particularly one-dimensional) entangled quantum states. Our model enjoys efficient learning by utilizing the density matrix renormalization group method which allows dynamic adjusting dimensions of the tensors, and offers an efficient direct sampling approach, Zipper, for generative tasks. We apply our method to generative modeling of several standard datasets including the principled Bars and Stripes, random binary patterns and the MNIST handwritten digits, to illustrate ability of our model, and discuss features as well as drawbacks of our model over popular generative models such as Hopfield model, Boltzmann machines and generative adversarial networks. Our work shed light on many interesting directions for future exploration on the development of quantum-inspired algorithms for unsupervised machine learning, which is of possibility of being realized by a quantum device.
- Aug 22 2017 quant-ph arXiv:1708.06057v1Aharonov, Jones, and Landau [Algorithmica 55, 395 (2009)] have presented a polynomial quantum algorithm for approximating the Jones polynomial. We investigate the bipartite entanglement properties in AJL's algorithm for three-strand braids. We re-describe AJL's algorithm as an equivalent algorithm which involves three work qubits in some mixed state coupled to a single control qubit. Furthermore, we use the Peres entanglement criterion to study the entanglement features of the state before measurements present in the re-described algorithm for all bipartitions. We show that the state is a product state relative to the bipartition between the first work qubit and the others. And it has no entanglement between the control qubit and work ones. We also prove a sufficient and necessary condition for its entanglement between the second (third) work qubit and the others. Moreover, we discuss the relation between its bipartite entanglement and elementary crossings in the three-strand braid group. We find that braids whose trace closures are topologically identical may have different entanglement properties in AJL's algorithm.
- We derive the spectral density of the equiprobable mixture of two random density matrices of a two-level quantum system. We also work out the spectral density of mixture under the so-called quantum addition rule. We use the spectral densities to calculate the average entropy of mixtures of random density matrices, and show that the average entropy of the arithmetic-mean-state of $n$ qubit density matrices randomly chosen from the Hilbert-Schmidt ensemble is never decreasing with the number $n$. We also get the exact value of the average squared fidelity. Some conjectures and open problems related to von Neumann entropy are also proposed.
- Jul 31 2017 quant-ph physics.optics arXiv:1707.09088v1High visibility temporal ghost imaging with classical light is possible when superbunching pseudothermal light is employed. In the numerical simulation, the visibility of temporal ghost imaging with pseudothermal light equaling ($4.7\pm 0.2$)\% can be increased to ($75\pm 8$)\% in the same scheme with superbunching pseudothermal light. The reasons for the difference in visibility and quality of the retrieved images in different situations are discussed in detail. It is concluded that high visibility and high quality temporal ghost image can be obtained by collecting large enough number of data points. The results are helpful to understand the difference between ghost imaging with classical light and entangled photon pairs. The superbunching pseudothermal light can be employed to improve the image quality in ghost imaging applications.
- Jul 14 2017 cond-mat.mes-hall quant-ph arXiv:1707.03977v2Periodic driving fields can induce topological phase transitions, resulting in Floquet topological phases with intriguing properties such as very large Chern numbers and unusual edge states. Whether such Floquet topological phases could generate robust edge state conductance much larger than their static counterparts is an interesting question. In this paper, working under the Keldysh formalism, we study two-lead transport via the edge states of irradiated quantum Hall insulators using the method of recursive Floquet-Green's functions. Focusing on a harmonically-driven Hofstadter model, we show that quantized Hall conductance as large as $8e^2/h$ can be realized, but only after applying the so-called Floquet sum rule. To assess the robustness of edge state transport, we analyze the DC conductance, time-averaged current profile and local density of states. It is found that co-propagating chiral edge modes are more robust against disorder and defects as compared with the remarkable counter-propagating edge modes, as well as certain symmetry-restricted Floquet edge modes. Furthermore, we go beyond the wide-band limit, which is often assumed for the leads, to study how the conductance quantization (after applying the Floquet sum rule) of Floquet edge states can be affected if the leads have finite bandwidths. These results may be useful for the design of transport devices based on Floquet topological matter.
- Jul 12 2017 quant-ph arXiv:1707.03178v1The hybrid quantum network, a universal form of quantum network which is aimed for quantum communication and distributed quantum computation, is that the quantum nodes in it are realized with different physical systems. This universal form of quantum network can combine the advantages and avoid the inherent defects of the different physical system. However, one obstacle standing in the way is the compatible photonic quantum interface. One possible solution is using non-degenerate, narrow-band, entangled photon pairs as the photonic interface. Here, for the first time, we generate nondegenrate narrow-band polarization-entangled photon pairs in cavity-enhanced spontaneous parametric down-conversion process. The bandwidths and central wavelengths of the signal and idler photons are 9 MHz at 935 nm and 9.5 MHz at 880 nm, which are compatible with trapped ion system and solid-state quantum memory system. The entanglement of the photon source is certified by quantum state tomography, showing a fidelity of 89.6% between the generated quantum state with a Bell state. Besides, a strong violation against Bell inequality with 2.36+/-0.03 further confirms the entanglement property of the photon pairs. Our method is suitable for the hybrid quantum network and will take a big step in this field.
