Spin cluster qubits in molecular and semiconductor systems
Electron spins are prototypical examples of two-level quantum systems and represent natural candidates for the encoding and manipulation of quantum bits. In the canonical approach, localized spins are individually addressed by oscillating magnetic fields in order to implement single-qubit gates, while the implementation of two-qubit gates is achieved by tuning the exchange interaction between neighboring spins. One of the technological challenges that this approach implies is the system control at the single-spin level, through localized magnetic and/or electric fields. This problem can be possibility to mitigated by encoding the qubit in the state of several spins, i.e. of a spin cluster. The singlet-triplet qubit [1] – which has been realized both in GaAs [2] and in Si [3] quantum dots – essentially follows this approach.
In this presentation, after briefly introducing the above concepts, I will overview the implementation of spin cluster qubits in molecular nanomagnets [4]. In particular, I will discuss the manipulation of scalar quantities (spin chirality, partial and total spin sums) induced by oscillating electric fields, through the modulation of the exchange interaction [5,6] and of other terms that enter the spin Hamiltonian, such as the g-factor or the axial anisotropy [7]. Unlike what happens in electron spin resonance, where the oscillating magnetic field induces transitions between different Zeeman levels, here one manipulates the spin within subspaces of given Sz. This allows to protect the qubits from the main decoherence mechanisms, such as those resulting from the hyperfine interactions. The connections with the semiconductor-based implementation of quantum computing will also be discussed.
References
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[3] B. M. Maune, M. G. Borselli, B. Huang, T. D. Ladd, P. W. Deelman, K. S. Holabird, A. A. Kiselev, I. Alvarado-Rodriguez, R. S. Ross, A. E. Schmitz, M. Sokolich, C. A. Watson, M. F. Gyure, and A. T. Hunter , Nature 481, 344 (2012).
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[6] F. Troiani, D. Stepanenko, and D. Loss, Phys. Rev. B 86, 161409 (2012).
[7] F. Troiani, arXiv:1907.08391 (2019).