Scalable integrated single-photon source

Abstract

Photonic qubits are key enablers for quantum information processing deployable across a distributed quantum network. An on-demand and truly scalable source of indistinguishable single photons is the essential component enabling high-fidelity photonic quantum operations. A main challenge is to overcome noise and decoherence processes to reach the steep benchmarks on generation efficiency and photon indistinguishability required for scaling up the source. We report on the realization of a deterministic single-photon source featuring near-unity indistinguishability using a quantum dot in an "on-chip" planar nanophotonic waveguide circuit. The device produces long strings of >100 single photons without any observable decrease in the mutual indistinguishability between photons. A total generation rate of 122 million photons per second is achieved, corresponding to an on-chip source efficiency of 84%. These specifications of the single-photon source are benchmarked for boson sampling and found to enable scaling into the regime of quantum advantage.

Fig. 1. Illustration of the single-photon source device.

A QD embedded in a photonic crystal waveguide is excited using a pulsed laser at the resonance wavelength of the QD. The emitted single-photon train is coupled to the waveguide with near-unity efficiency and outcoupled from the device using a grating outcoupler (see inset). Metal electrical contacts (shown in gold) are used for applying a gate voltage across the QD embedded in the 180-nm-thin membrane.

Fig. 2. Deterministic preparation of an excitation in the QD.

(A) Resonance fluorescence measured from a QD in a photonic crystal waveguide weakly excited using a continuous-wave tunable diode laser. The two bright lines are the charge plateaus of the fine structure split neutral exciton. (B) Pulsed resonance fluorescence measured with the QD tuned on resonance (V_g = 1.24 V) and excited with a mode-locked laser with a pulse width of 20 ps. (C) Lifetime of the resonantly excited QD exhibiting a single-exponential decay with a radiative decay rate of γ = 2.89 ns⁻¹. The black dotted curve is the instrument response function of the measurement setup. cts, counts. (D) Measured coincidence counts of the single-photon source in a Hanbury Brown and Twiss interferometer under π pulse excitation showing a strongly suppressed peak at time delay τ = 0. The small asymmetry visible as a double peak at τ = 0 ns is a noise artifact in the bias electronics of the superconducting nanowire single-photon detectors. The inset shows the integrated coincidence counts under each peak over a time scale of 50 μs that highlights the minimal bunching observed.

Fig. 3. Highly indistinguishable train of single photons.

(A) Schematic of a fiber-based unbalanced Mach-Zehnder interferometer with a variable fiber delay line (delay time, Δτ) in one arm used for Hong-Ou-Mandel interference measurements. SNSPD, superconducting nanowire single-photon detector. (B) Photon indistinguishability measured under π pulse excitation for photons generated with Δτ = 13.8 ns and Δτ = 786.6 ns and with the two input photons co- and cross-polarized by adjusting the half–wave plate in the fiber delay arm. (C) Photon indistinguishability between photon pairs in a temporal string of up to 115 photons reaching >96%, as illustrated by the interference of photon 1 with photons 2, 39, 77, and 115. (D) Estimate of boson sampling capabilities of QD sources. Top: Variational distance of an ideal boson sampler from the real scenario implemented with the present source (blue curve) and that from Wang et al. (25) (red curve). At a given N, higher trace distance requires more sampling events and hence longer time to validate the boson sampler, thereby inhibiting the scaling into quantum advantage. Bottom: Minimum source efficiency η_S required for validating boson sampling with N indistinguishable photons by detecting collision-free events in a fixed runtime of 30 days.