Electrically spin-injected ensembles of InGaAs/GaAs quantum dots emit circularly polarized single photons, with the emission helicity determined by the spin polarization of the injected electrons. A ferromagnetic CoFeB spin-injection layer with perpendicular magnetic anisotropy, combined with an MgO tunnel barrier, enables spin-polarized electron injection into the quantum-dot active region, resulting in circularly polarized photon emission at zero applied magnetic field. These devices realize direct spin-charge-photon conversion in an electrically driven, microelectronics-compatible quantum light source that operates without an external pump laser. Using an extended Hanbury Brown-Twiss interferometer with four detectors and time-tagged readout, we measure time-resolved photon-number statistics that resolve up to
∣2⟩ Fock states within a single excitation cycle. While single-photon (
n=∣1⟩) emission tracks the electrical excitation pulse as expected, the two-photon (
n=∣2⟩) statistics exhibit pronounced fluctuations at multiple temporal positions within the pulse. These fluctuations vanish upon demagnetization of the spin-injection layer, establishing a direct link between the multi-photon emission dynamics and the spin polarization of the injected carriers. The observations indicate that spin injection controls not only the helicity of the emitted photons but also higher-order temporal correlations of the emission. The behavior is phenomenologically captured by inserting both a variable excitation mechanism and a set of multi-emitter collapse operators within a stochastic wave function evolution. The platform thus opens a route to electrically driven, spin-controlled single and multi-photon Fock-state sources and provides a testbed for spin-conditioned multi-emitter dynamics in solid-state quantum light sources.
A single-photon source capable of emitting indistinguishable photons is a key element in schemes for scalable quantum information processing with linear optics. Whilst several groups have reported such sources, up until now an electrically driven source capable of making these protocols technologically viable has yet to be reported. We present the first demonstration of an electrically driven single-photon source emitting indistinguishable photons. Our sample consists of a layer of InAs/GaAs quantum dots embedded in the intrinsic region of a p-i-n microcavity diode. We test the indistinguishability of consecutive photons by carrying out a Hong-Ou- Mandel-type two-photon interference experiment whereby two identical photons arriving simultaneously at two input ports of a 50:50 beamsplitter exit together. The device was operated under two modes, continuous and pulsed current injection. In the former case, we measured a coherence time of up to 400 ps at low pump current - the longest reported under these excitation conditions. A two-photon interference visibility was measured, limited only by the timing resolution of our detection system and further suggesting a 100% overlap of photon wavepackets at the output beamsplitter. In the case of pulsed injection, we employed a two-pulse voltage sequence which allowed us to carry out temporal filtering of photons which had undergone dephasing. The characteristic Hong-Ou-Mandel "dip" was measured resulting in a visibility of 64 ± 4%.
The time dynamics of spin-injected, electrically contacted quantum dots were investigated with a focus on the time evolution of photon statistics. Photon statistics can provide insights into whether the device functions as an effective single-photon emitter or exhibits higher-order emissions. Through these investigations, we found that the shape of the electrical excitation pulse has a direct impact on photon statistics. Specifically, the rising edge of the pulse corresponds to a significantly higher number of higher-order photon states, which decay much faster than single photons associated with the falling edge of the electrical pulse. This relationship implies that the pulse shape can be tailored to optimize the device as either a better single-photon source or a generator of higher-order photon states, with potential applications in creating deterministic higher-order photon Fock states. The ability to easily modify the pulse shape is a unique feature of electrically excited quantum dots.