All-Optical Quantum Information Processing in the Ultrafast Regime

Abstract

Quantum computing offers a promising pathway to solving problems that are intractable for classical computers. Despite recent progress, the development of a scalable quantum information processor remains an open challenge. Several hardware platforms remain contenders; however, photonic systems have emerged as a strong candidate due to their ability to operate at room temperature and exceptional resilience to decoherence. State-of-the-art photonic quantum processors use path encoding, which often requires an exponentially increasing number of optical components, making these schemes unavoidably large or lossy in some cases. Time-bin encoding has recently proven to be a promising method to reduce the number of optical components in a photonic quantum processor by enabling operation along a single optical path. Yet, a major barrier to the widespread adoption of time-bin encoding is the challenge of maintaining phase stability across the entire device for extended periods of time. We overcome this obstacle here, by introducing a promising new contender in the photonic quantum computing landscape: ultrafast time-bin encoding.

The architecture proposed in this thesis encodes quantum information in the arrival time of photons on ultrafast timescales, processed using optically induced nonlinearities via Kerr gating and birefringent media. A major part of this thesis is the development of the optical Kerr gating technique, which is explored in three initial studies. We first introduce a characterization method, which also happens to enable the carving of ultrashort pulses from a continuous wave laser. With this technique, we demonstrate configurable all-optical Kerr gating down to a temporal resolution of 305 fs. We next investigate translating this high temporal resolution to the frequency domain, using our Kerr gate to measure time-stretched spectra. Finally, we employ two Kerr gates for measurement of the temporal profiles of entangled photons, subsequently allowing for verification of entanglement. Each of these studies provides valuable insight into the optimal operation of the Kerr gate for quantum information processing.

The Kerr gate is then deployed for the demonstration of our ultrafast time-bin encoding scheme. Operating first in the single photon regime, we implement both a specific quantum algorithm and a more generalized, programmable circuit. Within the large multimode interferometers presented in these two implementations, we see remarkable phase stability over extended periods of time, high fidelity operation, and low loss - all key ingredients for scalability in quantum information processing. Finally, these capabilities are extended to include multiphoton operation, which is a crucial requirement for any photonic quantum information processing system. Taken together, these results demonstrate the potential of ultrafast time-bin encoding for the development of scalable photonic quantum information processing.