On-demand Single-photon Sources Research Articles

Dissipative dynamics of optomagnonic nonclassical features via anti-Stokes optical pulses: squeezing, blockade, anti-correlation, and entanglement

We propose a feasible experimental model to investigate the generation and characterization of nonclassical states in a cavity optomagnonic system consisting of a ferromagnetic YIG sphere that simultaneously supports both the magnon mode and two whispering gallery modes of optical photons. The photons undergo the magnon-induced Brillouin light scattering, which is a well-established tool for the cavity-assisted manipulations of magnons as well as magnon spintronics. At first, we derive the desired interaction Hamiltonian under the influence of the anti-Stokes scattering process and then proceed to analyze the dynamical evolution of quantum statistics of photons and magnons as well as their intermodal entanglement. The results show that both photons and magnons generally acquire some nonclassical features, e.g., the strong antibunching and anti-correlation. Interestingly, the system may experience the perfect photon and magnon blockade phenomena, simultaneously. Besides, the nonclassical features may be protected against the unwanted environmental effects for a relatively long time, especially, in the weak driving field regime and when the system is initiated with a small number of particles. However, it should be noted that some fast quantum-classical transitions may occur in-between. Although the unwanted dissipative effects plague the nonclassical features, we show that this system can be adopted to prepare optomagnonic entangled states. The generation of entangled states depends on the initial state of the system and the interaction regime. The intermodal photon-magnon entanglement may be generated and pronounced, especially, if the system is initialized with low intensity even Schrödinger cat state in the strong coupling regime. The cavity-assisted manipulation of magnons is a unique and flexible mechanism that allows an interesting test bed for investigating the interdisciplinary contexts involving quantum optics and spintronics. Moreover, such a hybrid optomagnonic system may be used to design both on-demand single-photon and single-magnon sources and may find potential applications in quantum information processing.

Formation of a lateral p–n junction light-emitting diode on an n-type high-mobility GaAs/Al0.33Ga0.67As heterostructure

We have fabricated a device which includes two lateral p–n junctions on an n-type GaAs/Al0.33Ga0.67As heterostructure. A section of the n-type material has been converted to p-type by removing dopants and applying a voltage to a gate placed in this region. Controlled electroluminescence from both of the p–n junctions has been demonstrated by varying the applied bias voltages. An emission peak with a width of nm is observed around 812 nm. The electroluminescence seen from both junctions is considered to originate from the GaAs quantum well layer in the device. The lithographic techniques that we have developed are compatible for further integration of gated quantum devices such as single-electron pumps to build on-demand single-photon sources.

Generation of entangled-photons by a quantum dot cascade source in polarized cavities: Using cavity resonances to boost signals and preserve the entanglements.

Motivated by recent advances in the development of single photon emitters for quantum information sciences, here we design and formulate a quantum cascade model that describes cascade emission by a quantum dot (QD) in a cavity structure while preserving entanglement that stores information needed for single photon emission. The theoretical approach is based on a photonic structure that consists of two orthogonal cavities in which resonance with either the first or second of the two emitted photons is possible, leading to amplification and rerouting of the entangled light. The cavity-QD scheme uses a four-level cascade emitter that involves three levels for each polarization, leading to two spatially entangled photons for each polarization. By solving the Schrodinger equation, we identify the characteristic properties of the system, which can be used in conjunction with optimization techniques to achieve the "best" design relative to a set of prioritized criteria or constraints in our optical system. The theoretical investigations include an analysis of emission spectra in addition to the joint spectral density profile, and the results demonstrate the ability of the cavities to act as frequency filters for the photons that make up the entanglements and to modify entanglement properties. The results provide new opportunities for the experimental design and engineering of on-demand single photon sources.

Widely Tunable Single-Photon Frequency Conversion in a Nanomechanically Reconfigurable Si-Rich Nitride Waveguide

An on-demand single-photon source based on self-assembled semiconductor quantum dots (QDs) is a key building block for quantum communication and quantum information processing. To scale up, a challenge lies in frequency converting the emitted photons to the telecom wavelengths and eliminating the wavelength difference between multiple QDs. In this paper, we propose a novel nanomechanical silicon-rich nitride waveguide device, which can be reconfigured to realize efficient frequency conversion from the emission wavelength band of InAs/GaAs quantum dots at 900 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim$</tex-math></inline-formula> 950 nm to the telecom C-band around 1550 nm, by exploiting the Bragg scattering process in nonlinear four-wave mixing. Numerical results show that an optimal conversion efficiency over 99% and a signal-to-noise ratio up to 27 dB can be achieved when the waveguide loss is negligible, and the efficiency only reduces to 78% when a realistic loss of 1 dB/cm is considered. We conduct a detailed study on the unwanted noise processes, and provide design guidelines to improve the conversion efficiency and suppress the noises, e.g., by optimizing the power and wavelength of the pump laser.

