| Photonic quantum computing | |
|---|---|
| Name | Photonic quantum computing |
| Caption | Schematic of a photonic quantum circuit with single-photon sources, beam splitters, and detectors |
| Type | Quantum computing paradigm |
| Field | Quantum physics |
| Developer | Various academic groups and companies (e.g. University of Oxford, University of Bristol, MIT, Google Research, Xanadu (company), PsiQuantum) |
| Introduced | 1990s–2000s |
| Components | Photonic qubits, single-photon sources, beam splitters, interferometers, single-photon detectors |
Photonic quantum computing
Photonic quantum computing is a paradigm of quantum computing that encodes quantum information in states of light, typically using single photons, coherent states, or continuous-variable modes. It leverages quantum optical phenomena such as quantum superposition, quantum entanglement, and interference to implement quantum gates and algorithms, offering potential advantages in room-temperature operation, high-bandwidth communication, and integrability with existing photonic technologies.
Photonic quantum computing lies at the intersection of quantum optics and quantum information science. It exploits the bosonic nature of photons and the linear and nonlinear interactions available in optical media to realize qubits and continuous-variable (CV) encodings. Within Quantum physics, photonic platforms provide a testbed for foundational experiments (e.g., Bell tests, boson sampling) and practical implementations of quantum protocols such as quantum key distribution pioneered by researchers like Charles H. Bennett and Gilles Brassard and demonstrated in systems by groups at MIT and Caltech. Photonics is central to quantum communication infrastructure and interfaces between quantum processors and long-distance quantum networks.
Photonic encodings include discrete-variable qubits (polarization, time-bin, path) and CV encodings (quadrature amplitudes). Polarization qubits use orthogonal polarization modes of single photons, while time-bin qubits exploit temporal modes used in experiments at University of Bristol and NIST. CV photonic computing employs squeezed states and homodyne detection, concepts developed in squeezed light research by groups like S. L. Braunstein and labs including MPQ (Max Planck Institute of Quantum Optics). Bosonic encoding schemes such as dual-rail encoding and cluster states enable fault-tolerant designs adapted from theories by Raussendorf and Briegel for measurement-based quantum computation.
Architectures span linear optical quantum computing (LOQC), measurement-based quantum computing (MBQC) with photonic cluster states, continuous-variable quantum computing, and hybrid photonic-matter systems coupling photons to quantum dots, trapped ions, or superconducting qubits. LOQC, formalized by Knill, Laflamme and Milburn (KLM protocol), uses linear elements, ancillary photons, and postselection to effect non-deterministic gates. MBQC relies on generation of large photonic cluster states as pursued by teams at Xanadu (company), Weizmann Institute, and University of Oxford. Integrated photonics platforms (silicon photonics, silicon nitride, lithium niobate) provide scalable waveguide circuits developed by companies like PsiQuantum and academic groups at EPFL.
Single-photon and entangled-photon sources include spontaneous parametric down-conversion (SPDC) in nonlinear crystals and single-photon emission from quantum dots (e.g., work by E. B. Flagg and P. Michler). Deterministic sources are a focus for scalability; companies such as Xanadu (company) and PsiQuantum invest in engineered quantum emitters and photonic integration. Photonic circuits use beam splitters, phase shifters, and interferometers implemented in fiber or on-chip waveguides; programmable photonic processors have been demonstrated at MIT and University of Bristol. Single-photon detectors include superconducting nanowire single-photon detectors (SNSPDs) and avalanche photodiodes, with advances reported by groups at NIST and commercial suppliers like Single Quantum.
Quantum gates for photons are realized via linear optics and measurement-induced nonlinearities (KLM), or via deterministic interactions in nonlinear media and cavity quantum electrodynamics (cQED) coupling photons to atoms or quantum dots. Entanglement generation methods include SPDC, quantum-dot-mediated emission, and heralded schemes that underpin boson sampling experiments by teams at University of Bristol and UCLA. Dominant error sources are photon loss, mode mismatch, imperfect interference, detector inefficiency, and dephasing in matter-based interfaces. These errors affect fidelity in implementations of algorithms such as Shor's algorithm and protocols like quantum teleportation pioneered by experiments at Caltech and University of Geneva.
Error correction for photonic systems adapts bosonic codes (cat codes, GKP states by Gottesman, Kitaev, Preskill) and concatenated schemes for CV and discrete encodings. The GKP proposal enables encoding of qubits into harmonic oscillators, promising protection against small shift errors in phase space; experimental progress has been reported by groups at University of Sydney and Yale University. Fault-tolerant architectures require high-efficiency sources and detectors, multiplexing strategies for probabilistic gates, and integrated photonics to reduce loss. Scalable roadmaps by companies such as PsiQuantum emphasize deterministic sources and CMOS-compatible fabrication, while academic consortia (e.g., Quantum Flagship) coordinate standards and milestones.
Applications include quantum simulation, secure communication (quantum key distribution), photonic implementations of machine learning algorithms, and specialized tasks such as boson sampling that probe computational complexity (work by Aaronson and Arkhipov). Benchmarks include fidelity measures, quantum volume, and task-specific demonstrations: high-rate entangled-photon distribution by TREL and satellite QKD by Micius (satellite) teams; large-scale photonic boson sampling and Gaussian boson sampling experiments at University of Bristol, UCLA, and Zapata Computing collaborators; and scalable on-chip quantum photonic processors from Xanadu (company) and PsiQuantum. Recent milestones include demonstrations of high-purity single-photon sources, low-loss integrated circuits, and progress toward error-corrected photonic memory and repeater nodes for quantum networks.
Category:Quantum computing Category:Quantum optics Category:Photonic devices