Low Noise C-band Upconversion Detection for Demultiplexing Temporal Modes

Silver, Michael Lee

doi:https://doi.org/10.21985/n2-307f-j895

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Low Noise C-band Upconversion Detection for Demultiplexing Temporal Modes

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Quantum technologies have the capability of greatly increasing the security of communication systems. Many components are required for a full quantum network including memory, repeaters, routers, and detectors. The efficiencies of quantum communication technologies suffer greatly due to their inherent sensitivity to loss. Low-loss propagation and high detection efficiency can help create faster, more reliable quantum networks. Optical fibers are a convenient viable approach to low-loss propagation if the signals are in the range of 1530 to 1565 nm, the communications band (C-band). Efficient avalanche photodiode (APD) based single photon counters exist for the 400 to 900 nm wavelength range. Therefore, an efficient single photon detection system may employ both optical fibers and APDs. The issue then is converting a single photon from the low transmission loss C-band to the high efficiency visible range detection band. Quantum frequency conversion (QFC) is a nonlinear optical process in which the frequency of a single photon is changed through an interaction with a strong optical pump in a nonlinear medium. Using QFC a signal photon can be transmitted in an optical fiber, upconverted in a nonlinear medium, and then detected on a high efficiency single photon counter. A signal photon existing with a specific temporal mode shape has certain temporal pump profiles that will upconvert the signal photon with high efficiency. Given a set of photons in orthogonal temporal modes, different pumps can be designed that will upconvert them independently with high efficiency. By engineering these pumps to have high upconversion efficiency for one signal mode and low upconversion efficiency for other signal modes in the orthogonal set, we can selectively demultiplex single photons from a photon packet using QFC. Therefore, not only are the losses in the system greatly reduced by using QFC, but mode selectivity is also achieved which no other current technology is capable of. An upconversion detection system utilizing QFC can accomplish both photon counting and mode resolution, which most other detection technologies do not achieve. Using signal sets with d orthogonal temporal modes creates a d-dimensional Hilbert space, whereas most quantum communication techniques today use only a 2 -dimensional space by encoding on polarization. These polarization-entangled photon schemes are one example of a quantum bit, qubit, technology. By increasing the Hilbert space to contain d modes a qudit technology is built. The experiments presented herein focus on temporal mode multiplexing but the Hilbert space can be further expanded by including polarization, spatial mode, and wavelength multiplexing technologies. To demultiplex spatio-temporally overlapped orthogonal modes, multiple waveguides in series could provide multi-stage upconversions after which the signal photons could be detected on independent detectors. Due to the increased loss and complexity of such a system, it is desirable to use a single waveguide to upconvert independent temporal signals to different wavelengths which can then be separated into individual spatial paths for detection. This can be accomplished using a waveguide engineered to have multiple phase-matching peaks in it's transfer function profile. Other research has demonstrated the ability to upconvert signals in a waveguide with multiple phase-matching peaks but it has never been used for temporal mode demultiplexing. Controlling the phase and amplitude of a frequency comb through optical arbitrary waveform generation (OAWG) allows fine tuned control over temporal pulse profiles. OAWG is essential for efficient QFC mode selection for a given signal mode set. Technological restrictions of OAWG devices are sometimes a bottleneck for system data rates. By implementing dynamic OAWG (DOAWG) new options for pulse rates and symbol switching in adjacent pulses is available. We propose a scheme which utilizes both OAWG and QFC to build a high data rate upconversion qudit system which can easily be adapted into a DOAWG system for improved performance.

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