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arXiv:1110.4806 · Physical Review A

Environment-assisted error correction of single-qubit phase damping

Benjamin Trendelkamp-Schroer, Julius Helm, Walter T. Strunz

Published Dec 19, 2011• 14 citations
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Abstract

Open quantum system dynamics of random unitary type may in principle be fully undone. Closely following the scheme of environment-assisted error correction proposed by Gregoratti and Werner [J. Mod. Opt. 50, 915 (2003)], we explicitly carry out all steps needed to invert a phase-damping error on a single qubit. Furthermore, we extend the scheme to a mixed-state environment. Surprisingly, we find cases for which the uncorrected state is closer to the desired state than any of the corrected ones.

DOI
10.1103/physreva.84.062314

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Open quantum system dynamics of random unitary type may in principle be fully undone. Closely following the scheme of environment-assisted error correction proposed by Gregoratti and Werner [J. Mod. Opt. 50, 915 (2003)], we explicitly carry out all steps needed to invert a phase-damping error on a single qubit. Furthermore, we extend the scheme to a mixed-state environment. Surprisingly, we find cases for which the uncorrected state is closer to the desired state than any of the corrected ones.
In this talk, I will present an explicit scheme to fully recover a multiple-qubit state subject to a phase damping noise. We establish the theoretical framework and the operational procedure to restore an unknown initial quantum state for an N-qubit model interacting with either individual baths or a common bath. We give an explicit construction of the random unitary (RU) Kraus decomposition for an N-qubit model interacting with a common bath. We also demonstrate how to use only one unitary reversal operation to restore an arbitrary state with phase damping noise. In principle, the initial state can always be recovered with a success probability of 1. Interestingly, we found that non-RU decomposition can also be used to restore some particular entangled states. This may open a new path to restore a quantum state beyond the standard RU scheme.
In this paper, we propose an environment-assisted error correction scheme to fully recover a multiple-qubit state subject to a phase damping noise. We establish the theoretical framework and the operational procedure to restore an unknown initial quantum state for an NNN-qubit model interacting with either individual baths or a common bath. We give an explicit construction of the random unitary (RU) Kraus decomposition for an NNN-qubit model interacting with a common bath. We also demonstrate how to use only one unitary reversal operation to restore an arbitrary state with phase damping noise. In principle, the initial state can always be recovered with a success probability of 1. Interestingly, we found that non-RU decomposition can also be used to restore some particular entangled states. This may open a new path to restore a quantum entangled state beyond the standard RU scheme.
Several instructions need to be considered in order to transfer classical error correction techniques to the quantum regime. The quantum error correction field has been developed to face these issues. One of the common source of errors in quantum systems is environment decoherence. Due to interaction with the environment, the quantum states of a system entangle with the environment and are subject to decoherence. In this thesis, we mainly focused on the amplitude damping which is one of the most important models of decoherence processes. We showed that general two-qubit mixed states undergoing an amplitude damping, can be almost completely restored using a reversal procedure. This reversal procedure through CNOT and Hadamard gates could also protect the entanglement of two-qubit mixed states from general amplitude damping. Concurrence and fidelity are two measurements used in order to examine how our proposed scheme performs. Furthermore, to give generality to our scheme, we proposed a robust recovery scheme to protect the quantum states when the decay parameters or the input quantum states are not completely known.
The loss of the coherence happens due to the interaction between the desired system and its surroundings.Addressing decoherence is one of the main concepts in the study of quantum channels to access their potential for various scale.In this process the information of the system is penetrated into the environment.Therefore, the measurement of the environment could be used as the error correction method.Phase damping channels principally belong to random unitary channels and can be corrected by classical information.This paper presents a method to generate non-random unitary phase damping channels based on the Bloch vectors in two qubits systems.A phase damping channel which consisted of a two qubits system and a single qubit environment was investigated.The results demonstrated that the phase damping channels belong to random unitary dynamics if the three-dimensional tetrahedron volume spanned by the Bloch vectors in R 3 is not zero, or, the same hyperplane in R 3 was not pointed by the Bloch vectors.It is found that the Bell state belongs to random unitary class and can be corrected based on classical information obtained from environmental measurements.
It is commonly stated that decoherence in open quantum systems is due to growing entanglement with an environment. In practice, however, surprisingly often decoherence may equally well be described by random unitary dynamics without invoking a quantum environment at all. For a single qubit, for instance, pure decoherence (or phase damping) is always of random unitary type. Here, we construct a simple example of true quantum decoherence of two qubits: we present a feasible phase damping channel of which we show that it cannot be understood in terms of random unitary dynamics. We give a very intuitive geometrical measure for the positive distance of our channel to the convex set of random unitary channels and find remarkable agreement with the so-called Birkhoff defect based on the norm of complete boundedness.
It has recently been shown that there are efficient algorithms for quantum computers to solve certain problems, such as prime factorization, which are intractable to date on classical computers. The chances for practical implementation, however, are limited by decoherence, in which the effect of an external environment causes random errors in the quantum calculation. To combat this problem, quantum error correction schemes have been proposed, in which a single quantum bit (qubit) is ``encoded'' as a state of some larger number of qubits, chosen to resist particular types of errors. Most such schemes are vulnerable, however, to errors in the encoding and decoding itself. We examine two such schemes, in which a single qubit is encoded in a state of nnn qubits while subject to dephasing or to arbitrary isotropic noise. Using both analytical and numerical calculations, we argue that error correction remains beneficial in the presence of weak noise, and that there is an optimal time between error correction steps, determined by the strength of the interaction with the environment and the parameters set by the encoding.