Gerald S. Buller

Performance and design of InGaAs /InP photodiodes for single-photon counting at 1.55 microm.

The performance of selected, commercially available InGaAs/InP avalanche photodiodes operating in a photon-counting mode at an incident wavelength of 1.55 microm is described. A discussion on the optimum operating conditions and their relationship to the electric field distribution within the device is presented.

A learning scheme with coherent state receiver (Conference Presentation)

The laws of quantum mechanics pose stringent constraints on the amplification of a quantum signal. Deterministic amplification of an unknown quantum state always implies the addition of a minimal amount of noise. In principle, linear and noiseless amplification is allowed provided it works only probabilistically [1,2]. nnThe state comparison amplifier [3] is an approximate probabilistic amplifier that amplifies a coherent state chosen at random from a set of coherent states with known mean photon number. The amplification process works as follows: Alice picks uniformly at random an input state and passes it to Bob. He desires to amplify the state so he mixes it with a guess coherent state at a beam splitter in an attempt to achieve destructive interference in one of the output arms. This output is fed into an APD detector. nnThe lack of trigger at the detector is an imperfect indication that Bob’s guess is right and that the output contains the correct amplified state. On the other hand, if the first detector fires Bob knows that his guess was wrong but he can still correct the output by changing the input state for a second amplification stage via a feed-forward loop. nnIn summary, Bob declares success when both the detectors do not fire or when the first detector does fire and state correction is performed. We generalize this mechanism for an arbitrary number of input states and beam splitters, using an on-line learning strategy based on maximum a posteriori probability.nnThe success probability-fidelity product [2] of the SCAMP is the joint probability of success and of passing a measurement test on the output comparing it to right amplified state.nnOur figures of merit compare favorably with other schemes. The success probability-fidelity product of the SCAMP is always bigger than that of a USD based amplifier [2] that, when inconclusive, delivers a conveniently chosen random output. nnThe SCAMP can be realized with classical resources (i.e., lasers, linear optics and APD detectors), the ability to switch between input states on the fly requires delay lines and fast switching but it can still be achieved with classical resources and the loss introduced by the delay can be offset at the second stage. Similar systems, with no state correction, proved to achieve high-gain, high fidelity and high repetition rates, e.g. [4, 5]. nnDue to its simplicity, the system we propose might represent an ideal candidate either as a recovery station to counteract quantum signal degradation due to propagation in a lossy fibre or across the turbulent atmosphere or as a quantum receiver to improve the key-rate of continuous-variable quantum key distribution with discrete modulation. The system is also suitable for on-chip implementation. n[1] T.C. Ralph & A.P. Lund, Proceedings of the 9th QCMC Conference 2009. nn[2] S. Pandey, et al., Phys. Rev. A 88, 033852 (2013).nn[3] E. Eleftheriadou et al., Phys. Rev. Lett. 111, 213601 (2013). nn[4] R. Donaldson et al., Phys. Rev. Lett. 114, 120505 (2015). nn[5] R. Donaldson et al., in preparation.

Progress in experimental quantum digital signatures

There is ongoing research into information-theoretically secure digital signature schemes. Mathematically based approaches typically require additional resources such as anonymous broadcast and/or a trusted authority to achieve information-theoretical security. The principles of quantum mechanics can be applied to the problem to create the approach known as quantum digital signatures, which does not have these limitations. This presentation will provide an overview of the development of experimental quantum digital signatures. The evolution of experimental test-beds will be charted from small scale demonstrators to long distance implementations with commercial prototypes, along with overviews of the theoretical background of each stage.

Analysis of three-dimensional scenes using photon-starved data in cluttered target scenarios (Conference Presentation)

This paper investigates a new computational method for reconstruction and analysis of complex 3D scenes. In the presence of targets, Lidar waveforms usually consist of a series of peaks, whose positions and amplitudes depend on the distances of the targets and on their reflectivities, respectively. Inferring the number of surfaces or peaks, as well as their geometric and colorimetric properties becomes extremely difficult when the number of detected photons is low (e.g., short acquisition time) and the ambient illumination is high. In this work, we adopt a Bayesian approach to account for the intrinsic spatial organization of natural scenes and regularise the 3D reconstruction problem. The proposed model is combined with an efficient Markov chain Monte Carlo (MCMC) method to reconstruct the 3D scene, while providing measures of uncertainty (e.g., about target range and reflectivity) which can be used for subsequent decision making processes, such as object detection and recognition. Despite being an MCMC method, the proposed approach presents a competitive computational cost when compared to state-of-the-art optimization-based reconstruction methods, while being more robust to the lack of detected photons (empty or non-observed pixels). Moreover, it includes a multi-scale strategy which allows a quick recovery of coarse approximations of the 3D structures, while is often sufficient for object detection/recognition. We assess the performance of our approach via extensive experiments conducted with real, long-range (hundreds of meters) single-photon Lidar data. The results clearly demonstrate its benefits to infer complex scene content from extremely sparse photon counts.

Time-correlated single-photon counting for single and multiple wavelength underwater depth imaging (Conference Presentation)

A scanning depth imaging system is used for the investigation of three-dimensional image reconstruction and classification of targets in underwater environments. The system uses the Time-Correlated Single-Photon Counting (TCSPC) technique to measure single-photon time-of-flight. In this paper, we use both single and multiple wavelengths to interrogate underwater targets. This presentation will show laboratory measurements on several target scenarios, including targets in clutter. We demonstrate high resolution depth and intensity image reconstruction in highly scattering underwater scenarios, and show image reconstruction at up to nine attenuation lengths between transceiver and target. nThe system comprised a scanning transceiver unit, fiber coupled to a silicon single-photon avalanche diode (Si SPAD) and a supercontinuum laser system operating at the repetition rate of 19.5 MHz. An acousto-optic tunable filter (AOTF) is used to select an individual operational wavelength in the range 500 nm to 725 nm. The measurements used a range of system configurations, including both single wavelength and multiple wavelength measurements. Generally, the measurements used sub-milliwatt average optical power levels. nBespoke algorithms were developed to identify man-made objects hidden by marine vegetation in the scanned scene. Advanced statistical image processing methods were used to improve target discrimination and to reconstruct the target under different conditions, including reduced number of wavelengths and number of pixels, and reduced acquisition time. Particular attention will be given to the photon starved regime, which will be typical of data acquired at long distances in open ocean waters or in highly scattering environments.