Jeff S. Lundeen

Heralded generation of ultrafast single photons in pure quantum states

We present an experimental demonstration of heralded single photons prepared in pure quantum states from a parametric down-conversion source. It is shown that, through controlling the modal structure of the photon pair emission, one can generate pairs in factorable states and thence eliminate the need for spectral filters in multiple-source interference schemes. Indistinguishable heralded photons were generated in two independent spectrally engineered sources and Hong-Ou-Mandel interference observed between them without spectral filters. The measured visibility of 94.4% sets a minimum bound on the mean photon purity.

Optimal Quantum Phase Estimation

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.

Quantum phase estimation with lossy interferometers

We give a detailed discussion of optimal quantum states for optical two-mode interferometry in the presence of photon losses. We derive analytical formulae for the precision of phase estimation obtainable using quantum states of light with a definite photon number and prove that maximization of the precision is a convex optimization problem. The corresponding optimal precision, i.e., the lowest possible uncertainty, is shown to beat the standard quantum limit thus outperforming classical interferometry. Furthermore, we discuss more general inputs: states with indefinite photon number and states with photons distributed between distinguishable time bins. We prove that neither of these is helpful in improving phase estimation precision.

Classical dispersion-cancellation interferometry.

Even-order dispersion cancellation, an effect previously identified with frequency-entangled photons, is demonstrated experimentally for the first time with a linear, classical interferometer. A combination of a broad bandwidth laser and a high resolution spectrometer was used to measure the intensity correlations between anti-correlated optical frequencies. Only 14% broadening of the correlation signal is observed when significant material dispersion, enough to broaden the regular interferogram by 4250%, is introduced into one arm of the interferometer.

Measuring measurement : theory and practice

Recent efforts have applied quantum tomography techniques to the calibration and characterization of complex quantum detectors using minimal assumptions. In this work, we provide detail and insight concerning the formalism, the experimental and theoretical challenges and the scope of these tomographical tools. Our focus is on the detection of photons with avalanche photodiodes and photon-number resolving detectors and our approach is to fully characterize the quantum operators describing these detectors with a minimal set of well-specified assumptions. The formalism is completely general and can be applied to a wide range of detectors.

A characterization of the single-photon sensitivity of an electron multiplying charge-coupled device

We experimentally characterize the performance of the electron multiplying charge-coupled device (EMCCD) camera for the detection of single photons. The tests are done with the photon pairs generated from parametric downconversion (PDC). The gain, time response and noise performance of the EMCCD are characterized. In addition, we attempt to use the camera to measure the spatial correlations of PDC. The results reveal the capabilities and limits of the EMCCD as a single-photon-detector array for the applications of quantum optics, astronomy and microscopy.

A proposed testbed for detector tomography

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.

Heralded generation of two-photon NOON states for precision quantum metrology

We experimentally demonstrate a heralded source of high-purity two-photon NOON states derived from heralded single-photon sources.

A short perspective on long crystals: Broadband wave mixing and its application to ultrafast quantum optics

We present an overview of recently developed ideas in ultrafast nonlinear optics, and describe three applications where these ideas have had an impact. A closer look at three wave mixing of broadband electromagnetic fields in birefringent nonlinear crystals shows that not only phase matching, but also group velocity matching is important for understanding the process of up- and down-conversion with ultrashort laser pulses. In fact the higher-order dispersion of nonlinear crystalline materials provides an underused degree of freedom that allows tailoring the interaction so that it is suitable for a number of different applications. We analyse the processes of parametric downconversion for the production of pure single photon states, and upconversion for ultrashort pulse characterization and for quantum state and process tomography in molecules.

Homodyne state tomography with photon number resolving detectors

We introduce a complete tomographic reconstruction scheme geared toward low photon-number states. To demonstrate this method we reconstruct various single-mode coherent states.