| Of all the capabilities afforded by quantum information science, quantum cryptography currently shows the most promise for practical implementation. Accordingly, there has been a concerted effort to develop quantum cryptography schemes that mitigate the technical challenges associated with existing approaches. Among the successes in this effort are the development of noise-immune (alignment-free) schemes for polarization and time-bin qubits. A further advance is the development of a symmetric scheme for time-bin qubits in which neither of the participants (referred to as Alice and Bob) is required to make active changes to their setups. The term symmetric is used to describe quantum cryptography schemes in which a central source distributes some number of photons to both Alice and Bob, such that they share entanglement. This is in contrast to round-trip and one-way configurations, in which the photons move according to Bob → Alice → Bob, and Alice → Bob, respectively. In this thesis, we demonstrate that symmetry and noise-immunity can be combined in a single implementation, for both polarization and time-bin qubits. These two new quantum cryptography schemes exploit entanglement to achieve an extreme simplicity in the setups of Alice and Bob. That is, each party has an apparatus that operates passively and does not require precision alignment or stabilization.; The noise-immune symmetric scheme for polarization qubits requires a six-photon entangled state made up of three separately entangled pairs. The physical principle underlying the operation of the protocol is entanglement swapping, in which two particles that have never interacted are entangled by a remote operation on their respective twins. The theoretical efficiency of this scheme is 3/64, where efficiency is defined as the fraction of signals sent that lead to a shared bit. We prove the security of this scheme by demonstrating that, based on their joint measurements, Alice and Bob can verify the identity of the entangled state they share. Thus, Alice and Bob can extract a shared, secret cryptographic key, even if the signal source is under the control of the adversary.; The noise-immune symmetric scheme for time-bin qubits requires a four-photon entangled state made up of two separately entangled pairs. The physical principle underlying the operation of the protocol is entanglement purification, in which some number of non-maximally entangled photon pairs are converted into a smaller number of maximally entangled photon pairs. The theoretical efficiency of this scheme is 1/64. As in the case of the polarization scheme, we prove the security of this scheme by demonstrating that, based on their joint measurements, Alice and Bob verify the identity of the entangled state they share. |
