Ian Walmsley

Abstract:

A nonclassical light source is used to demonstrate experimentally the absolute efficiency calibration of a photon-number-resolving detector. The photon-pair detector calibration method developed by Klyshko for single-photon detectors is generalized to take advantage of the higher dynamic range and additional information provided by photon-number-resolving detectors. This enables the use of brighter twin-beam sources including amplified pulse pumped sources, which increases the relevant signal and provides measurement redundancy, making the calibration more robust.

Abstract:

We report an optical detector with tunable positive operator-valued measures. The device is based on a combination of weak-field homodyne techniques and photon-number-resolving detection. The resulting positive operator-valued measures can be continuously tuned from Fock-state projectors to a variety of phase-dependent quantum-state measurements by adjusting different system parameters such as local oscillator coupling, amplitude, and phase, allowing thus not only detection but also preparation of exotic quantum states. Experimental tomographic reconstructions of classical benchmark states are presented as a demonstration of the detector capabilities.

Abstract:

By using a systematic optimization approach, we determine quantum states of light with definite photon number leading to the best possible precision in optical two-mode interferometry. Our treatment takes into account the experimentally relevant situation of photon losses. Our results thus reveal the benchmark for precision in optical interferometry. Although this boundary is generally worse than the Heisenberg limit, we show that the obtained precision beats the standard quantum limit, thus leading to a significant improvement compared to classical interferometers. We furthermore discuss alternative states and strategies to the optimized states which are easier to generate at the cost of only slightly lower precision.

Abstract:

Measurement is the only part of a general quantum system that has yet to be characterised experimentally in a complete manner. Detector tomography provides a procedure for doing just this; an arbitrary measurement device can be fully characterised, and thus calibrated, in a systematic way without access to its components or its design. The result is a reconstructed POVM containing the measurement operators associated with each measurement outcome. We consider two detectors, a single-photon detector and a photon-number counter, and propose an easily realised experimental apparatus to perform detector tomography on them. We also present a method of visualising the resulting measurement operators.

Abstract:

The interference of single heralded photons from multiple parametric downconversion sources requires photon pairs in factorable states. Typically, these are selected from an ensemble of pairs by narrow filters that remove any exhibiting correlations. In order to eliminate these lossy filters, factorable photon pairs free from any spatio-temporal correlations must be created directly at each source. This requires careful engineering of the group velocity dispersion of the nonlinear crystal in which pair generation takes place. Several schemes have been proposed to do this in the plane-wave regime, but in a realistic experiment one must also take into account the effects of focusing on the two-photon state. Focusing leads to space-time coupling between the pump structure and the downconverted pairs that has the potential to reduce their factorability, but if carefully managed can actually increase it. In this paper, we consider some of the effects of focusing and their consequences for pure single photon generation.