Unambiguous coherent state identification: Searching a quantum database

TL;DR

This paper proposes a practical optical setup using beamsplitters for unambiguous identification of unknown coherent states, enabling quantum database search with better performance than existing finite-dimensional measurement methods.

Contribution

It introduces a beamsplitter-based scheme for unambiguous coherent state identification and demonstrates its advantage over generalized optimal measurements.

Findings

Single beamsplitter performs optimal unambiguous comparison for coherent states.

Proposed setup outperforms finite-dimensional optimal measurement generalizations.

Experimental configuration with 2N-1 beamsplitters enables quantum database search.

Abstract

We consider an unambiguous identification of an unknown coherent state with one of two unknown coherent reference states. Specifically, we consider two modes of an electromagnetic field prepared in unknown coherent states alpha_1 and alpha_2, respectively. The third mode is prepared either in the state alpha_1 or in the state alpha_2. The task is to identify (unambiguously) which of the two modes are in the same state. We present a scheme consisting of three beamsplitters capable to perform this task. Although we don't prove the optimality, we show that the performance of the proposed setup is better than the generalization of the optimal measurement known for a finite-dimensional case. We show that a single beamsplitter is capable to perform an unambiguous quantum state comparison for coherent states optimally. Finally we propose an experimental setup consisting of 2N-1 beamsplitters for unambiguous identification among N unknown coherent states. This setup can be considered as a search in a quantum database. The elements of the database are unknown coherent states encoded in different modes of an electromagnetic field. The task is to specify the two modes that are excited in the same, though unknown, coherent state.