Quantum repeaters with imperfect memories: cost and scalability

Understand

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 $\tau_c \ll L/c$.