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arXiv:quant-ph/0306131 · Physical Review A

Direct observation of photon pairs at a single output port of a beam-splitter interferometer

Giovanni Di Giuseppe, Mete Atatüre, Matthew D. Shaw, Alexander V. Sergienko, Bahaa E. A. Saleh, Malvin Carl Teich, Aaron Miller, Sae Woo Nam, John M. Martinis

Published Dec 16, 2003• 55 citations
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Abstract

Quantum theory predicts that two indistinguishable photons incident on a beam-splitter interferometer exit together (the pair emerges randomly from one port or the other). We use a special photon-number-resolving energy detector for a direct observation of this quantum-interference phenomenon. Simultaneous measurements from two such detectors, one at each beam-splitter-interferometer output port, confirm the absence of cross coincidences. Photon-number-resolving detectors are expected to find use in other quantum-optics and quantum-information-processing experiments.

DOI
10.1103/physreva.68.063817

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Quantum theory predicts that two indistinguishable photons incident on a beam-splitter interferometer exit together (the pair emerges randomly from one port or the other). We use a special photon-number-resolving energy detector for a direct observation of this quantum-interference phenomenon. Simultaneous measurements from two such detectors, one at each beam-splitter-interferometer output port, confirm the absence of cross coincidences. Photon-number-resolving detectors are expected to find use in other quantum-optics and quantum-information-processing experiments.
A two-photon interference experiment is presented in which an entangled pair of photons generated from a parametric down-conversion was incident on two input ports of a Mach-Zehnder interferometer. The experiment was carried out using two photon coincidence detection with two detectors at the two output ports of the interferometer. The measured coincidence counts exhibit an interference effect with visibility of 0.75 at simultaneous inputs and 0.38 at inputs with different arrival times according to the degree of photon number entanglement.
We demonstrate experimentally a phase measurement using general two-photon superposition states in a multiport interferometer shown schematically. A pair of photons, produced in the process of spontaneous parametric down-conversion, is transformed into the desired superposition state using beam splitters BS1 and BS2, provided that no photon escapes through the unmonitored port of BS2. In the signal arm, the photon pair experiences losses introduced by the beam splitter BS3 and monitored by a detector C, and subsequently probes the phase shift phi. The phase information is read out by sending the photons onto the beam splitter BS4, whose output ports are monitored by detectors A and B.
The interference of two single photons impinging on a beam splitter is measured in a time-resolved manner. Using long photons of different frequencies emitted from an atom-cavity system, a quantum beat with a visibility close to 100% is observed in the correlation between the photodetections at the output ports of the beam splitter. The time dependence of the beat amplitude reflects the coherence properties of the photons. Most remarkably, simultaneous photodetections are never observed, so that a temporal filter allows one to obtain perfect two-photon coalescence even for nonperfect photons.
The interference of two photons at a beam splitter is at the core of many quantum photonic technologies, such as quantum key distribution or linear-optics quantum computing. Observing high-visibility interference is challenging because of the difficulty of realizing indistinguishable single-photon sources. Here, we perform a two-photon interference experiment using phase-randomized weak coherent states with different mean photon numbers. We place a tight upper bound on the expected coincidences for the case when the incident wavepackets contain single photons, allowing us to observe the Hong-Ou-Mandel effect. We find that the interference visibility is at least as large as 0.995-0.013+0.005.
One of the most fundamental quantum features is the two-photon intensity correlation on a beam splitter, resulting in photon bunching into either output port. According to the conventional understanding of quantum mechanics, the origin of photon bunching on a beam splitter is in the indistinguishable characteristics between coincident photons, resulting in destructive quantum interference even without a clear definition of phase information of the paired photons. Here, a completely different approach is presented for the same quantum feature, where a strong mutual phase dependency is the essential requirement for the nonclassical feature of photon bunching. This definite phase relationship is now understood as the origin of this quantum feature resulting from the phase-basis superposition of the photon-BS system, resulting in photon indistinguishability. On behalf of a coherent model with a single input-port BS system, an extended scheme of a Mach-Zehnder interferometer is additionally analyzed for the validity of forbidden phase-basis superposition in the quantum feature.
Hong-Ou-Mandel interference is one of the clearest signatures of quantum behaviour: two identical photons meeting at a beam splitter always leave together. Here we demonstrate that this symmetry can be broken. Using a programmable integrated photonic processor, we interfere imbalanced two-photon states on a variable beam splitter and show that the output state of quantum interference can be continuously tuned, reaching the extreme case where destructive interference suppresses bunching at one output port. The asymmetry creates a forbidden bunching channel: photons may exit together at one port or separately at both ports, but cannot emerge together at the other port.