NASA is developing quantum metrology capabilities for potential space-based quantum components in future navigation and communications systems. Innate knowledge of component operation is key for the space qualification of these components. This paper focuses on the measurement and analysis of an important characteristic of an entanglement source, the joint spectrum. We describe a spectrometer based on dispersive optical fibers and present experimental measurements of the joint spectrum of a highly non-degenerate SPDC-based entanglement source that emits entangled photons in the near-infrared and telecommunications bands. How the analysis of such a joint spectrum could be applied to the modeling and simulation of entanglement swapping operations as possible extensions of quantum networks is examined. Lastly, we discuss how the separability of the two-photon state is quantified via Schmidt decomposition and how the degree of separability impacts the spectral purity of heralded single-photon emissions.
NASA is developing quantum metrology capabilities for potential space-based quantum components in future navigation and communications systems. Innate knowledge of component operation is key for the space qualification of these components. This paper focuses on the measurement and analysis of an important characteristic of an entanglement source, the joint spectrum. We describe a spectrometer based on dispersive optical fibers and present experimental measurements of the joint spectrum of a highly non-degenerate SPDC-based entanglement source that emits entangled photons in the near-infrared and telecommunications bands. How the analysis of such a joint spectrum could be applied to the modeling and simulation of entanglement swapping operations as possible extensions of quantum networks is examined. Lastly, we discuss how the separability of the two-photon state is quantified via Schmidt decomposition and how the degree of separability impacts the spectral purity of heralded single-photon emissions.
Joint spectral measurements are a powerful tool for characterising biphoton spectral correlation, which is crucial for quantum information and communication technologies. In these applications, highly pure biphoton states are essential in any time- and frequency-mode, often obviating the need for time-resolved measurements. Conversely, spectroscopy utilising entangled photon pairs is gaining significant attention for its ability to unveil molecular dynamics, a field that critically demands time-resolved capabilities. Here, we introduce a methodology for capturing a biphoton spectrum that comprises visible and near-infrared photons, resulting in a highly non-degenerate joint spectrum. Our system employs two non-scanning spectrographs: a fibre spectrometer for near-infrared photons and a delay-line-anode single-photon imager for visible photons. We successfully measure the joint spectral intensity by leveraging a time-tagging acquisition strategy. Furthermore, our approach uniquely enables time-resolved joint spectral measurements with respect to the laser synchronisation against the hundreds-of-picosecond instrument response function. Our methodology could advance heralded fluorescence spectroscopy using biphoton sources to investigate temporal dynamics of complex biological, chemical, and physical systems.
The frequency entangled biphoton source generated via spontaneous parametric down-conversion (SPDC) process has found important applications in the fields of quantum clock synchronization, quantum communication, quantum information processing, etc. As quantum technologies evolve, quantitative characterization of the frequency entanglement becomes necessary and has been implemented by measuring the spectral properties of the biphoton state. However, due to the high dark rate and low quantum efficiency of the InGaAs single-photon detectors, direct measurement of the spectral properties of the biphoton state at optical communication wavelength is hard to implement. In this paper, we report the measurement of the spectral properties of a biphoton state at optical communication wavelength which is generated from periodically poled potassium titanyl phosphate (PPKTP) pumped by an ultra-short pulsed optical source at 787 nm. Based on the coincidence measurement setup together with two infrared spectrometers, the spectra of the signal and idler photons are obtained with their center wavelengths being 1574.4 nm and 1574.9 nm, while their 3-dB bandwidths being 35.3 nm and 37.6 nm respectively. The joint spectrum of the photon pair is observed as well and shows a coincident-frequency entanglement and a joint spectrum bandwidth of 3 nm. According to the ratio of the single-photon spectral bandwidth to the joint spectral bandwidth of the photon pairs, the degree of frequency entanglement is quantified to be 12, denoting a relatively high quality of the entanglement.
We report the demonstration of a bulk, intrinsically phase-stable source of polarization- and time-energy-entangled photon pairs at 810nm and 1550nm, directly coupled into single-mode optical fibers. This highly non-degenerate wavelength combination is well suited for hybrid quantum communication networks, enabling low-loss transmission in optical fibers at 1550nm while maintaining efficient free-space propagation and detection at 810nm. The source is based on spontaneous parametric down-conversion in a periodically poled lithium niobate crystal embedded in a polarization Sagnac interferometer, providing inherent stability and dual-degree-of-freedom entanglement.
