Photons, or particles of light, exhibit quantum behaviors of their own allowing them to be utilized for qubits. Unlike trapped ions and neutral atoms that require cryogenic or laser cooling, photonic qubits can operate at room temperature, making them a promising candidate for building large-scale quantum computers. A leading manufacturer of photonic quantum computers, Psi Quantum, refrigerates the systems to about 4 Kelvin using liquid Helium to optimize the fault tolerance of its photodetectors, whereas competitor Xanadu does not cool the qubits, just the photodetectors to less than 100mK.
The main method of building photonic quantum computers is the KLM protocol developed in 2000 where a light source that produces as few coherent photons as possible travels through fiber optic cables where the qubit states are based on the photon’s modes of polarization. In general, photons can exhibit infinite modes of polarization; however, in the KLM protocol, the photons are either in vertical or horizontal modes as the probability of a photon occupying more than one mode is zero. Therefore, a qubit state can be represented by a photon being in either of these two modes where these states are known as Fock states.
To implement qubits and gates, linear optical elements such as mirrors, beam splitters, and phase shifters are used. Beam splitters can split a beam of photons into two or more separate beams, leading to superposition, and phase shifters can change the phase of the photons which becomes important when entangling them. The combinations of beam splitters, mirrors, and phase shifters lead to a wide variety in the creation of multi-qubit quantum gates. Creating entanglement is more difficult as photons do not interact with each other; however, through a series of phase shifters, photons traveling through different fiber optic cables, entangled states can be created where many of the states that do not become entangled are filtered out.
To determine these final states, photodetectors are used where in a given elementary quantum gate, the photon in the top mode channel corresponds to a state of 0, while a photon in the bottom channel corresponds to a state of 1. As the combination of beam splitters, mirrors, and phase shifters can lead to non-deterministic gates, measuring the fidelity or accuracy of these gates can be very difficult as the operations would have to be performed repeatedly and can become exponentially larger as the number of gates increases. To circumvent this, researchers have used a phenomenon known as quantum teleportation to decrease the time and required resources to determine the fidelity of these systems. Instead of doing these operations successively, each gate would be operated offline and the event signal would be sent back to the quantum circuit using quantum teleportation.
By using photons, these quantum computers can operate at room temperature and through a combination of a few linear optical tools, a universal set of quantum gates can be created. Although using photons as the source of qubits is a seemingly robust system, photons can escape to their environment like any system. However, this is a problem as photons are in a superposition of Fock states, photons escaping can change the state of the photons within the circuit. As phase shifters are used to change the phase of photons, phase decoherence can occur where the phase of the state can change due to factors such as including noise in the control signals that are used to manipulate the quantum system, imperfections in the physical components of the system, and other sources of noise and perturbation. This can become problematic, especially when it can lead to unentangled pairs or affect successive quantum gates in a circuit.
Regarding DiVincenzo’s Criteria, the photonic quantum computer again does theoretically fulfill all five criteria; yet, photon loss, phase decoherence, and efficiency of the quantum gates remain an issue. Simpler methods of photonic quantum computers are being developed to address these issues, hopefully leading to a scalable, fully functional quantum computer.