There are many architectures being explored in the current era of quantum computing. The following is a collection of such approaches gathered as personal notes per part of my role on the product team at Quantum Brilliance. This collection was written in February of 2023 so take care if referencing in the future.

## Superconducting qubits

Superconducting qubits are among the most advanced and widely used quantum computing architectures. They are built using superconducting circuits and operate at extremely low temperatures, near absolute zero. These qubits leverage the quantum properties of Josephson junctions, which allow for the creation of artificial atoms with quantized energy levels. Superconducting qubits are favored by many major players in the field, including IBM, Google, and Rigetti, due to their scalability and relatively long coherence times. They offer fast gate operations and the potential for high-density qubit arrays, making them promising for large-scale quantum computers.

- IBM
- Rigetti Computing

## Trapped-ion qubits

Trapped-ion quantum computers use individual ions (charged atoms) as qubits, held in place by electromagnetic fields. This architecture is known for its exceptionally long coherence times and high-fidelity operations. Trapped-ion systems can operate at or near room temperature, which is an advantage over some other architectures. Companies like IonQ and Honeywell (now part of Quantinuum) are at the forefront of trapped-ion quantum computing. While scaling up the number of qubits is challenging, recent advancements in chip-based ion traps are addressing this issue.

- IonQ
- Quantinuum (formerly Honeywell Quantum Solutions)
- Alpine Quantum Technologies

## Silicon spin qubits

Silicon spin qubits use the spin states of individual electrons or atomic nuclei in silicon as quantum bits. This approach leverages the well-established silicon manufacturing infrastructure of the semiconductor industry, potentially allowing for easier scaling and integration with classical electronics. Silicon spin qubits can operate at slightly higher temperatures than superconducting qubits and offer long coherence times. Companies like Intel and research groups worldwide are working on this promising technology.

- Intel
- Silicon Quantum Computing
- Quantum Motion

## Photonic qubits

Photonic quantum computing uses individual photons (particles of light) as qubits. This approach offers several advantages, including the ability to operate at room temperature and natural resistance to decoherence. Photonic qubits are especially promising for quantum communication and networking. However, creating deterministic two-qubit gates is challenging. Companies like Xanadu and PsiQuantum are developing photonic quantum computers, with potential applications in quantum simulation and machine learning.

- Xanadu
- PsiQuantum
- Quandela

## Neutral atom qubits

Neutral atom qubits use individual neutral atoms, often alkali metals like rubidium or cesium, trapped in optical lattices or tweezer arrays. This architecture offers excellent scalability, with the potential for creating large arrays of qubits. Neutral atom systems can operate at room temperature and provide long coherence times. Companies like AtomComputing and QuEra are developing this technology, which shows promise for quantum simulation and optimization problems.

- AtomComputing
- QuEra Computing
- ColdQuanta

## Topological qubits

Topological qubits are a theoretical type of qubit based on exotic quantum states of matter called anyons. These qubits would be inherently protected from decoherence, potentially offering a path to fault-tolerant quantum computing. Microsoft is the primary company pursuing this approach, although the existence of the necessary anyons (Majorana fermions) is still under debate in the scientific community. If realized, topological qubits could revolutionize quantum computing.

- Microsoft
- Bell Labs
- TU Delft (research institution)

## Diamond nitrogen-vacancy (NV) center qubits

Diamond NV center qubits use the quantum states of nitrogen-vacancy defects in diamond as qubits. These systems can operate at room temperature and offer long coherence times. While scaling up to many qubits is challenging, NV centers are particularly promising for quantum sensing and quantum networking applications. Companies like Quantum Brilliance are exploring this technology for room-temperature quantum computing.

- Quantum Brilliance
- Qnami
- Element Six (research division)

## Nuclear magnetic resonance (NMR) qubits

NMR quantum computing uses the nuclear spins of atoms in molecules as qubits, manipulated using radio-frequency pulses. This was one of the earliest implementations of quantum computing, but it faces significant scaling challenges. While NMR systems were crucial for early quantum algorithm demonstrations, they are less favored for large-scale quantum computing due to initialization and measurement difficulties.

- Quantum Biosystems
- Oxford Instruments (equipment provider)
- D-Wave Systems (early work)

## Quantum dots

Quantum dot qubits use the electronic or spin states of quantum dots (nanoscale semiconductor structures) as qubits. These can be implemented in various materials, including silicon and gallium arsenide. Quantum dots offer the potential for high-density qubit arrays and compatibility with existing semiconductor manufacturing techniques. Research in this area is ongoing, with potential applications in scalable quantum computing systems.