- The random magnetic field produced by nuclear spins has long been viewed as the dominating source of decoherence in the quantum-dot based spins. Here we obtain in both exact and analytical manner the dynamics of spin qubits coupled to nuclear spin environments via the hyperfine interaction, going beyond the weak system-bath interaction and Markovian approximation. We predict that the detailed-balance breaking produced by chemical potential gradient in nuclear baths leads to the rapid oscillations of populations, quantum coherence and entanglement, which are absent in the conventional case (i.e., Overhauser noise). This is attributed to the nonequilibrium feature of the system as shown in the relation between the oscillation period and the chemical potential imbalance. Our results reveal the essentiality of nonequilibriumness with detailed-balance breaking for enhancing the dynamical coherence and entanglement of spin qubits. Moreover, our exact solution explicitly demonstrates that the non-Markovian bath comprised by nuclear spins can preserve the collective quantum state, due to the recovery of coherence. Finally, we propose an experiment using ultracold trapped ions to observe these nonequilibrium and memory effects.
- Long-distance entanglement distribution is essential both for foundational tests of quantum physics and scalable quantum networks. Owing to channel loss, however, the previously achieved distance was limited to ~100 km. Here, we demonstrate satellite-based distribution of entangled photon pairs to two locations separated by 1203 km on the Earth, through satellite-to-ground two-downlink with a sum of length varies from 1600 km to 2400 km. We observe a survival of two-photon entanglement and a violation of Bell inequality by 2.37+/-0.09 under strict Einstein locality conditions. The obtained effective link efficiency at 1200 km in this work is over 12 orders of magnitude higher than the direct bidirectional transmission of the two photons through the best commercial telecommunication fibers with a loss of 0.16 dB/km.
- An arbitrary unknown quantum state cannot be precisely measured or perfectly replicated. However, quantum teleportation allows faithful transfer of unknown quantum states from one object to another over long distance, without physical travelling of the object itself. Long-distance teleportation has been recognized as a fundamental element in protocols such as large-scale quantum networks and distributed quantum computation. However, the previous teleportation experiments between distant locations were limited to a distance on the order of 100 kilometers, due to photon loss in optical fibres or terrestrial free-space channels. An outstanding open challenge for a global-scale "quantum internet" is to significantly extend the range for teleportation. A promising solution to this problem is exploiting satellite platform and space-based link, which can conveniently connect two remote points on the Earth with greatly reduced channel loss because most of the photons' propagation path is in empty space. Here, we report the first quantum teleportation of independent single-photon qubits from a ground observatory to a low Earth orbit satellite - through an up-link channel - with a distance up to 1400 km. To optimize the link efficiency and overcome the atmospheric turbulence in the up-link, a series of techniques are developed, including a compact ultra-bright source of multi-photon entanglement, narrow beam divergence, high-bandwidth and high-accuracy acquiring, pointing, and tracking (APT). We demonstrate successful quantum teleportation for six input states in mutually unbiased bases with an average fidelity of 0.80+/-0.01, well above the classical limit. This work establishes the first ground-to-satellite up-link for faithful and ultra-long-distance quantum teleportation, an essential step toward global-scale quantum internet.
- Quantum key distribution (QKD) uses individual light quanta in quantum superposition states to guarantee unconditional communication security between distant parties. In practice, the achievable distance for QKD has been limited to a few hundred kilometers, due to the channel loss of fibers or terrestrial free space that exponentially reduced the photon rate. Satellite-based QKD promises to establish a global-scale quantum network by exploiting the negligible photon loss and decoherence in the empty out space. Here, we develop and launch a low-Earth-orbit satellite to implement decoy-state QKD with over kHz key rate from the satellite to ground over a distance up to 1200 km, which is up to 20 orders of magnitudes more efficient than that expected using an optical fiber (with 0.2 dB/km loss) of the same length. The establishment of a reliable and efficient space-to-ground link for faithful quantum state transmission constitutes a key milestone for global-scale quantum networks.
- Based on the multi-level model, we have calculated light shifts for Zeeman states of hyperfine levels of cesium (Cs) 6S1/2 ground state and 6P3/2 excited state.The magic-wavelength linearly-polarized optical dipole trap (ODT) for Cs 6S1/2 F=4, mF=+4 - 6P3/2 F'=5, mF=+5 transition is experimentally constructed and characterized by using the laser-induced fluorescence spectra of trapped single Cs atoms. The magic wavelength is 937.7 nm which produces almost the same light shift for 6S1/2 F=4, mF=+4 ground state and 6P3/2 F'=5, mF=+5 excited state with linearly-polarized ODT laser beam. Compared to undisturbed Cs 6S1/2 F=4, mF=+4 - 6P3/2 F'=5, mF=+5 transition frequency in free space, the differential light shift is less than 0.7 MHz in a linearly-polarized 937.7 nm ODT, which is less than 1.2% of the trap depth. We also discussed influence of the trap depth and the bias magnetic field on the measurement results.
- Jun 21 2017 physics.atom-ph quant-ph arXiv:1706.06237v3We demonstrate a single-photon Rydberg excitation spectroscopy of cesium (Cs) atoms in a room-temperature vapor cell. Cs atoms are excited directly from 6S1/2 ground state to nP3/2 (n = 70 - 100) Rydberg states with a 318.6 nm ultraviolet (UV) laser,and Rydberg excitation spectra are obtained by transmission enhancement of a probe beam resonant to Cs 6S1/2, F = 4 - 6P3/2, F' = 5 transition as partial population on F = 4 ground state are transferred to Rydberg state .Analysis reveals that the observed spectra are velocity-selective spectroscopy of Rydberg state, from which the amplitude and linewidth influenced by lasers'Rabi frequency have been investigated. Fitting to energies of Cs nP3/2 (n = 70 -100) states, the determined quantum defect is 3.56671(42). The demodulated spectra can also be employed as frequency references to stabilize the UV laser frequency to specific Cs Rydberg transition.
