Quantum Dot Single Photon Sources Research Articles

The influence of solvent refractive index on the photoluminescence decay of thick-shell gradient-alloyed colloidal quantum dots investigated in a wide range of delay times.

Colloidal semiconductor quantum dots have many potential optical applications, including quantum dot light-emitting diodes, single-photon sources, or biological luminescent markers. The optical properties of colloidal quantum dots can be affected by their dielectric environment. This study investigated the photoluminescence (PL) decay of thick-shell gradient-alloyed colloidal semiconductor quantum dots as a function of solvent refractive index. These measurements were conducted in a wide range of delay times to account for both the initial spontaneous decay of excitons and the delayed emission of excitons that has the form of a power law. It is shown that whereas the initial spontaneous PL decay is very sensitive to the refractive index of the solvent, the power-law delayed emission of excitons is not. Our results seem to exclude the possibility of carrier self-trapping in the considered solvents and suggest the existence of trap states inside the quantum dots. Finally, our data show that the average exciton lifetime significantly decreases as a function of the solvent refractive index. The change in exciton lifetime is qualitatively modeled and discussed.

Programmable multiphoton quantum interference in a single spatial mode.

The interference of nonclassical states of light enables quantum-enhanced applications reaching from metrology to computation. Most commonly, the polarization or spatial location of single photons are used as addressable degrees of freedom for turning these applications into praxis. However, the scale-up for the processing of a large number of photons of these architectures is very resource-demanding due to the rapidly increasing number of components, such as optical elements, photon sources, and detectors. Here, we demonstrate a resource-efficient architecture for multiphoton processing based on time-bin encoding in a single spatial mode. We use an efficient quantum dot single-photon source and a fast programmable time-bin interferometer to observe the interference of up to eight photons in 16 modes, all recorded only with one detector, thus considerably reducing the physical overhead previously needed for achieving equivalent tasks. Our results can form the basis for a future universal photonics quantum processor operating in a single spatial mode.

Deterministic storage and retrieval of telecom light from a quantum dot single-photon source interfaced with an atomic quantum memory.

A hybrid interface of solid-state single-photon sources and atomic quantum memories is a long sought-after goal in photonic quantum technologies. Here, we demonstrate deterministic storage and retrieval of light from a semiconductor quantum dot in an atomic ensemble quantum memory at telecommunications wavelengths. We store single photons from an indium arsenide quantum dot in a high-bandwidth rubidium vapor-based quantum memory, with a total internal memory efficiency of (12.9 ± 0.4)%. The signal-to-noise ratio of the retrieved light field is 18.2 ± 0.6, limited only by detector dark counts.

A versatile single-photon-based quantum computing platform

AbstractQuantum computing aims at exploiting quantum phenomena to efficiently perform computations that are unfeasible even for the most powerful classical supercomputers. Among the promising technological approaches, photonic quantum computing offers the advantages of low decoherence, information processing with modest cryogenic requirements, and native integration with classical and quantum networks. So far, quantum computing demonstrations with light have implemented specific tasks with specialized hardware, notably Gaussian boson sampling, which permits the quantum computational advantage to be realized. Here we report a cloud-accessible versatile quantum computing prototype based on single photons. The device comprises a high-efficiency quantum-dot single-photon source feeding a universal linear optical network on a reconfigurable chip for which hardware errors are compensated by a machine-learned transpilation process. Our full software stack allows remote control of the device to perform computations via logic gates or direct photonic operations. For gate-based computation, we benchmark one-, two- and three-qubit gates with state-of-the art fidelities of 99.6 ± 0.1%, 93.8 ± 0.6% and 86 ± 1.2%, respectively. We also implement a variational quantum eigensolver, which we use to calculate the energy levels of the hydrogen molecule with chemical accuracy. For photon native computation, we implement a classifier algorithm using a three-photon-based quantum neural network and report a six-photon boson sampling demonstration on a universal reconfigurable integrated circuit. Finally, we report on a heralded three-photon entanglement generation, a key milestone toward measurement-based quantum computing.

Telecom‐Band Quantum Dots Compatible with Silicon Photonics for Photonic Quantum Applications

AbstractSilicon photonics is promising for quantum photonics applications owing to its large‐scale and high‐performance circuitry enabled by complementary‐metal‐oxide‐semiconductor fabrication processes. However, there is a lack of bright single‐photon sources (SPSs) capable of deterministic operation on Si platforms, which largely limits their applications. To this end, on‐Si integration of high‐performance solid‐state quantum emitters, such as semiconductor quantum dots (QDs), is greatly desired. In particular, it is preferable to integrate SPSs emitting at telecom wavelengths for fully leveraging the power of silicon photonics, including efficient chip‐to‐fiber coupling. In this review, recent progress and challenges in the integration of telecom QD SPSs onto silicon photonic platforms are discussed.

