Zachary D. Walton

Decoherence-Free Subspaces in Quantum Key Distribution

We demonstrate that two recent innovations in the field of practical quantum key distribution (one-way autocompensation and passive detection) are closely related to the methods developed to protect quantum computations from decoherence. We present a new scheme that combines these advantages, and propose a practical implementation of this scheme that is feasible using existing technology.

Generation of polarization-entangled photon pairs with arbitrary joint spectrum

We present a scheme for generating polarization-entangled photons pairs with arbitrary joint spectrum. Specifically, we describe a technique for spontaneous parametric down-conversion in which both the center frequencies and the bandwidths of the down-converted photons may be controlled by appropriate manipulation of the pump pulse. The spectral control offered by this technique permits one to choose the operating wavelengths for each photon of a pair based on optimizations of other system parameters (loss in optical fiber, photon counter performance, etc.). The combination of spectral control, polarization control, and lack of group-velocity matching conditions makes this technique particularly well suited for a distributed quantum information processing architecture in which integrated optical circuits are connected by spans of optical fiber.

Controllable frequency entanglement via auto-phase-matched spontaneous parametric down-conversion

A new method for generating entangled photons with controllable frequency correlation via spontaneous parametric down-conversion (SPDC) is presented. The method entails initiating counter-propagating SPDC in a single-mode nonlinear waveguide by pumping with a pulsed beam perpendicular to the waveguide.

Performance of photon-pair quantum key distribution systems

Abstract We analyse the quantitative improvement in performance provided by a novel quantum key distribution (QKD) system that employs a correlated photon source (CPS) and a photon-number resolving detector (PNR). Calculations suggest that given current technology, the CPS/PNR implementation offers an improvement of several orders of magnitude in secure bit rate over previously described implementations.

One-Way Entangled-Photon Autocompensating Quantum Cryptography

A quantum cryptography implementation is presented that uses entanglement to combine one-way operation with an autocompensating feature that has hitherto only been available in implementations that require the signal to make a round trip between the users. Using the concept of advanced waves, it is shown that this proposed implementation is related to the round-trip implementation in the same way that Ekerts two-particle scheme is related to the original one-particle scheme of Bennett and Brassard. The practical advantages and disadvantages of the proposed implementation are discussed in the context of existing schemes.

Symmetric autocompensating quantum key distribution

We present quantum key distribution schemes which are autocompensating (require no alignment) and symmetric (Alice and Bob receive photons from a central source) for both polarization and time-bin qubits. The primary benefit of the symmetric configuration is that both Alice and Bob may have passive setups (neither Alice nor Bob is required to make active changes for each run of the protocol). We show that both the polarization and the time-bin schemes may be implemented with existing technology. The new schemes are related to previously described schemes by the concept of advanced waves.

One-way autocompensating quantum cryptography via auto-phase-matched spontaneous parametric down-converstion

We present a new quantum cryptography implementation that uses frequency-correlated photon pairs to combine one-way operation with an autocompensating feature that has hitherto only been available in implementations that require the signal to make a round trip between the users. Furthermore, we describe a new scheme for creating frequency-correlated photon pairs (auto-phase-matched spontaneous parametric down-conversion). The new scheme offers several advantages over previous schemes, including the ability to generate frequency-correlated photon pairs regardless of the dispersion characteristics of the system.

Passive autocompensating quantum cryptography

A new quantum cryptography scheme is presented that combines passive detection and autocompensation, two desirable features that have hitherto only been demonstrated in separate implementations. A practical implementation based on a well-known down-conversion source is proposed.