Fast and ultra-low loss single-photon switching and routing are essential for photonic quantum computation and communication. To address this need in a scalable fashion, all-optical switches that can be fabricated in an ultra-low loss and mature Si$_{3}$N$_{4}$photonic integrated circuit (PIC) foundry platform are designed and optimized for sub-ns switching times suitable for deterministic quantum-dot single-photon sources. The working principle relies on cross-phase modulation (XPM) of the single photons with a 1550-nm pump pulse and is enhanced by a ring resonator. Two different designs of the primary switch element are theoretically studied, namely a ring resonator intensity switch (RRIS) based on resonance shifting due to XPM and a ring resonator phase switch (RRPS) acting as an all-optical phase shifter in a Mach–Zehnder interferometer. As a novel approach to speed up the switching, chirped pre-emphasis and wipe sections for the pump pulses are utilized. A design tool is established from analytical expressions and serves as starting point for further optimization using a dedicated travelling-wave model (TWM). The TWM demonstrates the feasibility of both designs to be driven by either the proposed pre-emphasis pulse shape or a train of chirped Gaussian pulses. While the RRPS turns out to require less pump energy, its operation is more sensitive to pump-power fluctuations. Insertion losses below 0.1 dB and a power consumption below 5 nJ at 1 GHz switching rates for both configurations prove the potential of this concept for scalable quantum photonic applications.
Fast and ultra-low loss single-photon switching and routing are essential for photonic quantum computation and communication. To address this need in a scalable fashion, all-optical switches that can be fabricated in an ultra-low loss and mature Si3N4photonic integrated circuit (PIC) foundry platform are designed and optimized for sub-ns switching times suitable for deterministic quantum-dot single-photon sources. The working principle relies on cross-phase modulation (XPM) of the single photons with a 1550-nm pump pulse and is enhanced by a ring resonator. Two different designs of the primary switch element are theoretically studied, namely a ring resonator intensity switch (RRIS) based on resonance shifting due to XPM and a ring resonator phase switch (RRPS) acting as an all-optical phase shifter in a Mach–Zehnder interferometer. As a novel approach to speed up the switching, chirped pre-emphasis and wipe sections for the pump pulses are utilized. A design tool is established from analytical expressions and serves as starting point for further optimization using a dedicated travelling-wave model (TWM). The TWM demonstrates the feasibility of both designs to be driven by either the proposed pre-emphasis pulse shape or a train of chirped Gaussian pulses. While the RRPS turns out to require less pump energy, its operation is more sensitive to pump-power fluctuations. Insertion losses below 0.1 dB and a power consumption below 5 nJ at 1 GHz switching rates for both configurations prove the potential of this concept for scalable quantum photonic applications.
Efficient single-photon routing and switching are crucial for optical quantum computing and communication. For this purpose, all-optical switches are designed for gigahertz bandwidths. The switching mechanism is based on the optical Kerr effect via cross-phase modulation (CPM) of the single-photon signal by a strong 1550-nm pump pulse. For energy-efficient switching, this nonlinear effect is exploited in a microresonator that can either be used directly as an intensity switch in a typical add–drop configuration or as a phase shifter in a Mach–Zehnder interferometer (MZI) structure. To speed up resonance build-up and quenching, a pre-emphasis build and an off-resonance wipe pulse are used. The proposed designs are verified by traveling-wave simulations which demonstrate that 0.1 dB insertion loss and ~1 ns switching windows can be achieved. For a scalable out-of-the-lab transfer, we investigate the feasibility of the proposed switch designs for fabrication in a mature photonic integrated circuit (PIC) platform. In particular, silicon nitride PICs have demonstrated record-low losses which makes them suitable for single-photon applications. By parametric modelling of the microresonator’s directional couplers based on Lumerical EME and 2.5-dimensional varFDTD simulations, the required power transmission coefficients for both signal and pump wavelength can be achieved. This results in an all-optical switch design ready for fabrication in a commercial PIC foundry which can potentially enable scalable architectures for quantum photonic applications.
Realizing photonic quantum computing at scale requires integrated circuits that combine ultra-low loss with fast, low-power, low-hysteresis, and low-crosstalk reconfiguration. These requirements are particularly challenging at visible wavelengths, where many quantum resource-state generators, including single-photon sources and quantum memories, naturally operate. Low-confinement silicon nitride waveguides offer the requisite loss performance, but conventional thermo-optic modulators dissipate significant static power, driving thermal crosstalk and precluding cryogenic operation. Visible-wavelength piezo-optomechanical circuits avoid these drawbacks, but existing demonstrations rely either on high-confinement waveguides with propagation losses of 35-100 dB/m, or on low-confinement platforms using foundry-incompatible PZT. Here we combine piezo-optomechanical actuation with a CMOS-foundry-fabricated, low-confinement silicon nitride platform, achieving 2.6 dB/m propagation loss at 780 nm, megahertz-scale modulation bandwidth, a half-wave voltage-length product of 2.8 Vm, and negligible hysteresis. We demonstrate reconfigurable Mach-Zehnder interferometers with 0.63 dB loss per spiral phase shifter, enabling deep, actively reconfigurable visible-wavelength quantum photonic circuits.
