The full theory of quantum mechanics emerged as a completely unexpected description of nature at a fundamental level. It portrays a world that is fundamentally probabilistic, where a single object can be in two places at once — superposition — and where two objects in remote locations can be instantaneously connected — entanglement. These unusual properties have been observed, and quantum mechanics remains the most successful theory ever developed, in terms of the precision of its predictions. Today, we are learning how to harness these surprising quantum effects to realize profoundly new quantum technologies.
All-optical systems come in two flavours: those that depend on nonlinear effects to execute gates, and those in which the only necessary nonlinearity is measurement, known as Linear Optics Quantum Computation (LOQC). Research on all-optical systems has focused on photon sources capable of generating precise numbers of photons with the necessary timing precision, gates based on measurement, and high-quality single-photon detectors. Measurement-based gates are inherently probabilistic in nature, though it has been shown that these gates can be built into a scalable feed-forward network.
Photons remain a promising vehicle for the development of next-generation quantum technology. Integrated quantum photonics, with its intrinsic phase stability and miniature devices, is necessary to bring linear optics to the large scale. Several integrated photonic platforms have emerged to solve this problem, including silica-on-silicon, direct-write glass, lithium niobate, silicon nitride and silicon-on-insulator. Silicon quantum photonics promises to simultaneously achieve the required functionality, performance, and scale.
Several important quantum optical functionalities have already been shown with high performance in silicon. Photon pairs can be generated using spontaneous four-wave mixing, and interfered with high visibility. Single-photon and pump-rejection spectral demultiplexers, as well as two-mode interferometers, have been demonstrated with very high extinction. The very high refractive index contrast of silicon-on-insulator waveguides yields micron-scale components, while miniature ring resonator SFWM sources, and quantum interferometric networks facilitate devices on a very large scale.
For any computation classical or quantum, there is a notion of a universal gate set. For classical computation, the NAND gate suffices, and for reversible classical computation, the TOFFOLI gate suffices. For quantum computation, one needs to append this with other gates. One standard gate set used in quantum computing consists of the CNOT gate; Hadamard gate denoted as H; phase gate, sometimes denoted as S; and its square root, denoted T. The TOFFOLI gate is a three-bit logic gate, where if the first two bits (the control bits) are set to 1, then it toggles the last bit (the target bit). The CNOT gate is similar except that it has a single control bit; i.e. it acts on two bits (control and target), and if the control bit is set to 1, then it toggles the target bit.
Analogous to the original condensed matter systems, topological photonic systems provide an avenue for the robust transport of light. Classical light experiments have demonstrated topologically protected edge states in both the microwave, and optical regimes with potential applications in robust optical delay lines, lasing in topological defects, and eventually topological insulator lasers. Recently, progress in topological photonics has extended into the quantum regime. Photons remain intrinsically well-isolated from the environment, even at room temperatures, but scattering loss, absorption, and phase errors are still hindering the scalability of photonic quantum computing and communication systems. Single-photon experiments in free space have demonstrated topological protection in discrete-time. However, the real advantage of using topological settings for quantum optics experiments should be in integrated platforms, where the topological protection can provide robustness against defects, inhomogeneities and disorder.
Topological quantum computing (TQC) represents a fault tolerant quantum computation scheme in which unitary quantum gates are realized by the braiding of so called anyons, satisfying non-abelian exchange statistics. These operations are only dependent on the topological class of the braiding group element, leading to TQC algorithms, protected by the energy gap of the system. Such quantum gates remain robust to disturbance and local noise if braiding procedures are (quasi-) adiabatic and if external perturbations do not close this gap.
From the first theoretical proposals to realize topologically protected transport of light, slightly over a decade ago, we have witnessed astonishing progress in topological photonics. Beyond the fascinating fundamental discoveries of new phases of light, the field has been driven by its potential to provide robust transport of classical and quantum light in the presence of fabrication imperfections. In particular, multiphoton quantum states—key to quantum information, communications, and computing—are particularly fragile and could benefit greatly from topologically protected transport.
Motivated primarily by the potential of topology to prevent the pressing issue of backscattering in integrated photonics, topology made its way into nanophotonics experiments. From a fundamental perspective, nanophotonics is an excellent platform to probe novel topological phases of classical and quantum light for various reasons. On the one hand, nanofabrication has reached a level of maturity that enables the realization of carefully designed lattices with relatively accurate control over the shape and the position of each element. This provides a playground for the creation of different lattice geometries, symmetries, and modulations that can be harnessed to generate nontrivial topologies. On the other hand, the unparalleled confinement available in nanophotonic structures enables strong nonlinearities that offer possibilities for optical tuning of topological properties and for the on-chip generation of correlated photons.
A quantum photonic interconnect must maintain quantum coherence between sub-systems and be capable of coherently interconverting between the preferred encodings in the platforms and media through which it connects. Perhaps, the most demanding requirement for an interconnect is the preservation of high-fidelity entanglement throughout any manipulation, interconversion and transmission processes within the full system. Distributing entanglement between integrated chips is a key requirement and a major technical challenge due to the highly fragile nature of entanglement and the potential for decoherence of quantum states transmitted between different chips. Path-encoding a photon across two waveguides—is a natural choice of robust encoding on-chip, however, polarisation, spatial mode, or time-bin encoding is typically more suitable in fibre and free space. Already there have been demonstrations of important features of quantum interconnect components, including on-chip entanglement generation and manipulation, photon detection, and on-chip interconversion of different photonic encoding.
Fully integrating the generation, manipulation, and detection of photons is an outstanding challenge for the field due to the unique material requirements for each distinct component. For example, epitaxially grown III‐V semiconductor quantum dots are a leading approach for the near‐deterministic generation of single photons in terms of purity, brightness, and indistinguishability. However, the loss per component of III‐V platforms is relatively high, and likely not at the level required for a large‐scale photonic quantum technology. In contrast, silicon photonics is unrivalled in terms of component density, scale, and compatibility with complementary metal–oxide–semiconductor (CMOS) electronics, with classical systems featuring over 1000 active components and integration with millions of transistors. Moreover, silicon‐photonic based quantum systems have demonstrated control of > 100 components as well as the generation of entangled states of light. However, methods to generate photons in silicon are based on spontaneous processes, such as four‐wave mixing, or are incompatible with deterministic solid‐state quantum emitters at visible or infrared wavelengths.
Hybrid integration provides a potential solution by incorporating disparate photonic technologies into a single integrated system that may not be otherwise compatible in a single fabrication process. Hybrid integration techniques include pick‐and‐place, wafer bonding, and epitaxial growth. In the context of quantum technologies, hybrid integration offers the tantalizing goal of bringing together quantum emitters, quantum memories, coherent linear and nonlinear operations, and single photon detection into a single quantum photonic platform