Demonstration of a quantum nondemolition sum gate

TL;DR

This paper reports the experimental demonstration of a quantum nondemolition sum gate for continuous-variable quantum computation, utilizing offline squeezed states, homodyne detection, and feedforward to achieve QND interactions.

Contribution

It presents the first experimental realization of a QND sum gate based solely on offline squeezed states, homodyne measurements, and feedforward, confirming theoretical predictions.

Findings

Successfully demonstrated a QND sum gate in the lab.

Verified the gate satisfies criteria for QND measurements in conjugate quadratures.

Shows potential for use in universal continuous-variable quantum computation.

Abstract

The sum gate is the canonical two-mode gate for universal quantum computation based on continuous quantum variables. It represents the natural analogue to a qubit C-NOT gate. In addition, the continuous-variable gate describes a quantum nondemolition (QND) interaction between the quadrature components of two light fields. We experimentally demonstrate a QND sum gate, employing the scheme by R. Filip, P. Marek, and U.L. Andersen [\pra {\bf 71}, 042308 (2005)], solely based on offline squeezed states, homodyne measurements, and feedforward. The results are verified by simultaneously satisfying the criteria for QND measurements in both conjugate quadratures.