Quantum repeaters with imperfect memories: cost and scalability

TL;DR

This paper analyzes how memory dephasing affects entanglement generation rates in quantum repeaters, showing that long coherence times are essential for scalable quantum communication over large distances.

Contribution

It provides a quantitative estimate of the maximum entanglement rate considering imperfect memories and introduces an optimized partial nesting protocol for quantum repeaters.

Findings

Rate scales polynomially with distance if coherence time is on the order of distance divided by light speed.

Rate degrades exponentially with the square root of distance over coherence time when coherence time is short.

Long coherence times are crucial for maintaining high entanglement distribution rates over large distances.

Abstract

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. We show that, for any given distance $L$, the polynomial scaling of rate with distance can only be achieved if quantum memories with coherence times on the order of $L/c$ or longer, with $c$ being the speed of light in the channel, are available. The above rate degrades as a power of $\exp[-\sqrt{(L/c)/ \tau_c}]$ with distance when the coherence time $\tau_c \ll L/c$.