Kim, JungsangCahall, Clinton T.2019-06-072019-06-072019https://hdl.handle.net/10161/18833<p>Quantum communication protocols, such as quantum key distribution (QKD), are practically important in the dawning of a new quantum information age where quantum computers can perform efficient prime factorization to render public key cryptosystems obsolete. QKD is a communication scheme that utilizes the quantum state of a single photons to transmit information, such as a cryptographic key, that is robust against adversaries including those with a quantum computer. In this thesis I describe the contributions that I have made to the development of high-rate, free-space quantum communication systems.</p><p>My effort is focused on building a robust quantum receiver for a high-dimensional time-phase QKD protocol where the data is encoded and secured using a single photon's timing and phase degrees of freedom. This type of communication protocol can encode information in a high-dimensional state, allowing the transmission of $>1$ bit per photon. To realize a successful implementation of the protocol a high-performance single-photon detection system must be constructed. My contribution to the field begins with the development of low-noise, low-power cryogenic amplifiers for a detection system using superconducting nanowire single-photon detectors (SNSPDs). Detector characteristics such as maximum count rate and timing resolution are heavily influenced by the design of the read-out circuits that sense and amplify the detection signal. I demonstrate a read-out system with a maximum count rate $>20\,$million counts-per-second and timing resolution as high as $35$\,ps. These results are achieved while maintaining a low power dissipation $<3$\,mW at 4\,K operation, enabling a scalable read-out circuit strategy. </p><p>A second contribution I make to the development of detection systems utilizing SNSPDs is extending the superb performance of these detectors to include photon number resolving capabilities. I demonstrate that SNSPDs exhibit multi-photon detection up to four photons where the absorbed photo number is encoded in the rise time of the electrical waveform generated by the detector. Additionally, our experiment agrees well with the predictions of a universal model for turn-on dynamics of SNSPDs. A feature our multi-photon detection system demonstrates high resolution between $n=1$ and $n>1$ photons with a bit-error-rate (BER) of $4.2\times10^{-4}$.</p><p>Finally, I extend the utility of the time-phase QKD protocol to free-space applications. Atmospheric turbulences cause spatial mode scrambling of the optical beam during transmission. Therefore, the quantum receiver, and most importantly the time-delay interferometer needed for the measurement of a phase encoding of a single photon, must support many spatial modes. I construct and characterize an interferometer with a 5\,GHz free spectral range that has a wide field-of-view and is passively a-thermal. The results of interferometer characterization are highlighted by a $>99\,\%$ single-mode, and $>98\,\%$ multi-mode interference visibility with negligible dependence on the spatial mode structure of the input beam and modest temperature fluctuations. Additionally, the interferometer displays a small path-length shift of 130\,nm/$^{\,\circ}$C, allowing for great thermal stability with modest temperature control.</p>Quantum physicsOpticsEngineeringcryogenic electronicsInterferometrymulti-photon detectionQuantum communicationQuantum key distributionsingle-photon detectorDissertation