Cavity-Enhanced 2D Material Quantum Emitters Deterministically Integrated with Silicon Nitride Microresonators.

Optically active defects in 2D materials, such as hexagonal boron nitride (hBN) and transition-metal dichalcogenides (TMDs), are an attractive class of single-photon emitters with high brightness, operation up to room temperature, site-specific engineering of emitter arrays with strain and irradiation techniques, and tunability with external electric fields. In this work, we demonstrate a novel approach to precisely align and embed hBN and TMDs within background-free silicon nitride microring resonators. Through the Purcell effect, high-purity hBN emitters exhibit a cavity-enhanced spectral coupling efficiency of up to 46% at room temperature, exceeding the theoretical limit (up to 40%) for cavity-free waveguide-emitter coupling and demonstrating nearly a 1 order of magnitude improvement over previous work. The devices are fabricated with a CMOS-compatible process and exhibit no degradation of the 2D material optical properties, robustness to thermal annealing, and 100 nm positioning accuracy of quantum emitters within single-mode waveguides, opening a path for scalable quantum photonic chips with on-demand single-photon sources.

Tunneling times of single photons

Although the group delay of classical pulses through a barrier may suggest superluminality, the information transfer is limited by the precursor, which propagates at the vacuum light speed. Single photons, however, have infinite tails, and the question of causality becomes meaningless. We solve this problem by introducing strictly localized states close to single photons, which are examples of optical states produced by on-demand single-photon sources. These states can be arbitrarily close to single photons while demonstrating causality for their leading edge.

Nanoscale Focus Pinspot for High-Purity Quantum Emitters via Focused-Ion-Beam-Induced Luminescence Quenching.

Epitaxially grown quantum dots (QDs), especially embedded in photonic structures, play an essential role in various quantum photonic systems as on-demand single-photon sources. However, these QDs often suffer from adjacent unwanted emitters, which contribute to the background noise of the QD emission and fundamentally limit the single-photon purity. In this paper, a nanoscale focus pinspot (NFP) technique using focused-ion-beam-induced luminescence quenching enables us to improve single-photon purity from site-controlled QD as a proof-of-concept experiment. The optical quality of the QD emission is not degraded while the signal-to-noise ratio of the QD is improved. Moreover, the QD after the NFP technique reveals the single-photon nature at further elevated temperatures owing to the reduced background noise. As the NFP technique is nondestructive, it retains the apparent physical structures and photonic functions, thereby indicating its promising potential for applying diverse high-purity quantum emitters, particularly integrated in photonic devices and circuits.

Investigation of the Time Behavior of the Second-Order Coherence Function of a Tunable Single-Photon Source

Single-photon sources are critical optical components in quantum communication, in particular, for security applications. One of the essential parameters that define these sources is the magnitude of the second-order coherence function, whose investigation reveals the state of the emitted photon. In this study, we indicate that the second-order coherence function varies over time when using two lasers and preparing coherent population trapping. The calculation is based on solving the master equation to find the density matrix corresponding to the emission dynamics and provide the second-order coherence function. The changes of the second-order coherence function can be estimated and the system behavior regarding photon emission can be predicted by solving the master equation based on the parameters obtained from the experimental results of a nitrogen vacancy (NV) in a diamond. Here we report, for the first time to the best of our knowledge, that the state of the emitted photons persists in the strong interaction of the aforementioned process. As using two lasers is a familiar method for controlling the single-photon source and the stability of the source is an essential point in a quantum network, this study can be considered to develop quantum network components such as memory and on-demand single-photon sources. Also, it suggests a method for tuning photon statistics while controlling the photon states.