Quantum signals such as entangled photons are typically at the sub-photon level and, thus, require near noise-less and low-loss components for their manipulation and processing. A common signal-manipulation tool is a 2 × 2 switch, and many applications of quantum switches also require fast switching rates and input signals in the fiber-friendly 1550-nm band. Traditional optical switching technology does not simultaneously meet all the desired metrics for a quantum switch. We demonstrate a low-noise fiber-optic photonic switch based on cross phase modulation that is fast (sub-ns), low loss (<;1 dB), and operates in the 1550-nm wavelength band.
A high-speed, low-loss modular cross-bar switching element suitable for single photons is demonstrated. Entanglement preserving voltage-driven switching of single photons is achieved with a unique combination of ultra-fast and low-loss characteristics (less than 50 ps 10%–90% transition time and greater than 90% transmission), and we present a roadmap towards even lower losses. The switch properties are well matched to the wavelength, bandwidth and lifetime of photons generated by spontaneous four-wave mixing sources also realized in silicon photonics. Such a switch can enable reconfigurability of each single photon and addressing of each time bin, and benefit future quantum photonic circuits.
In the past decades quantum optics has been at the forefront of quantum innovative technologies. For practical applications, scalable platforms for implementation of quantum optical circuits are vital. This thesis presents two new platforms for scalable implementation of quantum optical circuits, namely, modular approach and monolithic integration. Here, we take the first steps towards the integration of three main elements of every quantum optics circuits: Single-photon emitters, single-photon detectors, and quantum logics. Until now, most quantum optical circuits used separate platforms for single-photon generation and detection. The main challenge in the integration of these technologies, which have different requirements, has slowed down the research in the field. Here, we integrate sources and detectors by first fabricating the devices on their own platform and then transferring and combining them together. Plasma enhanced chemical vapor deposition of silicon nitride followed by etching optical waveguides connect these elements. Removing the Poissonian optical excitation field from the quantum circuit is necessary for integration. Classical optical excitation can be avoided if the sources are electrically pumped. However, fabrication of high-quality electrically pumped sources, suitable for integration, has been limited. The experiments described in chapter 4 are our first step towards addressing the mentioned problem. Defect-free nanowires are grown on <100> direction and their optoelectronic performance are characterized. Nanowire quantum dots, thanks to their waveguiding, purity, coherence and their potentials for deterministic integration with other optical circuits, are promising single-photon sources for on-chip quantum optics. However, precise control of the emission energy of the quantum dots by growth has not become possible yet. Chapter 5 describes a method for on-chip tuning of emission energy of nanowire quantum dots using strain fields. We show the emission energy of independent nanowire quantum dots can be brought into degeneracy without affecting their single-photon emission properties. The quantum optical components have to be routed and connected together to form functional circuits. On a chip, this is usually carried out using optical waveguides. Moreover, manipulation of single photons has to be done in a scalable fashion. Again optical waveguides and ring resonators are very good candidates for this task. Therefore, understanding the behavior of these circuits such as their losses, polarization dependence, and temperature behavior is important. The experiment described in chapter 6 studies the behavior of plasma enhanced silicon nitride waveguides in cryogenic temperatures. We concluded in this chapter that due to weak thermo-optic sensitivity of silicon nitride at cryogenic temperatures, the available thermal budget on the system should be carefully considered. An important step in achieving a scalable platform for quantum optical circuits is deterministic and efficient integration of single-photon sources. In chapter 7, we demonstrate successful integration of III-V nanowire quantum dots with silicon nitride waveguides. The nanowires are deterministically selected and transferred from the original growth chip to the new substrate where they are integrated with low-loss silicon nitride waveguides. Our measurements show that the integrated sources preserve their high quality emission properties. In chapter 8, we describe an alternative approach: amodular method for scalable quantum optics. The proposed technique is based on coupling the single-photon from sources into optical fibers where the photons can be processed and then fed into the single-photon detectors. This approach has high flexibility and is easier to implement but as described in the chapter, at the moment, losses in the interfaces between optical fibers and single-photon sources are a major limiting factor. We conclude the thesis with some possible future directions and exciting new results on integration of single-photon detectors with sources and waveguides. Finally, primary results on on-chip single-photon filtering and removal of the optical excitation field are demonstrated.