Two-photon interferences with degenerate and nondegenerate paired photons

Abstract

We generate narrow-band frequency-tunable entangled photon pairs from spontaneous four-wave mixing in three-level cold atoms and study their two-photon quantum interference after a beam splitter. We find that the path-exchange symmetry plays a more important role in the Hong-Ou-Mandel interference than the temporal or frequency indistinguishability, and observe coalescence interference for both degenerate and nondegenerate photons. We also observe a quantum beat in the same experimental setup using either slow or fast detectors. Quantum interference between single photons is one of the most important physical mechanisms for realizing linear optical quantum computation and information processing [1,2]. When two photons meet on a beam splitter (BS), their fourth-order interference can be observed as a coincidence correlation between two single-photon detectors, each placed at the output ports of the BS. In the well-known Hong-OuMandel (HOM) interference [3,4], it was found that two identical photons coalesce in the BS and go together into the same output port. This two-photon quantum interference effect has been confirmed from paired photons in spontaneous parametric down-conversion [3], four-wave mixing [5], and single photons from independent sources [6,7]. In these experiments, the indistinguishability between the two photons is the key for observing the HOM effect where the detectors are slow as compared to the bandwidth of the photons. When the two photons are in different frequencies, the two-photon interference can be measured as a quantum beat in time domain by two fast detectors [8,9].It was later realized that the HOM effect is not the interference between two individual photons, but the interference resulting from different two-photon Feynman paths [10,11]. Therefore, indistinguishability of the two photons (which holds in the early experiments) may not be the necessary condition for observing the two-photon interference. This explains why the HOM dip is immune from the groupvelocity dispersion [12]. The HOM interference has also been demonstrated using photons with distinguishable polarizations [13]. The HOM effect with photons of different colors was also predicted [13][14][15]. By adjusting the phase difference between the two-photon Feynman paths, one can even change the HOM dip into a peak using an entangled photon source [10,11,13]. In this Rapid Communication, we report an experimental demonstration of the HOM interference with nondegenerate photon pairs. We find that, while the photons may be distinguishable in their temporal and/or frequency degrees of freedom, the indistinguishable pathways from the spatialmode exchange symmetry leads to the HOM interference for both the degenerate and nondegenerate paired photons in our right-angle spontaneous four-wave mixing (SFWM) * dusw@ust.hk configuration. In addition to directly observing the quantum beat with fast detectors, we also show that the quantum beat between nondegenerate photons can be measured in the HOM setup by two slow detectors (without...

Two-photon interference with non-identical photons

“…In the same year, Ou et al reported a similar beating experiment with blue and green laser light beams [9]. The second-order interference with photons of different spectra was further studied with photons generated by spontaneous parametric down conversion [10][11][12][13][14][15][16], single-photon sources [17], laser and single-photon source [18], chirped laser pulses [19], and spontaneous Raman process [20], etc. These experiments can be categorized into two groups.…”

“…These experiments can be categorized into two groups. One group is that the two light beams of different spectra are incident to both input ports of a Hong-Ou-Mandel (HOM) interferometer [8][9][10][11][12][13][14][15][16]20]. The other group is that two light beams are incident to the two input ports of a HOM interferometer, respectively [17][18][19].…”

“…The other group is that two light beams are incident to the two input ports of a HOM interferometer, respectively [17][18][19]. The first group of experiments [8][9][10][11][12][13][14][15][16]20] can be easily understood in quantum mechanics. By adding more pathes, there can be indistinguishablealternatives for non-identical photons to trigger a two-photon coincidence count.…”

Measuring the Biphoton Temporal Wave Function with Polarization-Dependent and Time-Resolved Two-Photon Interference

“…1. We work with right-angle SFWM in laser-cooled 85 Rb atoms loaded in a magneto-optical trap [33]. The energy difference of the two hyperfine ground states j1i ¼ j5S 1=2 ; F ¼ 2i and j2i ¼ j5S 1=2 ; F ¼ 3i is ℏΔ 21 , with Δ 21 ¼ 2π × 3036 MHz.…”

Narrowband biphotons with polarization-frequency-coupled entanglement

“…1(a). We produce narrowband photon pairs from SFWM in lasercooled 85Rb atoms in a three-dimensional magneto-optical trap driven by two coherent (pump and coupling) laser fields [24]. The atomic cloud has a diameter of about 1.3 mm.…”