Towards optimal single-photon sources from polarized microcavities

Bright sources of indistinguishable single photons are strongly needed for the scalability of quantum information processing. Semiconductor quantum dots are promising systems to build such sources. Several works demonstrated emission of indistinguishable photons while others proposed various approaches to efficiently collect them. Here we combine both properties and report on the fabrication of ultrabright sources of indistinguishable single photons, thanks to deterministic positioning of single quantum dots in well-designed pillar cavities. Brightness as high as 0.79±0.08 collected photon per pulse is demonstrated. The indistinguishability of the photons is investigated as a function of the source brightness and the excitation conditions. We show that a two-laser excitation scheme allows reducing the fluctuations of the quantum dot electrostatic environment under high pumping conditions. With this method, we obtain 82±10% indistinguishability for a brightness as large as 0.65±0.06 collected photon per pulse.

A key ingredient for quantum photonic technologies is an on-demand source of indistinguishable single photons. State-of-the-art indistinguishable single-photon sources typically employ resonant excitation pulses with fixed repetition rates, creating a string of single photons with predetermined arrival times. However, in future applications, an independent electronic signal from a larger quantum circuit or network will trigger the generation of an indistinguishable photon. Further, operating the photon source up to the limit imposed by its lifetime is desirable. Here, we report on the application of a true on-demand approach in which we can electronically trigger the precise arrival time of a single photon as well as control the excitation pulse duration based on resonance fluorescence from a single InAs/GaAs quantum dot. We investigate in detail the effect of the finite duration of an excitation $\pi$ pulse on the degree of photon antibunching. Finally, we demonstrate that highly indistinguishable single photons can be generated using this on-demand approach, enabling maximum flexibility for future applications.

We demonstrate a high-purity source of indistinguishable single photons using a quantum dot embedded in a nanophotonic waveguide. The source features a near-unity internal coupling efficiency and the collected photons are efficiently coupled off-chip by implementing a taper that adiabatically couples the photons to an optical fiber. By quasi-resonant excitation of the quantum dot, we measure a single-photon purity larger than 99.4% and a photon indistinguishability of up to 94+-1% by using p-shell excitation combined with spectral filtering to reduce photon jitter. A temperature-dependent study allows pinpointing the residual decoherence processes notably the effect of phonon broadening. Strict resonant excitation is implemented as well as another mean of suppressing photon jitter, and the additional complexity of suppressing the excitation laser source is addressed. The study opens a clear pathway towards the long-standing goal of a fully deterministic source of indistinguishable photons, which is integrated on a planar photonic chip.

Efficient sources of indistinguishable single photons are a key resource for various applications in fields like quantum sensing, quantum metrology and quantum information processing. In this contribution we report on single photon generation based on III-V semiconductor quantum dots (QDs). To increase the emission efficiency of single photons, it is essential to tailor the radiative properties of the quantum dot emitters by engineering their photonic environment. We present optimized single photon emitters being based on both micropillar and photonics crystal cavities, for applications in a vertical platform and on-chip in-plane platform, respectively. Electrically driven single photon sources with self assembled semiconductor QDs embedded into GaAs/AlAs micropillar cavities emit on demand net rates of ~35 MHz single photons, thus being well exploitable in quantum key distribution systems. In order to establish also a spatially deterministic fabrication platform, position controlled quantum dots are integrated into p-i-n micropillar cavities and single photon emission of a coupled QD-micropillar diode system is observed. Efficient broadband coupling of single photons into photonic crystal waveguides provides the basis for all on-chip quantum information processing, and an according approach is reported.

In 2001, all-optical quantum computing became feasible with the discovery that scalable quantum computing is possible using only single-photon sources, linear optical elements, and single-photon detectors. Although it was in principle scalable, the massive resource overhead made the scheme practically daunting. However, several simplifications were followed by proof-of-principle demonstrations, and recent approaches based on cluster states or error encoding have dramatically reduced this worrying resource overhead, making an all-optical architecture a serious contender for the ultimate goal of a large-scale quantum computer. Key challenges will be the realization of high-efficiency sources of indistinguishable single photons, low-loss, scalable optical circuits, high-efficiency single-photon detectors, and low-loss interfacing of these components.

