Triggered Single-photon Source Research Articles

Electrically Pumped h-BN Single-Photon Emission in van der Waals Heterostructure.

Atomic defects in solids offer a versatile basis to study and realize quantum phenomena and information science in various integrated systems. All-electrical pumping of single defects to create quantum light emission has been realized in several platforms including color centers in diamond and silicon carbide, which could lead to the circuit network of electrically triggered single-photon sources. However, a wide conduction channel which reduces the carrier injection per defect site has been a major obstacle. Here, we realize a device concept to construct electrically pumped single-photon emission using a van der Waals stacked structure with atomic plane precision. Defect-induced tunneling currents across graphene and NbSe2 electrodes sandwiching an atomically thin h-BN layer allow robust and persistent generation of nonclassical light from h-BN. The collected emission photon energies range between 1.4 and 2.9 eV, revealing the electrical excitation of a variety of atomic defects. By analyzing the dipole axis of observed emitters, we further confirm that emitters are crystallographic defect structures of h-BN crystal. Our work facilitates implementing efficient and miniaturized single-photon devices in van der Waals platforms toward applications in quantum optoelectronics.

Absolute calibration of a single-photon avalanche detector using a bright triggered single-photon source based on an InGaAs quantum dot.

We apply an InGaAs quantum dot based single-photon source for the absolute detection efficiency calibration of a silicon single-photon avalanche diode operating in Geiger mode. The single-photon source delivers up to (2.55 ± 0.02) × 106 photons per second inside a multimode fiber at the wavelength of 929.8 nm for above-band pulsed excitation with a repetition rate of 80 MHz. The purity of the single-photon emission, expressed by the value of the 2nd order correlation function g(2)(τ = 0), is between 0.14 and 0.24 depending on the excitation power applied to the quantum dot. The single-photon flux is sufficient to be measured with an analog low-noise reference detector, which is traceable to the national standard for optical radiant flux. The measured detection efficiency using the single-photon source remains constant within the measurement uncertainty for different photon fluxes. The corresponding weighted mean thus amounts to 0.3263 with a standard uncertainty of 0.0022.

InP-based single-photon sources operating at telecom C-band with increased extraction efficiency

In this work, we demonstrate a triggered single-photon source operating at the telecom C-band with photon extraction efficiency exceeding any reported values in this range. The non-classical light emission with low probability of the multiphoton events is realized with single InAs quantum dots (QDs) grown by molecular beam epitaxy and embedded directly in an InP matrix. Low QD spatial density on the order of 5 × 108 cm−2 to ∼2 × 109 cm−2 and symmetric shape of these nanostructures together with spectral range of emission make them relevant for quantum communication applications. The engineering of extraction efficiency is realized by combining a bottom distributed Bragg reflector consisting of 25 pairs of InP/In0.53Ga0.37Al0.1 As layers and cylindrical photonic confinement structures. Realization of such a technologically non-demanding approach even in a non-deterministic fashion results in photon extraction efficiency of (13.3 ± 2) % into 0.4 numerical aperture detection optics at approximately 1560 nm emission wavelength, i.e., close to the center of the telecom C-band.

Radiometric characterization of a triggered narrow-bandwidth single-photon source and its use for the calibration of silicon single-photon avalanche detectors

The traceability of measurements of the parameters characterizing single-photon sources, such as photon flux and optical power, paves the way towards their reliable comparison and quantitative evaluation. In this paper, we present an absolute measurement of the optical power of a single-photon source based on an InGaAs quantum dot under pulsed excitation with a calibrated single-photon avalanche diode (SPAD) detector. For this purpose, a single excitonic line of the quantum dot emission with a bandwidth below 0.1 nm was spectrally filtered by using two tilted interference filters. Since high count rates are essential for many metrological applications, we optimized the setup efficiency by combining high overall transmission of the optical components with a geometrical enhancement of the extraction efficiency of a single quantum dot by a monolithic microlens to reach photon fluxes up to photons per second at the SPADs. A relative calibration of two SPAD detectors with a relative standard uncertainty of 0.7% was carried out and verified by the standard calibration method using an attenuated laser. Finally, an Allan deviation analysis was performed giving an optimal averaging time of 92 s for the photon flux.

