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arXiv:quant-ph/0204016 · Physical Review A

Encoded universality for generalized anisotropic exchange Hamiltonians

Jiří Vala, K. Birgitta Whaley

Published Aug 8, 2002• 21 citations
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Abstract

We derive an encoded universality representation for a generalized anisotropic exchange Hamiltonian that contains cross-product terms in addition to the usual two-particle exchange terms. The recently developed algebraic approach is used to show that the minimal universality-generating encodings of one logical qubit are based on three physical qubits. We show how to generate both single- and two-qubit operations on the logical qubits, using suitably timed conjugating operations derived from analysis of the commutator algebra. The timing of the operations is seen to be crucial in allowing simplification of the gate sequences for the generalized Hamiltonian to forms similar to that derived previously for the symmetric $(\mathrm{XY})$ anisotropic exchange Hamiltonian. The total number of operations needed for a controlled-$Z$ gate up to local transformations is five. A scalable architecture is proposed.

DOI
10.1103/physreva.66.022304

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We derive an encoded universality representation for a generalized anisotropic exchange Hamiltonian that contains cross-product terms in addition to the usual two-particle exchange terms. The recently developed algebraic approach is used to show that the minimal universality-generating encodings of one logical qubit are based on three physical qubits. We show how to generate both single- and two-qubit operations on the logical qubits, using suitably timed conjugating operations derived from analysis of the commutator algebra. The timing of the operations is seen to be crucial in allowing simplification of the gate sequences for the generalized Hamiltonian to forms similar to that derived previously for the symmetric (XY)(\mathrm{XY})(XY) anisotropic exchange Hamiltonian. The total number of operations needed for a controlled-ZZZ gate up to local transformations is five. A scalable architecture is proposed.
We study the quantum computational power of a generic class of anisotropic solid-state Hamiltonians. A universal set of encoded logic operations are found, which do away with difficult-to-implement single-qubit gates in a number of quantum-computer proposals, e.g., quantum dots and donor atom spins with anisotropic exchange coupling, quantum Hall systems, and electrons floating on helium. We show how to make the corresponding Hamiltonians universal by encoding one qubit into two physical qubits, and by controlling nearest-neighbor interactions.
We present a theoretical analysis of the paradigm of encoded universality, using a Lie algebraic analysis to derive specific conditions under which physical interactions can provide universality. We discuss the significance of the tensor product structure in the quantum circuit model and use this to define the conjoining of encoded qudits. The construction of encoded gates between conjoined qudits is discussed in detail. We illustrate the general procedures with several examples from exchange-only quantum computation. In particular, we extend our earlier results showing universality with the isotropic exchange interaction to the derivation of encoded universality with the anisotropic exchange interaction, i.e., to the XY model. In this case the minimal encoding for universality is into qutrits rather than into qubits as was the case for isotropic (Heisenberg) exchange. We also address issues of fault-tolerance, leakage and correction of encoded qudits.
We present a theoretical analysis of the paradigm of encoded universality, using a Lie algebraic analysis to derive specific conditions under which physical interactions can provide universality. We discuss the significance of the tensor product structure in the quantum circuit model and use this to define the conjoining of encoded qudits. The construction of encoded gates between conjoined qudits is discussed in detail. We illustrate the general procedures with several examples from exchange-only quantum computation. In particular, we extend our earlier results showing universality with the isotropic exchange interaction to the derivation of encoded universality with the anisotropic exchange interaction, i.e., to the XY model. In this case the minimal encoding for universality is into qutrits rather than into qubits as was the case for isotropic (Heisenberg) exchange. We also address issues of fault-tolerance, leakage and correction of encoded qudits.
We present a theoretical analysis of the paradigm of encoded universality, using a Lie algebraic analysis to derive specific conditions under which physical interactions can provide universality. We discuss the significance of the tensor product structure in the quantum circuit model and use this to define the conjoining of encoded qudits. The construction of encoded gates between conjoined qudits is discussed in detail. We illustrate the general procedures with several examples from exchange-only quantum computation. In particular, we extend our earlier results showing universality with the isotropic exchange interaction to the derivation of encoded universality with the anisotropic exchange interaction, i.e., to the XY model. In this case the minimal encoding for universality is into qutrits rather than into qubits as was the case for isotropic (Heisenberg) exchange. We also address issues of fault-tolerance, leakage and correction of encoded qudits.
In a previous publication [J. Kempe et al., Quantum Computation and Information (Rinton Press, Princeton, NJ, 2001), Vol. 1, special issue, p. 33] we showed that it is possible to implement universal quantum computation with the anisotropic XY-Heisenberg exchange acting as a single interaction. To achieve this we used encodings of the states of the computation into a larger Hilbert space. This proof is nonconstructive, however, and did not explicitly give the trade-offs in time that are required to implement encoded single-qubit operations and encoded two-qubit gates. Here we explicitly give the gate sequences needed to simulate these operations on encoded qubits and qutrits (three-level systems) and analyze the trade-offs involved. We also propose a possible layout for the qubits in a triangular arrangement.
The requirement of performing both single-qubit and two-qubit operations in the implementation of universal quantum logic often leads to very demanding constraints on quantum computer design. We show here how to eliminate the need for single-qubit operations in a large subset of quantum computer proposals: those governed by isotropic and XXZ, XY-type anisotropic exchange interactions. Our method employs an encoding of one logical qubit into two physical qubits, while logic operations are performed using an analogue of the NMR selective recoupling method.