results for au:Wang_P in:quant-ph

- 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.
- Jan 16 2018 quant-ph arXiv:1801.04681v1The non-Markovia dynamics of quantum evolution plays an important role in open quantum sytem. However, how to quantify non-Markovian behavior and what can be obtained from non- Markovianity are still open questions, especially in complex solid systems. Here we address the problem of quantifying non-Markovianity with entanglement in a genuine noisy solid state system at room temperature. We observed the non-Markovianity of quantum evolution with entanglement. By prolonging entanglement with dynamical decoupling, we can reveal the non-Markovianity usually concealed in the environment and obtain detailed environment information. This method is expected to be useful in quantum metrology and quantum information science.
- Nov 09 2017 quant-ph arXiv:1711.02911v1Because of the intrinsic robustness, adiabatic evolutions find a broad range of applications in quantum state engineering, quantum computing, and quantum simulation. According to the original form of the quantum adiabatic theorem, for a process to remain adiabatic, its rate of change at all times must be much smaller than the energy gap of the Hamiltonian. On the other hand, in order to avoid perturbations from the environment high rates of change are desirable. This tension imposes severe limitations on the practical use of adiabatic methods. Current approaches to obtain adiabatic evolution within system coherence times are shortcuts to adiabaticity by using intricate schemes of additional counterdiabatic driving fields which however can be exceedingly challenging to implement experimentally. Here, we investigate the roles of energy gaps in quantum adiabatic processes by using a nitrogen-vacancy (NV) centre in diamond. We demonstrate experimentally that energy gaps may vanish while maintaining adiabatic evolution, a result that challenges traditional views. This shows that dynamic phases are more fundamental than energy gaps in quantum adiabatic evolution, the key insight that allows us to overcome the limits on evolution times imposed by the traditional form of the adiabatic theorem and to achieve, within experimental uncertainties, unit fidelity quantum adiabatic processes in finite time. The results provide a deeper understanding on quantum adiabatic processes, as well as promising strategies and directions in the control of quantum systems.
- Nov 08 2017 quant-ph cond-mat.stat-mech arXiv:1711.02482v1Quantum measurements and phase transitions are seemingly uncorrelated topics, but here we show that phase transitions occur in sequential quantum measurements. We find that the probability distribution of the measurement results of a sequence of quantum measurements on a two-level system (e.g. a qubit) is equivalent to the Boltzmann distribution of a classical lattice spin model. So the measurement results present phase transitions similar to those in the lattice spin model. In sequential commuting positive-operator valued measurements, the probability distribution is mapped to a long-range Ising model in the weak-measurement regime, and a projective measurement emerges from a sequence of weak measurement when the strength or the number of measurements becomes above certain critical values, which correspond to a second-order ferromagnetic phase transition of the lattice spin model. These findings not only provide new insights on sequential quantum measurements, but may also have potential applications in quantum technologies.
- How to construct the $d+1$ dimensional geometry explicitly from the dual CFT$_d$ is a widely concerned problem. Specifically, given entanglement entropies of a CFT$_2$, which is purely expressed by two dimensional parameters, can we build the dual three dimensional geometry unambiguously? To do this, one must assume nothing is known about the three dimensional geometry and starts with the most general setup. In this paper, by identifying the UV and IR entanglement entropies of a perturbed usual CFT$_2$ with the geodesic lengths, we show that, the dual geometry is uniquely determined to be asymptotic AdS$_3$. The hidden dimension is generated by the energy cut-off of the CFT$_2$, according to the holographic principle. The pure AdS$_3$ is obtained by taking the massless limit. Our derivations apply to both static and covariant scenarios. Moreover, what deserves special attention is that the ratio of the numberical factors of the UV/IR entanglement entropies are crucial to have a dual geometry. We are led to conjecture a necessary condition of holographic CFT$_2$.
- Exceptional point in non-Hermitian system possesses fascinating properties. We present an exactly solvable attractor dynamics for the first time from a two-level time dependent non-Hermitian Hamiltonian. It allows a way to evolve to the coalescence state from a pure or mixed initial state through varying the imaginary parameter along a specific diabatic passage. Contrast to a chaotic attractor that is ultrasensitive to the initial condition, the designed attractor is insensitive to the initial conditions. The attractor-like behavior still exists for several adiabatic processes.
- Sep 18 2017 quant-ph cond-mat.str-el arXiv:1709.05086v1We study the edge modes of a finite-size Kitaev model on the square lattice with periodic boundary conditions in one direction and open boundary conditions in the other. Based on the fact that the Majorana representation of Kitaev model is equivalent to a brick wall model, the model in a finite-length cylindrical geometry is shown to support perfect Majorana bound states which is in strong localization limit, characterized by a edge-mode fermionic operator. In the framework of edge-mode pseudospin analysis, we find that the edge modes exhibit long-range maximal entanglement.
