New High-Efficiency Source of Photon Pairs for Engineering Quantum Entanglement

Abstract

We have constructed an efficient source of photon pairs using a waveguide-type nonlinear device and performed a twophoton interference experiment with an unbalanced Michelson interferometer. Parametric down-converted photons from the nonlinear device are detected by two detectors located at the output ports of the interferometer. Because the interferometer is constructed with two optical paths of different length, photons from the shorter path arrive at the detector earlier than those from the longer path. We find that the difference of arrival time and the time window of the coincidence counter are important parameters which determine the boundary between the classical and quantum regime. When the time window of the coincidence counter is smaller than the arrival time difference, fringes of high visibility (80± 10%) were observed. This result is only explained by quantum theory and is clear evidence for quantum entanglement of the interferometer's optical paths.Two-photon entanglement has attracted considerable interest for studying the nonlocal correlations of quantum theory [1][2][3][4], and many experiments have been performed [5][6][7][8][9]. The contradiction of local realism can be realized more clearly with multi-photon entanglement systems [10], which have been demonstrated experimentally in recent years [11]. We can expect these systems to be used for novel applications such as quantum cryptography [12], and quantum teleportation [13].Multi-photon entanglement systems can be generated by parametric down-conversion. Since the probability of generating multi-photon entangled systems decreases exponentially with the number of entangled photons, it becomes more difficult to conduct experiments with a large number of entangled photons [14]. One of the candidates for solving this difficulty is to make the ultrabright source of polarization-entangled photons proposed by Kwiat et al. [15]. The source is superior to other sources because nearly every pair of photons is polarization entangled. Since the total number of generated photon pairs is limited by the nonlinear susceptibility and phase matching condition of a nonlinear crystal, a remarkable increase in the number of photon pairs can not be expected if one uses bulk crystals. This is to be compared to the drastic improvement of the efficiency to generate photon pairs we present. Our method uses a waveguide type nonlinear device originally developed for type-I quasi-phase-matching frequency doubling. Using the newly developed source of photon pairs, we then perform a two-photon interference experiment and show that photon pairs are in the entangled state for interferometer's optical paths. Parametric down-converted photons from the nonlinear device are detected by two detectors located at the output ports of the interferometer. Because this interferometer is constructed with two optical paths of different length, photons from the shorter path arrive at the detector earlier than those from the longer path. When the time window of the coincidence counter is ...

Multi-Photon Entanglement and Quantum Non-Locality

“…Given that, more photonpair sources could be combined with polarizing beam-splitters to yield entangled states of arbitrary numbers of particles. The latest developments in photon-pair sources suggest that it should be possible in the near future to have sources with many orders of magnitude higher emission rates [36]. With these entanglement sources one would be able to implement some quantumcomputation algorithms using only entanglement and linear optics [35].…”

Heralded single photon sources: a route towards quantum communication technology and photon standards

“…Reference [11] reports the generation of pairs of coincident photons at 854 nm from a PPLN waveguide 1 cm long. The waveguide structure acts as a spatial filter to select a single transverse mode of PDC light, and without requiring narrow band filters or spatial filters, the photon bandwidth was less than 1 nm, for a CW pump source.…”

“…Moreover, lack of high brightness single photon sources, and imperfect detector characteristics such as detection efficiency, dark count rate, timing jitter, and dead-time, significantly limit the key rate production in QKD [4]. To meet the demands for higher-rate secret key production and pseudo deterministic pure single photon state sources, the quantum information community is presently engaged in a number of efforts aimed at improving QKD, including optimizing the quantum channels for minimum loss [5,6] (low loss optical storage and fast optical switches), improving detector efficiency [2,7,8], reducing detector timing jitter [9], reducing detector deadtime [10], and single-mode single-photon source engineering [11][12][13][14][15][16][17].…”

“…In this case correlated photons are generated in single modes directly, and the usual problem associated with the correlation between spatial and spectral degrees of freedom is solved. Further development of this single photon source could include modal engineering of the photon states and mode matching into singlemode fibers, given that the waveguide exhibits accurate modal control [11][12][13][51][52][53][54][55], considering a much more precise analysis by Schmidt decomposition of spatio-temporal state amplitude [56]. Sections 7 and 8 deal with a general overview of practical heralded photon sources for quantum communication and radiometry.…”

Reduced deadtime and higher rate photon-counting detection using a multiplexed detector array

“…Quantum cryptography [3] would particularly benefit from improved detectors, as that application in the form of Quantum Key Distribution (QKD), is currently significantly constrained by detector characteristics such as detection efficiency, dark count rate, timing jitter, and deadtime [4]. Because of demands for higher-rate secret key production, the quantum information community is presently engaged in a number of efforts aimed at improving QKD, including optimizing the quantum channels for minimum loss [5,6], improving detector efficiency [2,7,8], reducing detector timing jitter [9], reducing detector deadtime [10], and single-mode single-photon source engineering [11,12,13,14,15,16,17]. Moreover, with the exponential growth in multimode parametric downconversion (PDC) photon pair production rates now in the range of 2x10 6 s −1 [18] and the more recent development of χ (3) single-mode fiber-based sources with pair rates up to 10 7 s −1 [16,17], the need is clear for better photon-counting detection by all means possible, including improved deadtime.…”