N.C.S.R. "Demokritos"


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Hybrid electron-nuclear spin systems for applications in quantum computing

Electron and nuclear spins are ideal elements for quantum bits (qubits) because they are natural two-state systems with relatively long decoherence times. A fundamental challenge in the realization of a solid state quantum computer is the construction of fast and reliable two-qubit quantum gates. Of particular interest in this direction are hybrid systems of electron and nuclear spins, where the two qubits are coupled through the hyperfine interaction. They have well-defined EPR and NMR transitions that can be selectively manipulated by resonant microwave (mw) and radio frequency (rf) pulses (Fig.1). However, the significantly different gyromagnetic ratios of electron and nuclear spins do not allow for their coherent manipulation at the same time scale. While this difference has been utilized for the construction of quantum memories, the slow inversion of nuclear spins using rf pulses (typical period 10 μs for a proton) can be a severe obstacle for the efficient function of a two-qubit quantum gate.

The required extreme increment (about two orders of magnitude) of the Rabi nuclear frequency through a corresponding increment of the rf B2 field can be technically very demanding. Alternatively, one can use anisotropic hyperfine-coupled systems and exploit the pseudo-secular part of the hyperfine interaction which is typically stronger than the Rabi nuclear frequency, i.e. of the order of some MHz in organic radicals. Recently, using the stable radical .CH(COOH)2, S =1/2, in a γ-irradiated malonic acid single crystal, we showed that under certain conditions (i.e. exact cancellation, Fig.2b) it is possible to control the nuclear spin polarization in sub-microsecond time intervals and also to toggle active and passive state dynamics using only mw pulses (Fig.3). Interestingly, the inversion of the α-proton nuclear spin, I=1/2, was accomplished within 252 ns which implies approximately 50-times faster operation than the corresponding rf-based pulse sequence.

This new possibility of fully manipulating the proton nuclear spin using only mw pulses and time delays overcomes the asymmetry in relaxation times (which is an inherent property of hybrid electron-nuclear spin systems) and thus opens the way for the realization of high-fidelity quantum gates based on hybrid electron-nuclear spin systems. As a consequence of this achievement, we are currently exploring possible pulse sequences that are based on this concept in order to construct a complete set of universal quantum gates.

for more information: Georgios Mitrikas

Department of Materials Science, N.C.S.R. "Demokritos", 153 10 Aghia Paraskevi, Attiki, phone: +30 210 6503381, fax: +30 210 6519430
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