Joint Temporal Density Measurements for Two-Photon State Characterization

Abstract

We demonstrate a new technique for characterizing two-photon quantum states based on joint temporal correlation measurements using time-resolved single-photon detection by femtosecond upconversion. We measure for the first time the joint temporal density of a two-photon entangled state, showing clearly the time anti-correlation of the coincident-frequency entangled photon pair generated by ultrafast spontaneous parametric down-conversion under extended phase-matching conditions. The new technique enables us to manipulate the frequency entanglement by varying the down-conversion pump bandwidth to produce a nearly unentangled two-photon state that is expected to yield a heralded single-photon state with a purity of 0.88. The time-domain correlation technique complements existing frequency-domain measurement methods for a more complete characterization of photonic entanglement in quantum information processing.PACS numbers: 42.50. Dv, 42.79.Nv, 42.50.Ar, 42.65.Lm Spontaneous parametric down-conversion (SPDC) is a powerful method for generating two-photon states for quantum information processing (QIP). The joint quantum state can be engineered for specific QIP applications by tailoring its polarization, momentum, and spectral degrees of freedom. Ultrafast-pumped SPDC is of great interest because a well defined time of emission is desirable in clocked applications such as linear optics quantum computing (LOQC) [1]. In ultrafast SPDC, spectral engineering of the two-photon state can be accomplished by manipulating the crystal phase-matching function and the pump spectral amplitude [2, 3] to yield unique forms of two-photon frequency entanglement. For example, coincident-frequency entanglement with strong positive correlation between signal and idler emission frequencies can be used to improve time-of-flight measurements beyond the standard quantum limit [4,5]. On the other hand, one can utilize a two-photon state with negligible spectral correlations to implement a heralded source of pure-state single photons, which can be a valuable resource for LOQC [6,7].Characterizing the spectral correlations of a twophoton state can be done by measuring the joint spectral density (JSD) profile with tunable narrowband filtering of the signal and idler [6,7,8]. Hong-Ou-Mandel quantum interference [9] is also useful for quantifying the twophoton coherence bandwidth and the indistinguishability of the photon pair. However, the two measurements do not give the whole picture of the two-photon state. Both measurements are insensitive to the spectral phase and therefore cannot capture the time-domain dynamics unless the joint state is known to be transform limited. Moreover, JSD measurements in wavelength regions with low detector efficiency or high detector noise can be challenging due to long acquisition times and low signal-tonoise ratios. Frequency-domain techniques for estimating the spectral phase exist, but they are not simple to implement in practice [10].In ultrafast optics ultrashort pulses are routinely analyzed spectrall...

Generation of degenerate, factorizable, pulsed squeezed light at telecom wavelengths

“…If the two entangled squeezed modes are generated in a factorizable state, we should see that the joint spectral probability distribution is also factorizable [8,9]. In the lowsqueezing limit, where a single photon pair is generated, it was recently shown that it is possible to generate pairs with an effective mode number that is close to the ideal value of 1 [8][9][10][11][12][13][14][15]. Hong-Ou-Mandel interference provides a measure of the similarity of the spectral distributions of the signal and idler modes.…”

Fast time-domain measurements on telecom single photons

“…As spectral intensity measurements can be determined both by time-of-flight dispersive fiber spectrometers [8] and commercially available single photon sensitive spectrometers, one only needs a means of temporal intensity characterisation. Among these are homodyne detection [9,10], streak cameras [11,12] and ultrafast sampling using sum-frequency generation (SFG) [13,14]. Temporal measurements by upconversion sampling are particularly interesting because they can in principle be highly efficient and therefore of measure the temporal envelope with a short integration time.…”

Temporal and spectral manipulations of correlated photons using a time lens

“…However, direct JTI measurements are limited in time-resolution because of the detector jitter. While time-resolved frequency upconversion [37] and intensity modulation [8] schemes allow JTI measurements with picosecond resolution by effectively introducing narrow filters in time or frequency, they require a two-dimensional scan of the filter position(s) for a two-photon state and therefore, can be extremely slow. In the following, we demonstrate that a time-lens expands the two-photon temporal wavefunction while preserving the quantum correlations of the wavefunction.…”