Researchers link a single molecule’s spin to light at 4.5 K

Researchers have demonstrated a link between light and the spin of a single organic molecule held inside a carefully matched crystal. The study published in Nature on 28 October reports selective optical access to the molecule’s spin states and controlled spin behaviour at 4.5 kelvin, about −269 °C. A spin–photon interface connects a spin that holds quantum information with photons, the particles of light that can carry it between devices.

Earlier optical-readout experiments on ground-state molecular qubits measured ensembles—many molecules together—rather than addressed individual molecules, the authors said. Optical spin detection in single pentacene molecules came earlier, but used an excited-state spin. The new experiment combines individual access with a spin in the molecule’s ground state, its lowest-energy electronic configuration.

Why the molecule and crystal work together

The molecule is di([1,1′-biphenyl]-4-yl)carbene. Its central carbon and nearby carbon rings carry two unpaired electrons whose spins favour parallel alignment. Together they form a triplet, with three spin states in which quantum information can be encoded. The researchers' calculations place the alternative configuration with total spin zero 0.47 electronvolts higher in energy, supporting a stable triplet ground state.

The host crystal consists of the similarly shaped di([1,1′-biphenyl]-4-yl)methanone. It holds the carbene’s linked carbon rings nearly flat: their twist falls from about 36° in free space to about 6° in the crystal. That restraint reduces shape changes during optical excitation and directs more emission into the zero-phonon line, the sharp part of the spectrum emitted without exciting vibrations. The host’s edge-to-face molecular arrangement also reduces overlap between the carbene’s electron clouds and those of its neighbours, electronically isolating the spin-bearing molecule.

Finding and controlling one molecule

Rather than put reactive carbenes directly into the crystal, the team embedded precursor molecules. Focused laser light removed nitrogen from those precursors, creating carbenes where the light fell. Gradual activation kept their density low, while a laser tuned to a particular optical frequency selected a smaller subset. A photon-correlation test on an isolated bright spot gave g²(0) = 0.142, with uncertainties of −0.023 and +0.169. This measure of simultaneous photon arrivals was below the 0.5 threshold indicating a single emitter.

Optical excitation changes where the molecule’s spin density sits, changing the energy gaps between its spin states. As a result, different spin states absorb light at distinct frequencies. A tuned laser can select one state, and the resulting fluorescence reveals the spin’s response.

NVision Quantum Technologies, whose team developed the interface as part of its quantum-computer programme, says light prepares and reads the spin. Between laser pulses, microwaves rotate it back and forth between states, producing Rabi oscillations—the measured signature of controlled spin rotations. Light then resets the spin for another cycle.

How sharp the light is, and how long the spin lasts

The smallest separation between spin-selective optical transitions was much wider than an individual optical line, allowing the laser to distinguish the states. Repeated measurements also found little change in the optical frequencies over more than an hour.

NVision gives the following spin times. Coherence measures how long the spin preserves its quantum state; protective microwave pulses suppress disruption from its surroundings. Spin relaxation is a different measure, describing how long spin populations take to return towards equilibrium.

Analysis

Limits of the laboratory interface

The interface required cryogenic cooling, and its optical line remained broader than the limit set by the excited-state lifetime. The authors attribute much of that extra broadening to thermally activated molecular or crystal vibrations. They measured representative detected photon rates of about 2,000 per second, limited mainly by a fluorescence quantum yield of 10.8%—the fraction of absorbed photons that produce fluorescence—and also dependent on microwave driving.

Continuous illumination also eventually stops the molecules emitting, a process called photobleaching. Under the reported continuous laser and microwave conditions, the authors estimated a lifetime of about 119 hours, despite the molecules' chemical stability at cryogenic temperatures.

The tests needed to connect molecules

NVision presents this interface as a building block for a proposed quantum computer, not an operating network. It lists entanglement between separate molecules—creating a shared quantum state—and error correction among the further research steps.

• Optical cavities: NVision proposes using structures that confine light to accelerate photon emission. Faster emission would also broaden the optical lines, so the spin-selective transitions must remain distinguishable.
• Photonic chips: the next integration test is whether the molecular interface keeps its optical and spin properties on a fabricated chip.
• Nuclear-spin memories: the company proposes placing isotopes such as carbon-13 at selected positions in the molecule. Its research programme includes operations connecting the electron spin to these intended memory spins.

NVision says several next steps are already under way and expects further results in the coming months.