Glasgow-Led Team Reads Electron Spins in a Porous Crystal With Light, a Step Toward a 'Quantum Nose'

A team of researchers from Scotland and Japan has used light to read the spins of electrons inside a porous crystal for the first time, a result that points toward a new generation of quantum sensors. The work involved the University of Glasgow, the University of Tokyo, the University of Sheffield and Kobe University, and appears in the Journal of the American Chemical Society.

The crystals are metal-organic frameworks, or MOFs, which Dr. Sam Bayliss of the University of Glasgow's James Watt School of Engineering describes as molecular scaffolds. "They are rigid and porous structures that our colleagues synthesized at the University of Tokyo, and have great potential for sensing applications," he said. At Glasgow's Advanced Research Center, the team read the magnetic state of spins held inside one of these frameworks using a method called optically detected magnetic resonance, or ODMR. "It is the first time anyone has measured spin resonance this way in this type of material," Bayliss said.

ODMR has drawn attention as a way to read out spin qubits for quantum sensing. Electron spins that can be detected by light offer higher sensitivity and finer spatial resolution than electron spin resonance, which relies on microwave detection. The best-known example is the nitrogen-vacancy center in diamond. But because those spins live in defects in the crystal, there are limits to how well they can be controlled and how they interact with outside substances.

Spins derived from molecules are different. Their structures can be designed and their spin properties tuned chemically. MOFs go a step further by fixing the position and orientation of the spins inside a porous material. Because the pores can take in target chemicals, those chemicals can sit close to the spins and be detected through their interaction. MOFs have long been seen as promising for quantum sensing, but optical detection of spins inside them had not been shown before this study.

Dr. Alistair Inglis, who co-led the Glasgow contribution, said the signal carries more than information about the molecules. "The exciting thing is that spin resonance tells us not only about the molecules themselves, but also about the environment they exist in, which means these materials could be used as sensors," he said. "Because these MOFs are porous, we can, in principle, load target molecules into the scaffold and read them out using the same technique, offering sensing on a molecular scale."

Since MOFs can be designed in many ways by changing their ligands and crystal structures, the team says the demonstration could extend to many related materials. A library of such MOFs, each responding differently to particular chemicals, could work as a "quantum nose," identifying substances from their response patterns.

The next steps are practical ones. "The next step is pushing this to work at more practical temperatures and tuning the chemistry to make the signal stronger," Inglis said. The paper, by Miku Inoue and colleagues, is titled "Optically Addressable Spins in a Metal-Organic Framework" (DOI 10.1021/jacs.6c12867). Researchers from Saitama University, JEOL Ltd. and the Institute for Molecular Science also collaborated.