Heralded Nondestructive Quantum Entangling Gate with Single-Photon Sources

Heralded entangling quantum gates are an essential element for the implementation of large-scale optical quantum computation. Yet, the experimental demonstration of genuine heralded entangling gates with free-flying output photons in linear optical system, was hindered by the intrinsically probabilistic source and double-pair emission in parametric down-conversion. Here, by using an on-demand single-photon source based on a semiconductor quantum dot embedded in a micropillar cavity, we demonstrate a heralded controlled-NOT (CNOT) operation between two single photons for the first time. To characterize the performance of the CNOT gate, we estimate its average quantum gate fidelity of (87.8±1.2)%. As an application, we generated event-ready Bell states with a fidelity of (83.4±2.4)%. Our results are an important step towards the development of photon-photon quantum logic gates.

Purcell-enhanced single-photon emission from a strain-tunable quantum dot in a cavity-waveguide device

On-chip quantum photonics is a promising route toward the implementation of complex photonic architectures on a small footprint. Therefore, different photonic components demonstrated for off-chip operation must be realized in an integrated manner. An essential building block for the realization of this goal is the integration of efficient on-demand single-photon sources within waveguide circuits. Here, we address this challenge by demonstrating the Purcell-enhanced single-photon emission from an In(Ga)As quantum dot coupled to a high-Q cavity-waveguide device. The combination with a piezoelectric actuator further enables the strain-induced emission energy tuning of the quantum dot as well as the cavity mode. We observe wavelength shifts up to 0.85 nm for the quantum dot, with a differential tuning factor of four between emitter and cavity. This allows for the full compensation of the spectral mismatch between a selected quantum dot and the cavity resonance. A nearly twofold enhancement of the spontaneous emission rate is observed at resonance with the on-demand generation of single photons. This demonstration of a strain-tunable emitter in a waveguide-coupled cavity device represents an essential building block for large scale quantum photonic circuits, especially if combined in the future with miniaturization approaches based on recently developed micromachined piezoelectric actuators.

Deterministically fabricated strain-tunable quantum dot single-photon sources emitting in the telecom O-band

Most quantum communication schemes aim at the long-distance transmission of quantum information. In the quantum repeater concept, the transmission line is subdivided into shorter links interconnected by entanglement distribution via Bell-state measurements to overcome inherent channel losses. This concept requires on-demand single-photon sources with a high degree of multi-photon suppression and high indistinguishability within each repeater node. For a successful operation of the repeater, a spectral matching of remote quantum light sources is essential. We present a spectrally tunable single-photon source emitting in the telecom O-band with the potential to function as a building block of a quantum communication network based on optical fibers. A thin membrane of GaAs embedding InGaAs quantum dots (QDs) is attached onto a piezoelectric actuator via gold thermocompression bonding. Here, the thin gold layer acts simultaneously as an electrical contact, strain transmission medium, and broadband backside mirror for the QD-micromesa. The nanofabrication of the QD-micromesa is based on in situ electron-beam lithography, which makes it possible to integrate pre-selected single QDs deterministically into the center of monolithic micromesa structures. The QD pre-selection is based on distinct single-QD properties, signal intensity, and emission energy. In combination with strain-induced fine tuning, this offers a robust method to achieve spectral resonance in the emission of remote QDs. We show that the spectral tuning has no detectable influence on the multi-photon suppression with g(2)(0) as low as 2%–4% and that the emission can be stabilized to an accuracy of 4 μeV using a closed-loop optical feedback.

Planarized spatially-regular arrays of spectrally uniform single quantum dots as on-chip single photon sources for quantum optical circuits

A long standing obstacle to realizing highly sought on-chip monolithic solid state quantum optical circuits has been the lack of a starting platform comprising scalable spatially ordered and spectrally uniform on-demand single photon sources (SPSs) buried under a planar surface. In this paper, we report on the first realization of planarized SPS arrays based on a unique class of shape-controlled single quantum dots (SQDs) synthesized on mesa top (dubbed MTSQDs) using substrate-encoded size-reducing epitaxy (SESRE) on spatially regular arrays of patterned nanomesas with edge orientation chosen to drive symmetric adatom migration from the nanomesa sidewalls to the top, thereby enabling spatially selective growth. Specifically, on GaAs(001) square nanomesas with edges along ⟨100⟩, we synthesized binary GaAs/InAs/GaAs MTSQDs emitting around 1120 nm with 1.8 nm standard deviation and single photon emission purity &amp;gt;99.5%. SESRE based MTSQDs are shown for the first time to lend themselves to planarization of the surface morphology when grown on pedestal shape mesas. We demonstrate that the planarizing overgrowth process over arrays of InGaAs SQDs largely maintains the SQDs’ high single photon emission purity (&amp;gt;98%) and spectral uniformity (∼5 nm). Such planarized SQD arrays offer the long-sought platform for on-chip integration with light manipulating structures to realize quantum optical circuits.

