Physicists Control and Read a Single Molecule as a Qubit for Quantum Networks

A team from the German startup NVision Quantum Technologies has controlled and read out the quantum state of a single molecule, a result reported in Nature that makes molecules a credible candidate for the light-linked qubits that future quantum networks will need.

Quantum computers use qubits as their units of information. To connect them into a larger network over long distances, a qubit must be able to transmit its information using light. The leading candidates so far, including defects in diamond and other semiconductors, are plagued by stray defects and trapped electric charges nearby. These shift the frequency of the light they emit, which makes reliable links between distant devices difficult to build.

The paper, "A single-molecule spin-photon interface," is led by Simon Roggors. The team embedded a chemical compound in a crystal chosen for its stable surroundings and cooled it to 4.5 kelvin, about minus 269 degrees Celsius. Laser light converted the compound into carbene molecules, each of which has two unpaired electrons whose combined spin can store quantum information. Microwave pulses controlled that spin, and laser light read out the result.

The measurements showed the spin of an individual molecule keeps its quantum properties for about 2.2 milliseconds. The light emitted by individual molecules stayed remarkably stable in frequency for more than one hour, which is the property that conventional solid-state emitters struggle to deliver.

The authors write that the results "establish molecular qubits as a viable platform for single-emitter quantum optics." They also caution that the focus of the work was demonstrating an individually addressable molecular qubit, and that its properties should be judged against the requirements for a quantum-network node.

The appeal of molecules is that chemists can design them from the bottom up and tune their properties, and that they can be processed into thin films and combined with existing photonic chips. That could eventually let researchers link information stored in single molecules to photons, the step needed to connect quantum devices.

The result is an early step, not a finished network component. A spin lasting 2.2 milliseconds and a crystal at 4.5 kelvin are a long way from a deployable system, and the work does not demonstrate entanglement between two molecules. It does show that the frequency stability problem, which has dogged other platforms, can be avoided by changing the material.

The findings matter because networks of quantum machines are widely seen as the route to larger quantum computers and to communication links that are secure by design. Those networks depend on emitters whose photons are identical from one device to the next, so that light from two separate nodes can be interfered with each other. A molecule whose emission frequency stays put for an hour is a step toward that kind of consistency, though the team will need to show the same behavior across many molecules and at the scale of a real device.