We demonstrate the generation of broad-bandwidth optical frequency combs from a CMOS-compatible integrated microresonator. We characterize the comb quality using a novel self-referencing method and verify that the comb line frequencies are equidistant over a bandwidth of 115 nm (14.5 THz), which is nearly an order of magnitude larger than previous measurements.

A balanced cross correlator, the optical equivalent of a balanced microwave phase detector, is demonstrated. Its use in synchronizing an octave-spanning Ti:sapphire laser and a 30-fs Cr:forsterite laser yields 300-attosecond timing jitter measured from 10 mHz to 2.3 MHz. The spectral overlap between the two lasers is strong enough to permit direct detection of the difference in carrier-envelope offset frequency between the two lasers.

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...

We demonstrate a time-resolved single-photon detection technique based on ultrafast sum-frequency generation, providing femtosecond measurement capability for single photons in photonic quantum information processing. Noncollinear broadband upconversion in periodically poled MgO-doped stoichiometric lithium tantalate with an ultrafast pump and detection with a Si single-photon counter enable efficient detection of IR photons and temporal resolution of ~150 fs. We utilize the timing resolution to map the generation efficiency profile along the propagation axis of a periodically poled KTiOPO(4) crystal, revealing its local grating quality with millimeter resolution. We also apply the technique to two-photon coincidence measurements and directly demonstrate time anticorrelation between coincident-frequency entangled photons that are parametrically generated under extended phase-matching conditions.

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