Generation and characterization of polarization-entangled states using quantum dot single-photon sources

Single-photon sources based on semiconductor quantum dots find several applications in quantum information processing due to their high single-photon indistinguishability, on-demand generation, and low multiphoton emission. In this context, the generation of entangled photons represents a challenging task with a possible solution relying on the interference in probabilistic gates of identical photons emitted at different pulses from the same source. In this work, we implement this approach via a simple and compact design that generates entangled photon pairs in the polarization degree of freedom. We operate the proposed platform with single photons produced through two different pumping schemes, the resonant excited one and the longitudinal-acoustic phonon-assisted configuration. We then characterize the produced entangled two-photon states by developing a complete model taking into account relevant experimental parameters, such as the second-order correlation function, Hong–Ou–Mandel visibility, multiphoton emission and pump laser filtering. Our source shows long-term stability and high quality of the generated entangled states, thus constituting a reliable building block for optical quantum technologies.

Quantum key distribution using deterministic single-photon sources over a field-installed fibre link

Quantum-dot-based single-photon sources are key assets for quantum information technology, supplying on-demand scalable quantum resources for computing and communication. However, long-lasting issues such as limited long-term stability and source brightness have traditionally impeded their adoption in real-world applications. Here, we realize a quantum key distribution field trial using true single photons across an 18-km-long dark fibre, located in the Copenhagen metropolitan area, using an optimized, state-of-the-art, quantum-dot single-photon source frequency-converted to the telecom wavelength. A secret key generation rate of > 2 kbits/s realized over a 9.6 dB channel loss is achieved with a polarization-encoded BB84 scheme, showing remarkable stability for more than 24 hours of continuous operation. Our results highlight the maturity of deterministic single-photon source technology while paving the way for advanced single-photon-based communication protocols, including fully device-independent quantum key distribution, towards the goal of a quantum internet.

Optical Memory in a Microfabricated Rubidium Vapor Cell.

Scalability presents a central platform challenge for the components of current quantum network implementations that can be addressed by microfabrication techniques. We demonstrate a high-bandwidth optical memory using a warm alkali atom ensemble in a microfabricated vapor cell compatible with wafer-scale fabrication. By applying an external tesla-order magnetic field, we explore a novel ground-state quantum memory scheme in the hyperfine Paschen-Back regime, where individual optical transitions can be addressed in a Doppler-broadened medium. Working on the ^{87}Rb D_{2} line, where deterministic quantum dot single-photon sources are available, we demonstrate bandwidth-matching with hundreds of megahertz broad light pulses keeping such sources in mind. For a storage time of 80ns we measure an end-to-end efficiency of η_{e2e}^{80 ns}=3.12(17)%, corresponding to an internal efficiency of η_{int}^{0 ns}=24(3)%, while achieving a signal-to-noise ratio of SNR=7.9(8) with coherent pulses at the single-photon level.

Sub-50 fs, 0.5 W average power Nd-doped fiber amplifier at 920 nm.

We develop an all polarization-maintaining (PM) 920 nm Nd-doped fiber amplifier delivering a train of pulses with ∼0.53 W average power and sub-50 fs duration. The sub-50 fs pulse benefits from the pre-chirping management method that allows for over 60 nm broadening spectrum without pulse breaking in the amplification stage. By virtue of the short pulse duration, the pulse peak power can reach to ∼0.31 MW in spite of the moderate average power. These results represent a key step in developing high-peak-power pulse Nd-doped fiber laser systems at 920 nm, which will find important applications in fields such as biomedical imaging, ultrafast optical spectroscopy, and excitation of quantum-dot single photon sources.

Robust excitation of C-band quantum dots for quantum communication

Building a quantum internet requires efficient and reliable quantum hardware, from photonic sources to quantum repeaters and detectors, ideally operating at telecommunication wavelengths. Thanks to their high brightness and single-photon purity, quantum dot (QD) sources hold the promise to achieve high communication rates for quantum-secured network applications. Furthermore, it was recently shown that excitation schemes such as longitudinal acoustic phonon-assisted (LA) pumping provide security benefits by scrambling the coherence between the emitted photon-number states. In this work, we investigate further advantages of LA-pumped quantum dots with emission in the telecom C-band as a core hardware component of the quantum internet. We experimentally demonstrate how varying the pump power and spectral detuning with respect to the excitonic transition can improve quantum-secured communication rates and provide stable emission statistics regardless of network-environment fluctuations. These findings have significant implications for general implementations of QD single-photon sources in practical quantum communication networks.

