Hong–Ou–Mandel
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| Hong–Ou–Mandel | |
|---|---|
| Name | Hong–Ou–Mandel effect |
| Discovered | 1987 |
| Discoverers | C. K. Hong; Z. Y. Ou; Leonard Mandel |
| Field | Quantum optics |
| Related | Two-photon interference; beam splitter; entanglement |
Hong–Ou–Mandel
Introduction
The Hong–Ou–Mandel effect is a quantum optical interference phenomenon observed when two indistinguishable photons impinge on a 50:50 beam splitter, producing a characteristic dip in coincidence detection rates. First demonstrated by C. K. Hong, Z. Y. Ou, and Leonard Mandel in 1987, the effect underpins technologies in quantum information, photonic quantum computing, and quantum metrology. Its observation connects experiments performed with sources such as spontaneous parametric down-conversion and devices including fiber couplers, and has been reproduced across platforms developed by groups at institutions like Bell Laboratories, Caltech, and MIT.
Principle and Theoretical Background
The principle relies on bosonic two-photon interference and the quantum superposition of indistinguishable paths at a beam splitter described by unitary scattering matrices used in analyses at institutions such as Harvard and Stanford. When two photons from sources like spontaneous parametric down-conversion or quantum dots arrive simultaneously, probability amplitudes for both transmitted or both reflected outcomes interfere destructively, leading to photon bunching and suppression of coincident detections at detectors such as avalanche photodiodes used in labs at NIST and ETH Zurich. Theoretical treatments invoke creation and annihilation operators in the formalism developed in works by Glauber and others at Cambridge and Princeton, and relate to Hong, Ou and Mandel’s original quantum optical calculation and to later formulations by Dirac and Feynman regarding indistinguishable particles.
Experimental Implementation
Experimental implementation typically employs nonlinear crystals such as beta-barium borate (BBO) used in setups at Bell Labs and Caltech pumped by lasers from companies like Coherent and New Focus. Photon pairs from parametric down-conversion are routed through delay lines and fiber couplers to a 50:50 beam splitter fabricated by integrated photonics groups at IBM and Intel or free-space cube beam splitters from Thorlabs. Coincidence counting is performed with single-photon detectors developed at institutions including MIT Lincoln Laboratory and the Max Planck Institute, with timing electronics from Tektronix or Keysight to scan path-length differences and measure the coincidence dip.
Observations and Results
Observed signatures include the HOM dip in coincidence counts as a function of relative delay, with visibility quantified in experiments by groups at Oxford, Columbia, and the University of Vienna. High-visibility dips indicate near-perfect indistinguishability as achieved in experiments at UCSB and RIKEN using narrowband filtering, polarization control with wave plates from Edmund Optics, and spectral engineering techniques reported by researchers at the University of Geneva. Results have been used to benchmark single-photon sources such as nitrogen-vacancy centers studied at Caltech, semiconductor quantum dots at the University of Cambridge, and heralded photons at Stanford.
Applications and Implications
The effect underlies linear optical quantum computing proposals by Knill, Laflamme, and Milburn and practical implementations at companies like Xanadu and PsiQuantum, enabling two-photon gates and entanglement swapping protocols demonstrated by teams at IBM and Google Quantum AI. It informs quantum key distribution experiments performed by groups at ID Quantique and Toshiba Research, and enhances quantum metrology techniques pursued at NIST and LIGO by exploiting two-photon interference for precision timing and sensing. The HOM effect also has implications for boson sampling experiments led by researchers at MIT and the University of Science and Technology of China.
Variations and Extensions
Extensions include time-resolved HOM interferometry explored at Caltech and EPFL, polarization-based HOM experiments conducted at Yale and ETH Zurich, and multi-photon generalizations studied by groups at UCLA and the University of Tokyo. Integrated photonic implementations on silicon and lithium niobate platforms by Intel and HP Labs enable on-chip HOM interference, while hybrid systems combining trapped ions at Innsbruck or superconducting circuits at the University of Waterloo probe analogous interference phenomena. Theoretical extensions link HOM-type effects to Hong–Ou–Mandel interferometry in continuous-variable frameworks developed at the University of Tokyo and to entanglement witnesses used by researchers at Perimeter Institute.
Historical Development and Key Experiments
Key experiments include the original 1987 demonstration by Hong, Ou, and Mandel at the University of Rochester and subsequent high-visibility implementations at Bell Labs, Caltech, and NIST. Important milestones were the use of spontaneous parametric down-conversion pioneered at Bell Labs, integrated photonics realizations at the University of Cambridge and IBM Research, and single-photon source benchmarking at Harvard and the University of Geneva. Later notable experiments include demonstrations of HOM interference with independent lasers at Oxford, with quantum dot sources at the University of Cambridge, and multi-photon interference experiments at MIT and the University of Vienna, solidifying the effect’s role in quantum optics and quantum information science.