Probing Correlations, Indistinguishability and Entanglement in Microwave Two-Photon Interference

TL;DR

This paper demonstrates two-photon interference at microwave frequencies using independent single-photon sources, revealing quantum properties like indistinguishability and entanglement, and advancing microwave quantum communication and computation.

Contribution

It presents the first observation of Hong-Ou-Mandel interference with microwave photons from independent sources, including full entanglement characterization.

Findings

Successful observation of HOM effect in microwave domain

Resolved photon number states and coherence via quadrature detection

Characterized two-mode entanglement of output modes

Abstract

Interference at a beam splitter reveals both classical and quantum properties of electromagnetic radiation. When two indistinguishable single photons impinge at the two inputs of a beam splitter they coalesce into a pair of photons appearing in either one of its two outputs. This effect is due to the bosonic nature of photons and was first experimentally observed by Hong, Ou, and Mandel (HOM) [1]. Here, we present the observation of the HOM effect with two independent single-photon sources in the microwave frequency domain. We probe the indistinguishability of single photons, created with a controllable delay, in time-resolved second-order cross- and auto-correlation function measurements. Using quadrature amplitude detection we are able to resolve different photon numbers and detect coherence in and between the output arms. This measurement scheme allows us to observe the HOM effect and, in addition, to fully characterize the two-mode entanglement of the spatially separated beam splitter output modes. Our experiments constitute a first step towards using two-photon interference at microwave frequencies for quantum communication and information processing, e.g. for distributing entanglement between nodes of a quantum network [2, 3] and for linear optics quantum computation [4, 5].