We measure a spectral brightness of B = 4800 pair/s/mW/GHz, with fiber coupling efficiencies exceeding 0.48 at both wavelengths. The entanglement quality is characterized by high-visibility two-photon interference, yielding net visibilities of 0.995 in the polarization basis and 0.991 in the energy-time basis. These performances demonstrate a compact and robust entanglement source compatible with hybrid fiber/free-space quantum key distribution architectures, and suitable for future ground-to-satellite quantum communication links.
NASA Glenn Research Center’s quantum metrology approach is to combine measurements and models. Measurement results and models are subsequently integrated with NASA’s aerospace competency needs to provide an understanding of how spaceflight components work together in quantum network architectures. Trade studies and device measurements are performed within NASA’s Quantum Metrology Laboratory (NQML) whereas dynamic quantum network modeling occurs via the NASA Quantum Communications Analysis Suite (NQCAS) simulation tool. In illustrating the synthesis of the network model and metrology for quantum network development, we have focused on the evaluation of a degenerate Spontaneous Parametric Down Conversion (SPDC) source. Here we present an overview of Hong-Ou-Mandel and Joint Spectral Intensity measurements of the degenerate SPDC source. Results of these experiments are input into NQCAS to evaluate source suitability for entanglement swapping. This demonstrates the technology development approach of coupling of quantum measurement and free space quantum network models.
The ability to determine the joint spectral properties of photon pairs produced by the processes of spontaneous parametric downconversion (SPDC) and spontaneous four-wave mixing (SFWM) is crucial for guaranteeing the usability of heralded single photons and polarization-entangled pairs for multi-photon protocols. In this paper, we compare six different techniques that yield either a characterization of the joint spectral intensity or of the closely related purity of heralded single photons. These six techniques include: (i) scanning monochromator measurements, (ii) a variant of Fourier transform spectroscopy designed to extract the desired information exploiting a resource-optimized technique, (iii) dispersive fibre spectroscopy, (iv) stimulated-emission-based measurement, (v) measurement of the second-order correlation function g(2) for one of the two photons, and (vi) two-source Hong–Ou–Mandel interferometry. We discuss the relative performance of these techniques for the specific cases of a SPDC source designed to be factorable and SFWM sources of varying purity, and compare the techniques’ relative advantages and disadvantages.
This thesis describes work on single-photon pair creation, entanglement and entanglement measurement; single-photon spectral narrowing for atomic interaction; and single-photon detection and spectral analysis. The systems developed exploit the interactions of light beams in non-linear crystal materials. Greatly non-degenerate time-bin entangled photon pair sources are implemented based on the quantum process of spontaneous-parametric-down-conversion. The non-degenerate wavelengths correspond to an atomic transition (895 nm) and a telecommunications band (1310 nm). Interference-fringe-visibilities of over 78% are achieved, verifying entanglement. An efficient and low-noise single-photon up-conversion detector based on sum-frequency-generation is implemented. Photons at 1310 nm are converted to the visible region and efficiently detected using a visible single-photon detector. An overall detection efficiency of over 32% is achieved with a dark-count noise rate of 2500 s-1. The detector is further developed to achieve an inter-symbol-interference free detection rate at twice the limit of the type of visible detector used. The detector is adapted into an up-conversion spectrometer for wavelengths near 1310 nm. A sensitivity of -126 dBm is achieved, corresponding to photon fluxes of approximately 500 s-1. The up-conversion spectrometer is further adapted for correlated biphoton spectroscopy by incorporating a frequency entangled and time correlated source. In this scheme, the spectral function near 895 nm of a remote object can be reproduced by locally measuring the 1310 nm with the up-conversion spectrometer and monitoring the coincidence detection rates. To achieve linewidths feasible for atomic interaction, a very narrow linewidth non-degenerate single-photon pair source is implemented based on spontaneous-parametric-down-conversion embedded in a singly-resonant cavity which is locked to and resonating at the frequency corresponding to a target atomic transition. Linewidths as narrow as 28 MHz are achieved. The brightness of the source within the modes is increased by two-orders compared to the single-pass implementations.