The photon-emission efficiencies and photon indistinguishabilities of a single-photon source, which employs a cavity coupled with a quantum dot, are studied under above-band and resonant excitations. The results are obtained by solving master equations and by applying the quantum regression theorem. According to the study, the photon indistinguishability increases with the Purcell factor under resonant excitation, which is consistent with the increase in emission efficiency; however, these two figures of merit are inconsistent for the above-band excitation scheme. Moreover, the efficiencies, defined as the average photon number emitted in one excitation cycle, are almost the same for the two different excitation schemes, whereas the excitation power needed to reach that efficiency is much lower under resonant excitation than that for above-band excitation. These results will be helpful in improving the performances of the applications concerning indistinguishability and efficiency.

The combination of integrated nanophotonic circuits and single quantum emitters holds great promise for scalable quantum information processing (QIP) and for the realization of photonic quantum networks. Efficient and deterministic sources of highly-pure and indistinguishable single photons have been demonstrated using quantum dots (QDs) in GaAs nanostructures [1], enabling the integration with planar circuitry. However, the efficient out-coupling of light from the chip into optical fibres, required for communication with distant quantum nodes and photon storage, remains a challenging task. Here, we report a spot-size converter for the end-fire coupling between suspended GaAs waveguides with embedded QDs and lensed fibres.

In a long-held preconception, photons play a central role in present-day quantum technologies. But what are sources producing photons one by one good for precisely? Well, in opposition to what many suggest, we show that single-photon sources are not helpful for point to point quantum key distribution because faint laser pulses do the job comfortably. However, there is no doubt about the usefulness of sources producing single photons for future quantum technologies. In particular, we show how single-photon sources could become the seed of a revolution in the framework of quantum communication, making the security of quantum key distribution device-independent or extending quantum communication over many hundreds of kilometers. Hopefully, these promising applications will provide a guideline for researchers to develop more and more efficient sources, producing narrowband, pure and indistinguishable photons at appropriate wavelengths.

On-chip single-photon sources are key components for integrated photonic quantum technologies. Semiconductor quantum dots can exhibit near-ideal single-photon emission, but this can be significantly degraded in on-chip geometries owing to nearby etched surfaces. A long-proposed solution to improve the indistinguishablility is to use the Purcell effect to reduce the radiative lifetime. However, until now only modest Purcell enhancements have been observed. Here we use pulsed resonant excitation to eliminate slow relaxation paths, revealing a highly Purcell-shortened radiative lifetime (22.7 ps) in a waveguide-coupled quantum dot-photonic crystal cavity system. This leads to near-lifetime-limited single-photon emission that retains high indistinguishablility (93.9%) on a timescale in which 20 photons may be emitted. Nearly background-free pulsed resonance fluorescence is achieved under π-pulse excitation, enabling demonstration of an on-chip, on-demand single-photon source with very high potential repetition rates.

In recent years, single-photon sources (SPSs) based on the emission of a single semiconductor quantum dot (QD) have been actively developed. While the purity and indistinguishability of single photons are already close to ideal values, the high brightness of SPSs remains a challenge. The widely used resonant excitation with cross-polarization filtering usually leads to at least a two-fold reduction in the single-photon counts rate, since single-photon emission is usually unpolarized, or its polarization state is close to that of the exciting laser. One of the solutions is the use of polarization-selective microcavities, which allows one to redirect most of the QD emission to a specific polarization determined by the optical mode of the microcavity. In the present work, elliptical micropillars with distributed Bragg reflectors are investigated theoretically and experimentally as a promising design of such polarization-selective microcavities. The impact of ellipticity, ellipse area and verticality of the side walls on the splitting of the optical fundamental mode is investigated. The study of the near-field pattern allows us to detect the presence of higher-order optical modes, which are classified theoretically. The possibility of obtaining strongly polarized single-photon QD radiation associated with the short-wavelength fundamental cavity mode is shown.

The performance of fiber Bragg grating (FBG) devices is determined by spatial distribution of FBG parameters. For many applications, like chirped FBG used for dispersion compensation, it is sufficient to control these parameters with spatial resolution of around 1 mm. We improve the sensitivity and accuracy of the side-diffraction FBG characterization at the cost of reducing the spatial resolution and demonstrate simultaneous measurement of fiber grating refractive index modulation with accuracy of 2/spl middot/10/sup -6/, grating period variation with accuracy of 5 pm, and spatial resolution of /spl sim/1 mm. The group delay ripple of a chirped FBG calculated from our side-diffraction characterization is in a good agreement with spectroscopic measurements.