- In most realistic models for quantum chaotic systems, the Hamiltonian matrices in unperturbed bases have a sparse structure. We study correlations in eigenfunctions of such systems and derive explicit expressions for some of the correlation functions with respect to energy. The analytical results are tested in several models by numerical simulations. An application is given for a relation between transition probabilities.
- May 25 2017 quant-ph arXiv:1705.08696v2We report the experimental demonstration of polarization squeezed beam at 795 nm by combining a quadrature squeezed beam with a coherent beam. The quadrature squeezed beam is generated by a degenerate optical parametric amplifier based on a PPKTP crystal. Stokes parameter squeezing of -3.8 dB and anti squeezing of +5.0 dB is observed. This polarization squeezed beam resonant to rubidium D1 line has potential applications in quantum information networks and precise measurement beyond the shot noise limit.
- May 19 2017 quant-ph physics.atom-ph arXiv:1705.06404v1A velocity-selective spectroscopy technique for studying the spectra from hot Rydberg gases is presented. This method provides high-resolution measurement of the spectrum interval. Based on this method, the velocity-selective hyperfine splitting of intermediate states is measured, as well as the Doppler-free fine-structure splitting of the Rydberg states via two-photon Rydberg excitation in a room-temperature 133Cs vapor cell. The experiment data and theoretical predictions show excellent agreement.
- Apr 27 2017 quant-ph arXiv:1704.08142v1We investigate the behavior of the maximal violations of the CHSH inequality and V$\grave{e}$rtesi's inequality under the local filtering operations. An analytical method has been presented for general two-qubit systems to compute the maximal violation of the CHSH inequality and the lower bound of the maximal violation of V$\acute{e}$rtesi's inequality over the local filtering operations. We show by examples that there exist quantum states whose non-locality can be revealed after local filtering operation by the V$\acute{e}$rtesi's inequality instead of the CHSH inequality.
- Apr 12 2017 quant-ph physics.optics arXiv:1704.03301v1Quantum sensors with solid state electron spins have attracted considerable interest due to their nanoscale spatial resolution.A critical requirement is to suppress the environment noise of the solid state spin sensor.Here we demonstrate a nanoscale thermometer based on silicon carbide (SiC) electron spins.We experimentally demonstrate that the performance of the spin sensor is robust against dephasing due to a self protected machenism. The SiC thermometry may provide a promising platform for sensing in a noisy environment ,e.g. biological system sensing.
- Apr 12 2017 quant-ph arXiv:1704.03268v2A 2.8-dB polarization squeezing of the Stokes operator S2 for the rubidium D1 line (795 nm) is achieved, with the lowest squeezing band at an audio frequency of 2.6 kHz. Two methods are applied for improving the squeezing towards low frequencies: an orthogonal-polarized locking beam that precludes residual unwanted interference and quantum noise locking method that stabilizes the relative phase between the squeezed vacuum and the local oscillator. The squeezing level is limited by absorption-induced losses at short wavelengths, here is 397.5 nm. The generated polarization squeezed light can be used in a quantum enhanced magnetometer to increase the measurement sensitivity.
- Apr 06 2017 quant-ph cond-mat.mes-hall arXiv:1704.01300v1Velleytronics as a new electronic conception is an emerging exciting research field with wide potential applications, which is attracting great research interests for their extraordinary properties. The localized electronic spins by optical generation of valley polarization with spin-like quantum numbers are promising candidates for implementing quantum-information processing in solids. It is expected that a single qubit preparation can be realized optically by using combination of left- and right-circularly polarized lights. Significantly in a series of experiments, this has already been well achieved by linearly polarized laser representing equal weights of left- and right-circular components resulting in formation of a valley exciton as a specific pseudo-spin qubit with equal amplitudes for spin up and spin down. Further researches on the control of valley pseudospin using longitudinal magnetic field and optical Stark effect have been reported. However, a general qubit preparation has not yet been demonstrated. Moreover as a platform for quantum information processing, the precise readout of a qubit state is necessary, for which the state tomography is a standard method in obtaining all information of a qubit state density matrix.
- Apr 03 2017 quant-ph physics.comp-ph arXiv:1703.10743v1A new method for compiling quantum algorithms is proposed and tested for a three qubit system. The proposed method is to decompose a a unitary matrix U, into a product of simpler U j via a neural network. These U j can then be decomposed into product of known quantum gates. Key to the effectiveness of this approach is the restriction of the set of training data generated to paths which approximate minimal normal subRiemannian geodesics, as this removes unnecessary redundancy and ensures the products are unique. The two neural networks are shown to work effectively, each individually returning low loss values on validation data after relatively short training periods. The two networks are able to return coefficients that are sufficiently close to the true coefficient values to validate this method as an approach for generating quantum circuits. There is scope for more work in scaling this approach to larger quantum systems.
- Mar 17 2017 quant-ph arXiv:1703.05402v1Efficiently characterising quantum systems, verifying operations of quantum devices and validating underpinning physical models, are central challenges for the development of quantum technologies and for our continued understanding of foundational physics. Machine-learning enhanced by quantum simulators has been proposed as a route to improve the computational cost of performing these studies. Here we interface two different quantum systems through a classical channel - a silicon-photonics quantum simulator and an electron spin in a diamond nitrogen-vacancy centre - and use the former to learn the latter's Hamiltonian via Bayesian inference. We learn the salient Hamiltonian parameter with an uncertainty of approximately $10^{-5}$. Furthermore, an observed saturation in the learning algorithm suggests deficiencies in the underlying Hamiltonian model, which we exploit to further improve the model itself. We go on to implement an interactive version of the protocol and experimentally show its ability to characterise the operation of the quantum photonic device. This work demonstrates powerful new quantum-enhanced techniques for investigating foundational physical models and characterising quantum technologies.