- The Lorentz symmetry and the space and time translational symmetry are fundamental symmetries of nature. Crystals are the manifestation of the continuous space translational symmetry being spontaneously broken into a discrete one. We argue that, following the space translational symmetry, the continuous Lorentz symmetry should also be broken into a discrete one, which further implies that the continuous time translational symmetry is broken into a discrete one. We deduce all the possible discrete Lorentz and discrete time translational symmetries in 1+1-dimensional spacetime, and show how to build a field theory or a lattice field theory that has these symmetries.
- Jun 14 2017 quant-ph cond-mat.mes-hall arXiv:1706.03939v1We demonstrate an optically induced polarization detection (OIPD) technique for mesoscopic magnetic resonance spectroscopy and imaging. Our method employs a single spin in highly purified diamond as the magnetic quantum sensor, allowing us to detect the spectra of polarized electron spin magnetization of a pentacene crystal with the size of tens of micrometers. We detected the magnetic resonance spectra of polarized electron spins, measured its relaxation time and observed the electron spin polarization. This is the first application of NV-based magnetic resonance to sense polarized electron spins. Compared to thermal distribution, the polarization of these electron spins is improved a thousandfold by optical pumping. The method can be extended to sense polarized nuclear spin magnetization at mesoscale with ultrahigh polarization by employing dynamic nuclear polarization.
- Jun 13 2017 quant-ph arXiv:1706.03482v1Searching for new particles beyond the standard model is crucial for understanding several fundamental conundrums in physics and astrophysics. Amongst them, axions or similar hypothetical pseudoscalar bosons would mediate electron-nucleon interactions. While previous experiments set stringent upper bounds of this interaction strength with force range over $20 ~\mu$m, experimental searching at shorter force range remains elusive. We develop a method that utilizes a near-surface Nitrogen-vacancy center as a quantum sensor to explore such interaction. New constraints for axion-mediated electron-nucleon coupling, $g_s^Ng_p^e$, have been set for the force range $0.1 $--$23~\mu$m. The obtained upper bound of the interaction at $20~\mu$m, $g_s^Ng_p^e < 6.24\times10^{-15}$, is two orders of magnitude more stringent than that set by earlier experiments. Our method can be further extended to investigate other spin-dependent interactions and opens the door for the single-spin quantum sensor to explore new physics beyond the standard model.
- May 26 2017 quant-ph physics.chem-ph arXiv:1705.09201v1Water is the most important solvent in nature. It is a crucial issue to study interactions among water molecules. Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful tools to detect magnetic interactions for the structure analysis of a molecule with broad applications. But conventional NMR spectroscopy requires macroscopic sample quantities with hampers in investigating nanoscale structures. Through quantum control of a single spin quantum sensor, magnetic resonance spectroscopy of nanoscale organic molecules and single molecules has been achieved. However, the measurement of the dipolar interaction of nuclear spins within a molecule at nanoscale and the analysis of its structure remain a big challenge. Here we succeed in detecting the NMR spectrum from an ice crystal with (6-nanometer)$^3$ detection volume. More importantly, the magnetic dipolar coupling between two proton nuclear spins of a water molecule was recorded. The resolved intra-molecule magnetic dipolar interactions are about 15 kHz and 33 kHz with spectral resolution at a few kHz. Analysis of the interaction-resolved NMR spectroscopy provides a spatial view of nanoscale ice crystal, from which the orientation of a water-molecule bond is derived and further the length of the bond can be got. This work enables NMR spectroscopy applications in single molecule structure analysis, provides a further tool for nanocrystalline and confined water research.
- Apr 14 2017 quant-ph arXiv:1704.04115v1We study the connection between a family of non-Hermitian Hamiltonians H and Hermitian ones H based on exact solutions. In general, for a dynamic process in a non-Hermitian system H, there always exists a parallel dynamic process governed by the corresponding Hermitian conjugate Hamiltonian H\dag. We show that a linear superposition of the two parallel dynamics is exactly equivalent to the time evolution of a state under a Hermitian Hamiltonian H. It reveals a novel connection between non-Hermitian and Hermitian systems.
- Dec 26 2016 quant-ph arXiv:1612.07944v1Single-shot readout of qubits is required for scalable quantum computing. Nuclear spins are superb quantum memories due to their long coherence times but are difficult to be read out in single shot due to their weak interaction with probes. Here we demonstrate single-shot readout of a weakly coupled $^{13}$C nuclear spin, which is unresolvable in traditional protocols. We use dynamical decoupling pulse sequences to selectively enhance the entanglement between the nuclear spin and a nitrogen-vacancy center electron spin, tuning the weak measurement of the nuclear spin to a strong, projective one. A nuclear spin coupled to the NV center with strength 330 kHz is read out in 200 ms with fidelity 95.5\%. This work provides a general protocol for single-shot readout of weakly coupled qubits and therefore largely extends the range of physical systems for scalable quantum computing.
