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arXiv:quant-ph/0404101 · Physical Review A

Exact solutions for universal holonomic quantum gates

Vahid Karimipour, N. Majd

Published Jul 30, 2004• 3 citations
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Abstract

We show how one can implement any local quantum gate on specific qubits in an array of qubits by carrying adiabatically a Hamiltonian around a closed loop. We find the exact form of the loop and the Hamiltonian for implementing general one and two qubits gates. Our method is analytical and is not based on numerical search in the space of all loops.

DOI
10.1103/physreva.70.012320

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We show how one can implement any local quantum gate on specific qubits in an array of qubits by carrying adiabatically a Hamiltonian around a closed loop. We find the exact form of the loop and the Hamiltonian for implementing general one and two qubits gates. Our method is analytical and is not based on numerical search in the space of all loops.
Nonadiabatic holonomic quantum computation provides the means to perform fast and robust quantum gates by utilizing the resilience of non-Abelian geometric phases to fluctuations of the path in state space. While the original scheme [E. Sj\"oqvist et al., New J. Phys. 14, 103035 (2012)] needs two loops in the Grassmann manifold (i.e., the space of computational subspaces of the full state space) to generate an arbitrary holonomic one-qubit gate, we propose single-loop one-qubit gates that constitute an efficient universal set of holonomic gates when combined with an entangling holonomic two-qubit gate. Our one-qubit gate is realized by dividing the loop into path segments, each of which is generated by a $\mathrm{\ensuremath{\Lambda}}$-type Hamiltonian. We demonstrate that two path segments are sufficient to realize arbitrary single-loop holonomic one-qubit gates. We describe how our scheme can be implemented experimentally in a generic atomic system exhibiting a three-level $\mathrm{\ensuremath{\Lambda}}$-coupling structure by utilizing carefully chosen laser pulses.
We study the construction of holonomy loops numerically in a realization-independent model of holonomic quantum computation. The aim is twofold. First, we present our technique of finding the suitable loop in the control manifold for any one-qubit and two-qubit unitary gates. Second, we develop the formalism further and add a penalty term for the length of the loop, thereby aiming to minimize the execution time for the quantum computation. Our method provides a general means by which holonomy loops can be realized in an experimental setup. Since holonomic quantum computation is adiabatic, optimizing with respect to the length of the loop may prove crucial.
Adiabatic quantum gate implementation generally takes longer time, which is disadvantageous in view of decoherence. In this report we implement several essential one-qubit quantum gates nonadiabatically by making use of a dynamical invariant associated with a Hamiltonian. Moreover we require that these gates be holonomic, that is, the dynamical phases associated with the gates vanish. Our implementation is based on our recent work [J. Phys. Soc. Jpn. 83, 034001 (2014)] and the gate parameters required for the implementations are found by numerical optimization.
In the holonomic approach to quantum computation, information is encoded in a degenerate eigenspace of a parametric family of Hamiltonians and manipulated by the associated holonomic gates. These are realized in terms of the non-Abelian Berry connection and are obtained by driving the control parameters along adiabatic loops. We show how it is possible for a specific model to explicitly determine the loops generating any desired logical gate, thus producing a universal set of unitary transformations. In a multipartite system unitary transformations can be implemented efficiently by sequences of local holonomic gates. Moreover, a conceptual scheme for obtaining the required Hamiltonian family, based on frequently repeated pulses, is discussed, together with a possible process whereby the initial state can be prepared and the final one can be measured.