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Optical quantum information science and technologies require the capability to generate, control, and detect single or multiple quanta of light. The need to detect individual photons has motivated the development of a variety of novel and refined single-photon detectors (SPDs) with enhanced detector performance. Superconducting nanowire single-photon detectors (SNSPDs) and single-photon avalanche diodes (SPADs) are the top-performer in this field, but alternative promising and innovative devices are emerging. In this review article, we discuss the current state-of-the-art of one such alternative device capable of single-photon counting: the resonant tunneling diode (RTD) single-photon detector. Due to their peculiar photodetection mechanism and current-voltage characteristic with a region of negative differential conductance, RTD single-photon detectors provide, theoretically, several advantages over conventional SPDs, such as an inherently deadtime-free photon-number resolution at elevated temperatures, while offering low dark counts, a low timing jitter, and multiple photon detection modes. This review article brings together our previous studies and current experimental results. We focus on the current limitations of RTD-SPDs and provide detailed design and parameter variations to be potentially employed in next-generation RTD-SPD to improve the figure of merits of these alternative single-photon counting devices. The single-photon detection capability of RTDs without quantum dots is shown. [ABSTRACT FROM AUTHOR]

We create a large exciton-polariton condensate and employ a Michelson interferometer setup to characterize the short- and longdistance behavior of the first order spatial correlation function. Our experimental results show distinct features of both the twodimensional and nonequilibrium characters of the condensate. We find that the gaussian short-distance decay is followed by a power: law decay at longer distances, as expected for a two-dimensional condensate. The exponent of the power law is measured in the range 0.9-1.2. larger than is possible in equilibrium. We compare the experimental results to a theoretical model to understand the features required to observe a power law and to clarify the influence of external noise on spatial coherence in nonequilibrium phase transitions. Our results indicate that Berezinskii-KosterlitzThouless-like phase order survives in open-dissipative systems. [ABSTRACT FROM AUTHOR]

Future communication and computation technologies that exploit quantum information require robust and well-isolated qubits. Electron spins in III-V semiconductor quantum dots, although promising candidate qubits, see their dynamics limited by undesirable hysteresis and decohering effects of the nuclear spin bath. Replacing electrons with valence-band holes should suppress the hyperfine interaction and consequently eliminate strong nuclear effects. Such suppression was recently observed in optical initialization and coherent population trapping experiments, but complete control over the phase of an arbitrary hole superposition-the essence of a hole-based qubit-has not yet been achieved. Using picosecond optical pulses, we now demonstrate complete coherent control of a single hole qubit and examine both free-induction and spin-echo decay. In moving from electrons to holes, we observe the effects of the reduced hyperfine interactions in the re-emergence of hysteresis-free dynamics, while obtaining similar coherence times limited by non-nuclear mechanisms. These results demonstrate the potential of optically controlled quantum-dot hole qubits. [ABSTRACT FROM AUTHOR]

Micropillar cavities are potential candidates for high-efficiency single-photon sources and are testbeds for cavity quantum electrodynamics experiments. In both applications a high quality (Q) factor is desired. It was recently shown that the Q of high-Q semiconductor micropillar cavities exhibit pronounced quasi-periodic variations in the regime from 1 to 4 \mum, and a detailed understanding of the variational behavior of the Q is required. Here, we study the origin of these variations using a multi-mode Fabry-Perot model appropriate for this regime. We analyze in detail contributions to the effective reflectivity of the fundamental mode arising from coupling to scattering channels involving higher-order cavity modes and propagating Bloch modes in the distributed Bragg reflectors (DBRs). We show how these weak contributions lead to strong variations of the Q factor, and we relate the average periodicity of these variations to the thickness of the DBRs and the derivative of the effective indices of the guided Bloch modes. We also examine the influence of various geometrical parameters, including the number of DBR layers pairs, the amplitude of the corrugation of the pillar sidewalls and the number of etched layer pairs in the bottom DBR on the Q versus diameter relation. Comparisons are made between extensive numerical simulations and experimental measurements, and a good qualitative agreement is found. [ABSTRACT FROM PUBLISHER]

We show that four-terminal measurements of the differential conductance of field effect transistors (FETs) can provide important insights into the transport mechanism, and in particular can reveal the presence of ballistic transport. Measurements and simulations of purposely fabricated AlGaAs–GaAs heterostructure FETs show that ballistic transport results in a pronounced peak in the derivative of the differential conductance versus the gate voltage, which splits into two peaks with increasing drain-to-source voltage. Analyzing the four-probe conductance, ballistic electron transport through the channel is revealed as the origin of the observed peak splitting. [Copyright &y& Elsevier]

We compare two regimes indicative of polariton Iasing and photon lasing of a planar GaAs/GaAlAs microcavity with zero detuning between the bare cavity mode and the quantum-well exciton. In particular, we investigate the cavity emission subsequent to nonresonant pulsed excitation. For the ground state emission from the lower energy-momentum dispersion branch we find a two-threshold behavior in the input-output curve where each transition is accompanied by characteristic changes of the in-plane mode dispersion. We demonstrate that the thresholds are unambiguously evidenced in the photon statistics of the emission based on the second-order correlation function. Moreover, the distinct two-threshold behavior is confirmed in the evolution of the emission pulse duration. Our findings show that a comprehensive study of spectral and temporal characteristics of the emission from a semiconductor microcavity can be used to characterize the different emission regimes. [ABSTRACT FROM AUTHOR]