Offir Cohen

Fast and highly resolved capture of the joint spectral density of photon pairs

Controlling the spatial and spectral–temporal properties of photon pairs produced in artificially structured materials is fundamental to the realization of numerous photonic quantum information applications. Tailoring the joint spectral properties of photon pairs is of particular importance for applications relying on time–energy entanglement, high-visibility interference, and heralding. Yet measuring the joint spectral properties is a time-consuming task requiring coincidence counting, typically resulting in low-resolution spectra with a poor signal-to-noise ratio. In this work we capture the joint spectral correlations of photon pairs that would be produced in optical fibers with unprecedented speed, resolution, and signal-to-noise ratio, using a scheme based on stimulated four-wave mixing. We also illustrate that this technique can be used in engineering joint spectral correlations, making it a powerful tool for studying quantum states.

State engineering of photon pairs produced through dual-pump spontaneous four-wave mixing

We study theoretically the joint spectral properties of photon-pairs produced through spontaneous four-wave mixing (SFWM) with two spectrally distinct pump pulses in optical fibers. We show that, due to the group velocity difference between the pulses, the signature of the interaction can be significantly different from spontaneous parametric down-conversion or SFWM with a single pump pulse. Specifically, we study the case where temporal walk-off between the pumps enables a gradual turn-on and turn-off of the interaction. By utilizing this property, we develop a new approach towards tailoring the spectral correlations within the generated photon pairs, demonstrating the ability to produce factorable photon-pair states, and hence heralded single photons in a pure wave-packet. We show that the use of two pumps is advantageous over single-pump SFWM approaches towards this goal: the usage of the dual-pump configuration enables, in principle, the creation of completely factorable states without any spectral filtering, even in media for which single-pump SFWM tailoring techniques are unsatisfactory, such as standard polarization-maintaining fiber.

Polarization-entangled photon-pair generation in commercial-grade polarization-maintaining fiber

We demonstrate a fiber-based source of polarization-entangled photon pairs at visible wavelengths suitable for integration with local quantum-processing schemes. The photons are created through birefringent phase-matching in spontaneous four-wave mixing inside a Sagnac interferometer. We address entanglement due to temporal distinguishability of the photons to enable the generation of a spectrally unfiltered polarization-entangled photon-pair state with 95.86±0.10% fidelity to a maximally entangled Bell state, evaluated with a tomographic state reconstruction without applying any corrections or background subtractions. Owing to the large birefringence of the fiber, photons are created far detuned from the pump, where Raman contamination is negligible. This source’s spatial mode and ability to produce spectrally uncorrelated photons make it suitable for implementing quantum information protocols over free-space and fiber-based networks.

Heralded generation of single photons in pure quantum states

Sources of single photons in pure quantum states are essential to many quantum optics applications. We explore a technique, known as group-velocity matching, that enables the heralded generation of pure single photons through nonlinear optical processes. It reduces or eliminates the requirement of spectral filtering that conventional sources rely on to achieve high purity, therefore increasing both heralding efficiency and photon flux. Implementation in both spontaneous parametric downconversion and spontaneous four-wave mixing is studied as we discuss several recent experimental realizations demonstrating purity of more than 0.84 for the heralded photons.

Measuring vibrational coherence lifetimes in liquid methanol using transient coherent Raman scattering

We demonstrate the measurement of vibrational coherence in liquids using transient coherent Raman scattering. This technique measures the coherence lifetime of vibrational states through the interference of time-delayed coherent Raman-scattered photons using low-power, non-resonant optical pulses. We measure the vibrational lifetime of the 1033 cm−1 mode in liquid methanol. The resulting lifetime agrees with frequency-domain lineshape measurements. This technique is a complementary and in some cases simpler alternative to standard nonlinear spectroscopy techniques.

Polarization-entangled photon generation in a standard polarization-maintaining fiber

We demonstrate the generation of polarization-entangled photon pairs in a standard polarization-maintaining fiber using a Sagnac interferometer. This sources spatial mode is well matched to fiber-based quantum information networks.

Observation of coherence oscillations of single ensemble excitations in methanol

We demonstrate coherence measurements of single-photon-level collective excitations of vibrational states using transient coherent spontaneous Raman scattering in liquid methanol. We observe the decay of the 1033  cm−1 mode and coherence oscillations due to simultaneous excitation of the 2834 and 2944  cm−1 modes. The coherence life-times and oscillation frequencies agree with frequency-domain line-shape measurements and femtosecond coherent anti-Stokes Raman scattering measurements. The demonstrated technique is complementary to and, in some cases, simpler than traditional stimulated spectroscopy techniques in that it does not require more than one laser and is free of nonresonant background.

Nonlinear optics for high-order frequency conversion: applied attosecond science

We show that it is possible to use of a train of counterpropagating light pulses to enhance the coherent upconversion of intense femtosecond lasers into the extreme ultraviolet (EUV) region of the spectrum. This all optical quasi-phase-matching uses interfering beams to scramble the quantum phase of the generated EUV light, suppressing the contribution of out-of-phase emission. Selective enhancement of up to 600X is observed at photon energies of ~70 eV using argon gas and ~ 150 eV using helium gas.

Storage of ultra-broadband pulses in hot atomic barium vapor

We demonstrate the potential for an ultra-broadband quantum memory in hot atomic barium vapor using an off-resonance Raman interaction. It may enable storage of THz-bandwidth photons for high-speed quantum information processing in the telecom range.

Multidimensional tomography of an entangled photon-pair source using stimulated emission

We perform a multidimensional characterization of a polarization-entangled photon-pair source using stimulated emission tomography (SET). We measure the frequency-resolved polarization density matrix, which is composed of thousands of individual polarization density matrices, each corresponding to a different frequency pair. The measurement exhibits detailed information about correlations that would be difficult to observe using traditional quantum state tomography. This demonstration exhibits the power of SET to characterize a source of quantum states with multi-dimensional correlations and hyper-entanglement. The SET technique can be applied to a variety of photon-pair-based sources for the optimization and engineering of quantum states.