On-Demand, Room Temperature Single-Photon Generation with an Electrically-Pumped Colloidal Quantum Dot

Single-photon sources are in great demand for quantum information technologies. A perfect single photon source emits one and only one photon at a time. This photon 'anti-bunching' is characterized by the second-order intensity correlation function, g⁽²⁾(τ), with g⁽²⁾ at time zero of &lt; 0.5. Single-photon emission has been observed in a wide variety of systems (1), but there is still a need for electrically-pumped single-photon sources capable of operating at room temperature with stable emission. Other desired characteristics include: 1) ease of operation, 2) suppression of background noise, 3) scalability using established micro-fabrication techniques, and 4) ease of integration with photonic resonators and waveguides for on-chip manipulation of single photons. However, current room temperature electrically-driven single photon sources (based, e.g., on defects in diamond or silicon carbide) suffer from poor single-photon purity and low determinicity in photon emission.<br/><br/>Here we describe the fabrication and characterization of colloidal quantum dot (QD) LEDs (QLED) which exhibit high-purity single photon emission at room temperature. QDs have received extensive attention due to their size-tunable emission color, reproducible and scalable solution-based synthesis, narrow emission linewidths and high emission quantum yields approaching 100%. In our devices, single photon emission is achieved by dilution of the QD concentration by ~5 orders of magnitude below that of a normal QLED. Further, to meticulously control electron/hole injection we apply a novel LED design that features an insulating interlayer embedded with QDs into which current is focused, while emission from a single QD is isolated <i>via</i> an aperture in the optical path to the detectors.<br/><br/>At room temperature, stable non-blinking electroluminescence is observed with a single QD linewidth as narrow as ~25 meV, which is equivalent to the room-temperature thermal energy (<i>k_BT</i> = 25 meV). At higher biases, we realize dual-band emission due to the band-edge (1S) and the excited-state (1P) transitions. This unusual emission regime is a direct result of a novel device design which allows us to flow ultrahigh currents through a single QD and achieve high steady-state QD occupancies of more than 2 excitons per QD. The measured g⁽²⁾(0) values are of ~0.2 under low biases, indicating high single-photon purity. The g⁽²⁾(0) dip shows an exponential decay with a lifetime of 4.5 ns, indicating that emitting species are negatively charged excitons (negative trions) as dictated by intentionally imbalanced charge injection. As was demonstrated previously, negative charging helps suppress intensity fluctuations and accelerates emission rates (2).<br/><br/>Finally, to establish that our single-dot QLEDs are suitable for the deterministic, on-demand generation of single photons, we employ pulsed electrical excitation with pulse widths varied from 3-50 µs. We are able to vary the number of photons emitted per pulse from &lt;&lt; 1 to &gt;&gt; 1 by changing the pulse width, the repetition rate and the applied voltage. This wide-range control over the photon number signifies a transition from a regime of quasi-random photon emission to the deterministic generation of single photons. This proof-of-concept demonstration paves the way for the near-future use of single-dot QLEDs as inexpensive, spectrally tunable, room temperature, and deterministic sources of single photons for a range of emerging quantum technologies.<br/><br/>1. Lin, X., Dai, X., Pu, C. <i>et al.</i> Electrically-driven single-photon sources based on colloidal quantum dots with near-optimal antibunching at room temperature. <i>Nature Communications</i> <b>8, </b>1132 (2017). https://doi.org/10.1038/s41467-017-01379-6<br/>2. Galland, C., Ghosh, Y., Steinbrück, A. <i>et al.</i> Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots. <i>Nature</i> <b>479, </b>203–207 (2011). https://doi.org/10.1038/nature10569