Demonstration of InP/InAsP/InP axial heterostructure nanowire array vertical LEDs

Semiconductor nanowires (NWs), which have nanoscale footprints, enable us to realize various quantum structures with excellent position and size controllability, utilizing a wide range of materials for heterostructures. In addition, enhancing light extraction and controlling spontaneous emission by modifying their size and shape are possible. Thus, NWs are promising materials for nanoscale light sources applicable from visible to telecommunication bands. In this study, we grew InP/InAsP/InP axial heterostructure NWs, where the InAsP layer was embedded to serve as an active layer, by selective-area growth and demonstrated vertical NW array light-emitting diodes (LEDs) as a step towards realizing on-demand single photon sources. The NW array LEDs showed rectifying characteristics and electroluminescence originating from the embedded InAsP layer in the near-infrared region.

Proof-of-principle demonstration of compiled Shor's algorithm using a quantum dot single-photon source.

We report a proof-of-principle demonstration of Shor's algorithm with photons generated by an on-demand semiconductor quantum dot single-photon source for the first time. A fully compiled version of Shor's algorithm for factoring 15 has been accomplished with a significantly reduced resource requirement that employs the four-photon cluster state. Genuine multiparticle entanglement properties are confirmed to reveal the quantum character of the algorithm and circuit. The implementation realizes the Shor's algorithm with deterministic photonic qubits, which opens new applications for cluster state beyond one-way quantum computing.

Highly customized 1010 nm, ns-pulsed Yb-doped fiber amplifier as a key tool for on-demand single-photon generation.

The development of highly customized technical devices is a decisive feature of technically complex setups, as frequently observed in quantum experiments. This paper describes the development and realization of an Yb-doped all-fiber amplifier system designed for such a special application, more specifically, an on-demand single-photon source based on four-wave mixing with rubidium Rydberg atoms. The laser is capable of generating bandwidth-limited configurable nanosecond pulses up to peak powers of >100 W and with pulse repetition frequencies (PRF) between 50 Hz and 1 MHz at selectable wavelengths (1008-1024 nm). Especially the amplification of the 1010 nm reference seed at the lower edge of the amplification range for Yb-based fibers is challenging and tends to produce amplified spontaneous emission (ASE) at higher wavelengths. To achieve high ASE suppression, particularly at low pulse repetition frequencies, two acousto-optical modulators (AOM) are utilized both for pulse picking and for temporal filtering. The synchronization between pulse repetition frequency and AOM driver signal allows pulse amplitude fluctuations to be kept below 1%, while ASE is suppressed by at least 85 dB (PRF = 1 MHz) and 65 dB (PRF = 1 kHz).

Tunable Microwave Single-Photon Source Based on Transmon Qubit with High Efficiency

Single-photon sources are of great interest because they are key elements in different promising applications of quantum technologies. Here we demonstrate a highly efficient tunable on-demand microwave single-photon source based on a transmon qubit with the intrinsic emission efficiency above 98$\%$. The high efficiency ensures a negligible pure dephasing rate and the necessary condition for generation of indistinguishable photons. We provide an extended discussion and analysis of the efficiency of the photon generation. To further experimentally confirm the single-photon property of the source, correlation functions of the emission field are also measured using linear detectors with a GPU-enhanced signal processing technique. Our results experimentally demonstrate that frequency tunability and negligible pure dephasing rate can be achieved simultaneously and show that such a tunable single-photon source can be good for various practical applications in quantum communication, simulations and information processing in the microwave regime.