To support the development of a U.K. quantum technology industry, NPL has been developing techniques to perform accurate and precise measurements of key parameters of components used in quantum photonic systems. This resulted in the first standardised methods for quantifying the performance of quantum communications hardware and is important for security evaluation. Attenuated coherent states are used in current hardware, but a single-photon source based on a single emitter could be more efficient. The emission from these single-photon sources is frequently characterised by the second-order correlation, g2(τ). A g2(0) < 0.5 indicates the channel consists of mostly single photons. Reported values of g2(0) never reach zero, as expected for a perfect single-photon source; these measurements cannot definitively ascribe the additional coincidences at τ = 0 to an uncorrelated background, or a second dimmer single-photon source. Higher order correlation functions provide additional discrimination between these contributions. We have performed g3(τ) measurements on NV centres in nanodiamond. We have developed a model which predicts the form of the measured histograms and have found that it fits the data well. In one example these results show no three-fold coincidences detected over an 18-hour period. This allows us to ascribe what fraction of the additional coincidences at g2(0) are due to a second single-photon source. This technique has immediate applicability for optimising single-photon sources, which can enable a range of photonic quantum technologies.

In biology as well as in quantum information protocols, efficient light sources based on nanoemitters are of great interest. For quantum cryptography, single photons sources, with high quantum yield but also efficient emission collection, are needed. Colloidal CdSe nanocrystals (NCx) are very good candidates for these purposes, as they are versatile and efficient single photon sources at the individual scale. For quantum gates, coherence time of the emission is crucial but its measurement, at cryogenic temperature, remains challenging for single CdSe NCx as they suffer of phase diffusion. A recent experiment proved that their coherence time is larger than 200 ps. But the measurement was limited by the collection efficiency, especially low when the sample is inside a cryostat, and an objective with a quite low numerical aperture has to be used. This paper proposes a configuration for which emission efficiency and far field collection of single NCx can be increased. A confocal microscope is used to collect part of the fluorescence of single quantum dots located at various distances from the gold surface and to measure the corresponding far field intensity. The electromagnetic interaction between a single NCx and its optical environment is highlighted by time-resolved fluorescence measurements at room temperature. Not only the possibility of emission control by coupling to surface plasmon of a metallic structure is demonstrated but also optimization of the far field collection efficiency, which is of great importance for applications in biology and quantum information.

Based on the quantum dot-photon molecular coupling system, we proposed an efficient and stable single photon source. The system uses photon molecules formed by the coupling of two optical microcavities A and B, one of which contains a quantum dot. The excitation laser act on the quantum dot in cavity A, and through the interaction between the light field in cavity A and cavity B, single photon is generated in cavity B and output extraction is performed therein. The system maintains good sub-Poisson characteristics over a wide range of dot-cavity coupling strength variation (0.25κ<;g<;κ) and at low cavity tunneling strength (J≥0.5κ) as well. It also has excellent robustness and is easy to construct. Under the optimal experimental parameters that can be realized, the zero-delay second-order correlation function g <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (0) of the system can be as low as 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> with a relatively large intra cavity photon number. We theoretically explains the reason why the system produces sub-Poisson light, and analyzed the impact of system parameters change. Meanwhile, the system spatially separates the output light from the excitation light, making it easy to extract single photons, and has potential application value for easier integration. The model proposed in this paper will bring important guiding value to the efficient and integrated single photon sources in future quantum information processing.

Efficient generation of single photons is one of the key challenges of building photonic quantum technology, such as quantum computers and long-distance quantum networks. Photon source multiplexing—where successful pair generation is heralded by the detection of one of the photons, and its partner is routed to a single mode output—has long been known to offer a concrete solution, with output probability tending toward unity as loss is reduced. Here, we present a temporally multiplexed integrated single photon source based on a silicon waveguide and a low-loss fibre switch and loop architecture, which achieves enhancement of the single photon output probability of 4.5 ± 0.5, while retaining g (2)(0) = 0.01.