Photonic Quantum Computing (PQC) uses an individual particle of light, known as a photon, as a qubit, the fundamental unit of quantum information. Several properties make this modality attractive, including:
As with all modalities, however, photonics is not without its challenges. The two primary challenges are:
For more information, a diginomica article titled “Is phototonic quantum computing the answer to commercial quantum use? Maybe” delves a little deeper into what photonic qubits are, what their advantages are, and how cryogenics is involved. There are also some comparisons of photonic qubits vs superconducting qubits and trapped ion qubits.
Several paradigms of photonic qubits are currently being researched. However, three in particular are arguably the most common. These include squeezed states, dual-rail encoding, and qumodes.
Entangled photonic qubits typically have one of two potential sources. Photons can be created as entangled pairs before computation commences. Photons can also become entangled during computation through the use of interferometry.
In comparing photonic qubits vs superconducting qubits and trapped ion qubits, gate operations are executed quite differently. The latter receive pulses of microwaves or lasers, respectively, and they interact with each other directly. Superconducting qubits are frozen in place, whereas ions are mostly stationary, but they move around for their interactions.
Though the former can interact through interferometry, they mostly interact with linear optical components and photodetectors. As opposed to passively receiving manipulation, photons actively go get manipulated. They cannot be held in place, so they physically move through their circuits at the speed of light.
Photonic operations include:
The photonic quantum computing landscape continues to change. A Stanford University team, for example, has proposed a paradigm in which photons in a loop are controlled by a single atom. You can read more about this one in an article titled “Stanford engineers propose a simpler design for quantum computers.” But, again, this is just one example, and it is not alone in being a proposed alternative to current implementations.