Short answer: Molecules can host spin qubits whose properties are tuned by chemistry, and clock transitions, where the spin is protected from magnetic noise, can lengthen their coherence. A 2016 Nature paper reported coherence times of up to 8.4 microseconds at 5 kelvin in a holmium molecular nanomagnet at high concentration. A 2022 Nature Chemistry paper reported raising the phase memory time from 1.0 to 12 microseconds in a lutetium complex, and a 2022 Physical Review X paper reported host-matrix engineering of chromium-based qubits. These are coherence times of microseconds, not milliseconds.
The problem
The 2016 Nature abstract explains that the strongest source of decoherence for spin qubits is the magnetic dipolar interaction. To minimize it, spins are typically diluted in a diamagnetic matrix, with a typical ratio of one magnetic molecule per 10,000 matrix molecules. The authors point to a contradiction: dilution reduces decoherence but also prevents the interactions needed for quantum operations. They propose chemically designing molecules with crystal field ground states that have large tunnelling gaps, giving optimal operating points, or atomic clock transitions, where the spin dynamics are protected from dipolar decoherence.
2016: a holmium nanomagnet
The authors illustrate the idea with a holmium molecular nanomagnet. They report long coherence times, up to 8.4 microseconds at 5 kelvin, at unusually high concentrations. They say the finding opens avenues for quantum computing with molecular spin qubits, which is their outlook.
2022: a large hyperfine clock transition
The Nature Chemistry abstract reports one of the largest hyperfine interactions for a molecular system, with an isotropic value of 3,467 MHz (uncertainty 50), and a very large associated clock transition. They achieve this through chemical control of s-orbital mixing into the spin-bearing d orbital in a series of lanthanum(II) and lutetium(II) complexes. More s-orbital character reduces spin-orbit coupling, which reduces spin-lattice relaxation, and enhances the electron-nuclear Fermi contact interaction, which maximizes the hyperfine interaction. This produces a 9 GHz clock transition and, for one lutetium(II) complex, an increase in phase memory time from 1.0 (uncertainty 0.4) to 12 (uncertainty 1) microseconds. The authors say the findings suggest strategies for molecular quantum technologies, akin to trapped ion systems.
2022: host engineering for optical molecular qubits
The Physical Review X abstract concerns optically addressable molecular spins, which combine a long-lived qubit with a spin-optical interface. The authors insert chromium(IV)-based molecular qubits into a nonisostructural host matrix, which generates noise-insensitive clock transitions through a transverse zero-field splitting that are absent in an isostructural host. They report spin coherence times that exceed a value that did not render in the retrieved text, for optically addressable molecular qubits in a nuclear and electron-spin-rich environment. They model the dependence of coherence on transverse zero-field splitting from first principles and verify it with four molecular systems, and they investigate optical linewidth and spin-lattice relaxation. They suggest molecular qubits could serve nanoscale sensing in noisy environments.
Limits
The coherence times here are on the microsecond scale, and the abstracts make no comparison with other platforms. None of them reports a quantum gate, and none discusses vanadyl complexes, so this post does not either.
FAQ
What is a clock transition?
An operating point where the spin energy is insensitive to magnetic noise, per the 2016 abstract.
How long did coherence last?
Up to 8.4 microseconds at 5 kelvin in 2016, and 12 microseconds in one 2022 complex.
Why use molecules?
Chemistry lets researchers tailor qubit properties, the authors say.
What did host engineering do?
It created clock transitions in chromium-based molecular qubits.
Are molecular qubits competitive?
The abstracts do not make that comparison.
All field notes