Username: preskill

Name: John Preskill
University: Caltech
Occupation: Professor
Discipline: quantum information
Website: http://www.theory.caltech.edu/~preskill

Papers SciTed

Scites: 14
0911.0581[abs pdf who comments(0)]
Title: Fast Decoders for Topological Quantum Codes
Authors: Guillaume Duclos-Cianci, David Poulin
Scites: 19
0905.2292[abs pdf who comments(0)]
Title: A new physical principle: Information Causality
Authors: M. Pawlowski, T. Paterek, D. Kaszlikowski, V. Scarani, A. Winter, M. Zukowski
Scites: 13
0812.2385[abs pdf who comments(0)]
Title: Quantum mechanical evolution towards thermal equilibrium
Authors: Noah Linden, Sandu Popescu, Anthony J. Short, Andreas Winter
Scites: 20
0807.4935[abs pdf who comments(3)]
Title: Quantum Communication With Zero-Capacity Channels
Authors: Graeme Smith, Jon Yard
Scites: 13
0802.1919[abs pdf who comments(0)]
Title: Random Quantum Circuits are Approximate 2-designs
Authors: Aram Harrow, Richard Low
Scites: 14
0711.4114[abs pdf who comments(0)]
Title: Quantum weak coin flipping with arbitrarily small bias
Authors: Carlos Mochon
Scites: 5
0710.1624[abs pdf who comments(0)]
Title: Hamiltonian Formulation of Quantum Error Correction and Correlated Noise
Authors: E. Novais, Eduardo R. Mucciolo, Harold U. Baranger
Scites: 10
0709.3603[abs pdf who comments(0)]
Title: Threshold lower bounds for Knill's Fibonacci scheme
Authors: Panos Aliferis
Scites: 6
0705.4067[abs pdf who comments(0)]
Title: The Complexity of Quantum Systems on a One-dimensional Chain
Authors: Sandy Irani
Scites: 9
0708.1337[abs pdf who comments(0)]
Title: Quantum Graphical Models and Belief Propagation
Authors: Matthew Leifer, David Poulin
Scites: 13
0708.0827[abs pdf who comments(0)]
Title: Simulating Quantum Correlations with Finite Communication
Authors: Oded Regev, Ben Toner
Scites: 15
0709.2090[abs pdf who comments(0)]
Title: On the Complexity of Computing Zero-Error and Holevo Capacity of Quantum Channels
Authors: Salman Beigi, Peter W. Shor
Scites: 12
0705.4077[abs pdf who comments(0)]
Title: The power of quantum systems on a line
Authors: Dorit Aharonov, Daniel Gottesman, Sandy Irani, Julia Kempe
Scites: 5
0705.0292[abs pdf who comments(0)]
Title: Entropy scaling and simulability by Matrix Product States
Authors: Norbert Schuch, Michael M. Wolf, Frank Verstraete, J. Ignacio Cirac
Scites: 5
0704.3661[abs pdf who comments(0)]
Title: Complementarity, distillable secret key, and distillable entanglement
Authors: Masato Koashi
Scites: 3
0704.3719[abs pdf who comments(0)]
Title: A holographic proof of the strong subadditivity of entanglement entropy
Authors: Matthew Headrick, Tadashi Takayanagi

Comments

0812.4622 preskill [2009-01-06 11:31:18]
It seems to me that there may be another reason why (logarithmic) confinement of defects due to Coulombic interactions should not be regarded as "topological order at nonzero temperature", irrespective of how if it realized by an underlying microscopic Hamiltonian.

Consider the toric code on a plane with two large punctures. We can encode a qubit in the sector with zero total electric charge and zero total magnetic charge: the encoded |0> is the state with nontrivial Z2 electric charge and trivial Z2 magnetic charge in each hole, and the encoded |+> is the state with nonzero Z2 magnetic charge and trivial Z2 electric charge in each hole. The qubit is protected because the energy splitting becomes exponentially small as the radius r of each hole gets large and the distance R between the holes also gets large.

If we now introduce a Coulombic attractive interaction between defects, there is a contribution to the qubit's energy that scales like log(R/r) and depends on the charge. This might be acceptable if the energy splitting were very stable, but in fact the energy depends on the details of the geometry of the hole's boundary. Interactions with the bath that excite fluctuations of the boundary can drive decoherence of the qubit; it is not topologically protected.

This may be a fundamental problem. On the one hand, we want infinite-range interactions between defects to prevent logical errors due to long-distance diffusion of the defects. On the other hand, infinite-range interactions induce nontopological contributions to the qubit's energy, interfering with topological protection.

Stronger confinement of the defects would not seem to improve the situation. In either the gapped Higgs phase (with linearly confined magnetic charges and unconfined electric defects) or the gapped confinement phase (linearly confined electric charges and unconfined magnetic defects) there is no surviving topological order and hence no topological quantum memory.

And yet ... as Barbara says we might regard the scheme with active error correction (in which the defects are periodically identified and paired to prevent the formation of long error chains that cause logical errors) as an existence proof for 2D topological memory that can be stabilized at nonzero temperature. This scheme is realizable in principle and we have nice ways to model it. Yes .. but it is much different than stabilizing the topological memory with Coulombic pairwise interactions between defects. Isn't it?

Statistics

Papers SciTed: 16
Average Scites for those papers: 11.00
Number of comments: 1

History

[2009-11-04 09:07:16] preskill voted for 0911.0581
[2009-05-15 06:59:54] preskill voted for 0905.2292
[2009-01-06 11:31:18] preskill commented on 0812.4622
[2008-12-19 17:54:19] preskill voted for 0812.2385
[2008-08-01 13:48:20] preskill voted for 0807.4935
[2008-02-18 11:52:39] preskill voted for 0802.1919
[2007-11-28 10:44:50] preskill voted for 0711.4114
[2007-10-15 17:10:18] preskill voted for 0710.1624
[2007-09-25 06:50:30] preskill voted for 0709.3603
[2007-09-15 07:56:24] preskill voted for 0705.4067
[2007-09-15 07:49:07] preskill voted for 0708.1337
[2007-09-15 07:48:16] preskill voted for 0708.0827
[2007-09-15 07:42:37] preskill voted for 0709.2090
[Prior to 7/11] preskill voted for 0704.3719
[Prior to 7/11] preskill voted for 0704.3661
[Prior to 7/11] preskill voted for 0705.0292
[Prior to 7/11] preskill voted for 0705.4077