The ultrafast detection of single photons is currently restricted by the limited time resolution (a few picoseconds) of the available single-photon detectors. Optical gates offer a faster time resolution, but so far they have been applied mostly to ensembles of emitters. Here, we demonstrate through a semi-analytical model that the ultrafast time-resolved detection of single quantum emitters can be possible using an optical Kerr shutter at gigahertz rates under focused illumination. This technique provides sub-picosecond time resolution, while keeping a gate efficiency at around 85%. These findings lay the ground for future experimental investigations on the ultrafast dynamics of single quantum emitters, with implications for quantum nanophotonics and molecular physics.
The ultrafast detection of single photons is currently restricted by the limited time resolution (a few picoseconds) of the available single-photon detectors. Optical gates offer a faster time resolution, but so far they have been applied mostly to ensembles of emitters. Here, we demonstrate through a semi-analytical model that the ultrafast time-resolved detection of single quantum emitters can be possible using an optical Kerr shutter at gigahertz rates under focused illumination. This technique provides sub-picosecond time resolution, while keeping a gate efficiency at around 85%. These findings lay the ground for future experimental investigations on the ultrafast dynamics of single quantum emitters, with implications for quantum nanophotonics and molecular physics.
The generation and efficient detection of ultrafast single photons is at the forefront of modern optical science, and it offers an unprecedented set of capabilities, for instance, in the area of quantum information science, high-sensitivity detection, and remote sensing [1] - [3] . In addition, it provides a powerful analysis tool in the life sciences at the single-molecule level [4] . The detection of single photons is currently restricted by the limited time resolution (a few picoseconds) of the available single-photon detectors.
The modern information era is built upon technologies such as telecommunications, cryptography, high-performance computing, and, more recently, artificial intelligence (AI). At the core of all these systems are electronic circuits, whose performance is fundamentally limited by the physical speed of electrons. Photonic technologies, by contrast, rely on light quanta—single photons—that propagate orders of magnitude faster and offer intrinsically higher bandwidth and enhanced security. Replacing electronic components with optical counterparts promises major advances in communication speed, data transfer, secure information processing, and quantum technologies. Central to these developments are ultrabright single-photon sources (SPSs), including quantum dots and color centers in solid-state hosts. However, the brightest SPSs currently require cryogenic temperatures, limiting their practicality in real-world applications. This dissertation proposes an alternative route toward achieving bright, room temperature single-photon emission by exploiting high-repetition-rate excitation. Advances in laser technology now allow solid-state lasers to operate at gigahertz repetition rates. By combining such excitation with hybrid quantum systems—where a quantum emitter is coupled to a nanophotonic structure to enhance its radiative emission through the Purcell effect—ultrabright SPSs can, in principle, be realized at room temperature. At these high radiative rates, however, the fluorescence lifetime becomes extremely short, well below the temporal resolution of conventional single-photon detectors, which are further constrained by cost, cryogenic requirements, bulkiness, and limited synchronization capabilities at gigahertz rates. To address this limitation, this dissertation demonstrates an optical Kerr gate (OKG) operating at a 1-GHz repetition rate with sub-picosecond temporal resolution. The OKG is a third-order (χ(3)) nonlinear optical process that does not require phase matching conditions and is therefore suitable for broadband fluorescence detection. Kerr gating was achieved using two distinct Kerr media: bulk bismuth-borosilicate (BBS) glass and two-dimensional thin graphite films. In both cases, OKG signals were generated using sub-nanojoule gate-pulse energies—remarkably low compared with traditional OKG implementations that rely on millijoule-level amplified pulses, more than six orders of magnitude higher. The temporal resolution achieved was 175 ± 1 fs with BBS glass (∼ 3% gating efficiency) and 141 ± 6 fs with thin graphite films (∼ 10% efficiency). This compact, room-temperature technique requires no cryocooler, vacuum system, or bulky amplification stages. The demonstrated OKG platform can be directly integrated into a standard confocal fluorescence microscope, enabling ultrafast fluorescence measurements from quantum emitters and paving the way for practical room-temperature ultrabright SPSs.
Ultrafast quantum optics with time-frequency entangled photons is at the forefront of progress towards future quantum technologies. However, to unravel the time domain structure of entangled photons and exploit fully their rich dimensionality, a single-photon detector with sub-picosecond temporal resolution is required. Here, we present ultrafast single-photon detection using an optical Kerr gate composed of a photonic crystal fiber (PCF) placed inside a Sagnac interferometer. A near-rectangle temporal waveform of a heralded single-photon generated via spontaneous parametric down-conversion is measured with temporal resolution as high as 224 ± 9 fs. The large nonlinearity and long effective interaction length of the PCF enables maximum detection efficiency to be achieved with only 30.5 mW gating pulse average power, demonstrating an order-of-magnitude improvement compared to optical gating with sum-frequency generation. Also, we discuss the trade-off relationship between detection efficiency and temporal resolution.
Single photons are a natural platform for quantum technologies as they support entanglement in many degrees of freedom and are inherently well protected from a detrimental type of noise called decoherence. In order to fully exploit photons for quantum technologies, it will be necessary to be able to shape, control, and measure their properties in all degrees of freedom: space, time, frequency, and polarization. \n \n The time-frequency degree of freedom of light is of particular interest for quantum information tasks as it supports various encodings, including frequency bins and time bins, and is intrinsically robust for propagation through long-distance fibre links. Applications which harness quantum correlations in this degree of freedom, referred to as energy-time entanglement, include dispersion cancellation and high-dimensional quantum key distribution. However, detection of this entanglement and observation of these effects requires time resolution beyond the capabilities of current photon detectors. Thus, for operations on ultrafast time scales, more powerful and complex methods are required. \n \n In this thesis, we use a nonlinear technique known as optical gating to surpass the limitations in current detectors and measure single photon pairs, improving the time resolution by two orders of magnitude. When a single photon enters a nonlinear medium at the same time as a strong laser pulse, it may upconvert to a higher energy photon. The strong laser pulse effectively acts as an ultrafast shutter or gate and this creates a high resolution snapshot of the photon in time. Optical gating in conjunction with single photon spectrometers then allow us to measure both the spectral and temporal features of photons on subpicosecond time scales. \n \n These high resolution measurements enable us to explore a host of quantum effects which remained previously inaccessible. First, we directly observe energy-time entanglement, analogous to Einstein-Rosen-Podolsky correlations but \\emph{in frequency and time}. Then, with full control over the dispersion of each photon, we observe nonlocal dispersion cancellation on femtosecond time scales. We also explore ultrafast interferometry in the quantum domain by temporally resolving two-photon interference from a Franson interferometer and observe a violation of the CHSH-Bell inequality. Finally, we show that, using these measurements, it is possible to reconstruct a two-photon energy-time entangled state. Such measurement capabilities will be essential to precisely control both the spectral and temporal shape of entangled photons and to develop new applications in quantum state engineering.
We offer a perspective on recent advances in picosecond-timescale all-optical switching with applications in quantum optics. The switch is based on polarization rotation in standard single-mode fiber via the optical Kerr effect. By using ultrafast laser pulses and short (∼10 cm) fibers, this technique can achieve a switching duration of ≲1 ps, at the repetition rate of 80 MHz or above. This high repetition rate is well-suited to quantum optics where experiments operate in the photon-counting regime. The switch efficiency can be ≳99% with a noise floor of just ∼10−4 photons/pulse, enabling high fidelity operations on quantum states of light, with negligible generation of spurious noise photons. We highlight the capabilities of this technique in four early applications: switching of heralded single photons, time-bin to polarization conversion of photonic qubits, noise gating for quantum key distribution, and pulse carving.