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Efficient Quantum Frequency Conversion for Advanced Optical Communications


OBJECTIVE: Conceive and develop methods and techniques for substantially improving the performance of optical signal processing in nonlinear optical devices. Of particular interest is developing technologies suitable for quantum information processing such as near-100%-efficient quantum frequency conversion. Areas for development include achieving nonlinear processes with precisely tailored phase-matching spectra, reducing power requirements for driving such processes, developing single and few spatial-mode waveguides with ultralow level of background noise, reducing end-to-end loss, and robust packaging of devices with input and/or output fiber coupling. DESCRIPTION: Frequency conversion that preserves the quantum states of the converted signals, i.e., the so-called"quantum frequency conversion"(QFC), has found numerous applications due to its ability to manipulate coherent or nonclassical state light. For example, advanced optical communications applications, including single-photon quantum communications and quantum information processing for provably secure data links, can benefit from operation in both the visible/near- IR wavelengths (400800 nm) and the telecommunications C-band (15301565 nm). Visible and near-IR wavelengths enable coupling to atomic systems / quantum dots (e.g., for quantum memory) and detection via fast, high-efficiency, room-temperature single-photon detectors; but, unfortunately, visible/near-IR photons cannot be transmitted over long distances through single-mode fiber. C-band photons can be transmitted over long distances with extremely low loss using the existing single-mode fiber telecommunications grid, but can be neither coupled to atomic systems nor easily detected. In principle, QFC provides the ideal solution: coherent transfer between visible photons (for photon processing, storage, and detection) and telecommunications photons (for long-haul transmission through fiber). Perfecting this technology could bridge the distance gap that limits the state-of-the-art in provably secure communications, thus opening the door for a revolutionary capability for the DoD. Thus far, QFC has been demonstrated in various nonlinear optical media, such as crystals, waveguides, microresonators, and optical fibers. In such systems, quasi-phase matching (QPM) has been an enabling technology to allow efficient nonlinear interactions to be designed over a large wavelength span. In nonlinear crystals or waveguides, QPM is usually achieved via periodic poling, whereas in nonlinear microresonators, it can be realized utilizing the modal dispersion of the cavity modes. While some applications of QPM have become commercialized, others are emerging that place more stringent requirements on device fabrication and processing. For example, QFC may prove to be important for a host of quantum information processing applications [1]. For some such applications it would be advantageous to obtain a narrow phase matching peak, enabling a potential direct interface to quantum memory, for example. On the other hand, a broad phase matching bandwidth is useful when up-converting a high-rate single-photon channel since it allows the channel to be time-multiplexed to multiple detectors by exploiting multi-wavelength pumps [2]. In addition to reaching higher speed performance, such a system allows high efficiency Si avalanche photodiode (APD) single-photon counters to be used at a wider variety of wavelengths, including the telecom bands where single-photon counters tend to have poor efficiency, limiting the reach of quantum communications. Optical up- conversion with a tunable laser followed by Si APDs can also be used to build an extremely sensitive spectrometer [3]. In such a case a narrow band single phase matching peak is highly desirable because a narrow band leads to higher resolution while a single peak reduces unwanted cross talk. Quantum applications are in general highly sensitive to very small (i.e., single photon) light leakage levels from scattering processes such as Raman scattering [4]. Moreover, quantum systems are particularly sensitive to loss but most waveguide based frequency converters have significant (>3dB) loss especially when coupled to standard optical fibers. In addition to quantum applications, classical applications in instrumentation are also emerging such as measuring the jitter of attosecond optical pulses [5]. Such applications can benefit from waveguides that support dual polarizations and Type-II nonlinear interactions. Type-II phase matching can also be employed for pre- screening waveguides prior to poling to improve yield and uniformity [6]. These emerging applications demand a new generation of nonlinear waveguides with different and more stringent performance metrics. Thus it is of great practical importance that these new waveguides can be made with high yield, so the cost is not a limiting factor. Innovative solutions are sought to the problem of robust quantum frequency conversion. Successful solutions should simultaneously address all of the following critical technological challenges. (1) Low loss. Nonlinear waveguides are limited by coupling loss (from single-mode fiber for the C-Band signal, to single-mode fiber or free space for the visible signal), insertion loss (to and from the waveguide itself) and transmission loss (per unit length) through the frequency conversion medium; the net loss from all sources limits device performance and it is this net loss that should be compared to program milestones. (2) Ability to tailor the phase-matching bandwidths as detailed above. (3) Wavelength selectability. A completely successful solution should provide the flexibility to couple any wavelength in the visible/near-IR to any wavelength in the C-band. Note that this flexibility need not be in real time, but should instead reflect a flexibility in the fabrication process that allows any one device to be tailored for any two specific wavelengths. For high-energy photons near 400-nm, multiple stage frequency conversion may be required. (4) Efficiency. The waveguide nonlinearity should be sufficiently strong to provide near 100%-efficient coherent (entanglement-preserving) frequency conversion. (5) Robustness and portability. The ultimate goal of this SBIR is to develop a packaged, robust, portable, room-temperature technology that can be inserted into an existing system. PHASE I: Develop a detailed computer model for the performance of a quantum frequency conversion technology which fulfills all of the above requirements. Perform a detailed simulation and preliminary experimental evaluation of end- to-end device performance, and predict the technology"s loss, noise, speed, wavelength flexibility, efficiency, and robustness under a variety of realistic conditions and for the target wavelengths (400800 nm and 15301565 nm). A plan for reducing end-to-end losses from all sources to<1.5 dB should be identified. The impact of Raman photons shall be considered and methods to achieve low noise shall be identified. Based on the simulation results, each team should make quantitative predictions about PHASE II and PHASE III device performance in each of the five categories listed above. Using these predictions, establish appropriate PHASE II technical milestones on a six-month (all milestones must include a quantitative target) incremental basis. PHASE II: Construct prototype devices whose end-to-end performance effectively demonstrates the contractor's technical approach. Using these devices, test all PHASE I predictions for device performance. In particular, singly peaked phase matching bandwidths of<0.2 nm must be demonstrated. Losses and scattering effects must be minimized making the devices suitable for quantum applications, possibly those in a cascaded environment. Low end-to-end losses (<1.5 dB) through the nonlinear material must be demonstrated. Methods to maintain acceptable yields must also be developed. After characterizing the prototype devices, further refine and extend the PHASE I models. Finalize a design for a production system capable of delivering tailored devices as final products, and use the refined models to predict the performance of the second-generation (PHASE III) devices. PHASE II deliverables will include device prototypes operating in multiple operational regimes (e.g., at different source and target wavelengths), intermediate reports quantifying progress towards fulfilling the proposed technical milestones, and a final report summarizing all simulations, models, device designs, quantitative predictions for PHASE III device performance, and proposed quantitative milestones for PHASE III. PHASE III: Secure optical communications are critical to both the DoD mission and to many commercial systems. DARPA is currently funding a major effort to create macroscopic quantum communications systems capable of combining quantum security with classical telecommunications speeds and distances (The QUINESS Program). Deployment of this technology will require exactly the type of robust frequency conversion devices described in this SBIR. More broadly, any sensing or communications system which requires detection of ultra-low levels of telecommunications-band radiation will benefit from a frequency conversion technology that enables use of superior visible-light single-photon detectors. The new breed of nonlinear frequency conversion is foreseen to be used in a variety applications including single photon detection for quantum information or lidar, and classical measurements such as low photon spectroscopy or highly accurate laser pulse jitter measurements.

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