- Intel
- HRL Laboratories
- TU Delft (research institution)

## Majorana fermion qubits

Majorana fermion qubits are a type of topological qubit based on exotic quasiparticles called Majorana fermions. These theoretical particles are their own antiparticles and could offer inherent protection against decoherence. Microsoft is leading research in this area, although the experimental realization of Majorana fermions in a form suitable for quantum computing is still a subject of intense research and debate.

- Microsoft
- QuTech (research institution)
- University of Maryland (research institution)

## Cavity QED (Quantum Electrodynamics) qubits

Cavity QED qubits involve the interaction between atoms or artificial atoms and photons in a high-quality optical or microwave cavity. This architecture allows for strong coupling between matter and light, enabling quantum information processing. Cavity QED systems can be used to create hybrid quantum systems, combining the advantages of different qubit types.

- Yale Quantum Institute (research)
- ETH Zurich (research)
- JILA (research institution)

## Molecular qubits

Molecular qubits use the quantum states of molecules as qubits. This approach offers the potential for chemically identical qubits and the ability to leverage molecular design for specific quantum computing applications. Molecular qubits can potentially operate at higher temperatures than some other architectures. Research in this area is still in early stages but shows promise for quantum sensing and computing applications.

- IBM Research
- University of Chicago (research)
- CQT Singapore (research institution)

## Rare-earth ion qubits

Rare-earth ion qubits use the electronic or nuclear spin states of rare-earth ions doped into crystal hosts. These systems offer exceptionally long coherence times and can be manipulated using optical techniques. While scaling is challenging, rare-earth ion qubits are particularly promising for quantum memories and repeaters in quantum communication networks.

- Australian National University (research)
- Caltech (research)
- Delft University of Technology (research)

## Electron spin qubits

Electron spin qubits use the spin states of individual electrons as qubits, often in semiconductor structures. These can be implemented in various materials, including silicon and gallium arsenide. Electron spin qubits offer fast operation times and the potential for high-density qubit arrays. They are being researched for scalable quantum computing systems, with potential integration with classical electronics.

- QuTech
- HRL Laboratories
- Princeton University (research)

## Quantum annealing

Quantum annealing is a specialized form of quantum computing optimized for solving certain classes of optimization problems. It uses quantum fluctuations to traverse an energy landscape and find low-energy states corresponding to optimal or near-optimal solutions. D-Wave Systems is the primary company producing quantum annealers, which have found applications in areas like logistics, finance, and materials science.

- D-Wave Systems
- Fujitsu
- NEC (research)

## Linear optical quantum computing

Linear optical quantum computing uses linear optical elements like beam splitters and phase shifters to manipulate photonic qubits. This approach is promising for quantum communication and certain quantum algorithms. While creating deterministic two-qubit gates is challenging, recent advancements in photon sources and detectors are making this architecture more viable for quantum computing applications.

- Xanadu
- Quandela
- PsiQuantum

## Rydberg atom qubits

Rydberg atom qubits use highly excited electronic states of neutral atoms. These states have exaggerated properties, including strong, long-range interactions that can be used for multi-qubit gates. Rydberg atom systems offer the potential for fast gate operations and are being explored for quantum simulation and quantum many-body physics studies.

- QuEra Computing
- Pasqal
- University of Wisconsin-Madison (research)

## Semiconductor qubits (other than silicon)

This category includes various semiconductor-based qubit implementations beyond silicon, such as gallium arsenide or indium arsenide quantum dots. These systems leverage different material properties and can offer advantages in terms of spin-orbit coupling or nuclear spin-free environments. Research in this area contributes to the broader understanding of solid-state quantum systems.

- HRL Laboratories
- University of New South Wales (research)
- CEA-Leti (research institution)

## Spin-photon hybrid qubits

Spin-photon hybrid qubits combine the advantages of spin qubits (long coherence times) with those of photonic qubits (easy transmission). These systems often use spins in solid-state defects or quantum dots coupled to optical cavities. They are particularly promising for creating quantum networks and distributed quantum computing systems.

- EPFL (research institution)
- University of Chicago (research)
- Delft University of Technology (research)

## Flux qubits (a type of superconducting qubit)

Flux qubits are a specific type of superconducting qubit that use the quantized magnetic flux in a superconducting loop as the basis for quantum states. They offer certain advantages in terms of anharmonicity and coupling strengths, making them useful for certain quantum computing and quantum simulation applications. Flux qubits are often used in conjunction with other types of superconducting qubits in heterogeneous quantum processor designs.

- D-Wave Systems
- MIT Lincoln Laboratory (research)
- NEC (research)