- Nov 21 2016 quant-ph physics.optics arXiv:1611.05997v2Squeezed-state interferometry plays an important role in quantum-enhanced optical phase estimation, as it allows the estimation precision to be improved up to the Heisenberg limit by using ideal photon-number-resolving detectors at the output ports. Here we show that for each individual $N$-photon component of the phase-matched coherent $\otimes$ squeezed vacuum input state, the classical Fisher information always saturates the quantum Fisher information. Moreover, the total Fisher information is the sum of the contributions from each individual $N$-photon components, where the largest $N$ is limited by the finite number resolution of available photon counters. Based on this observation, we provide an approximate analytical formula that quantifies the amount of lost information due to the finite photon number resolution, e.g., given the mean photon number $\bar{n}$ in the input state, over $96$ percent of the Heisenberg limit can be achieved with the number resolution larger than $5\bar{n}$.
- Nov 03 2016 quant-ph arXiv:1611.00572v1We studied the critical dynamics of spectral singularities. The system investigated is a coupled resonator array with a side-coupled loss (gain) resonator. For a gain resonator, the system acts as a wave emitter at spectral singularities. The reflection probability increased linearly over time. The rate of increase is proportional to the width of the incident wave packet, which served as the spectral singularity observer in the experiment. For a lossy resonator, the system acts as a wave absorber. The emission and absorption states at spectral singularities coalesce in a finite parity-time (PT ) symmetric system that combined by the gain and loss structures cut from corrresponding scattering systems at spectral singularities; in this case, the PT -symmetric system is at an exceptional point with a 2 * 2 Jordan block. The dynamics of the PT -symmetric system exhibit the characteristic of exceptional points and spectral singularities.
- Sep 23 2016 cond-mat.dis-nn quant-ph arXiv:1609.06939v1We study a one-dimensional quasiperiodic system described by the off-diagonal Aubry-André model and investigate its phase diagram by using the symmetry and the multifractal analysis. It was shown in a recent work (\it Phys. Rev. B \bf 93, 205441 (2016)) that its phase diagram was divided into three regions, dubbed the extended, the topologically-nontrivial localized and the topologically-trivial localized phases, respectively. Out of our expectation, we find an additional region of the extended phase which can be mapped into the original one by a symmetry transformation. More unexpectedly, in both "localized" phases, most of the eigenfunctions are neither localized nor extended. Instead, they display critical features, that is, the minimum of the singularity spectrum is in a range $0<\gamma_{min}<1$ instead of $0$ for the localized state or $1$ for the extended state. Thus, a mixed phase is found with a mixture of localized and critical eigenfunctions.
- Aug 18 2016 quant-ph cond-mat.mes-hall arXiv:1608.04890v1Anyons are exotic quasiparticles obeying fractional statistics,whose behavior can be emulated in artificially designed spin systems.Here we present an experimental emulation of creating anyonic excitations in a superconducting circuit that consists of four qubits, achieved by dynamically generating the ground and excited states of the toric code model, i.e., four-qubit Greenberger-Horne-Zeilinger states. The anyonic braiding is implemented via single-qubit rotations: a phase shift of \pi related to braiding, the hallmark of Abelian 1/2 anyons, has been observed through a Ramsey-type interference measurement.
- Nonequilibrium steady state (NESS) is a quasistationary state, in which exist currents that continuously produce entropy, but the local observables are stationary everywhere. We propose a theory of NESS under the framework of quantum chaos. In an isolated quantum system, there exist some initial states for which the thermodynamic limit and the long-time limit are noncommutative. The density matrix $\hat \rho$ of these states displays a universal structure. Suppose that $\alpha$ and $\beta$ are different eigenstates of the Hamiltonian with energies $E_\alpha$ and $E_\beta$, respectively. $<\alpha|\hat \rho|\beta>$ behaves as a random number which approximately follows the Laplace distribution with zero mean. In thermodynamic limit, the variance of $<\alpha|\hat \rho|\beta>$ is a smooth function of $\left|E_\alpha-E_\beta\right|$, scaling as $1/(E_\alpha-E_\beta)^2$ in the limit $\left|E_\alpha-E_\beta\right|\to 0$. If and only if this scaling law is obeyed, the initial state evolves into NESS in the long time limit. We present numerical evidence of our hypothesis in a few chaotic models. Furthermore, we find that our hypothesis implies the eigenstate thermalization hypothesis (ETH) in a bipartite system.
