W. S. Kolthammer

Interfacing GHz-bandwidth heralded single photons with a warm vapour Raman memory

Broadband quantum memories, used as temporal multiplexers, are a key component in photonic quantum information processing, as they make repeat-until-success strategies scalable. We demonstrate a prototype system, operating on-demand, by interfacing a warm vapour, high time-bandwidth-product Raman memory with a travelling wave spontaneous parametric down-conversion source. We store single photons and observe a clear influence of the input photon statistics on the retrieved light, which we find currently to be limited by noise. We develop a theoretical model that identifies four-wave mixing as the sole important noise source and point towards practical solutions for noise-free operation.

High-fidelity polarization storage in a gigahertz bandwidth quantum memory

We demonstrate a dual-rail optical Raman memory inside a polarization interferometer; this enables us to store polarization-encoded information at GHz bandwidths in a room-temperature atomic ensemble. By performing full process tomography on the system, we measure up to 97 ± 1% process fidelity for the storage and retrieval process. At longer storage times, the process fidelity remains high, despite a loss of efficiency. The fidelity is 86 ± 4% for 1.5 μs storage time, which is 5000 times the pulse duration. Hence, high fidelity is combined with a large time-bandwidth product. This high performance, with an experimentally simple setup, demonstrates the suitability of the Raman memory for integration into large-scale quantum networks.

Large scale quantum walks by means of optical fiber cavities

We demonstrate a platform for implementing quantum walks that overcomes many of the barriers associated with photonic implementations. We use coupled fiber-optic cavities to implement time-bin encoded walks in an integrated system. We show that this platform can achieve very low losses combined with high-fidelity operations, enabling an unprecedented large number of steps in a passive system, as required for scenarios with multiple walkers. Furthermore the platform is reconfigurable, enabling variation of the coin, and readily extends to multidimensional lattices. We demonstrate variation of the coin bias experimentally for three different values.

Quantum interference of multiple on-chip heralded sources of pure single photons

We demonstrate the generation of heralded single photons in silica photonic chips with a preparation efficiency of 80% and single photon purity of 0.86. We show multiple indistinguishable photon sources on a single chip with HOM-dip visibilities of up to 95%.

On-chip III-V monolithic integration of heralded single photon sources and beamsplitters

We demonstrate a monolithic III-V photonic circuit combining a heralded single photon source with a beamsplitter, at room temperature and telecom wavelength. Pulsed parametric down-conversion in an AlGaAs waveguide generates counterpropagating photons, one of which is used to herald the injection of its twin into the beamsplitter. We use this configuration to implement an integrated Hanbury-Brown and Twiss experiment, yielding a heralded second-order correlation gher(2)(0)=0.10±0.02 that confirms single-photon operation. The demonstrated generation and manipulation of quantum states on a single III-V semiconductor chip opens promising avenues towards real-world applications in quantum information.

Experimental characterisation of a broadband multimode squeezed light source in the high gain regime

Recent advances in the design of broadband parametric down-conversion (PDC) sources have opened the possibility to attain a regime of high squeezing previously unacessible in the pulsed domain [1]. However, characterising a PDC source is a challenging task when multiple spectral modes are being squeezed simultaneously and one wants to determine the number of squeezers and their relative strengths. Moreover, as one approaches the high squeezing regime, a number of nonlinear effects that are not usually considered when the source is operated at low pump power become relevant [2, 3].

A reconfigurable modular system for on-chip quantum optics

Quantum optics experiments are increasingly taking an integrated format for the benefits of phase stability and scalability. Silica has proved an advantageous material for this work due to its low propagation loss and high-efficiency coupling to optical fibre, and a number of key on-chip experiments have already been carried out with up to four photons using this material [1]. Fabrication imperfections create a number of difficulties in moving to more complex setups since the classical performance of the network must be characterised before an analysis of its quantum properties can be carried out, and this process becomes greatly more challenging as the circuit complexity grows.

Frequency-multiplexed single-photon sources using electro-optic frequency translation

Heralded single-photon sources are a standard experimental tool for studying quantum optics and quantum information in few-photon systems. These sources use spontaneous parametric wave mixing to probabilistically emit pairs of photons. One photon of the pair, the heralding photon, is detected to indicate the presence of the other. In theory, the maximal probability of emitting a single pair of photons is 1/4. In practice, the typical probability is restricted to a few percent to suppress multi-pair emission. This is a major obstacle to experiments utilizing multiple single photons, which require simultaneous output from numerous independent sources. A potential solution is to multiplex many heralded sources and route an emitted photon to the predetermined output mode [1, 2]. A number of multiplexing schemes have been investigated; however the operation of many identical heralded sources along with fast efficient routing remains a challenge. A recently reported frequency-multiplexing scheme is based on a single continuous-wave parametric down-conversion (PDC) process that generates photon pairs with highly correlated frequencies [3]. This way different frequency channels in one physical component act as independent sources. However the heralding and routing by nonlinear frequency conversion required a number of components (detectors and pump lasers, respectively) that increased in proportion to the number of effective heralded sources.

Photonic Networked Quantum Information Technologies

Hybrid light-matter networks offer the promise for delivering robust quantum information processing technologies, from sensor arrays to quantum simulators. New sources, detectors and memories illustrate progress towards build a resilient, scalable photonic quantum network.

High-efficiency Bragg grating enhanced on-chip photon-number-resolving detectors

Summary form only given. The recent trend towards integration of quantum optics experiments has produced a demand for on-chip single photon detectors with high quantum efficiencies. In previous work we demonstrated integrated photon number resolving detectors for use at telecommunications wavelengths [1], here we outline developments of this design which have enabled improvements in the quantum efficiency, permitting an on-chip detection efficiency of 92% to be obtained in the device of Fig. 1.