Tools for the performance optimization of single-photon quantum key distribution

Quantum light sources emitting triggered single photons or entangled photon pairs have the potential to boost the performance of quantum key distribution (QKD) systems. Proof-of-principle experiments affirmed these prospects, but further efforts are necessary to push this field beyond its current status. In this work, we show that temporal filtering of single-photon pulses enables a performance optimization of QKD systems implemented with realistic quantum light sources, both in experiment and simulations. To this end, we analyze the influence of temporal filtering of sub-Poissonian single-photon pulses on the expected secret key fraction, the quantum bit error ratio, and the tolerable channel losses. For this purpose, we developed a basic QKD testbed comprising a triggered solid-state single-photon source and a receiver module designed for four-state polarization coding via the BB84 protocol. Furthermore, we demonstrate real-time security monitoring by analyzing the photon statistics, in terms of g(2)(0), inside the quantum channel by correlating the photon flux recorded at the four ports of our receiver. Our findings are useful for the certification of QKD and can be applied and further extended for the optimization of various implementations of quantum communication based on sub-Poissonian quantum light sources, including measurement-device-independent schemes of QKD as well as quantum repeaters. Our work represents an important contribution towards the development of QKD-secured communication networks based on quantum light sources.

Instabilities in nanodiamond nitrogen-vacancy centre single photon sources under prolonged pulsed excitation

Colour centres in nanodiamonds provide robust sources of fluorescence and can be used as triggered sources of single photons at room temperature. However, practical devices require stability over thousands of hours of operation, and the use of strong pulsed optical excitation, placing significant burden on the robustness of the emitters that requires bespoke testing. In this work we report the response of single NV centres in nanodiamonds of 50 nm and 100 nm diameter to accelerated lifetime testing, exciting the defects close to saturation around 1013 times to simulate the minimum operational lifetime of a practical device. For nanodiamonds 50 nm in diameter, observed changes in the fluorescence intensity and lifetime suggest a progressive size reduction as a result of the pulsed laser excitation, combined with the introduction of non-radiative centres on or near the nanodiamond surface which affect the quantum efficiency of the NV centre and ultimately lead to photobleaching of the emission. We find examples of NV centres in 100 nm nanodiamonds for which triggered single photon emission remains stable for over these accelerated lifetime tests, demonstrating their suitability for use in practical devices.

Near-Unity Indistinguishability Single Photon Source for Large-Scale Integrated Quantum Optics.

Integrated single photon sources are key building blocks for realizing scalable devices for quantum information processing. For such applications highly coherent and indistinguishable single photons on a chip are required. Here we report on a triggered resonance fluorescence single photon source based on In(Ga)As/GaAs quantum dots coupled to single- and multimode ridge waveguides. We demonstrate the generation of highly linearly polarized resonance fluorescence photons with 99.1% (96.0%) single photon purity and 97.5% (95.0%) indistinguishability in case of multimode (single mode) waveguide devices fulfilling the strict requirements imposed by multi-interferometric quantum optics applications. Our integrated triggered single photon source can be readily scaled up, promising a realistic pathway for on-chip linear optical quantum simulation, quantum computation, and quantum networks.