- Apr 19 2016 quant-ph cond-mat.mes-hall arXiv:1604.04757v1Topological numbers can characterize the transition between different topological phases, which are not described by Landau's paradigm of symmetry breaking. Since the discovery of quantum Hall effect, more topological phases have been theoretically predicted and experimentally verified. However, it is still an experimental challenge to directly measure the topological number of various predicted topological phases. In this paper, we demonstrate quantum simulation of topological phase transition of a quantum wire (QW) using a single nitrogen-vacancy (NV) center in diamond. Deploying quantum algorithm of finding eigenvalues, we can reliably extract both the dispersion relations and topological numbers.
- The fairness of a secure multi-party quantum key agreement (MQKA) protocol requires that all involved parties are entirely peer entities and can equally influence the outcome of the protocol to establish a shared key wherein no one can decide the shared key alone. However, it is found that parts of the existing MQKA protocols are sensitive to collusion attacks, i.e., some of the dishonest participants can collaborate to predetermine the final key without being detected. In this paper, a multi-party QKA protocol resisting collusion attacks is proposed. Different from previous QKA protocol resisting $N-1$ coconspirators or resisting $1$ coconspirators, we investigate the general circle-type MQKA protocol which can be secure against $t$ dishonest participants' cooperation. Here, $t < N$. We hope the results of the presented paper will be helpful for further research on fair MQKA protocols.
- Feb 09 2016 quant-ph arXiv:1602.02455v1Quantum mechanics provides a statistical description about nature, and thus would be incomplete if its statistical predictions could not be accounted for some realistic models with hidden variables. There are, however, two powerful theorems against the hidden-variable theories showing that certain quantum features cannot be reproduced based on two rationale premises of classicality, the Bell theorem, and noncontextuality, due to Bell, Kochen and Specker (BKS) . Tests of the Bell inequality and the BKS theorem are both of fundamental interests and of great significance . The Bell theorem has already been experimentally verified extensively on many different systems , while the quantum contextuality, which is independent of nonlocality and manifests itself even in a single object, is experimentally more demanding. Moreover, the contextuality has been shown to play a critical role to supply the `magic' for quantum computation, making more extensive experimental verifications in potential systems for quantum computing even more stringent. Here we report an experimental verification of quantum contextuality on an individual atomic nuclear spin-1 system in solids under ambient condition. Such a three-level system is indivisible and thus the compatibility loophole, which exists in the experiments performed on bipartite systems, is closed. Our experimental results confirm that the quantum contextuality cannot be explained by nonlocal entanglement, revealing the fundamental quantumness other than locality/nonlocality within the intrinsic spin freedom of a concrete natural atomic solid-state system at room temperature.
- Dec 15 2015 cond-mat.stat-mech quant-ph arXiv:1512.04350v1We propose a new quantity called modulus fidelity to measure the closeness of two quantum pure states. Especially, we use it to investigate the closeness of eigenstates of quantum many-body systems. When the system is integrable, the modulus fidelity of neighbor eigenstates displays a large fluctuation. But the modulus fidelity is close to a constant when system becomes non-integrable with fluctuation reduced drastically. Average modulus fidelity of neighbor eigenstates increases with the increase of parameters that destroy the integrability, which also indicates the integrable-chaos transition. In non-integrable case, it is found two eigenstates are closer to each other if their level spacing is small. We also show that the closeness of eigenstates in non-integrable domain is the underlying mechanism of \empheigenstate thermalization hypothesis (ETH) which explains the thermalization in nonintegrable system we studied.
- Nov 17 2015 cond-mat.mes-hall quant-ph arXiv:1511.04939v2Building a quantum repeater network for long distance quantum communication requires photons and quantum registers that comprise qubits for interaction with light, good memory capabilities and processing qubits for storage and manipulation of photons. Here we demonstrate a key step, the coherent transfer of a photon in a single solid-state nuclear spin qubit with an average fidelity of 98% and storage over 10 seconds. The storage process is achieved by coherently transferring a photon to an entangled electron-nuclear spin state of a nitrogen vacancy centre in diamond, confirmed by heralding through high fidelity single-shot readout of the electronic spin states. Stored photon states are robust against repetitive optical writing operations, required for repeater nodes. The photon-electron spin interface and the nuclear spin memory demonstrated here constitutes a major step towards practical quantum networks, and surprisingly also paves the way towards a novel entangled photon source for photonic quantum computing.
- Sep 03 2015 quant-ph arXiv:1509.00484v4One-way quantum computing is experimentally appealing because it requires only local measurements on an entangled resource called a cluster state. Record-size, but non-universal, continuous-variable cluster states were recently demonstrated separately in the time and frequency domains. We propose to combine these approaches into a scalable architecture in which a single optical parametric oscillator and simple interferometer entangle up to ($3\times 10^3$ frequencies) $\times$ (unlimited number of temporal modes) into a new and computationally universal continuous-variable cluster state. We introduce a generalized measurement protocol to enable improved computational performance on this new entanglement resource.
