Energetic Suppression of Decoherence in Exchange-Only Quantum Computation

Universal quantum computation requiring only the Heisenberg exchange interaction and suppressing decoherence via an energy gap is presented. The combination of an always-on exchange interaction between the three physical qubits comprising the encoded qubit and a global magnetic field generates an energy gap between the subspace of interest and all other states. This energy gap suppresses decoherence. Always-on exchange couplings greatly simplify hardware specifications and the implementation of inter-logical-qubit gates. A controlled phase gate can be implemented using only three Heisenberg exchange operations all of which can be performed simultaneously.Comment: 4 pages, 4 figure

A Scalable Architecture for Coherence-Preserving Qubits

We propose scalable architectures for the coherence-preserving qubits introduced by Bacon, Brown, and Whaley [Phys. Rev. Lett. {\bf 87}, 247902 (2001)]. These architectures employ extra qubits providing additional degrees of freedom to the system. We show that these extra degrees of freedom can be used to counter errors in coupling strength within the coherence-preserving qubit and to combat interactions with environmental qubits. The presented architectures incorporate experimentally viable methods for inter-logical-qubit coupling and can implement a controlled phase gate via three simultaneous Heisenberg exchange operations. The extra qubits also provide flexibility in the arrangement of the physical qubits. Specifically, all physical qubits of a coherent-preserving qubit lattice can be placed in two spatial dimensions. Such an arrangement allows for universal cluster state computation.Comment: 4 pages, 4 figure

Quantum Cellular Automata Pseudo-Random Maps

Quantum computation based on quantum cellular automata (QCA) can greatly reduce the control and precision necessary for experimental implementations of quantum information processing. A QCA system consists of a few species of qubits in which all qubits of a species evolve in parallel. We show that, in spite of its inherent constraints, a QCA system can be used to study complex quantum dynamics. To this aim, we demonstrate scalable operations on a QCA system that fulfill statistical criteria of randomness and explore which criteria of randomness can be fulfilled by operators from various QCA architectures. Other means of realizing random operators with only a few independent operators are also discussed.Comment: 7 pages, 8 figures, submitted to PR