A stable wavelength-tunable triggered source of single photons and cascaded photon pairs at the telecom C-band

The implementation of fiber-based long-range quantum communication requires tunable sources of single photons at the telecom C-band. Stable and easy-to-implement wavelength-tunability of individual sources is crucial to (i) bring remote sources into resonance, (ii) define a wavelength standard, and (iii) ensure scalability to operate a quantum repeater. So far, the most promising sources for true, telecom single photons are semiconductor quantum dots, due to their ability to deterministically and reliably emit single and entangled photons. However, the required wavelength-tunability is hard to attain. Here, we show a stable wavelength-tunable quantum light source by integrating strain-released InAs quantum dots on piezoelectric substrates. We present triggered single-photon emission at 1.55 μm with a multi-photon emission probability as low as 0.097, as well as photon pair emission from the radiative biexciton–exciton cascade. We achieve a tuning range of 0.25 nm which will allow us to spectrally overlap remote quantum dots or tuning distant quantum dots into resonance with quantum memories. This opens up realistic avenues for the implementation of photonic quantum information processing applications at telecom wavelengths.

Triggered high-purity telecom-wavelength single-photon generation from p-shell-driven InGaAs/GaAs quantum dot.

We report on the experimental demonstration of triggered single-photon emission at the telecom O-band from In(Ga)As/GaAs quantum dots (QDs) grown by metal-organic vapor-phase epitaxy. Micro-photoluminescence excitation experiments allowed us to identify the p-shell excitonic states in agreement with high excitation photoluminescence on the ensemble of QDs. Hereby we drive an O-band-emitting GaAs-based QD into the p-shell states to get a triggered single photon source of high purity. Applying pulsed p-shell resonant excitation results in strong suppression of multiphoton events evidenced by the as measured value of the second-order correlation function at zero delay of 0.03 (and ~0.005 after background correction).

Suppression of single-cesium-atom heating in a microscopic optical dipole trap for demonstration of an 852-nm triggered single-photon source

We investigate single cesium (Cs) atom heating owing to the momentum accumulation process induced by the resonant pulsed excitation in a microscopic optical dipole trap formed by a strongly focused 1064 nm laser beam. The heating depends on the trap frequency which restricts the maximum repetition rate of pulsed excitation. We experimentally verify the heating of a single atom and then demonstrate how to suppress it with an optimized pulsed excitation/cooling method. The typical trap lifetime of single Cs atom is extended from 108 +/- 6 us to 2536 +/- 31 ms, and the corresponding number of excitation increases from ~ 108 to ~ 360000. In applying this faster cooling method, we use the trapped single Cs atom as a triggered single-photon source at an excitation repetition rate of 10 MHz. The second-order intensity correlations of the emitted single photons are characterized by implementing Hanbury Brown and Twiss setup, and clear anti-bunching effect has been observed.

High on/off ratio nanosecond laser pulses for a triggered single-photon source

An 852 nm nanosecond laser pulse chain with a high on/off ratio is generated by chopping a continuous-wave laser beam using a Mach–Zehnder-type electro-optic intensity modulator (MZ-EOIM). The detailed dependence of the MZ-EOIM’s on/off ratio on various parameters is characterized. By optimizing the incident beam polarization and stabilizing the MZ-EOIM temperature, a static on/off ratio of is achieved. The dynamic on/off ratios versus the pulse repetition rate and the pulse duty cycle are measured and discussed. The high-on/off-ratio nanosecond pulsed laser system was used in a triggered single-photon source based on a trapped single cesium atom, which reveals clear antibunching.

Plasmonic nanoantenna based triggered single-photon source

Highly integrated single photon sources are key components in future quantum-optical circuits. Whereas the probabilistic generation of single photons can routinely be done by now, their triggered generation is a much greater challenge. Here, we describe the triggered generation of single photons in a hybrid plasmonic device. It consists of a lambda-type quantum emitter coupled to a multimode optical nanoantenna. For moderate interaction strengths between the subsystems, the description of the quantum optical evolution can be simplified by an adiabatic elimination of the electromagnetic fields of the nanoantenna modes. This leads to an insightful analysis of the emitter's dynamics, entails the opportunity to understand the physics of the device, and to identify parameter regimes for a desired operation. Even though the approach presented in this work is general, we consider a simple exemplary design of a plasmonic nanoantenna, made of two silver nanorods, suitable for triggered generation of single photons.