- Aug 03 2015 quant-ph arXiv:1507.08775v1Enhancing the polarization of nuclear spins surrounding the nitrogen-vacancy (NV) center in diamond has attracted widespread attention recently due to its various applications. Here we present an analytical theory and comprehensive understanding on how to optimize the dynamic nuclear polarization by an optically pumped NV center near the ground state level anticrossing. Our results not only provide a parameter-free explanation and a clearly physics picture for the recently observed polarization dependence on the magnetic field for strongly coupled $^{13}$C nuclei [H. J. Wang \textitet al., Nat. Commun. 4, 1 (2013)], but also demonstrate the possibility to strongly polarize weakly coupled $^{13}$C nuclei under weak optical pumping and suitably chosen magnetic field. This allows sensitive magnetic control of the $^{13}$C nuclear spin polarization for NMR applications and significant suppression of the $^{13}$C nuclear spin noise to prolong the NV spin coherence time.
- Jul 24 2015 quant-ph arXiv:1507.06385v1Dephasing and relaxation of the nuclear spins coupled to the nitrogen-vacancy (NV) center during optical initialization and readout is an important issue for various applications of this hybrid quantum register. Here we present both an analytical description and a numerical simulation for this process, which agree reasonably with the experimental measurements. For the NV center under cyclic optical transition, our analytical formula not only provide a clear physics picture, but also allows controlling the nuclear spin dissipation by tuning an external magnetic field. For more general optical pumping, our analytical formula reveals significant contribution to the nuclear spin dissipation due to electron random hopping into/out of the $m=0$ (or $m=\pm1$) subspace. This contribution is not suppressed even under saturated optical pumping and/or vanishing magnetic field, thus providing a possible solution to the puzzling observation of nuclear spin dephasing in zero perpendicular magnetic field [M. V. G. Dutt \textitet al., Science \textbf316, 1312 (2007)]. It also implies that enhancing the degree of spin polarization of the nitrogen-vacancy center can reduce the effect of optical induced nuclear spin dissipation.
- Jul 17 2015 quant-ph arXiv:1507.04535v3The precise calculations of the Wigner's d-matrix are important in various research fields. Due to the presence of large numbers, direct calculations of the matrix using the Wigner's formula suffer from loss of precision. We present a simple method to avoid this problem by expanding the d-matrix into a complex Fourier series and calculate the Fourier coefficients by exactly diagonalizing the angular-momentum operator $J_{y}$ in the eigenbasis of $J_{z}$. This method allows us to compute the d-matrix and its various derivatives for spins up to a few thousand. The precision of the d-matrix from our method is about $10^{-14}$ for spins up to $100$.
- Jun 17 2015 quant-ph arXiv:1506.04826v1We experimentally resolve several weakly coupled nuclear spins in diamond using a series of novelly designed dynamical decoupling controls. Some nuclear spin signals, hidden by decoherence under ordinary dynamical decoupling controls, are shifted forward in time domain to the coherence time range and thus rescued from the fate of being submerged by the noisy spin bath. In this way, more and remoter single nuclear spins are resolved. Additionally, the field of detection can be continuously tuned on sub-nanoscale. This method extends the capacity of nanoscale magnetometry and may be applicable in other systems for high-resolution noise spectroscopy.
- Mar 03 2015 quant-ph arXiv:1503.00243v1We present a general theory for using an optically pumped diamond nitrogen-vacancy center as a tunable, non-equilibrium bath to control a variety of nuclear spin dynamics (such as dephasing, relaxation, squeezing, polarization, etc.) and the nuclear spin noise. It opens a new avenue towards engineering the dissipative and collective nuclear spin evolution and solves an open problem brought up by the $^{13}$C nuclear spin noise suppression experiment [E. Togan \textitet al., Nature 478, 497 (2011)].
- Sep 05 2014 quant-ph cond-mat.mes-hall arXiv:1409.1482v1We develop a quantum theory for a variety of nuclear spin dynamics such as dephasing, relaxation, squeezing, and narrowing due to the hyperfine interaction with a generic, dissipative electronic system. The first-order result of our theory reproduces and generalizes the nonlinear Hamiltonian for nuclear spin squeezing [M. S. Rudner, Phys. Rev. Lett. 107, 206806 (2011)]. The second-order result of our theory provides a good explanation to the experimentally observed $^{13}$C nuclear spin bath narrowing in diamond nitrogen-vacancy center [E. Togan, Nature 478, 497 (2011)
- Apr 16 2014 quant-ph cond-mat.mes-hall arXiv:1404.3870v3Compared with the quantum trajectory equation, the quantum Bayesian approach has the advantage of being more efficient to infer quantum state under monitoring, based on the integrated output of measurement. For weak measurement of qubits in circuit quantum electrodynamics(cQED), properly accounting for the measurement backaction effects within the Bayesian framework is an important problem of current interest.Elegant work towards this task was carried out by Korotkov in "bad-cavity" and weak-response limits (arXiv:1111.4016).In the present work, based on insights from the cavity-field states (dynamics) and the help of an effective QTE, we generalize the results of arXiv:1111.4016 to more general system parameters.The obtained Bayesian rule is in full agreement with Korotkov's result in limiting cases and as well holds satisfactory accuracy in non-limiting cases in comparison with the QTE simulations. We expect the proposed Bayesian rule to be useful for future cQED measurement and control experiments.
