Quantum Communication through an Unmodulated Spin Chain

TLDR

The authors propose using an unmodulated, unmeasured spin chain as a channel for short‑distance quantum communication. The scheme transmits a quantum state by initializing it on one spin of the chain and later retrieving it on a distant spin, with analytical expressions for fidelity and entanglement derived and applied to an open chain with nearest‑neighbor couplings. The protocol achieves better‑than‑classical fidelity for qubit transfer across chains up to 80 spins in reasonable time, with fidelity given by an inverse discrete cosine transform or Bessel series, and supports distillable entanglement over arbitrary distances.

Abstract

We propose a scheme for using an unmodulated and unmeasured spin chain as a channel for short distance quantum communications. The state to be transmitted is placed on one spin of the chain and received later on a distant spin with some fidelity. We first obtain simple expressions for the fidelity of quantum state transfer and the amount of entanglement sharable between any two sites of an arbitrary Heisenberg ferromagnet using our scheme. We then apply this to the realizable case of an open ended chain with nearest neighbor interactions. The fidelity of quantum state transfer is obtained as an inverse discrete cosine transform and as a Bessel function series. We find that in a reasonable time, a qubit can be directly transmitted with better than classical fidelity across the full length of chains of up to 80 spins. Moreover, our channel allows distillable entanglement to be shared over arbitrary distances.