- Mar 16 2017 quant-ph arXiv:1703.05169v1Quantum phase estimation is a fundamental subroutine in many quantum algorithms, including Shor's factorization algorithm and quantum simulation. However, so far results have cast doubt on its practicability for near-term, non-fault tolerant, quantum devices. Here we report experimental results demonstrating that this intuition need not be true. We implement a recently proposed adaptive Bayesian approach to quantum phase estimation and use it to simulate molecular energies on a Silicon quantum photonic device. The approach is verified to be well suited for pre-threshold quantum processors by investigating its superior robustness to noise and decoherence compared to the iterative phase estimation algorithm. This shows a promising route to unlock the power of quantum phase estimation much sooner than previously believed.
- Mar 14 2017 quant-ph arXiv:1703.04479v1In this work, we present a method for targeted, maskless, and scalable fabrication of single silicon vacancy (VSi) defect arrays in silicon carbide (SiC) using focused ion beam. The resolution of implanted VSi defects is limited to a few tens of nanometers, defined by the diameter of the ion beam. Firstly, we studied the photoluminescence (PL) spectrum and optically detected magnetic resonance (ODMR) of the generated defect spin ensemble, confirming that the synthesized centers were in the desired defect state. Then we investigated the fluorescence properties of single VSi defects and our measurements indicate the presence of a photostable single photon source. Finally, we find that the Si++ ion to VSi defect conversion yield increases as the implanted dose decreases. The reliable production of VSi defects in silicon carbide could pave the way for its applications in quantum photonics and quantum information processing.
- Mar 07 2017 quant-ph arXiv:1703.01852v2Quantum correlations and quantum coherence are of fundamental significance for the quantum theory, and are also cornerstones for theory of quantum computation and quantum information. Searching physically meaningful and mathematically rigorous quantifiers of them are long-standing concerns of the community of quantum information processing. Many different measures have been introduced up to now. We review in this paper the various notions of quantum correlations shared by bipartite and multipartite systems, the various measures of quantum coherence for a single quantum system, and their relationships in different settings. We try to provide a full review about the progress of quantum coherence, including the applications in many-body systems. For quantum correlations part, we focus on geometric quantum discord and measurement-induced nonlocality. All of these measures are formulated in a resource theoretic point of view, and can be defined as the closest (pseudo) distance of the considered state to the set of prescribed resource-free states. We discuss the interrelations between quantum coherence and quantum correlations established in an operational way, and the fundamental characteristics of quantum coherence such as their complementarity under different basis sets, their duality with path information of an interference experiment, their distillation under different operations, and some new viewpoints of the superiority of the quantum algorithms from the perspective of quantum coherence. Additionally, we review properties of geometric quantum correlations and quantum coherence under noisy quantum channels. Finally, the main progresses for the study of quantum correlations and quantum coherence in the relativistic settings are reviewed.
- Mar 07 2017 quant-ph arXiv:1703.01452v2Photons propagating in Laguerre-Gaussian modes have characteristic orbital angular momentums, which are fundamental optical degrees of freedom. The orbital angular momentum of light has potential application in high capacity optical communication and even in quantum information processing. In this work, we experimentally construct a ring cavity with 4 lenses and 4 mirrors that is completely degenerate for Laguerre-Gaussian modes. By measuring the transmission peaks and patterns of different modes, the ring cavity is shown to supporting more than 31 Laguerre-Gaussian modes. The constructed degenerate cavity opens a new way for using the unlimited resource of available angular momentum states simultaneously.
- Feb 15 2017 quant-ph arXiv:1702.04130v1High-quality entangled photon pairs generated via spontaneous parametric down-conversion have made great contributions to the modern quantum information science and the fundamental tests of quantum mechanics. However, the quality of the entangled states decreases sharply when moving from biphoton to multiphoton experiments, mainly due to the lack of interactions between photons. Here, for the first time, we generate a four-photon Greenberger-Horne-Zeilinger state with a fidelity of $98\%$, which is even comparable to the best fidelity of biphoton entangled states. Thus, it enables us to demonstrate an ultrahigh-fidelity entanglement swapping---the key ingredient in various quantum information tasks. Our results push the fidelity of multiphoton entanglement generation to a new level and would be useful in some demanding tasks, e.g., we successfully demonstrate the genuine multipartite nonlocality of the observed state in the nonsignaling scenario by violating a novel Hardy-like inequality, which requires very high state-fidelity.
- Feb 14 2017 quant-ph arXiv:1702.03508v2The initial entanglement shared between inertial and accelerated observers degrades due to the influence of the Unruh effect. Here, we show that the Unruh effect can be completely eliminated by the technique of partial measurement. The lost entanglement could be entirely retrieved or even amplified, which is dependent on whether the optimal strength of reversed measurement is \emphstate-independent or \emphstate-dependent. Our work provides a novel and unexpected method to recover the lost entanglement under Unruh decoherence and exhibits the ability of partial measurement as an important technique in relativistic quantum information.