- Mar 27 2014 physics.optics quant-ph arXiv:1403.6631v2One of the most scalable methods for continuous-variable quantum computing is to encode quantum information in the quantum optical frequency comb generated by an optical parametric oscillator (OPO). The scalability is limited by the quasiphasematching bandwidth and dispersion of the OPO. We study both factors in this article. The results show that 3200 qumodes are probably entangled in our recent demonstration of a record 60-qumode entanglement.
- Sep 26 2013 cond-mat.mes-hall quant-ph arXiv:1309.6548v1Single-molecule nuclear magnetic resonance (NMR) is a crown-jewel challenge in the field of magnetic resonance spectroscopy and has important applications in chemical analysis and in quantum computing. Recently, it becomes possible to tackle this grand challenge thanks to experimental advances in preserving quantum coherence of nitrogen-vacancy (NV) center spins in diamond as a sensitive probe and theoretical proposals on atomic-scale magnetometry via dynamical decoupling control. Through decoherence measurement of NV centers under dynamical decoupling control, sensing of single $^{13}\textbf{C}$ at nanometer distance has been realized. Toward the ultimate goal of structure analysis of single molecules, it is highly desirable to directly measure the interactions within single nuclear spin clusters. Here we sensed a single $^{13}\textbf{C}$-$^{13}\textbf{C}$ nuclear spin dimer located about 1 nm from the NV center and characterized the interaction between the two nuclear spins, by measuring NV center spin decoherence under various dynamical decoupling control. From the measured interaction we derived the spatial configuration of the dimer with atomic-scale resolution. These results demonstrate that central spin decoherence under dynamical decoupling control is a feasible probe for NMR structure analysis of single molecules.
- Sep 17 2013 quant-ph arXiv:1309.4105v2Cluster states with higher-dimensional lattices that cannot be physically embedded in three-dimensional space have important theoretical interest in quantum computation and quantum simulation of topologically ordered condensed-matter systems. We present a simple, scalable, top-down method of entangling the quantum optical frequency comb into hypercubic-lattice continuous-variable cluster states of a size of about 10^4 quantum field modes, using existing technology. A hypercubic lattice of dimension D (linear, square, cubic, hypercubic, etc.) requires but D optical parametric oscillators with bichromatic pumps whose frequency splittings alone determine the lattice dimensionality and the number of copies of the state.
- May 14 2013 quant-ph cond-mat.mes-hall arXiv:1305.2457v1Universal sensing the motion of mechanical resonators with high precision and low back-action is of paramount importance in ultra-weak signal detection which plays a fundamental role in modern physics. Here we present a universal scheme that transfer mechanically the motion of the resonator not directly measurable to the one can be precisely measured using mechanical frequency conversion. Demonstration of the scheme at room temperature shows that both the motion imprecision and the back-action force are below the intrinsic level of the objective resonator, which agree well with our theoretical prediction. The scheme developed here provides an effective interface between an arbitrary mechanical resonator and a high quantum efficient displacement sensor, and is expected to find extensive applications in high-demanding mechanical-based force measurements.
- Oct 19 2012 cond-mat.mes-hall quant-ph arXiv:1210.5050v2We consider the correlated parallel transport through two quantum dots which are tunnel-coupled to the ends of a semiconductor nanowire where the Majorana bound states (MBSs) may emerge under proper conditions. In terms of the cross-correlation of currents, we reveal unusual behaviors originated from the nonlocal MBSs, including such as the distinct symmetry and antisymmetry of the spectral density in response to the dot-level modulations, and the vanished cross correlation occurred when any of the dot-levels is in resonance with the Majorana zero mode.
- Oct 04 2012 quant-ph arXiv:1210.0961v1Quantum mechanics provides a statistical description about nature, and thus would be incomplete if its statistical predictions could not be accounted for by some realistic models with hidden variables. There are, however, two powerful theorems against the hidden-variable theories showing that certain quantum features cannot be reproduced based on two rationale premises of locality, Bell's theorem, and noncontextuality, due to Bell, Kochen and Specker (BKS). Noncontextuality is independent of nonlocality, and the contextuality manifests itself even in a single object. Here we report an experimental verification of quantum contextuality by a single spin-1 electron system at room temperature. Such a three-level system is indivisible and then we close the compatibility loophole which exists in the experiments performed on bipartite systems. Our results confirm the quantum contextuality to be the intrinsic property of single particles.
