| photonic qubit | |
|---|---|
| Name | Photonic qubit |
| Caption | Single-photon polarisation qubit illustration |
| Type | Quantum bit |
| Used in | Quantum computing, Quantum communication |
| Encoding | Polarization, time-bin, path, frequency, Orbital angular momentum |
| Implemented by | Single-photon source, Integrated photonics |
| Readout | Single-photon detector, Quantum state tomography |
photonic qubit
A photonic qubit is a two-level quantum information carrier encoded in properties of a single photon or photonic mode. Photonic qubits are central to experimental Quantum physics and Quantum information because photons are low-decoherence carriers that enable long-distance quantum communication and scalable approaches to quantum computing using linear optics and integrated platforms.
A photonic qubit is defined by two orthogonal quantum states of light that form a basis for a two-dimensional Hilbert space. Realizations commonly use single photons produced by deterministic or probabilistic sources; coherent-state encodings and single-rail/dual-rail modes are also used. Practical implementations rely on laboratory hardware developed at institutions such as Institute of Photonic Sciences and companies like Xanadu (company) and PsiQuantum. Photonic qubits are often manipulated in free-space or guided-wave setups using components standard in optical engineering and deployed in integrated circuits fabricated in platforms like silicon photonics and lithium niobate.
Photonic qubits exploit multiple degrees of freedom. Polarization qubits use orthogonal polarization states (e.g., horizontal/vertical) and are common in experiments by groups such as those at University of Vienna and Oak Ridge National Laboratory. Time-bin encoding stores information in early/late pulse arrival and is robust for fiber links used in quantum key distribution systems like those developed by ID Quantique. Path-encoded (dual-rail) qubits use spatial modes separated by beam splitters, central to the KLM protocol for linear-optical quantum computing. Frequency-bin qubits use discrete spectral modes and are compatible with wavelength-division multiplexing in telecom networks. Orbital angular momentum (OAM) encodes information in helical phasefronts and has been demonstrated by research groups at University of Glasgow and University of Ottawa for high-dimensional extensions.
Single-photon generation methods include spontaneous parametric down-conversion (SPDC) in nonlinear crystals, pioneered in experiments cited by Paul Kwiat, and quantum emitters such as semiconductor quantum dots (e.g., work at EPFL and Niels Bohr Institute), nitrogen-vacancy centers in diamond, and trapped ions used as deterministic sources. Heralded photons from SPDC remain widely used in foundational demonstrations. Manipulation employs passive linear-optical elements: beam splitters, phase shifters, and polarizing beam splitters, as formalized in the Hong–Ou–Mandel effect and the linear optics model of computation. Active control uses electro-optic modulators, acousto-optic modulators, and fast switches for time-bin and frequency control. Integrated photonics platforms—silicon, silicon nitride, indium phosphide, and lithium niobate—provide stable interferometers and scalable arrays; prominent integrated efforts include projects at IBM Research and Quantum Photonics Laboratory, University of Bristol.
Detection of photonic qubits relies on single-photon detectors: superconducting nanowire single-photon detectors (SNSPDs) offering high efficiency and low jitter, avalanche photodiodes (APDs), and transition-edge sensors (TES) with photon-number resolution. Interferometric measurements using Mach–Zehnder and Michelson architectures perform basis rotations and enable Bell-state analysis in entanglement experiments by teams like NIST. Quantum state tomography reconstructs the qubit density matrix from projective measurements and is employed to assess fidelity in demonstrations from academic groups and commercial providers. Homodyne and heterodyne detection extend measurement to continuous-variable encodings used in hybrid photonic architectures.
==Applications in quantum information (quantum communication, computing, metrology) == Photonic qubits underpin practical quantum communication protocols such as BB84 and entanglement-based quantum key distribution (QKD), demonstrated in field trials by China Academy of Engineering Physics and companies like Toshiba Research Europe. Entanglement distribution across fiber and satellite links (e.g., experiments by Micius (satellite)) leverages photonic qubits for global-scale quantum networks. In quantum computing, linear-optical quantum computing schemes (Knill–Laflamme–Milburn or KLM protocol) and measurement-based quantum computing (cluster states) use photonic qubits for gate and resource-state implementations; research from Perimeter Institute and University of Bristol advanced these directions. Photonic qubits also serve precision tasks in quantum metrology and sensing, including interferometric phase estimation and quantum-enhanced imaging, with applications in gravitational-wave detectors and optical clocks at institutions such as National Institute of Standards and Technology.
Key challenges include photon loss in transmission and coupling, which directly reduces protocol success probabilities and requires quantum error correction or repeater strategies such as those proposed in quantum repeater architectures by Sangouard et al.. Decoherence arises from polarization mode dispersion, chromatic dispersion, and thermal fluctuations in integrated devices. Indistinguishability between independently generated photons limits two-photon interference visibility; improvements require spectral-temporal engineering and cavity or Purcell-enhanced emitters as developed in Harvard University and Caltech research. Scalability issues demand integrated multiplexing, low-loss switches, and deterministic sources to approach fault-tolerant thresholds. Security of photonic implementations also faces side-channel and implementation attacks, motivating hardware and protocol standards from bodies like ETSI and ongoing research in device-independent QKD.
Category:Quantum information science Category:Quantum optics