- Feb 14 2017 quant-ph arXiv:1702.03493v1Network centrality has important implications well beyond its role in physical and information transport analysis; as such, various quantum walk-based algorithms have been proposed for measuring network vertex centrality. In this work, we propose a continuous-time quantum walk algorithm for determining vertex centrality, and show that it generalizes to arbitrary graphs via a statistical analysis of randomly generated scale-free and Erdős-Rényi networks. As a proof of concept, the algorithm is detailed on a 4-vertex star graph and physically implemented via linear optics, using spatial and polarization degrees of freedoms of single photons. This paper reports the first successful physical demonstration of a quantum centrality algorithm.
- Feb 02 2017 quant-ph arXiv:1702.00349v1Quantum entanglement is crucial for simulating and understanding exotic physics of strongly correlated many-body systems, such as high--temperature superconductors, or fractional quantum Hall states. The entanglement of non-identical particles exhibits richer physics of strong many-body correlations and offers more opportunities for quantum computation, especially with neutral atoms where in contrast to ions the interparticle interaction is widely tunable by Feshbach resonances. Moreover, the inter-species entanglement forms a basis for the properties of various compound systems, ranging from Bose-Bose mixtures to photosynthetic light-harvesting complexes. So far, the inter-species entanglement has only been obtained for trapped ions. Here we report on the experimental realization of entanglement of two neutral atoms of different isotopes. A ${}^{87}\mathrm{Rb}$ atom and a ${}^{85}\mathrm{Rb}$ atom are confined in two single--atom optical traps separated by 3.8 $\mu$m. Creating a strong Rydberg blockade, we demonstrate a heteronuclear controlled--NOT (C--NOT) quantum gate and generate a heteronuclear entangled state, with raw fidelities $0.73 \pm 0.01$ and $0.59 \pm 0.03$, respectively. Our work, together with the technologies of single--qubit gate and C--NOT gate developed for identical atoms, can be used for simulating any many--body system with multi-species interactions. It also has applications in quantum computing and quantum metrology, since heteronuclear systems exhibit advantages in low crosstalk and in memory protection.
- Jan 16 2017 quant-ph arXiv:1701.03565v2The Einstein-Podolsky-Rosen (EPR) pair of qubits plays a critical role in many quantum protocol applications such as quantum communication and quantum teleportation. Due to interaction with the environment, an EPR pair might lose its entanglement and can no longer serve as useful quantum resources. On the other hand, it has been suggested that introducing disorder into environment might help to prevent thermalization and improve the preservation of entanglement. Here, we theoretically investigate the time evolution of quantum entanglement of an EPR pair in a random-field XXZ spin chain model in the Anderson localized (AL) and many-body localized (MBL) phase. We find that the entanglement between the qubits decreases and approaches to a plateau in the AL phase, but shows a power-law decrease after some critical time determined by the interaction strength in the MBL phase. Our findings, on one hand, shed lights on applying AL/MBL to improve quantum information storage; on the other hand, can be used as a practical indicator to distinguish the AL and MBL phase.
- Jan 09 2017 quant-ph arXiv:1701.01537v1Quantum image processing has been a hot topic. The first step of it is to store an image into qubits, which is called quantum image preparation. Different quantum image representations may have different preparation methods. In this paper, we use GQIR (the generalized quantum image representation) to represent an image, and try to decrease the operations used in preparation, which is also known as quantum image compression. Our compression scheme is based on JPEG (named from its inventor: the Joint Photographic Experts Group) --- the most widely used method for still image compression in classical computers. We input the quantized JPEG coefficients into qubits and then convert them into pixel values. Theoretical analysis and experimental results show that the compression ratio of our scheme is obviously higher than that of the previous compression method.
- Nov 23 2016 quant-ph arXiv:1611.07126v2Random numbers are indispensable for a variety of applications ranging from testing physics foundation to information encryption. In particular, nonlocality tests provide a strong evidence to our current understanding of nature -- quantum mechanics. All the random number generators (RNG) used for the existing tests are constructed locally, making the test results vulnerable to the freedom-of-choice loophole. We report an experimental realization of RNGs based on the arrival time of cosmic photons. The measurement outcomes (raw data) pass the standard NIST statistical test suite. We present a realistic design to employ these RNGs in a Bell test experiment, which addresses the freedom-of-choice loophole.
- Nov 14 2016 quant-ph arXiv:1611.03511v5The efficient calculation of Hamiltonian spectra, a problem often intractable on classical machines, can find application in many fields, from physics to chemistry. Here, we introduce the concept of an "eigenstate witness" and through it provide a new quantum approach which combines variational methods and phase estimation to approximate eigenvalues for both ground and excited states. This protocol is experimentally verified on a programmable silicon quantum photonic chip, a mass-manufacturable platform, which embeds entangled state generation, arbitrary controlled-unitary operations, and projective measurements. Both ground and excited states are experimentally found with fidelities >99%, and their eigenvalues are estimated with 32-bits of precision. We also investigate and discuss the scalability of the approach and study its performance through numerical simulations of more complex Hamiltonians. This result shows promising progress towards quantum chemistry on quantum computers.
- Oct 25 2016 quant-ph arXiv:1610.07107v1In this paper, we investigate the simulation of continuous-time quantum walks on specific classes of graphs, for which it is possible to fast-forward the time-evolution operator to achieve constant-time simulation complexity and to perform the simulation exactly, while maintaining $\mbox{poly}(\mbox{log}(n))$ efficiency. In particular, we discuss two classes of composite graphs, commuting graphs and Cartesian product of graphs, that contain classes of graphs which can be simulated in this fashion. This allows us to identify new families of graphs that we can efficiently simulate in a quantum circuit framework, providing practical and explicit means to explore quantum-walk based algorithms in laboratories.
