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arXiv:quant-ph/0406096 · Physical Review Letters

Quantum Beat of Two Single Photons

Thomas Legero, Tatjana Wilk, Markus Hennrich, Gerhard Rempe, Axel Kuhn

Published Aug 13, 2004• 289 citations
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Abstract

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.

DOI
10.1103/physrevlett.93.070503

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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.
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We demonstrate fourth-order quantum beat between sunlight and single photons from a quantum dot. With a fast time-resolved detection system, we observed high-visibility quantum beat between the independent photons of different frequencies from the two astronomically separated light sources. The temporal dynamics of the beat oscillation indicate the coherent behavior of the interfering photons, and the raw visibility of two-photon interference shows violation of the classical limit with a frequency mismatch of three-times the line width.
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.
The interference of single photons from independent sources is an essential tool in quantum information processing. However, the interfering of photons with long temporal states in a time-resolved manner has rarely been studied. This is because without transmitting spectral filters or coupling to a cavity mode single photons generated in traditional nonlinear crystals suffer from a short temporal profile below 1 ns. With spectral correlation maintained in the biphotons generated from spontaneous four-wave mixing process in cold atom clouds, here we demonstrate the temporal interference of two frequency-tunable long photons from two independent cold atomic sources. We observe and analyze the interference of frequency-mismatched photons, where the phenomenon of the quantum beat at megahertz separation is displayed. Our paper provides more details for the quantum beat of two independent narrow-band single photons, which may find potential application in frequency-encoded photonic qubits in quantum information processing.
Two-photon interference (TPI) is a fundamental phenomenon in quantum optics and plays a crucial role in optical quantum technologies. TPI is commonly considered as quantum interference with an upper bound of 100% for both the TPI and the beat visibility in contrast to its classical counterpart with a maximum visibility of 50%. However, this is not always the case. Here, we report a simultaneous observation of quantum and classical TPI of single photons with ultralong coherence time, which is five orders of magnitude longer than the photon correlation time. We observe a TPI visibility of 94.3% ± 0.2% well above the 50% classical limit indicating the quantum feature, and a beat visibility of 50% reflecting the classical feature. In addition an anti-bunching central dip due to single-photon anti-correlation, we also observe two bunching side peaks in cross correlation curves for indistinguishable photons. Using the general wave superposition theory and the quantum field approach, we derive the same cross correlation functions, which fully reproduce and explain the experiments. Our results reveal that the quantum and classical features of TPI result from the same interference, as the fourth-order temporal interference arises from the second-order temporal interference between two photons on the timescale of the photon coherence time.