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arXiv:0904.0956 · Physical Review A

Entanglement sudden death in a quantum memory

Yaakov S. Weinstein

Published Aug 6, 2009• 9 citations
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Abstract

I explore the entanglement dynamics of multiqubit systems suitable for use as quantum memories: decoherence free subspaces and noiseless subsystems. The aim of this exploration is to determine how a complete loss of entanglement affects the ability of these error-avoidance techniques to protect quantum information. I utilize negativity and concurrence as entanglement measures and find no correlation between complete system loss of entanglement and the fidelity of the stored quantum information. These results complement previous results in which quantum protocols not explicitly based on entanglement exhibit little correlation between entanglement sudden death and the accuracy of the given protocol.

DOI
10.1103/physreva.80.022310

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I explore the entanglement dynamics of multiqubit systems suitable for use as quantum memories: decoherence free subspaces and noiseless subsystems. The aim of this exploration is to determine how a complete loss of entanglement affects the ability of these error-avoidance techniques to protect quantum information. I utilize negativity and concurrence as entanglement measures and find no correlation between complete system loss of entanglement and the fidelity of the stored quantum information. These results complement previous results in which quantum protocols not explicitly based on entanglement exhibit little correlation between entanglement sudden death and the accuracy of the given protocol.
When two entangled qubits, each owned by Alice and Bob, undergo separate decoherence, the amount of entanglement is reduced, and often, weak decoherence causes complete loss of entanglement, known as entanglement sudden death. Here we show that it is possible to apply quantum measurement reversal on a single-qubit to avoid entanglement sudden death, rather than on both qubits. Our scheme has important applications in quantum information processing protocols based on distributed or stored entangled qubits as they are subject to decoherence.
I explore entanglement dynamics in a three qubit system comparing the ability of entanglement witnesses to detect tripartite entanglement to the phenomenon of entanglement sudden death (ESD). Using a system subject to dephasing I invoke entanglement witnesses to detect tripartite Greenberger-Horne-Zeilinger (GHZ)-type and WWW-type entanglement and compare the evolution of their detection capabilities with the evolution of the negativity, bipartite concurrence, and tripartite negativity. Interestingly, I find a state in which there is no concurrence or tripartite negativity but there is entanglement. Finally, I utilize a three qubit quantum error correction code to address how ESD affects the abilities of quantum error correction.
We investigate qubit entanglement dynamics in a two-qubit cavity system with stochastically fluctuating coupling strength, comparing white noise (Markovian) and Ornstein-Uhlenbeck noise (non-Markovian) processes. We analytically solve the system dynamics for both Fock and coherent cavity states and systematically characterize how initial qubit configurations affect entanglement generation and preservation. Our results reveal distinct dynamical regimes: in the Fock-state cavity, initial states in the single-excitation subspace generate robust qubit entanglement in decoherence-free subspaces, while initially entangled states in the double-excitation subspace exhibit inevitable decay without protective symmetries. For coherent cavities, we identify a critical photon-number threshold in the intermediate coherent-state amplitude regime where dynamics transition from low-frequency to high-frequency entanglement oscillations, while universal entanglement loss occurs at large amplitudes. Remarkable resilience against entanglement sudden death emerges under strong non-Markovian noise with long correlation times, where repeated revivals occur. These findings establish cavity photon statistics and environmental memory time as key control parameters for engineering qubit entanglement dynamics in qubit-cavity systems, revealing how system-environment correlations govern quantum information preservation.
I explore the entanglement dynamics of a three-qubit system in an initial XXX state undergoing decoherence including the possible exhibition of entanglement sudden death. To quantify entanglement I utilize negativity measures and make use of appropriate entanglement witnesses. The negativity results are then extended to XXX states with an arbitraty number of qubits. I also demonstrate nonstandard behavior of the tripartite negativity entanglement metric: its sudden appearance after some amount of decoherence, followed quickly by its disappearance. Finally, I solve for a lower bound on the three-qubit XXX-state concurrence, demonstrate when this bound goes to 000, and outline simplifcations for the calculation of higher-order XXX-state concurrences.
Decoherence, often caused by unavoidable coupling with the environment, leads to degradation of quantum coherence. For a multipartite quantum system, decoherence leads to degradation of entanglement and, in certain cases, entanglement sudden death. Tackling decoherence, thus, is a critical issue faced in quantum information, as entanglement is a vital resource for many quantum information applications including quantum computing, quantum cryptography, quantum teleportation, and quantum metrology. Here, we propose and demonstrate a novel scheme to protect entanglement from decoherence. Our entanglement protection scheme makes use of the quantum measurement itself for actively battling against decoherence and it can effectively circumvent even entanglement sudden death (ESD).