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Japanese Journal of Applied Physics

Optical Pumping System for a Qubit Initializer in a Solid-state NMR Quantum Computer

Atsushi Goto, Ryo Miyabe, Kenjiro Hashi, Tadashi Shimizu, Shinobu Ohki, Giyuu Kido, Susumu Machida

Published May 15, 2003• 13 citations
Publisher

Abstract

We present an optical pumping NMR system working at low temperatures developed for the qubit initialization in a solid-state NMR quantum computer. Demonstrations of the initialization experiments of 31 P nuclear spins in InP:Fe at 4.2 K are also presented, where the signal enhancements of 31 P by two orders of magnitude compared to that at thermal equilibrium are achieved.

DOI
10.1143/jjap.42.2864

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We present an optical pumping NMR system working at low temperatures developed for the qubit initialization in a solid-state NMR quantum computer. Demonstrations of the initialization experiments of 31 P nuclear spins in InP:Fe at 4.2 K are also presented, where the signal enhancements of 31 P by two orders of magnitude compared to that at thermal equilibrium are achieved.
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Current experiments in liquid-state nuclear magnetic resonance quantum computing are limited by low initial polarization. To address this problem, we have investigated the use of optical pumping techniques to enhance the polarization of a 2-qubit NMR quantum computer (13C and 1H in 13CHCl3). To efficiently use the increased polarization, we have generalized the procedure for effective pure state preparation. With this new, more flexible scheme, an effective pure state was prepared with polarization-enhancement of a factor of 10 compared to the thermal state. An implementation of Grover's quantum search algorithm was demonstrated using this new technique.
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We propose a decoupling-free nuclear-spin quantum computer installed on a quantum electron spin chain with a singlet ground state and a finite spin gap. Qubits are I=1/2I=1/2I=1/2 nuclear spins implanted periodically along the quantum spin chain. A magnetic field gradient is applied parallel to the chain, which allows individual access to each qubit. A single-qubit operation (rotation gate) is realized with the rf field tuned to the nuclear Larmor frequency at the qubit of interest, while a two-qubit operation (controlled-NOT gate) is achieved using the Suhl-Nakamura interaction through a packet of triplet magnons, which are excited by a microwave tuned to the spin gap energy (SN(SN(SN gate). The interaction can be switched off by turning off the microwave, and a decoupling-free quantum computer is realized. The initialization is achieved with an optical pumping qubit initializer, which has a multilayered structure of the quantum spin chain and a semiconductor. Spin polarizations created by the optical pumping in the semiconducting layers are transferred to the spin chain layers through a cross polarization and a spin diffusion. The scheme allows us to separate the initialization process from the computation, enabling us to optimize the latter independently of the former.
An experimental implementation of an ensemble quantum algorithm is presented in which the thermal state is used as an initial state of the algorithm. More specifically, we use four active qubits to demonstrate the solution of the Deutsch-Jozsa problem using a nuclear magnetic resonance quantum computer. For the implemented algorithm the number of molecules in the NMR sample is irrelevant to the number of qubits available to an NMR quantum computer, and the initial state is chosen to be the state of thermal equilibrium, thereby avoiding the preparation of pseudopure states and the resulting exponential loss of signal as the number of qubits increases. As expected, measured spectra demonstrate a clear distinction between constant and balanced functions.