Optical qubits are a type of qubits that are the basic building blocks of quantum computing. In a classical computer, the basic unit of information is a bit, which can be either a 0 or a 1. In a quantum computer, the basic unit of information is a qubit, which can be in a superposition of both 0 and 1 states. This allows quantum computers to perform certain tasks much faster than classical computers.

Optical qubits are qubits that are based on the properties of light. They can be created using various physical systems, such as light polarization or an electromagnetic wave’s phase. One example of an optical qubit is a photon, which is a particle of light. Photons can be polarized in different directions, which can be used to represent different quantum states.

A quantum computer can be built using quantum optical technologies like photonics or superconducting circuits. In a photonic quantum computer, optical qubits are created using photons. These photons are typically generated by a laser and then manipulated using optical elements such as beam splitters, phase shifters, and detectors. The photons can be used to perform operations such as entanglement, which is a key feature of quantum computing.

One advantage of using optical qubits is that photons are relatively easy to manipulate and control. They can travel long distances without being significantly affected by noise or interference, which makes them attractive for building large-scale quantum computing systems. Additionally, optical qubits can be generated at a high rate, which is important for certain quantum algorithms that require many qubits to be processed in parallel.

Another approach to building an optical quantum computer is using superconducting circuits. This approach creates qubits using tiny circuits made from superconducting materials. These circuits can be used to create microwave pulses, which can be used to manipulate the state of the qubits. Superconducting qubits can be coupled together to create entangled states, which is important for performing certain quantum algorithms.

All in all, optical qubits are a type of qubit that can be created using the properties of light. They are a promising approach to building a quantum computer, as they are relatively easy to manipulate and control and can be generated at a high rate. Optical quantum technologies, such as photonics and superconducting circuits, offer two potential paths toward building an optical quantum computer. While many challenges are still to overcome in building a large-scale quantum computing system, optical qubits offer a promising path forward.

Most of the photonic quantum computers are composed of photonic components such as:

State preparation (state prep):

 In quantum computing, state preparation refers to the process of initializing a quantum system into a specific quantum state that can be used as input for a quantum algorithm. In optical quantum computing, state preparation involves creating a stream of photons that are in a specific quantum state, such as a superposition of two states or entangled states between multiple photons. This process is crucial because the performance of quantum algorithms depends on the quality of the initial quantum state.

Multiplexer:

 A multiplexer is a device that combines several input signals into a single output signal. In the context of optical quantum path. This is useful for increasing the density of information that can be processed in each space and simplifying the routing of optical signals within a quantum computer.

Stitchers:

 Stitchers are devices that are used to combine multiple streams of photons into a single optical path, much like a multiplexer. However, stitchers are different from multiplexers in that they allow for more complex routing of photons within the optical system. Specifically, stitchers can be used to route photons between different optical components, such as waveguides and photodetectors, in a way that minimizes signal loss and interference.

Optical Quantum Processing Unit (QPU):

 A quantum processing unit (QPU) is the core processing unit in a quantum computer, analogous to the central processing unit (CPU) in a classical computer. In optical quantum computing, a QPU consists of an array of optical components, such as waveguides, phase shifters, and beam splitters, that can be used to manipulate the quantum state of photons. The QPU is responsible for implementing quantum algorithms by performing operations such as entangling photons, applying quantum gates, and measuring the resulting quantum states.

There are several companies that are currently leading the development of optical quantum machines, which are devices that use optical qubits to perform quantum operations. Some of the prominent ones are:

• PsiQuantum: PsiQuantum is a startup that is focused on developing photonic quantum computing technologies. The company is working on building a one million-qubit quantum computer using optical qubits that are created using silicon photonics. PsiQuantum has raised over $665 million, at the time of writing this article, in funding and has partnerships with companies such as GlobalFoundries and Applied Materials. 
• Xanadu: Xanadu is a startup that is focused on developing quantum computing technologies based on photonics. The company has built an 8-qubit photonic quantum computer and is also developing software tools to make it easier for developers to write quantum algorithms. Xanadu has raised over $100 million in funding and has partnerships with companies such as Boeing and National Instruments. The Canadian government recently designated a $40 million fund to Xanadu research to scale up and commercially solve industry grad problems.
• QuTech: QuTech is a research center in the Netherlands that is focused on developing quantum technologies. The Center collaborates with the Delft University of Technology and the Netherlands Organization for Applied Scientific Research (TNO). QuTech is working on developing photonic quantum processors using optical qubits that are created using integrated photonics.
• HRL Laboratories: HRL Laboratories is a research center that is focused on developing advanced technologies for defense, aerospace, and other applications. The Center is developing photonic quantum processors using optical qubits created using indium phosphide (InP) technology. HRL has partnerships with companies such as Lockheed Martin and Boeing.
• Raytheon: Raytheon is a defense and aerospace company working on developing quantum optical technologies for various applications, including cryptography and communications. The company is developing photonic quantum processors using optical qubits created using various technologies, including diamond color centers and superconducting nanowires.

Concurrently, in collaboration with researchers from the China’s National University of Defense Technology, the University of Bristol has successfully demonstrated a photonic quantum processor on a silicon photonics chip. This processor can generate two photonic qubits and conduct arbitrary two-qubit unitary operations, including entangling operations. The quantum processor was fabricated using mature Complementary Metal Oxide Semiconductor (CMOS) compatible processing, incorporating over 200 photonic components. The researchers programmed the processor to execute 98 distinct two-qubit unitary operations, with an average quantum process fidelity of 93 with a ±4.5% margin of error.

Other companies and institutions are actively developing optical quantum machines, including IBM, Google, and the University of Bristol. While the field of optical quantum computing is still in its early stages, there is a lot of excitement and investment in this area. We can expect to see continued progress and innovation from these and other companies in the years to come.