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arXiv:1201.2232 · Journal of Physics A Mathematical and Theoretical

Entanglement amplification via local weak measurements

Yukihiro Ota, Sahel Ashhab, Franco Nori

Published Sep 25, 2012• 25 citations
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Abstract

We propose a measurement-based method to produce a maximally-entangled state from a partially-entangled pure state. Our goal can be thought of as entanglement distillation from a single copy of a partially-entangled state. The present approach involves local two-outcome weak measurements. We show that application of these local weak measurements leads to a probabilistic amplification of entanglement. In addition, we examine how the probability to find the maximally-entangled state is related to the entanglement of the input state. We also study the application of our method to a mixed initial state. We show that the protocol is successful if the separable part of the mixed initial state fulfils certain conditions.

DOI
10.1088/1751-8113/45/41/415303

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We propose a measurement-based method to produce a maximally-entangled state from a partially-entangled pure state. Our goal can be thought of as entanglement distillation from a single copy of a partially-entangled state. The present approach involves local two-outcome weak measurements. We show that application of these local weak measurements leads to a probabilistic amplification of entanglement. In addition, we examine how the probability to find the maximally-entangled state is related to the entanglement of the input state. We also study the application of our method to a mixed initial state. We show that the protocol is successful if the separable part of the mixed initial state fulfils certain conditions.
We demonstrate a method for the optimal purification of the asymmetric state, the symmetric state and the state from partially entangled pure states using two-outcome weak measurements. We obtain two different final states by setting different measurement strengths for the same initial state. The total success probability is related to the initial pure state parameters. We also show that the entanglement amplification can be achieved both for mixed state and mixed state by applying this local weak measurements. The approach requires a small cost because a lot of copies and two-body or multibody measurements are not needed.
Abstract We present a comprehensive approach to transform a partially entangled pure state into a maximally entangled state, utilizing only local operations and a single copy of the entangled state. Our approach eliminates the need for multiple copies of entangled states, bilateral operations, and the time-consuming partial-collapse weak measurements. Integrating ancilla qubits, rotation gates, and CNOT gates, we simplify the process of enhancing entanglement, achieving significant efficiency using only a single partially entangled state. Assuming perfect local operations, we showcase the efficacy of our method through both mathematical analysis and the practical implementation of the corresponding circuit in Qiskit. Furthermore, we explore the impact of imperfect local operations, specifically focusing on the CNOT gate and measurement, for a comprehensive analysis. Additionally, we address the case of unknown parameters in the partially entangled pure state and introduce a robust method to effectively handle this uncertainty, ensuring reliable entanglement fidelity optimization in practical situations. We extend our investigation to the application of our method to mixed initial entangled states. Our results demonstrate the reliability of our method in increasing entanglement even in the presence of phase damping and amplitude damping. Additionally, we illustrate its effectiveness in transforming specific mixed initial states in the presence of depolarization into states with enhanced entanglement.
We propose a scheme for distillation of free bipartite entanglement from bipartite bound entangled states. The crucial element of our scheme is an ancillary system that is coupled to the initial bound entangled state via appropriate weak measurements. We show that in this protocol free entanglement can be always generated with nonzero probability by using a single copy of the bound entangled state. We also derive a lower bound on the entanglement cost of the protocol and conclude that, on average, applying weaker measurements results in relatively higher values of free entanglement as well as lower costs.
We extend the idea of entanglement concentration protocol for pure states (Phys. Rev. Lett. 88 , 187903) to the case of entanglement distillation for mixed states. The scheme works only with particle statistics and local operations, without the need of any other interactions. We show that the maximally entangled state can be distilled out when the initial state is pure, otherwise the entanglement distilled depends on the off-diagonal element of density matrix of the initial state. Because of the requirement for density matrix, the entanglement distilled is always less than one, and this result is the same for both fermionic and bosonic particles. The distillation efficiency is a product of the diagonal elements of the initial state, it takes the maximum 50%, the same as that in the case for pure states.
We propose a new protocol for the weak measurement of any observable with remote pre and postselections. We show that if two parties share a pure entangled state, then by using local operations and classical communication they can preselect and postselect at distant locations leading to the weak value of an observable as a shift in the pointer of the apparatus at one location in the process of the weak measurement. This can be achieved with either sharing of a pure maximally or non-maximally entangled state. We generalize the protocol for realizing the weak value of any observable with remote pre and postselection of mixed states. Finally, we show how the weak value is modified in the remote pre and postselection setting if Alice and Bob share a mixed entangled state.
Entanglement distillation is an indispensable ingredient in extended quantum communication networks. Distillation protocols are necessarily non-deterministic and require non-trivial experimental techniques such as noiseless amplification. We show that noiseless amplification could be achieved by performing a post-selective filtering of measurement outcomes. We termed this protocol measurement-based noiseless linear amplification (MBNLA). We apply this protocol to entanglement that suffers transmission loss of up to the equivalent of 100km of optical fibre and show that it is capable of distilling entanglement to a level stronger than that achievable by transmitting a maximally entangled state through the same channel. We also provide a proof-of-principle demonstration of secret key extraction from an otherwise insecure regime via MBNLA. Compared to its physical counterpart, MBNLA not only is easier in term of implementation, but also allows one to achieve near optimal probability of success.