Analysis of the time-energy entanglement of down-converted photon pairs by correlated single photon interference

TL;DR

This paper introduces a novel method using correlated single photon interference with weak coherent light to characterize the time-energy entanglement of down-converted photon pairs, overcoming limitations of direct timing measurements.

Contribution

It presents a new technique for reconstructing the two-photon wavefunction by analyzing frequency-dependent interference patterns, providing a direct map of entanglement.

Findings

Successfully reconstructed the two-photon wavefunction

Demonstrated the method's ability to reveal phase and amplitude information

Provided a practical approach to characterize entanglement without high temporal resolution

Abstract

The time-energy entanglement of down-converted photon pairs is particularly difficult to characterize because direct measurements of photon arrival times are limited by the temporal resolution of photon detection. Here, we explore an alternative possibility of characterizing the temporal coherence of two-photon wavefunctions using single photon interference with weak coherent light. Specifically, this method makes use of the fact that down-conversion generates a coherent superposition of vacuum and two-photon states, so that the coincidence count rates for photon pairs after interference with weak coherent light are given by a superposition of the two-photon wavefunction from the down-conversion with the product wavefunction defined by the weak coherent references. By observing the frequency dependent interference pattern, it is possible to reconstruct the amplitudes and the phases of the two-photon wavefunction within the bandwidth of the reference pulses used in the single photon interferences. The correlated single photon interferences therefore provide a direct map of the entangled two-photon wavefunction generated in the down-conversion process.