- We present a numerical method for studying the real time dynamics of a small interacting quantum system coupled to an infinite fermionic reservoir. By building an orthonormal basis in the operator space, we turn the Heisenberg equation of motion into a system of linear differential equations, which is then solved iteratively by constructing excitation operators. The application of our method depends on a layer structure in the operator space, which help us to turn an infinite linear system into a series of small systems. We apply the method to investigate the decoherence dynamics of quantum impurity models in the Kondo regime with a non-Markovian reservoir. Taking full account of environmental back-actions and electron-electron interactions, we find that the coexistence of the Kondo correlation and a non-Markovian reservoir induces coherence ringings, which will be suppressed by either driving the system away from the particle-hole symmetric point or changing the reservoir into a Markovian one.
- Aug 21 2012 cond-mat.mes-hall quant-ph arXiv:1208.3738v2It was predicted by Tewari [Phys. Rev. Lett. \bf 100, 027001 (2008)] that a teleportationlike electron transfer phenomenon is one of the novel consequences of the existence of Majorana fermion, because of the inherently nonlocal nature. In this work we consider a concrete realization and measurement scheme for this interesting behavior, based on a setup consisting of a pair of quantum dots which are tunnel-coupled to a semiconductor nanowire and are jointly measured by two point-contact detectors. We analyze the teleportation dynamics in the presence of measurement backaction and discuss how the teleportation events can be identified from the current trajectories of strong response detectors.
- May 08 2012 quant-ph arXiv:1205.1307v1To implement reliable quantum information processing, quantum gates have to be protected together with the qubits from decoherence. Here we demonstrate experimentally on nitrogen-vacancy system that by using continuous wave dynamical decoupling method, not only the coherence time is prolonged by about 20 times, but also the quantum gates is protected for the duration of controlling time. This protocol shares the merits of retaining the superiority of prolonging the coherence time and at the same time easily combining with quantum logic tasks. It is expected to be useful in task where duration of quantum controlling exceeds far beyond the dephasing time.
- May 03 2012 cond-mat.mes-hall quant-ph arXiv:1205.0322v2We consider an experimentally feasible setup to demonstrate the existence and coherent dynamics of Majorana fermion. The transport setup consists of a quantum dot and a tunnel-coupled semiconductor nanowire which is anticipated to generate Majorana excitations under some conditions. For transport under finite bias voltage, we find that a subtraction of the source and drain currents can expose the essential feature of the Majorana fermion, including the zero-energy nature by gate-voltage modulating the dot level. Moreover, coherent oscillating dynamics of the Majorana fermion between the nanowire and the quantum dot is reflected in the shot noise via a spectral dip together with a pronounced zero-frequency noise enhancement effect. Important parameters, e.g. for the Majorana's mutual interaction and its coupling to the quantum dot, can be extracted out in experiment using the derived analytic results.
- Apr 11 2012 quant-ph physics.optics arXiv:1204.2107v1In this Letter, a linear scheme to generate polarization entanglement at 1.5 um based on commercial polarization maintained dispersion shifted fiber (PM-DSF) is proposed. The birefringent walk-off effect of the pulsed pump light in the PM-DSF provides an effective way to suppress the vector scattering processes of spontaneous four wave mixing. A 90 degree offset of fiber polarization axes is introduced at the midpoint of the fiber to realize the quantum superposition of the two correlated photon states generated by the two scalar processes on different fiber polarization axes, leading to polarization entanglement generation. Experiments of the indistinguishable property on single side and two-photon interference in two non-orthogonal polarization bases are demonstrated. A two photon interference fringe visibility of 89\pm3% is achieved without subtracting the background counts, demonstrating its great potential in developing highly efficient and stable fiber based polarization-entangled quantum light source at optical communication band.
- May 25 2011 cond-mat.mes-hall quant-ph arXiv:1105.4793v1Decoherence of quantum objects is critical to modern quantum sciences and technologies. It is generally believed that stronger noises cause faster decoherence. Strikingly, recent theoretical research discovers the opposite case for spins in quantum baths. Here we report experimental observation of the anomalous decoherence effect for the electron spin-1 of a nitrogen-vacancy centre in high-purity diamond at room temperature. We demonstrate that under dynamical decoupling, the double-transition can have longer coherence time than the single-transition, even though the former couples to the nuclear spin bath as twice strongly as the latter does. The excellent agreement between the experimental and the theoretical results confirms the controllability of the weakly coupled nuclear spins in the bath, which is useful in quantum information processing and quantum metrology.
