Single Photons Beamed on Demand with Record Speed of 67.4 Picoseconds

Scientists are continually striving to create highly reliable single-photon sources for applications in quantum cryptography and computation. Robert Behrends, Martin v. Helversen, and Pratim K. Saha, along with colleagues from the Institute of Physics and Astronomy at the Technical University Berlin, have now demonstrated a novel quantum dot single-photon source operating at the crucial 1550nm telecom wavelength. Their research details a Purcell-enhanced quantum dot exhibiting record-low decay times and exceptionally high indistinguishability of emitted photons, achieved through resonant two-photon excitation and asymmetric cavity design. This advancement significantly improves the performance of deterministic light sources, paving the way for more practical and efficient quantum technologies.

High-visibility biexciton emission from a quantum dot in the telecom C-band

Scientists have achieved a two-photon interference (TPI) visibility of 90% for biexciton photons emitted from a cavity-enhanced indium arsenide/indium phosphide quantum dot, representing a significant leap forward in deterministic single-photon source technology. This breakthrough surpasses the typical limit of approximately 60% for these systems, demonstrating nearly perfect indistinguishability of the emitted photons and paving the way for advanced quantum communication protocols.
The research centres on a novel quantum dot device operating in the telecom C-band, a crucial wavelength range for compatibility with existing optical fibre infrastructure. This work details the creation of a highly efficient single-photon source with a record-low biexciton emitter decay time of 67.4(2) picoseconds under resonant two-photon excitation.

Strong suppression of multi-photon emissions was observed, with measured values of g(2)X(0) = 0.006(1) and g(2)XX(0) = 0.007(1) for exciton and biexciton emissions respectively, indicating a highly reliable source of single photons. The asymmetric Purcell enhancement of the biexciton-exciton cascade is key to this achievement, manipulating the radiative properties to optimise photon indistinguishability.

Furthermore, researchers demonstrated stimulated two-photon excitation in the telecom C-band by introducing a second, timed laser pulse coinciding with biexciton emission energy. This technique improved the TPI visibility of the exciton photons to 0.69(3), compared to 0.61(4) achieved with conventional two-photon excitation, although both values remain slightly below theoretical predictions due to existing dephasing effects.

The advances presented in this study hold considerable promise for implementing sophisticated quantum communication schemes reliant on deterministic light sources. The quantum dot device was fabricated using indium arsenide/indium phosphide, grown via molecular-beam epitaxy and integrated within a circular Bragg grating cavity.

This cavity design selectively enhances the biexciton transition, leading to the observed improvements in photon indistinguishability and exceeding the performance of standard biexciton-exciton cascades lacking photonic engineering. The combination of ultra-fast biexciton decay and tailored cavity interaction represents a significant step towards realising practical, high-performance quantum light sources for future quantum technologies.

Resonant two-photon excitation and characterisation of InAs/InAlGaAs quantum dot emission dynamics

A cavity-enhanced InAs/InAlGaAs dot single-photon source operating in the telecom C-band was investigated using resonant two-photon excitation (TPE). The study achieved a record-low biexciton emitter decay time of 67.4(2) ps, facilitated by the device’s construction and excitation scheme. Detailed spectral analysis was performed, comprising the neutral exciton (X) and biexciton (XX) states, alongside a charged biexcitonic transition, with state assignment based on polarization and excitation-power-dependent measurements.

Second-order cross-correlation measurements confirmed the cascaded nature of the XX- and X- emission. Rabi rotations of the XX and X states were demonstrated in the excitation-power domain, reaching up to 6π, indicating coherent excitation. These rotations exhibited damping attributable to phonon pure-dephasing, and all subsequent measurements were conducted under π-pulse conditions unless otherwise noted.

Multiphoton suppression was quantified via second-order autocorrelation measurements in a Hanbury Brown-Twiss (HBT) type setup, revealing strongly suppressed coincidences at zero time delay, confirming on-demand single-photon preparation. To determine the g(2)(0) values, histograms were fitted with double-sided exponential functions to account for emitter blinking effects and accurately define integration intervals.

This yielded g(2) XX(0) = 0.007(1) and g(2) X (0) = 0.006(1) for the XX and X states, respectively. Mono-exponential decay functions fitted to the correlation peaks provided decay constants of T XX 1 = 67.4(2) ps and T X 1 = 544(2) ps. Deconvolution, accounting for the 57.9(1) ps full width at half maximum instrument response of the superconducting nanowire detector system, resulted in a 64(1) ps biexciton lifetime.

Hong-Ou-Mandel (HOM) type experiments were then conducted to explore the two-photon interference (TPI) visibilities. Moderate spectral filtering of 70 μeV (0.14nm) was applied to remove phonon sidebands while preserving the zero-phonon-line emission. This methodology enabled the observation of a TPI visibility of 90(3)% for the biexciton photons, exceeding the typical limit for these systems and indicating nearly perfect indistinguishability of the emitted photons.

High-visibility two-photon interference and rapid decay dynamics from a cavity-enhanced quantum dot

Researchers demonstrate a two-photon interference (TPI) visibility of 90% for biexciton photons, exceeding typical limits for these systems and indicating nearly perfect indistinguishability of the emitted photons. This achievement was obtained using a cavity-enhanced InAs/InAlGaAs dot single-photon source operating in the telecom C-band.

The study reports a record-low biexciton emitter decay time of 67.4(2) picoseconds under resonant two-photon excitation. Measurements reveal a strong suppression of multiphoton emission associated with both exciton and biexciton emission. Second-order autocorrelation measurements yielded g(2)(0) values of 0.007(1) for the biexciton and 0.006(1) for the exciton, confirming on-demand single-photon preparation.

Fitting mono-exponential decay functions to the correlation peaks determined biexciton and exciton decay constants of 67.4(2) ps and 544(2) ps, respectively. The observed asymmetric Purcell enhancement of the biexciton-exciton cascade enables the high TPI visibility. Hong-Ou-Mandel type experiments, utilising a moderate spectral filter of 70 μeV, resulted in a raw TPI visibility of 0.90(3) for the biexciton photons and 0.61(4) for the exciton photons.

The lifetime ratio between the exciton and biexciton states is greater than 8, contributing to the enhanced performance. This work presents a significant step towards implementing advanced schemes utilising deterministic light sources in quantum technologies.

High indistinguishability telecom-band photons from resonant biexciton emission

Researchers have achieved a record two-photon interference (TPI) visibility of 90% for photons emitted from a biexciton in a quantum dot system operating in the telecom C-band. This demonstrates nearly perfect indistinguishability of the emitted photons and represents a significant advance in the development of deterministic light sources.

The system utilises a cavity-enhanced indium arsenide/quantum dot structure, exhibiting a remarkably short biexciton decay time of 67.4 picoseconds under resonant two-photon excitation. This breakthrough facilitates the generation of highly indistinguishable photon pairs directly within the telecom C-band, a crucial wavelength range for compatibility with standard fibre-optic networks and direct fibre pigtailing.

Stimulated two-photon excitation was also successfully implemented, further enhancing the indistinguishability of exciton photons. While the current device demonstrates exceptional performance, the authors acknowledge that dephasing effects limit the theoretical maximum visibility. Future research will focus on utilising deterministic fabrication technologies to create cavity designs with inverted photonic engineering, aiming to achieve even brighter and more versatile C-band photon sources at multi-GHz rates. This work therefore establishes a clear path towards advanced quantum communication and computation schemes reliant on deterministic single-photon emission.