Deterministic photon source interfaced with a programmable silicon-nitride integrated circuit

We develop a quantum photonic platform that interconnects a high-quality quantum dot single-photon source and a low-loss photonic integrated circuit made in silicon nitride. The platform is characterized and programmed to demonstrate various multiphoton applications, including bosonic suppression laws and photonic entanglement generation. The results show a promising technological route forward to scale-up photonic quantum hardware.

Low-loss grating coupler based on inter-layer mode interference in a hybrid silicon nitride platform.

Surface grating couplers are an important component for interfacing photonic integrated circuits with optical fibers. However, conventional coupler designs typically provide limited performance due to low directionality and poor fiber-to-grating field overlap. The efficiency can be improved by using non-uniform grating structures at the expense of small critical dimensions complicating the fabrication process. While uniform gratings can alleviate this constraint, they produce an exponentially decaying near-field with the Gaussian fiber mode overlap limited to a theoretical maximum of 80%. In this work, we propose a uniform grating coupler that circumvents this field overlap limitation. This is achieved by leveraging inter-layer mode interference through a virtual directional coupler effect in a hybrid amorphous-silicon (α-Si) on silicon nitride (Si3N4) platform. By optimizing the inter-layer gap and grating geometry, a near-Gaussian profile of the out-radiated beam is achieved, resulting in an unprecedented grating-to-fiber overlap of 96%. The full three-dimensional (3D) finite-difference time-domain (FDTD) simulations show a high directionality of 84% and a record coupling loss of -1.27 dB with a 1-dB bandwidth of 20 nm for the uniform grating coupler design. Our device is designed for a wavelength of 950 nm aimed for use in hybrid quantum photonic integrated circuits using III-V quantum dot single photon sources.

On-Chip Hong–Ou–Mandel Interference from Separate Quantum Dot Emitters in an Integrated Circuit

Scalable quantum photonic technologies require the low-loss integration of many identical single-photon sources with photonic circuitry on a chip. Relatively complex quantum photonic circuits have already been demonstrated; however, sources used so far relied on parametric down-conversion which has a probabilistic nature that intrinsically limits its efficiency and scalability. Quantum emitter-based single-photon sources are free of this limitation, but frequency matching of multiple emitters within a single circuit remains challenging. In this work, we demonstrate a key component in this regard in the form of a fully monolithic GaAs circuit combining two frequency-matched quantum dot single-photon sources interconnected with a low-loss on-chip beamsplitter connected via single-mode ridge waveguides. This device enabled us to perform a two-photon interference experiment on-chip with a visibility reaching 66%. Our device could be further scaled up, providing a clear path to increasing the complexity of quantum circuits toward fully scalable integrated quantum technologies.

CMOS-compatible integration of telecom band InAs/InP quantum-dot single-photon sources on a Si chip using transfer printing

We report the hybrid integration of a telecom band InAs/InP quantum-dot (QD) single-photon source on a CMOS-processed Si photonics chip using transfer printing. The integration technique allows for the assembly of photonic components in a pick-and-place operation and therefore can introduce them on Si photonics chips after completing the entire CMOS-compatible fabrication processes. We demonstrate telecom single-photon generation from an InAs/InP QD integrated on Si and its coupling into a waveguide. We also demonstrate the integration of a QD on a fiber-pigtailed Si chip and single-photon output through the optical fiber, showing a novel pathway for modularizing solid-state quantum light sources.

Analysis and Design of Low-Loss and Fast All-Optical Switch Elements on Silicon Nitride for Integrated Quantum Photonics

Fast and ultra-low loss single-photon switching and routing are essential for photonic quantum computation and communication. To address this need in a scalable fashion, all-optical switches that can be fabricated in an ultra-low loss and mature Si <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> N <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{4}$</tex-math></inline-formula> photonic integrated circuit (PIC) foundry platform are designed and optimized for sub-ns switching times suitable for deterministic quantum-dot single-photon sources. The working principle relies on cross-phase modulation (XPM) of the single photons with a 1550-nm pump pulse and is enhanced by a ring resonator. Two different designs of the primary switch element are theoretically studied, namely a ring resonator intensity switch (RRIS) based on resonance shifting due to XPM and a ring resonator phase switch (RRPS) acting as an all-optical phase shifter in a Mach–Zehnder interferometer. As a novel approach to speed up the switching, chirped pre-emphasis and wipe sections for the pump pulses are utilized. A design tool is established from analytical expressions and serves as starting point for further optimization using a dedicated travelling-wave model (TWM). The TWM demonstrates the feasibility of both designs to be driven by either the proposed pre-emphasis pulse shape or a train of chirped Gaussian pulses. While the RRPS turns out to require less pump energy, its operation is more sensitive to pump-power fluctuations. Insertion losses below 0.1 dB and a power consumption below 5 nJ at 1 GHz switching rates for both configurations prove the potential of this concept for scalable quantum photonic applications.