852-nm triggered single-photon source based on trapping and manipulation of a single cesium atom confined in a microscopic optical dipole trap

Single-atom-based single-photon source has several advantages, such as narrow bandwidth, wavelength matching with the absorption line of the same atomic ensemble, and insensitivity to the environment disturbing, and it is very important not only for basic researches in quantum optic field but also for applications in quantum information processing. In this paper, we report the generation of a 10-MHz-repetition-rate triggered single-photon source at 852 nm based on a trapped single cesium atom in a far-off-resonance microscopic optical dipole trap (FORT). To generate an optical dipole trap, a far-red-detuned 1064 nm laser beam is tightly focused by using a high numerical aperture lens, a typical trap depth is 2 mK and trap waist is 2.3 m. To obtain a maximum probability of pulsed excitation, the frequency of the pulsed laser should be resonant with the atomic energy levels and the trapped single atom must be excited with a -pulse. However, the interaction between the FORT laser and the atoms causes AC Stark shifts of the atomic energy levels. Thus, in order to demonstrate the resonant pulsed excitation, it is important to calculate and measure the shift of 6S1/2|Fg=4,mF=+4-6P3/2|Fe=5,mF=+5 cyclical transition in the FORT. For a two-level system, the probability of pulsed excitation can be described by Rabi oscillations with a characteristic Rabi frequency . With an optimized time sequence, we experimentally demonstrate the Rabi oscillation between the ground state and the excited state, and the peak power of -pulse laser is about 1.25 mW. We also measure the temporal envelope of single photons after a -pulse excitation. A gated pulsed excitation and cooling technique are used to reduce the possibility that atoms are heated by -pulse laser. The typical trapping lifetime of single cesium atom is extended from~108 ups to~2536 ms. The corresponding number of excitations is improved from 108 to 360000. The second-order intensity correlations of the emitted single-photon are characterized by implementing Hanbury Brown-Twiss setup. The statistics shows a strong anti-bunching with a value of 0.09 for the second-order correlation at zero delay. In the future, we will perform a Hong-Ou-Mandel two-photon interference experiment to analyze the indistinguishability of the single photons. We will also trap single atoms in a magic-wavelength optical dipole trap where the ground and the excited states have the same shift.

Investigation of the silicon vacancy color center for quantum key distribution.

Single photon sources (SPS) are crucial for quantum key distribution. Here we demonstrate a stable triggered SPS at 738 nm with linewidth less than 5 nm at room temperature based on a negatively charged single silicon vacancy color center. Thanks to the short photon duration of about 1.3-1.7 ns, by using high repetition pulsed excitation at 30 MHz, the triggered single photon source generates 16.6 kcounts/s. And we discuss the feasibility of this triggered SPS in the application of quantum key distribution.

Improving the performance of bright quantum dot single photon sources using temporal filtering via amplitude modulation.

Single epitaxially-grown semiconductor quantum dots have great potential as single photon sources for photonic quantum technologies, though in practice devices often exhibit nonideal behavior. Here, we demonstrate that amplitude modulation can improve the performance of quantum-dot-based sources. Starting with a bright source consisting of a single quantum dot in a fiber-coupled microdisk cavity, we use synchronized amplitude modulation to temporally filter the emitted light. We observe that the single photon purity, temporal overlap between successive emission events, and indistinguishability can be greatly improved with this technique. As this method can be applied to any triggered single photon source, independent of geometry and after device fabrication, it is a flexible approach to improve the performance of systems based on single solid-state quantum emitters, which often suffer from excess dephasing and multi-photon background emission.

