Blueprint for a Molecular-Spin Quantum Processor

Abstract

The implementation of a universal quantum processor still poses fundamental issues related to error mitigation and correction, which demand investigation of also platforms and computing schemes alternative to the main stream. A possibility is offered by employing multilevel logical units (qudits), naturally provided by molecular spins. Here we present the blueprint of a molecular spin quantum processor consisting of single molecular nanomagnets, acting as qudits, placed within superconducting resonators adapted to the size and interactions of these molecules to achieve a strong single spin-to-photon coupling. We show how to implement a universal set of gates in such a platform and to readout the final qudit state. Singlequdit unitaries (potentially embedding multiple qubits) are implemented by fast classical drives, while an alternative scheme is introduced to obtain two-qubit gates via resonant photon exchange. The latter is compared to the dispersive approach, finding in general a significant improvement. The performance of the platform is assessed by realistic numerical simulations of gate sequences, such as Deutsch-Josza and quantum simulation algorithms. The very good results demonstrate the feasibility of the molecular route towards a universal quantum processor.

Proof-of-Concept Quantum Simulator Based on Molecular Spin Qudits

“…To further increase the scalability, the electronic spins can be used to activate an effective communication between distant qudits mediated by photons in superconducting resonators, after having swapped quantum information from the nuclear spins. This is made possible by the specific choice of molecular qudits as elementary units.…”

“…Being controllable quantum objects, magnetic molecules have attracted considerable attention as molecular qubits, − thanks to the remarkable possibilities of engineering their Hamiltonian and the long coherence times (from hundreds of μs to ms) reported in Cu or VO complexes. − Moreover, the possibility of controlling their quantum state by electric fields − and the blueprint of a magnetic quantum processor have been recently shown. These results are very interesting, but what makes magnetic molecules really potentially disruptive for quantum technologies is the fact that they naturally provide multilevel quantum systems, i.e., qudits with large number of states. − Indeed, the use of qudits as elementary units of computation − can simplify or improve quantum algorithms − and quantum sensing protocols .…”

Molecular nanomagnets: a viable path toward quantum information processing?

“…A similar tuning of single-qubit transition frequency can be achieved by applying a longitudinal microwave field. (f) Photon-mediated resonant interaction between molecular qubits within coplanar wave-guide resonators, exploiting and auxiliary level |e⟩ and the tunability of the resonator frequency (by a SQUID) to bring it into resonance with specific transitions, thus implementing a cφ gate after two subsequent photon emissions and absorptions [21]. S = S 12 + s 3 ) and the energy spectrum results in two lowenergy doublets |0, 1/2, M⟩ and |1, 1/2, M⟩ split by δ = J − J ′ and a higher energy S = 3/2.…”

“…These experimental issues must be interfaced with theoretical efforts mainly devoted to the development of a blueprint for a molecular spin quantum processor where quantum information can be initialized, processed and readout [21] thus achieving DiVincenzo criteria [22].…”

Semi-empirical Haken–Strobl model for molecular spin qubits

“…Electron spins in molecules have emerged as promising qubit systems [1][2][3][4][5] due to their relatively long intrinsic relaxation and decoherence timescales, comparable with traditional solid-state spin systems such as NV-centers [6,7]. Molecular spin qubits can be assembled into highly stable crystalline structures with tunable qubit densities [8][9][10] and can be integrated into other solid-state platforms such as superconducting resonators for controlling two-qubit interactions [11][12][13][14][15][16]. The vibrational and spin environment of molecular qubits can also be engineered using synthetic chemistry strategies [17][18][19].…”