Near-unity efficiency and photon indistinguishability for the “hourglass” single-photon source using suppression of the background emission

An on-going challenge within scalable optical quantum information processing is to increase the collection efficiency ε and the photon indistinguishability η of the single-photon source toward unity. Within quantum dot-based sources, the prospect of increasing the product εη arbitrarily close to unity was recently questioned. In this work, we discuss the influence of the trade-off between efficiency and indistinguishability in the presence of phonon-induced decoherence, and we show that the photonic “hourglass” design allows for improving εη beyond the predicted maximum for the standard micropillar design subject to this trade-off. This circumvention of the trade-off is possible thanks to control of the spontaneous emission into background radiation modes, and our work highlights the importance of engineering of the background emission in future pursuits of near-unity performance of quantum dot single-photon sources.

Enhancing quantum cryptography with quantum dot single-photon sources

Quantum cryptography harnesses quantum light, in particular single photons, to provide security guarantees that cannot be reached by classical means. For each cryptographic task, the security feature of interest is directly related to the photons’ non-classical properties. Quantum dot-based single-photon sources are remarkable candidates, as they can in principle emit deterministically, with high brightness and low multiphoton contribution. Here, we show that these sources provide additional security benefits, thanks to the tunability of coherence in the emitted photon-number states. We identify the optimal optical pumping scheme for the main quantum-cryptographic primitives, and benchmark their performance with respect to Poisson-distributed sources such as attenuated laser states and down-conversion sources. In particular, we elaborate on the advantage of using phonon-assisted and two-photon excitation rather than resonant excitation for quantum key distribution and other primitives. The presented results will guide future developments in solid-state and quantum information science for photon sources that are tailored to quantum communication tasks.

Quantifying n -Photon Indistinguishability with a Cyclic Integrated Interferometer

We report on a universal method to measure the genuine indistinguishability of n-photons - a crucial parameter that determines the accuracy of optical quantum computing. Our approach relies on a low-depth cyclic multiport interferometer with N = 2n modes, leading to a quantum interference fringe whose visibility is a direct measurement of the genuine n-photon indistinguishability. We experimentally demonstrate this technique for a 8-mode integrated interferometer fabricated using femtosecond laser micromachining and four photons from a quantum dot single-photon source. We measure a four-photon indistinguishability up to 0.81$\pm$0.03. This value decreases as we intentionally alter the photon pairwise indistinguishability. The low-depth and low-loss multiport interferometer design provides an efficient and scalable path to evaluate the genuine indistinguishability of resource states of increasing photon number.

Violation of Bell's inequality with quantum-dot single-photon sources

We investigate the possibility of realizing a loophole-free violation of Bell's inequality using deterministic single-photon sources. We provide a detailed analysis of a scheme to achieve such violations over long distances with immediate extensions to device-independent quantum key distribution. We investigate the effect of key experimental imperfections that are unavoidable in real-world single-photon sources including the finite degree of photon indistinguishability, single-photon purity, and the overall source efficiency. We benchmark the performance requirements to state-of-the-art deterministic single-photon sources based on quantum dots in photonic nanostructures and find that experimental realizations appear to be within reach. We also evaluate the requirements for a post-selected version of the protocol, which relaxes the demanding requirements with respect to the source efficiency.

Numerical optimization of single-mode fiber-coupled single-photon sources based on semiconductor quantum dots.

We perform extended numerical studies to maximize the overall photon coupling efficiency of fiber-coupled quantum dot single-photon sources emitting in the near-infrared and O-band and C-band. Using the finite element method, we optimize the photon extraction and fiber-coupling efficiency of quantum dot single-photon sources based on micromesas, microlenses, circular Bragg grating cavities and micropillars. The numerical simulations which consider the entire system consisting of the quantum dot source itself, the coupling lens, and the single-mode fiber, yield overall photon coupling efficiencies of up to 83%. Our work provides objectified comparability of different fiber-coupled single-photon sources and proposes optimized geometries for the realization of practical and highly efficient quantum dot single-photon sources.