Long Coherence Times in Nuclear Spin-Free Vanadyl Qubits

Abstract

Quantum information processing (QIP) offers the potential to create new frontiers in fields ranging from quantum biology to cryptography. Two key figures of merit for electronic spin qubits, the smallest units of QIP, are the coherence time (T), the lifetime of the qubit, and the spin-lattice relaxation time (T), the thermally defined upper limit of T. To achieve QIP, processable qubits with long coherence times are required. Recent studies on (PhP-d)[V(CS)], a vanadium-based qubit, demonstrate that millisecond T times are achievable in transition metal complexes with nuclear spin-free environments. Applying these principles to vanadyl complexes offers a route to combine the previously established surface compatibility of the flatter vanadyl structures with a long T. Toward those ends, we investigated a series of four qubits, (PhP)[VO(CS)] (1), (PhP)[VO(β-CS)] (2), (PhP)[VO(α-CS)] (3), and (PhP)[VO(CSO)] (4), by pulsed electron paramagnetic resonance (EPR) spectroscopy and compared the performance of these species with our recently reported set of vanadium tris(dithiolene) complexes. Crucially we demonstrate that solutions of 1-4 in SO, a uniquely polar nuclear spin-free solvent, reveal T values of up to 152(6) μs, comparable to the best molecular qubit candidates. Upon transitioning to vanadyl species from the tris(dithiolene) analogues, we observe a remarkable order of magnitude increase in T, attributed to stronger solute-solvent interactions with the polar vanadium-oxo moiety. Simultaneously, we detect a small decrease in T for the vanadyl analogues relative to the tris(dithiolene) complexes. We attribute this decrease to the absence of one nuclear spin-free ligand, which served to shield the vanadium centers against solvent nuclear spins. Our results highlight new design principles for long T and T times by demonstrating the efficacy of ligand-based tuning of solute-solvent interactions.

Mapping Magnetic Properties and Relaxation in Vanadium(IV) Complexes with Lanthanides by Electron Paramagnetic Resonance

“…As the first step in that direction, we have studied the magnetic properties of V-Ln heterometallic complexes. It has been found that the T m values in complexes of vanadium(IV) with diamagnetic lanthanides are generally comparable to those exhibited by other vanadium-based qubits specially designed for this purpose [3][4][5][6][7][8][9]15]. First of all, this owes to the nuclear spin-free first coordination sphere of the VO 6 moiety.…”

“…We dissolved two representative compounds (I and III) in glass-forming water/glycerol mixture (~50/50% v/v) and recorded two-pulse echo-detected (ED) EPR spectra in such frozen (glassy) solutions ( Figure 5). Note that mostly vanadium-based candidates for spin qubits were investigated in the solid crystalline state; however, a few earlier works used dissolution as well [3,4,7,13,14]. In particular, this approach allowed obtaining very long decoherence times at low temperatures, especially in nuclear spin-free solvents.…”

“…The search for molecular spin qubits with suitable physical properties is one of the main targets on the way to the implementation of quantum spin technologies and quantum computing [1,2]. Among other candidates for molecular spin qubits, vanadium-based complexes attracted special attention during the past decade due to their long decoherence times, both at cryogenic and room temperatures [3][4][5][6][7][8][9][10][11][12][13][14][15]. The vanadium(IV) ion has an electron spin S = 1/2 and a nuclear spin I = 7/2 for stable isotope 51 V of nearly 100% natural abundance.…”

“…These properties determine the intrinsically longer electron spin relaxation times compared to high-spin (S > 1/2) ions, and, at the same time, provide a set of nuclear spin states for manipulation of multiple coherences. Some previous works were concerned with the frozen solutions of vanadium(IV) complexes [3,4,7,13,14], while the others considered solid-state compounds and their relaxation properties. The longest decoherence time (also called dephasing, phase memory, sometimes transverse relaxation time) at room temperature T m~1 µs was obtained for vanadyl phthalocyanine complexes with nuclear spin-free ligands in the solid-state [6].…”

A Bis‐Binuclear NiII Complex with Easy and Hard Axes of Magnetization: Complementary Experimental and Theoretical Insights

“…However, for such a system to be a plausible double qubit, the coherence time of each qubit must be large enough to be able to perform quantum operations. Some molecular complexes have promising coherence times to be considered as possible candidates for qubits, but no measurements have been carried out on nickel complexes yet. Such studies were beyond the scope of the work reported in this paper; we have focused on the design of Ni II binuclear complexes and the analysis of their magnetic anisotropy and exchange coupling with a view to developing potential candidates for use as qubits.…”