- Oct 14 2016 quant-ph cond-mat.mes-hall arXiv:1610.03978v1Color centers in silicon carbide have increasingly attracted attention in recent years owing to their excellent properties such as single photon emission, good photostability, and long spin coherence time even at room temperature. As compared to diamond which is widely used for holding Nitrogen-vacancy centers, SiC has the advantage in terms of large-scale, high-quality and low cost growth, as well as advanced fabrication technique in optoelectronics, leading to the prospects for large scale quantum engineering. In this paper, we report experimental demonstration of the generation of nanoscale $V_{Si}$ single defect array through ion implantation without the need of annealing. $V_{Si}$ defects are generated in pre-determined locations with resolution of tens of nanometers. This can help in integrating $V_{Si}$ defects with the photonic structures which, in turn, can improve the emission and collection efficiency of $V_{Si}$ defects when it is used in spin photonic quantum network. On the other hand, the defects are shallow and they are generated $\sim 40nm$ below the surface which can serve as critical resources in quantum sensing application.
- Oct 03 2016 quant-ph arXiv:1609.09622v1We study, in the paradigm of open quantum systems, the dynamics of quantum coherence of a static polarizable two-level atom which is coupled with a thermal bath of fluctuating electromagnetic field in the absence and presence of boundaries. The purpose is to find the conditions under which the decoherence can be inhibited effectively. We find that without boundaries, quantum coherence of the two-level atom inevitably decreases due to the effect of thermal bath. However, the quantum decoherence, in the presence of a boundary, could be effectively inhibited when the atom is transversely polarizable and near this boundary. In particular, we find that in the case of two parallel reflecting boundaries, the atom with a parallel dipole polarization at arbitrary location between these two boundaries will be never subjected to decoherence provided we take some special distances for the two boundaries.
- Sep 23 2016 quant-ph arXiv:1609.06798v1A novel solid state based charge qubit is presented. The system consists of a one-dimensional wire with a pair of qubits embedded at its center. It is shown that the system supports collective states localized in the left and right sides of the wire and therefore, as a whole, performs as a single qubit. The couplings between the ground and excited states of the two central qubits are inversely proportional making them fully asynchronized and allowing for coherent manipulation and gate operations. Initialization and measurement devices, such as leads and charge detectors, connected to the edges of the wire are modeled by a continuum of energy states. The coupling to the continuum is discussed using the effective non-Hermitian Hamiltonian. At weak continuum coupling, all internal states uniformly acquire small decay widths. This changes dramatically as the coupling strength increases: the width distribution undergoes a sharp restructuring and is no longer uniformly divided among the eigenstates. Two broad resonances localized at the ends of the wire are formed. These superradiant states (analogous to Dicke states in quantum optics), effectively protect the remaining internal states from decaying into the continuum and hence increase the lifetime of the qubit. Environmental noise is introduced by considering random Gaussian fluctuations of electronic energies. The interplay between decoherence and superradiance is studied by solving the stochastic Liouville equation. In addition to increasing the lifetime, the emergence of the superradiant states increases the qubit coherence.
- Spin-parity effect in violation of bell's inequalities for entangled states of parallel polarizationSep 19 2016 quant-ph arXiv:1609.04811v2Bell inequalities (BIs) derived in terms of quantum probability statistics are extended to general bipartite-entangled states of arbitrary spins with parallel polarization. The original formula of Bell for the two-spin singlet is slightly modified in the parallel configuration, while, the inequality for- mulated by Clauser-Horne-Shimony-Holt remains not changed. The violation of BIs indeed resulted from the quantum non-local correlation for spin-1=2 case. However, the inequalities are always satisfied for the spin-1 entangled states regardless of parallel or antiparallel polarizations of two spins. The spin parity effect originally demonstrated with the antiparallel spin-polarizations (Mod. Phys. Lett. B28, 145004) still exists for the parallel case. The quantum non-locality does not lead to the violation for integer spins due to the cancellation of non-local interference effects by the quantum statistical-average. Again the violation of BIs seems a result of the measurement induced nontrivial Berry-phase for half-integer spins.
- Sep 05 2016 physics.optics quant-ph arXiv:1609.00394v1Imperfections in integrated photonics manufacturing have a detrimental effect on the maximal achievable visibility in interferometric architectures. These limits have profound implications for further photonics technological developments and in particular for quantum photonics technologies. Active optimisation approaches, together with reconfigurable photonics, have been proposed as a solution to overcome this. In this paper, we demonstrate an ultra-high (>60 dB) extinction ratio in a silicon photonic device consisting of cascaded Mach-Zehnder interferometers, in which additional interferometers function as variable beamsplitters. The imperfections of fabricated beamsplitters are compensated using an automated progressive optimization algorithm with no requirement for pre-calibration. This work shows the possibility of integrating and accurately controlling linear-optical components for large-scale quantum information processing and other applications.
