Deterministic quantum dot single-photon sources: operational principles and state-of-the-art specifications

TL;DR

The paper addresses the bottleneck of scalable, deterministic quantum light sources by focusing on quantum dots embedded in planar photonic crystal waveguides. It articulates the operational principles that yield broadband emission control and strong light-matter coupling, enabling near-unity internal coupling factors ($\beta$) and robust resonant excitation. The work reports state-of-the-art metrics including $g^{(2)}(0)\approx0.1\%$, raw HOM visibility $V\approx97.1\%$, and system efficiencies up to $\eta_S\approx55\%$, with $\beta>98.4\%$, demonstrating high-purity, high-indistinguishability photons in a scalable platform. These results position planar QD waveguide sources as a mature, commercially viable route for multi-photon entanglement and scalable photonic quantum information processing, with ongoing improvements and potential telecom-band extensions such as O-band operation.

Abstract

Non-classical states of light play a fundamental role in quantum technology. From photonic quantum computers and simulators, to quantum communication and sensing, quantum states of light enable performing tasks that may outperform their best classical counterparts. Semiconductor quantum dots embedded in photonic nanostructures offer the most advanced classes of quantum light sources. Importantly, the underlying physics processes determining device performance are today fully understood, and dedicated engineering projects are currently advancing these sources towards real-world quantum technology applications. We review the performance of deterministic single-photon sources based on quantum dots in photonic crystal waveguides, the approach with the highest performance specs since it intrinsically combines suppression of leaky modes and Purcell enhancement to slow-light waveguide mode. Furthermore, we present prototype data from sources that today are commercially available and with performance metrics approaching the ideal.

Figures (6)

• Figure 1: Illustration of operational principle and chip layout of planar QD single-photon sources. a) Illustration of a QD single-photon source based on a planar photonic crystal waveguide. A pulsed laser repetitively excites the QD that subsequently emits single photon pulses into a photonic crystal waveguide that routes the train of photons to an outcoupling grating for high-efficiency coupling to a single-mode optical fibre. b) Image of single-photon source chip mounted on a printed circuit board (PCB). The external connectors on the PCB allow electrical access to the chip, enabling spectral tuning and the suppression of electrical noise. Insets: (i) Optical microscope image showing the overall chip layout, which consists of multiple blocks of nanostructures arranged in a grid. (ii) SEM image of a representative block containing an array of individual photonic nanostructures, allowing operation of multiple QD sources. (iii) High-magnification SEM view of a single photonic nanostructure.
• Figure 2: Full experimental setup for extracting single photons from the chip. A pulsed laser, with repetition rate $R_\mathrm{L}$ is sent via a beamsplitter into the cryostat. The polarisation is controlled by quarter- and half-wave plates after which the light is focused by an objective lens onto the single-photon chip, which is cooled to $\sim 4$K. The internal efficiency of the QD emitter is $\eta_\mathrm{QD}$, which specifies the probability of emitting a photon within the zero-phonon line and into the waveguide after being excited by the laser pulse. The collected photon subsequently propagates from the QD, through the chip and into free space with an efficiency $\eta_\mathrm{chip}$. The out-coupled light is sent upward, collected by the same lens and transmitted by the beamsplitter thereby separating the single-photon emission from the input light. Here, it encounters the first lens of the free space optics, where the brightness is measured. $\eta_\mathrm{optics}$ is the total transmission efficiency of the free-space optics, which controls photon polarisation, implements phonon sideband filtering, and finally couples the single photons into an optical fibre. The complete setup is fully automatized and can therefore be operated "hands free" constituting a full plug-and-play single-photon source system.
• Figure 3: Single photon purity and Hong-Ou-Mandel (HOM) visibility of state-of-the-art planar single-photon sources. a) Measurement of second order autocorrelation function $g^{(2)}(\tau)$ versus time delay, where the suppression of coincidences at zero time delay signals single-photon emission. The purity is quantified by the minor residual coincidences detected at zero time delay (see inset) and we find $g^{(2)}(0){=}\left(0.1{\pm}0.1\right)\%$. b) Histogram of coincidences versus time delays resulting from a HOM experiment where subsequently emitted photons from the QD are interfered on a symmetric beamsplitter. The indistinguishability is quantified by the minor residual coincidences detected at zero time delay (see inset) and we find $V = \left(97.1\pm0.1\right)\%$. Note that this constitutes an experimental lower bound with no corrections for multi-photon contributions or measurement setup asymmetries.
• Figure 4: Data of high-performance single-photon sources. Multiple sources are characterised under $\pi$-pulse excitation, yielding an average single-photon purity (dashed blue line) exceeding $99\%$, and an average raw HOM visibility (dashed green line) above $95\%$. Source $\mathrm{S}3$ corresponds to the maximum values quoted in the main text.
• Figure 5: Demonstration of blinking free high-efficiency single-photon source. a) Long time-scale autocorrelation measurement. The data is binned with a 25 ns bin width, corresponding to twice the repetition rate of the laser, avoiding aliasing effects from data sampling. Since the peak area remains equal throughout the 10$\mu s$ collection run, this measurement shows an absence of blinking processes, which would manifest as a decay of the peak area. b) Observation of Rabi oscillations in resonant excitation of QD devices. Directly measured (blue points) single photon count rates versus excitation power resulting from the coherent driving of source $\mathrm{S}1$. The detected count rate at $\pi$-pulse excitation here is $25.5$ MHz, slightly differing from the maximum value reported in the main text due to a different detection efficiency condition. The red curve is a fit to the data.