Electrically Driven Quantum Dot Micropillar Light Sources

We report on light sources based on electrically pumped quantum dot (QD) micropillar cavities. The low-mode-volume high-quality microstructures feature pronounced cavity quantum electrodynamics (cQED) effects that are exploited for the realization of efficient single-photon sources and low-threshold microlasers. The compact and electrically driven devices are of special interest for applications in the field of quantum communication. In particular, operated as electrically triggered single-photon sources, the QD micropillars can act as building blocks for quantum key distribution and quantum repeaters. On the other hand, the electrically pumped microlasers with in-plane emission via whispering gallery modes or emission normal to the sample's surface are predestinated for integrated light sources in future photonic networks. The devices are based on doped high-quality factor GaAs/AlAs microcavity structures with InGaAs QDs in the active layer. A lateral injection scheme leaves the upper facet of the micropillars free of any absorbing metal and allows for efficient light output under electrical pumping of low-mode-volume micropillars with diameters between 1 and 20 μm. Due to cQED effects, triggered single-photon emission with high photon extraction efficiency up to 62% and a low multiphoton emission probability (g <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> (0) = 0.16) are realized for moderate-quality (Q ) factor samples. The efficient coupling of spontaneous emission into the lasing mode in high- Q micropillars results in ultralow laser threshold currents of less than 10 μA at cryogenic temperatures. Our paper demonstrates the high potential of electrically driven QD micropillars to act as integrated light sources in future communication systems.

Recent advances on single photon sources based on single colloidal nanocrystals

AbstractSingle colloidal quantum dots (QDs) are increasingly exploited as triggered sources of single photons. This review reports on recent results on single photon sources (SPS) based on colloidal quantum dots, whose size, shape and optical properties can be finely tuned by wet chemistry approach. First, we address the optical properties of different colloidal nanocrystals, such as dots, rods and dot in rods and their use as single photon sources will be discussed. Then, we describe different techniques for isolation and positioning single QDs, a major issue for fabrication of single photon sources, and various approaches for the embedding single nanocrystals inside microcavities. The insertion of single colloidal QDs in quantum confined optical systems allows one to improve their overall optical properties and performances in terms of efficiency, directionality, life time, and polarization control. Finally, electrical pumping of colloidal nanocrystals light emitting devices and of NC-based single photon sources is reviewed.

Wavelength Tunable Triggered Single-Photon Source from a Single CdTe Quantum Dot on Silicon Substrate

Triggered single-photon emission from a single CdTe quantum dot (QD) grown on Si(001) substrate is demonstrated for the first time. The emission wavelength of QDs can be tuned in a wide spectral range (more than 8 meV) using a focused laser beam. A nearly perfect single-photon emission from the exciton lines is preserved even after energy tuning. The lifetime is also measured before and after laser processing, and no appreciable change is observed.

Quantum key distribution with triggering parametric down-conversion sources

Parametric down-conversion (PDC) sources can be used as triggered single photon sources in quantum key distribution (QKD). Recently, there have been several practical proposals of the decoy state QKD with triggering PDC sources. In this paper, we generalize the passive decoy state idea, originally proposed by Mauerer and Silberhorn. The generalized passive decoy state idea can be applied to cases where either threshold detectors or photon-number-resolving detectors are used. The decoy state protocol proposed by Adachi, Yamamoto, Koashi and Imoto (AYKI) can be treated as a special case of the generalized passive decoy state method. By simulating a recent PDC experiment, we compare various practical decoy state protocols with the infinite decoy state protocol and also compare the cases using threshold detectors and photon-number-resolving detectors. Our simulation result shows that with the AYKI protocol, one can achieve a key generation rate that is close to the theoretical limit of an infinite decoy state protocol. Furthermore, our simulation result shows that a photon-number-resolving detector appears to be not very useful for improving QKD performance in this case. Although our analysis is focused on QKD with PDC sources, we emphasize that it can also be applied to QKD setups with other triggered single photon sources.

Room temperature triggered single-photon source in the near infrared

We report the realization of a solid-state triggered single-photon source with narrow emission in the near infrared at room temperature. It is based on the photoluminescence of a single nickel–nitrogen NE8 colour centre in a chemical vapour deposited diamond nanocrystal. Stable single-photon emission has been observed in the photoluminescence under both continuous-wave and pulsed excitations. The realization of this source represents a step forward in the application of diamond-based single-photon sources to quantum key distribution (QKD) under practical operating conditions.