- Sep 02 2016 quant-ph arXiv:1609.00173v1A major advantage in using Szegedy's formalism over discrete-time and continuous-time quantum walks lies in its ability to define a unitary quantum walk on directed and weighted graphs. In this paper, we present a general scheme to construct efficient quantum circuits for Szegedy quantum walks that correspond to classical Markov chains possessing transformational symmetry in the columns of the transition matrix. In particular, the transformational symmetry criteria do not necessarily depend on the sparsity of the transition matrix, so this scheme can be applied to non-sparse Markov chains. Two classes of Markov chains that are amenable to this construction are cyclic permutations and complete bipartite graphs, for which we provide explicit efficient quantum circuit implementations. We also prove that our scheme can be applied to Markov chains formed by a tensor product. We also briefly discuss the implementation of Markov chains based on weighted interdependent networks. In addition, we apply this scheme to construct efficient quantum circuits simulating the Szegedy walks used in the quantum Pagerank algorithm for some classes of non-trivial graphs, providing a necessary tool for experimental demonstration of the quantum Pagerank algorithm.
- Aug 30 2016 quant-ph arXiv:1608.07941v1The second-order temporal interference of classical and nonclassical light at an asymmetrical beam splitter is discussed based on two-photon interference in Feynman's path integral theory. The visibility of the second-order interference pattern is determined by the properties of the superposed light beams, the ratio between the intensities of these two light beams, and the reflectivity of the asymmetrical beam splitter. Some requirements about the asymmetrical beam splitter have to be satisfied in order to ensure that the visibility of the second-order interference pattern of nonclassical light beams exceeds classical limit. The visibility of the second-order interference pattern of photons emitted by two independent single-photon sources is independent of the ratio between the intensities. These conclusions are important for the researches and applications in quantum optics and quantum information when asymmetrical beam splitter is employed.
- The goal of this paper is to calculate exactly the average of uncertainty-product of two bounded observables and to establish its typicality over the whole set of finite dimensional quantum pure states. Firstly, we investigate the average uncertainty of an observable over isospectral density matrices with a fixed spectrum. By letting the isospectral density matrices be of rank-one, we get the average uncertainty of an observable restricted to pure quantum states. Physically, the ensemble of a large number of particles as a closed system is represented by mixed state $\rho$. When we measure observable $A$ at a mixed state $\rho$ with many repetitions, we suggest that each time we measure $A$ at a point within the isospectral density matrices, i.e. the unitary orbit $\cU_\rho$ of $\rho$. Thus it is suitable for taking average of uncertainty of observable $A$ over the whole unitary orbit $\cU_\rho$. Based on this result, we finally get the calculation of the average of uncertainty-product over the whole set of mixed quantum states. These results can help us check how large the gap is between the uncertainty-product and any obtained lower bounds about the uncertainty-product. Although our method in the present paper cannot give a tighter lower bound of uncertainty-product for bounded observables, it can help us drop any one that is not tighter than the known one substantially.
- Aug 05 2016 quant-ph arXiv:1608.01592v1We present an analytical lower bound of multipartite concurrence based on the generalized Bloch representations of density matrices. It is shown that the lower bound can be used as an effective entanglement witness of genuine multipartite entanglement. Tight lower and upper bounds for multipartite tangles are also derived. Since the lower bounds depend on just part of the correlation tensors, the result is experimentally feasible.
- Aug 04 2016 quant-ph arXiv:1608.01086v1Quantum digital signature (QDS) is an approach to guarantee the nonrepudiation, unforgeability and transferability of a signature with the information-theoretical security. All previous experimental realizations of QDS relied on an unrealistic assumption of secure channels and the longest distance is only several kilometers. Here, we have experimentally demonstrated a recently proposed QDS protocol without any secure channel. Exploiting the decoy state modulation, we have successfully signed one bit message through up to 102 km optical fiber. Furthermore, we continuously run the system to sign the longer message "USTC" with 32 bit at the distance of 51 km. Our results pave the way towards the practical application of QDS.
- Jul 26 2016 quant-ph arXiv:1607.07149v2Circulant matrices are an important family of operators, which have a wide range of applications in science and engineering related fields. They are in general non-sparse and non-unitary. In this paper, we present efficient quantum circuits to implement circulant operators using fewer resources and with lower complexity than existing methods. Moreover, our quantum circuits can be readily extended to the implementation of Toeplitz, Hankel, and block circulant matrices. Efficient quantum algorithms to implement the inverses and products of circulant operators are also provided.
- Jul 18 2016 cond-mat.stat-mech quant-ph arXiv:1607.04382v1We propose a feasible and effective approach to study quantum thermal transport through anharmonic systems. The main idea is to obtain an \it effective harmonic Hamiltonian for the anharmonic system by applying the self-consistent phonon theory. Using the effective harmonic Hamiltonian we study thermal transport within the framework of nonequilibrium Green's function method using the celebrated Caroli formula. We corroborate our quantum self-consistent approach using the quantum master equation that can deal with anharmonicity exactly, but is limited to the weak system-bath coupling regime. Finally, in order demonstrate its strength we apply the quantum self-consistent approach to study thermal rectification in a weakly coupled two segment anharmonic system.
- Jul 12 2016 quant-ph arXiv:1607.02673v3Various quantum-walk based algorithms have been proposed to analyse and rank the centrality of graph vertices. However, issues arise when working with directed graphs --- the resulting non-Hermitian Hamiltonian leads to non-unitary dynamics, and the total probability of the quantum walker is no longer conserved. In this paper, we discuss a method for simulating directed graphs using PT-symmetric quantum walks, allowing probability conserving non-unitary evolution. This method is equivalent to mapping the directed graph to an undirected, yet weighted, complete graph over the same vertex set, and can be extended to cover interdependent networks of directed graphs. Previous work has shown centrality measures based on the CTQW provide an eigenvector-like quantum centrality; using the PT-symmetric framework, we extend these centrality algorithms to directed graphs with a significantly reduced Hilbert space compared to previous proposals. In certain cases, this centrality measure provides an advantage over classical algorithms used in network analysis, for example by breaking vertex rank degeneracy. Finally, we perform a statistical analysis over ensembles of random graphs, and show strong agreement with the classical PageRank measure on directed acyclic graphs.
