Abstract
When a noisy communication channel is used multiple times, the errors occurring at different times generally exhibit correlations. Classically, these correlations do not affect the evolution of individual particles: a single classical particle can only traverse the channel at a definite moment of time, and its evolution is insensitive to the correlations between subsequent uses of the channel. In stark contrast, here we show that a single quantum particle can sense the correlations between multiple uses of a channel at different moments of time. In an extreme example, we show that a channel that outputs white noise when the particle is sent at a definite time can exhibit correlations that enable a perfect transmission of classical bits when the particle is sent at a superposition of two times. In contrast, we show that, in the lack of correlations, a single particle sent at a superposition of two times undergoes an effective channel with classical capacity of at most 0.16 bits. When multiple transmission lines are available, time correlations can be used to simulate the application of quantum channels in a coherent superposition of alternative causal orders, and even to provide communication advantages that are not accessible through the superposition of causal orders.
Highlights
Quantum communication enables new possibilities that were unthinkable in the classical world, notably including secure key distribution [1,2]
Here we show that a single quantum particle can sense the correlations between multiple uses of the same quantum communication channel
Single particle can carry from a sender to a receiver, beating the ultimate limit achievable in the lack of correlations. We demonstrate this effect with an extreme example, in which a single quantum particle carries one bit of classical information through a transmission line that completely erases information at every definite time step. This phenomenon witnesses the presence of correlations between different uses of the transmission line: in the lack of correlations, we show that the number of bits that can be reliably transmitted by sending a single particle at a superposition of two different times does not exceed 0.16
Summary
Quantum communication enables new possibilities that were unthinkable in the classical world, notably including secure key distribution [1,2]. We demonstrate this effect with an extreme example, in which a single quantum particle carries one bit of classical information through a transmission line that completely erases information at every definite time step This phenomenon witnesses the presence of correlations between different uses of the transmission line: in the lack of correlations, we show that the number of bits that can be reliably transmitted by sending a single particle at a superposition of two different times does not exceed 0.16. The errors occurring on successive uses of the same transmission line are correlated (yellow lines), so the particle experiences correlated errors across the two branches (red and blue) of the superposition These time correlations are a resource that can be used to mimic the use of quantum channels in a superposition of orders, and even to achieve larger communication advantages.
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