single-ion magnet*

Implementing quantum computation with spins faces the challenge of increasing the number of qubits while keeping errors under control. Even the simplest algorithm implies coupling two or more qubits in a controlled manner. However, dipolar interactions are also an important source of decoherence [1]. Here, we explore a way to scale up quantum resources, without introducing additional decoherence, by integrating several electron spin qubits in a single magnetic ion with spin S > ½. This approach is illustrated with a [Gd(H₂O)P₅W₃₀O₁₁₀]^12- polyoxometalate single-ion magnet [2]. Electron paramagnetic resonance experiments have been performed on molecules diluted in a crystal of the diamagnetic isostructural derivative [Y(H₂O)P₅W₃₀O₁₁₀]^12-. The seven allowed transitions between the 2S+1=8 spin states have been separately addressed and its spin coherence T₂ and spin-lattice relaxation T₁ rates measured. Rabi oscillations have been observed for all transitions. The spin states of each Gd³⁺ ion can then be mapped onto the states of three addressable qubits (or, alternatively, of a d = 8-level molecular “qudit”), for which the seven allowed transitions form a universal set of operations. Within this scheme, one of the coherent oscillations observed experimentally provides an implementation of a controlled-controlled-NOT (or Toffoli) three-qubit gate. We also propose a way to implement a simple quantum error correction code using this single-ion "processor". Our findings [3] open prospects for developing more complex and robust quantum computation schemes based on molecular spin qubits.

[1] A. Morello et al, Phys. Rev. Lett., 97, 207206 (2006).
[2] M. J. Martínez-Pérez et al, Phys Rev. Lett. 108, 247213 (2012).
[3] M. D. Jenkins et al, Phys. Rev. B 95, 064423 (2017).