- A profound quest of statistical mechanics is the origin of irreversibility - the arrow of time. New stimulants have been provided, thanks to unprecedented degree of control reached in experiments with isolated quantum systems and rapid theoretical developments of manybody localization in disordered interacting systems. The proposal of (many-body) eigenstate thermalization (ET) for these systems reinforces the common belief that either interaction or extrinsic randomness is required for thermalization. Here, we unveil a quantum thermalization mechanism challenging this belief. We find that, provided one-body quantum chaos is present, as a pure many-body state evolves the arrow of time can emerge, even without interaction or randomness. In times much larger than the Ehrenfest time that signals the breakdown of quantum-classical correspondence, quantum chaotic motion leads to thermal [Fermi-Dirac (FD) or Bose-Einstein (BE)] distributions and thermodynamics in individual eigenstates. Our findings lay dynamical foundation of statistical mechanics and thermodynamics of isolated quantum systems.
- Jun 23 2016 physics.atom-ph quant-ph arXiv:1606.06978v2We investigate single cesium (Cs) atom heating owing to the momentum accumulation process induced by the resonant pulsed excitation in a microscopic optical dipole trap formed by a strongly focused 1064 nm laser beam. The heating depends on the trap frequency which restricts the maximum repetition rate of pulsed excitation. We experimentally verify the heating of a single atom and then demonstrate how to suppress it with an optimized pulsed excitation/cooling method. The typical trap lifetime of single Cs atom is extended from 108 +/- 6 us to 2536 +/- 31 ms, and the corresponding number of excitation increases from ~ 108 to ~ 360000. In applying this faster cooling method, we use the trapped single Cs atom as a triggered single-photon source at an excitation repetition rate of 10 MHz. The second-order intensity correlations of the emitted single photons are characterized by implementing Hanbury Brown and Twiss setup, and clear anti-bunching effect has been observed.
- Jun 20 2016 quant-ph physics.atom-ph arXiv:1606.05580v2We demonstrate that the coherence of a single mobile atomic qubit can be well preserved during a transfer process among different optical dipole traps (ODTs). This is a prerequisite step in realizing a large-scale neutral atom quantum information processing platform. A qubit encoded in the hyperfine manifold of $^{87}$Rb atom is dynamically extracted from the static quantum register by an auxiliary moving ODT and reinserted into the static ODT. Previous experiments were limited by decoherences induced by the differential light shifts of qubit states. Here we apply a magic-intensity trapping technique which mitigates the detrimental effects of light shifts and substantially enhances the coherence time to $225 \pm 21\,\mathrm{ms}$. The experimentally demonstrated magic trapping technique relies on the previously neglected hyperpolarizability contribution to the light shifts, which makes the light shift dependence on the trapping laser intensity to be parabolic. Because of the parabolic dependence, at a certain "magic" intensity, the first order sensitivity to trapping light intensity variations over ODT volume is eliminated. We experimentally demonstrate the utility of this approach and measure hyperpolarizability for the first time. Our results pave the way for constructing a scalable quantum-computing architectures with single atoms trapped in an array of magic ODTs.
- Jun 17 2016 quant-ph arXiv:1606.04974v1We present a Mathematica package, QSWalk, to simulate the time evaluation of Quantum Stochastic Walks (QSWs) on arbitrary directed and weighted graphs. QSWs are a generalization of continuous time quantum walks that incorporate both coherent and incoherent dynamics and as such, include both quantum walks and classical random walks as special cases. The incoherent component allows for quantum walks along directed graph edges. The dynamics of QSWs are expressed using the Lindblad formalism, originally developed for open quantum systems, which frames the problem in the language of density matrices. For a QSW on a graph of $N$ vertices, we have a sparse superoperator in an $N^2$-dimensional space, which can be solved efficiently using the built-in MatrixExp function in Mathematica. We illustrate the use of the QSWalk package through several example case studies.
- Jun 01 2016 quant-ph arXiv:1605.09729v1Quantum image processing (QIP) means the quantum based methods to speed up image processing algorithms. Many quantum image processing schemes claim that their efficiency are theoretically higher than their corresponding classical schemes. However, most of them do not consider the problem of measurement. As we all know, measurement will lead to collapse. That is to say, executing the algorithm once, users can only measure the final state one time. Therefore, if users want to regain the results (the processed images), they must execute the algorithms many times and then measure the final state many times to get all the pixels' values. If the measurement process is taken into account, whether or not the algorithms are really efficient needs to be reconsidered. In this paper, we try to solve the problem of measurement and give a quantum image matching algorithm. Unlike most of the QIP algorithms, our scheme interests only one pixel (the target pixel) instead of the whole image. It modifies the probability of pixels based on Grover's algorithm to make the target pixel to be measured with higher probability, and the measurement step is executed only once. An example is given to explain the algorithm more vividly. Complexity analysis indicates that the quantum scheme's complexity is $O(2^{n})$ in contradistinction to the classical scheme's complexity $O(2^{2n+2m})$, where $m$ and $n$ are integers related to the size of images.