Can Two-Photon Interference be Considered the Interference of Two Photons?

Pittman, T. B.; Strekalov, Dmitry; Migdall, Alan L.; Rubin, Morton H.; Sergienko, A. V.; Shih, Yanhua

doi:10.1103/physrevlett.77.1917

Cited by 229 publications

(170 citation statements)

References 7 publications

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“…The interferometers erase the time information and temporally distinguishable events (at t and t + τ respectively) interfere 10,13,22 . Changing the relative phase α − β between the interferometers leads to interference fringes in the coincidence count rates.…”

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confidence: 99%

Entangling independent photons by time measurement

et al. 2007

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Entanglement is at the heart of quantum physics, both for its conceptual foundations and for applications in quantum communication. Remarkably, entanglement can be 'swapped': if we prepare two independent entangled pairs A1-A2 and B1-B2, a joint measurement on A1 and B1 (called a 'Bellstate measurement', BSM) has the effect of projecting A2 and B2 onto an entangled state, although these two particles have never interacted nor share any common past 1,2 . Entanglement swapping with photon pairs has already been experimentally demonstrated 3-6 using pulsed sources-where the challenge was to achieve sufficiently sharp synchronization of the photons in the BSM-but never with continuous-wave sources, as originally proposed 2 . Here, we present an experiment where the coherence time of the photons exceeds the temporal resolution of the detectors. Hence, photon timing can be obtained by the detection times, and pulsed sources can be replaced by continuous-wave sources, which do not require any synchronization 6,7 . This allows for the first time the use of completely autonomous sources, an important step towards real-world quantum networks with truly independent and distant nodes.The BSM is the essential element in an entanglement-swapping experiment. Linear optics allows the realization of only a partial BSM 8 by coupling the two incoming modes on a beam splitter and observing a suitable detection pattern in the outgoing modes. Such a measurement is successful in at most 50% of the cases. Still, a successful partial BSM entangles two photons that were, up to then, independent. The physics behind this realization is the bosonic character of photons. It is therefore crucial that the two incoming photons are indistinguishable: they must be identical in their spectral, spatial, polarization and temporal modes at the beam splitter; spectral overlap is achieved by the use of similar filters, spatial overlap by the use of single-mode optical fibres and polarization is matched by a polarization controller. In addition, the temporal resolution must be unambiguous: detection at a time t ± t d , where t d is the temporal resolution of the detector, must single out a unique time mode. In previous experiments, synchronized pulsed sources created both of the photons at the same time and path lengths had to be matched to obtain the required temporal overlap. The pulse length, that is, the coherence length of the photons, was τ c t d (typically τ c < 1 ps), but two subsequent pulses were separated by more than t d (ref. 9). The drawback of such a realization is that the two sources cannot be totally autonomous, because of the indispensable synchronization. For the case where τ c > t d (ref. 10), the detectors always single out a unique time mode. As a benefit, we can give up the pulsed character of the sources and the synchronization between them. By implementing this, we realize for the first time the entanglement swapping scheme as originally proposed in ref. 2.The experimental scheme is shown in Fig. 1. Each of the two nonlinear cr...

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Entangling independent photons by time measurement

et al. 2007

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“…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.…”

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“…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.…”

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Two-photon interferences with degenerate and nondegenerate paired photons

et al. 2012

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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...

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“…phenomenon, IFPS with an ideal 50:50 coupler does not rely on the intrinsic properties of the photon pair, but on only the indistinguishability of the sources [25,26]. Hence, IFPS has the potential to provide universal deterministic separation independent of the photon properties, allowing a single monolithically integrated device to separate any arbitrary two-photon state |ψ into different waveguides.…”

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confidence: 99%

Deterministic separation of arbitrary photon pair states in integrated quantum circuits

Marchildon

Helmy

2016

Laser & Photonics Reviews

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Entangled photon pairs generated within integrated devices must often be spatially separated for their subsequent manipulation in quantum circuits. Separation that is both deterministic and universal can in principle be achieved through anticoalescent two-photon quantum interference. However, such interference-facilitated pair separation (IFPS) has not been extensively studied in the integrated setting, where the strong polarization and wavelength dependencies of integrated couplers -as opposed to bulk-optics beamsplitters -can have important implications for performance beyond the identical-photon regime. This paper provides a detailed review of IFPS and examines how these dependencies impact separation fidelity and interference visibility. Focus is given to IFPS mediated by an integrated directional coupler. The analysis applies equally to both on-chip and in-fiber implementations, and can be expanded to other coupler architectures such as multimode interferometers. When coupler dispersion is present, the separation performance can depend on photon bandwidth, spectral entanglement, and the linearity of the dispersion. Under appropriate conditions, reduction in the separation fidelity due to loss of non-classical interference can be perfectly compensated for by classical wavelength demultiplexing effects. This work informs the design as well as the performance assessment of circuits for achieving universal photon pair separation for states with tunable arbitrary properties.

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Can Two-Photon Interference be Considered the Interference of Two Photons?

Cited by 229 publications

References 7 publications

Entangling independent photons by time measurement

Entangling independent photons by time measurement

Two-photon interferences with degenerate and nondegenerate paired photons

Deterministic separation of arbitrary photon pair states in integrated quantum circuits

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