Recent advances in nanowire quantum dot (NWQD) single-photon emitters

Future development of quantum technologies is dependent upon physical implementation of quantum systems. Photonic platforms have gained significant attention owing to the appealing properties that they offer, namely small probabilities of decoherence emergence, as well as being readily manipulable. In this regard, single-photon emitters play an integral role, and advancements in the design of single-photon sources, providing an anti-bunching photon correlation, have transitioned from its formerly proof-of-concept status to engineering attempts, and this has realized thanks to the ever-increasing improvements in the development of promising material platforms. In the race toward the realization of on-demand single-photon sources, solid-state host systems and in particular the quantum dot (QD)-based schemes, have very-well taken advantage of the recent headways in the bottom-up nanofabrication procedures. In this contribution, a review is presented on the recent progress and the existing challenges in fabrication of single-photon emitters, based on QD-containing nanowires (NWs) and their principles of operation. More specifically, we will consider how variation in values of different parameters, such as the purity, height and diameter of the NW waveguides, the electric-dipole orientation of the embedded QD, as well as different waveguide geometries, fabrication techniques and conditions exploited in their manufacturing, will affect the desired outputs in the system, i.e., the narrow photoluminescence spectrum, ideal single-photon emission of sufficient lifetime, and also the Gaussian profile in the far-field distribution. Additional focus is also given to different areas that the NWQD single-photon emitters have found application for quantum information processing purposes.

Indistinguishable Photons from Deterministically Integrated Single Quantum Dots in Heterogeneous GaAs/Si3N4 Quantum Photonic Circuits.

Silicon photonics enables scaling of quantum photonic systems by allowing the creation of extensive, low-loss, reconfigurable networks linking various functional on-chip elements. Inclusion of single quantum emitters onto photonic circuits, acting as on-demand sources of indistinguishable photons or single-photon nonlinearities, may enable large-scale chip-based quantum photonic circuits and networks. Toward this, we use low-temperature in situ electron-beam lithography to deterministically produce hybrid GaAs/Si3N4 photonic devices containing single InAs quantum dots precisely located inside nanophotonic structures, which act as efficient, Si3N4 waveguide-coupled on-chip, on-demand single-photon sources. The precise positioning afforded by our scalable fabrication method furthermore allows observation of postselected indistinguishable photons. This indicates a promising path toward significant scaling of chip-based quantum photonics, enabled by large fluxes of indistinguishable single-photons produced on-demand, directly on-chip.

Theory and Limits of On-Demand Single-Photon Sources Using Plasmonic Resonators: A Quantized Quasinormal Mode Approach

Quantum emitters coupled to plasmonic resonators are known to allow enhanced broadband Purcell factors, and such systems have been recently suggested as possible candidates for on-demand single photon sources, with fast operation speeds. However, a true single photon source has strict requirements of high efficiency (brightness) and quantum indistinguishability of the emitted photons, which can be quantified through two-photon interference experiments. To help address this problem, we employ and extend a recently developed quantized quasinormal mode approach, which rigorously quantizes arbitrarily lossy open system modes, to compute the key parameters that accurately quantify the figures of merit for plasmon-based single photon sources. We also present a quantized input-output theory to quantify the radiative and nonradiative quantum efficiencies. We exemplify the theory using a nanoplasmonic dimer resonator made up of two gold nanorods, which yields large Purcell factors and good radiative output beta factors. Considering an optically pulsed excitation scheme, we explore the key roles of pulse duration and pure dephasing on the single photon properties, and show that ultrashort pulses (sub-ps) are generally required for such structures, even for low temperature operation. We also quantify the role of the nonradiative beta factor both for single photon and two-photon emission processes. Our general approach can be applied to a wide variety of plasmon systems, including metal-dielectrics, and cavity-waveguide systems, without recourse to phenomenological quantization schemes.

Numerical analyses of emission of a single-photon pulse based on single-atom–cavity quantum electrodynamics

We numerically investigate an on-demand single-photon source, which is implemented with a strongly coupled atom-cavity system, proposed by Kuhn {\it et al}., Appl. Phys. B \textbf{69}, 373 (1999). In the scheme of Kuhn {\it et al}., a $\Lambda$-type three-level atom is captured in a single-mode optical cavity. Considering the three atomic levels, the ground state $u$, the first excited state $g$ accompanying the cavity mode, and the second excited state $e$, in the $\Lambda$-configuration, we assume that a classical field and a quantized cavity field lead to the transition between $u$ and $e$ and that between $e$ and $g$, respectively. The classical light pulse rising sufficiently slowly triggers an adiabatic process of the system and lets a single photon of the cavity mode emerge. We simulate this adiabatic evolution and transmission of the single photon through an imperfect mirror of the cavity using the master equation. We concentrate on examining physical properties of the efficiency of single-photon generation, the fluctuation of the duration of the photon emission, and the time of the emission measured from a peak of the trigger pulse. We find a function that approximates to the efficiency closely and the upper bound of the fluctuation of the duration.