We present a quantum network that distributes entangled photons between the University of Illinois Urbana-Champaign and a public library in Urbana. The network allows members of the public to perform measurements on the photons. We describe its design and implementation and outreach based on the network. Over 400 instances of public interaction have been logged with the system since it was launched in November 2023.

We develop a Hamiltonian formalism to study energy and position (momentum) correlations between a single Stokes photon and a single material excitation that are created as a pair in the spontaneous Raman scattering process. Our approach allows for intuitive separation of the effects of spectral linewidth, chromatic dispersion, and collection angle on these correlations, and we compare the predictions of the model to experiment. These results have important implications for the use of Raman scattering in quantum protocols that rely on spectrally unentangled photons and collective excitations.

We experimentally demonstrate a dual-pump spontaneous four-wave mixing photon-pair source for which we quantify the noise to determine the generation probability and collection efficiency directly, and that generates photons in pure quantum states. © 2019 The Author(s)

We experimentally study the generation of photon pairs via spontaneous four-wave mixing with two distinct laser pulses. We find that the dual-pump technique enables new capabilities: 1) a new characterization methodology to measure noise contributions, source brightness and photon-collection efficiencies directly from raw photon-count measurements; 2) an enhanced ability to generate heralded single photons in a pure quantum state; and 3) the ability to derive upper and lower bounds on heralded-photon quantum state purity from measurements of photon-number statistics even in the presence of noise. Such features are highly valuable in photon-pair sources for quantum applications.

We present 1D and 3D models that take into account Stokes-photon-excitation pair correlations in Raman scattering, revealing nontrivial dependence of the photon statistics on linewidth, dispersion and collection angle. © 2019 The Author(s)

Towards delayed-choice generation of single photons in pure quantum states, we measure and model the purity of Stokes photons scattered from sapphire, measuring a maximum purity of 0.99±0.03 and high quantum correlation with anti-Stokes photons.

We experimentally tailor the joint spectra of photon pairs produced via dual-pump spontaneous four-wave mixing, achieving a joint spectral intensity without side-lobes. This work presents a new route towards generating spectrally uncorrelated photon pairs.

We measure the quantum-state purity of Raman-scattered photons from sapphire, achieving a purity of 1.00 ± 0.03 and quantitative agreement with a new theoretical model of photon-phonon correlations that includes dispersion and finite excitation lifetime.

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.

We demonstrate characterization of the joint spectral density of photon pairs and the energy-resolved reconstruction of the polarization density matrix of entangled photon pairs using stimulated emission tomography. The stimulated photons can be detected with high signal-to-noise ratio, allowing the efficient, high-resolution measurement of correlations within and between degrees of freedom. Stimulated emission tomography can thus be used for swift multidimensional characterization of photonic quantum information processing systems.

Single photons in pure quantum states are an important resource for quantum applications in communication, cryptography, and computation, but despite major progress in the field, realization of a source of high-purity, deterministic single photons remains a challenge. One approach to implementing delayed-choice single photon generation is the Duan-Lukin-Cirac-Zoller (DLCZ) protocol [1], wherein delayed anti-Stokes photons are created via spontaneous Raman scattering. In this process a pump photon inelastically scatters, creating a Stokes photon together with a material excitation, which can be converted deterministically into an anti-Stokes photon within the excitation lifetime. Although this protocol has been demonstrated in various media, the purity of the photonic quantum states generated through this process has been only partially explored [2]. In solid-state media, the material excitation may take the form of a phonon mode; in a simple picture, the larger the ratio is between the bandwidth of the pump photon and the linewidth of the phonon mode, the higher the purity of the emitted photons will be. We present preliminary data of the purity of Stokes photons as a function of material thickness for c-axis single-crystal sapphire (α-Al2O3), which presents low fluorescent background [3] and is amenable to waveguide geometries. For free-space propagation and a pump bandwidth of approximately 8 nm, Stokes photons heralding the phonon mode at 746.6 cm−1 with linewidth of 11.0 cm−1 [4] exhibit purities of 0.99±0.03 and 0.87±0.04, for crystal lengths of 1 mm and 8 mm, respectively. For a pump bandwidth of 1.2 nm and 8 mm crystal length, the purity decreases to 0.61±0.06. These results are in qualitative agreement with a simplified theoretical model that takes into account material dispersion and the phonon frequency and linewidth. We also measure the second-order coherence crosscorrelation between Stokes and anti-Stokes photons and find a value of 340±10 for 8 mm crystal length, indicating a high degree of quantum correlation between Stokes and anti-Stokes photons. This and the near-unity measured purities lay the groundwork for implementation of the DLCZ protocol in dispersive media and for efficient, delayed-choice production of pure single photons. This work is supported in part by NSF Grant Nos. 1521110 and 1640968.

We demonstrate the measurement of photon-pair joint spectral correlations in optical fiber through stimulated four-wave mixing. This method enables us to study correlations more easily, precisely and quickly than with traditional coincidence counting measurements.

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 lifetimes 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. (C) 2014 Optical Society of America

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 degradation due to temporal distinguishability of the photons to enable the generation of a spectrally unfiltered polarization-entangled photon-pair state with $95.86\pm0.10%$ 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.

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. (C) 2014 Optical Society of America

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.

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.

We show one can engineer the spectral correlations of photon-pairs produced in optical fibers by employing the temporal walk-off between two distinct pumps in spontaneous four-wave mixing to create completely factorable states without spectral filtering.