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Quantum Information and Measurement (QIM)

Quantum memory decoherence-mitigating architecture for quantum repeaters

Siddhartha Santra, Liang Jiang, Vladimir S. Malinovsky

Published Jan 1, 2019• 0 citations
Publisher

Abstract

We propose a nested quantum repeater architecture that mitigates quantum memory decoherence by optimizing the memory access time. This maximizes the distillable entanglement rate in the remote entangled state for technologically feasible repeater parameter regimes.
DOI
10.1364/qim.2019.s1d.4

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We propose a quantum repeater protocol and architecture that mitigates decoherence of the entangled states by optimizing the quantum memory access time. The protocol maximizes the rate of distillable entanglement in the average accessed state at all nesting levels. The achievable rate is higher by orders of magnitude in comparison to a canonical protocol that does not optimize the access time. The advantage of the proposed design is observed for all nesting levels of the repeater for technologically feasible memory quality, entanglement generation and swapping success probabilities.
Abstract We propose a quantum repeater protocol and architecture that mitigates decoherence of the entangled states by optimizing the quantum memory buffer time. The protocol maximizes the rate of distillable entanglement in the average accessed state at all nesting levels. The achievable rate is higher by orders of magnitude in comparison to a canonical protocol that does not optimize the buffer time. The advantage of the proposed design is observed for all nesting levels of the repeater for technologically feasible memory quality, entanglement generation and swapping success probabilities.
Quantum repeaters enable us to distribute entanglement between remote parties by relying on a network of quantum memory units that exhibit efficient coupling to light, scalability, and long coherence times. Entanglement is initially distributed between nearest neighbors and then extended to the far-end nodes using entanglement swapping techniques. For real-time applications, such as quantum key distribution, the above tasks must be repeated successively, according to a proper protocol, to generate entangled states at a certain rate. This paper studies a number of such protocols and the interplay between the rate of entanglement generation, the number of employed memories, and the coherence time of memory units.
Memory dephasing and its impact on the rate of entanglement generation in quantum repeaters is addressed. For systems that rely on probabilistic schemes for entanglement distribution and connection, we estimate the maximum achievable rate per employed memory for our optimized partial nesting protocol, when a large number of memories are being used in each node. The above rate scales polynomially with distance, LLL, if quantum memories with infinitely long coherence times are available or if we employ a fully fault-tolerant scheme. For memories with finite coherence times and no fault-tolerant protection, the above rate optimistically degrades exponentially in L\sqrt{L}L​, regardless of the employed purification scheme. It decays, at best, exponentially in LLL if no purification is used.
Quantum communication enables the implementation of tasks that are unachievable with classical resources. However, losses on the communication channel preclude the direct long-distance transmission of quantum information in many relevant scenarios. In principle, quantum repeaters allow one to overcome losses. However, realistic hardware parameters make long-distance quantum communication a challenge in practice. For instance, in many protocols an entangled pair is generated that needs to wait in quantum memory until the generation of an additional pair. During this waiting time the first pair decoheres, impacting the quality of the final entanglement produced. At the cost of a lower rate, this effect can be mitigated by imposing a cutoff condition. For instance, a maximum storage time for entanglement after which it is discarded. In this article, we optimize the cutoffs for quantum repeater chains. First, we develop an algorithm for computing the probability distribution of the waiting time and fidelity of entanglement produced by repeater chain protocols which include a cutoff. Then, we use the algorithm to optimize cutoffs in order to maximize the secret-key rate between the end nodes of the repeater chain. In this article, we find that the use of the optimal cutoff extends the parameter regime for which secret key can be generated and, moreover, significantly increases the secret-key rate for a large range of parameters.
First-generation quantum repeaters are essential in distributing entanglement over long distances since they could mitigate losses in optical fiber. Repeaters split the total distance into shorter, easier to generate links. Then, entanglement swapping establishes long range entanglement by performing local measurements at the repeaters and heralding their results. In this work, we develop an analytical model that estimates the impact of decoherence during heralding for a given protocol. Furthermore, we identify a competition between two phenomena: as repeater count increases, so does entanglement generation, but the additive impact of memory decoherence on final entanglement rate increases too. Therefore, we find that an optimal number of repeaters exists. These findings could lead the design of future quantum networks, both in the inter-repeater distance and swapping timings.