- Generating and stabilizing the GHZ state in circuit QED: Joint measurement, Zeno effect and feedbackJan 25 2011 quant-ph arXiv:1101.4327v2In solid-state circuit QED system, we extend the previous study of generating and stabilizing two-qubit Bell state [Phys. Rev. A 82, 032335 (2010)], to three-qubit GHZ state. In dispersive regime, we employ the homodyne joint readout for multiple qubits to infer the state for further processing, and in particular use it to stabilize the state directly by means of an alternate-flip-interrupted Zeno (AFIZ) scheme. Moreover, the state-of-the-art feedback action based on the filtered current enables not only a deterministic generation of the pre-GHZ state in the initial stage, but also a fast recovery from the environment-caused degradation in the later stabilization process. We show that the proposed scheme can maintain the state with high fidelity if the efficient quantum measurement and rapid single-qubit rotations are available.
- Dec 27 2010 cond-mat.mes-hall quant-ph arXiv:1012.5395v2We study the evolution of reduced density matrix of an impurity coupled to a Fermi sea after the coupling is switched on at time $t=0$. We find the non-diagonal elements of the reduced density matrix decay exponentially, and the decay constant is the impurity level width $\Gamma$. And we study the information transfer rate between the impurity and the Fermi sea, which also decays exponentially. And the decay constant is $k\Gamma$ with $k=2\sim 4$. Our results reveal the relation between information transfer rate and decoherence rate.
- Jun 17 2010 cond-mat.quant-gas quant-ph arXiv:1006.3250v2We demonstrate clear collective atomic recoil motion in a dilute, momentum-squeezed, ultra-cold degenerate fermion gas by circumventing the effects of Pauli blocking. Although gain from bosonic stimulation is necessarily absent because the quantum gas obeys Fermi-Dirac statistics, collective atomic recoil motion from the underlying wave-mixing process is clearly visible. With a single pump pulse of the proper polarization, we observe two mutually-perpendicular wave-mixing processes occurring simultaneously. Our experiments also indicate that the red-blue pump detuning asymmetry observed with Bose-Einstein condensates does not occur with fermions.
- We study the decay of survival probability at quantum phase transitions (QPT). The semiclassical theory is found applicable in the vicinities of critical points with infinite degeneracy. The theory predicts a power law decay of the survival probability for relatively long times in systems with d=1 and an exponential decay in systems with sufficiently large d, where d is the degrees of freedom of the underlying classical dynamics. The semiclassical predictions are checked numerically in four models.
- Jul 22 2009 quant-ph arXiv:0907.3642v2It is exponentially hard to simulate quantum systems by classical algorithms, while quantum computer could in principle solve this problem polynomially. We demonstrate such an quantum-simulation algorithm on our NMR system to simulate an hydrogen molecule and calculate its ground-state energy. We utilize the NMR interferometry method to measure the phase shift and iterate the process to get a high precision. Finally we get 17 precise bits of the energy value, and we also analyze the source of the error in the simulation.
- Jun 08 2009 quant-ph arXiv:0906.1129v1Multi-normal-mode splitting peaks are experimentally observed in a system with Doppler-broadened two-level atoms inside a relatively long optical cavity. In this system, the atoms-cavity interaction can reach the ``superstrong coupling" condition with atoms-cavity coupling strength $g\sqrt{N}$ to be near or larger than the cavity free-spectral range $\Delta_{FSR}$. In such case, normal-mode splitting can occur in many cavity longitudinal modes to generate the multi-normal-mode splitting peaks, which can be well explained by the linear dispersion enhancement due to the largely increased atomic density in the cavity. Many new interesting phenomena might come out of this superstrong atoms-cavity coupling regime.
- Jan 10 2008 quant-ph arXiv:0801.1400v1The standard Landau-Ginzburg scenario of phase transition is broken down for quantum phase transition. It is difficult to find an order parameter to indicate different phases for quantum fluctuations. Here, we suggest a topological description of the quantum phase transition for the XY model. The ground states are identified as a specialized U(1) principal bundle on the base manifold $S^2$. And then different first Chern numbers of U(1) principal bundle on the base manifold $S^2$ are associated to each phase of quantum fluctuations. The particle-hole picture is used to parameterized the ground states of the XY system. We show that a singularity of the Chern number of the ground states occurs simultaneously with a quantum phase transition. The Chern number is a suitable topological order of the quantum phase transition.
- Jul 27 1998 quant-ph arXiv:quant-ph/9807071v1We briefly review the development and theory of an experiment to investigate quantum computation with trapped calcium ions. The ion trap, laser and ion requirements are determined, and the parameters required for simple quantum logic operations are described
- Aug 29 1997 quant-ph arXiv:quant-ph/9708050v1The development and theory of an experiment to investigate quantum computation with trapped calcium ions is described. The ion trap, laser and ion requirements are determined, and the parameters required for quantum logic operations as well as simple quantum factoring are described.