Abstract
Photons have been a flagship system for studying quantum mechanics, advancing quantum information science, and developing quantum technologies. Quantum entanglement, teleportation, quantum key distribution, and early quantum computing demonstrations were pioneered in this technology because photons represent a naturally mobile and low-noise system with quantum-limited detection readily available. The quantum states of individual photons can be manipulated with very high precision using interferometry, an experimental staple that has been under continuous development since the 19th century. The complexity of photonic quantum computing devices and protocol realizations has raced ahead as both underlying technologies and theoretical schemes have continued to develop. Today, photonic quantum computing represents an exciting path to medium- and large-scale processing. It promises to put aside its reputation for requiring excessive resource overheads due to inefficient two-qubit gates. Instead, the ability to generate large numbers of photons—and the development of integrated platforms, improved sources and detectors, novel noise-tolerant theoretical approaches, and more—have solidified it as a leading contender for both quantum information processing and quantum networking. Our concise review provides a flyover of some key aspects of the field, with a focus on experiment. Apart from being a short and accessible introduction, its many references to in-depth articles and longer specialist reviews serve as a launching point for deeper study of the field.
Highlights
There is intense research under way in the development of deterministic optical quantum gates,[3,4,5] which could take photonic quantum computing (PQC) in a new direction
We look at photon detection and generation tools, and integrated waveguide technology—and some new intermediate quantum computing demonstrations that these enable
We will concentrate on photonic[10] quantum computing (PQC) that relies on qubits encoded in discrete variables, noting, that quantum computing with continuous variables has become an important part of linear optical quantum computing (LOQC).[11,12,13,14]
Summary
A qubit can be encoded as probability amplitudes corresponding to the photon occupation of two modes of some degree of freedom of the optical field. The use of multiple single photons is required for circuits with two-qubit gates and beyond It is natural, to implement one qubit per photon, with a dual-rail encoding. The KLM scheme essentially works by using nonclassical interference to generate a phase shift that is nonlinear with respect to the photon number, conditioned on photons appearing at certain heralding modes. These operations are built into nondeterministic logical gates. The KLM scheme theoretically allowed for a resource-efficient implementation of two- and multiqubit gates—unlike encoding a single photon across many modes, the resource scaling was not exponential in the number of qubits, but rather linear.
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