Beating the One-half Limit of Ancilla-free Linear Optics Bell Measurements

TL;DR

This paper demonstrates that active linear optics with Gaussian squeezing can unambiguously discriminate Bell states with success probabilities exceeding 50%, surpassing previous passive linear optics limits, and provides feasible experimental schemes.

Contribution

It introduces active linear-optical schemes with squeezing to beat the 50% limit for Bell state discrimination without nonlinear interactions or auxiliary entanglement.

Findings

Achieved 64.3% success probability in dual-rail encoding.

Achieved 62.5% success probability in single-rail encoding.

Proposed experimentally feasible schemes for improved Bell measurement success rates.

Abstract

We show that optically encoded two-qubit Bell states can be unambiguously discriminated with a success probability of more than 50% in both single-rail and dual-rail encodings by using active linear-optical resources that include Gaussian squeezing operations. These results are in contrast to the well-known upper bound of 50% for unambiguous discrimination of dual-rail Bell states using passive, static linear optics and arbitrarily many vacuum modes. We present experimentally feasible schemes that improve the success probability to 64.3% in dual-rail and to 62.5% in single-rail for a uniform random distribution of Bell states. Conceptually, this demonstrates that neither interactions that induce nonlinear mode transformations (such as Kerr interactions) nor auxiliary entangled photons are required to go beyond the one-half limit. We discuss the optimality of our single-rail scheme, and talk about an application of our dual-rail scheme in quantum communication.