Lab-Made Molecule Could Become Building Block For Quantum Networks

Single Molecule Shows Promise as Tiny Memory for Quantum Computers

A single molecule, chilled inside a crystal to about minus 452 degrees Fahrenheit, kept a fragile piece of quantum information alive for roughly 2.2 thousandths of a second, with help from protective microwave pulses. Fleeting as that sounds (a blink lasts dozens of times longer), researchers could read the information back just by shining a laser on the molecule and watching whether it glowed. Chemists made the molecule in a lab, which hints at a different way to build quantum computers and communication networks.

Quantum networks, which could one day link quantum computers the way the internet links laptops, need a part that does two jobs: store a quantum bit, or qubit, long enough to be useful, and exchange information with the outside world through particles of light called photons. Defects in diamond have done both. Putting those defects on chips that run on light, though, usually means etching the host crystal, which can strain it and make its light less stable, according to a new paper in Nature.

Molecules made through ordinary chemistry might avoid that problem. This one is a carbene, a carbon-based compound. The researchers say no single molecule had combined bright light, steady color, and a long-lived spin (a tiny internal magnet that can point in different directions) before this experiment.

Crystal Trap Makes an Unstable Molecule a Stable Qubit

Carbenes are built around a carbon atom missing some of its usual bonds, so they grab onto neighbors fast. To keep one around, a team based at NVision Quantum Technologies and Ulm University in Germany started with a calmer precursor molecule and grew it inside a crystal made of a similarly shaped compound. The precursor slotted in like a puzzle piece dropped into a matching puzzle, holding it in place.

After cooling the crystal to 4.5 Kelvin, colder than any naturally occurring spot on Earth, the team pointed a focused laser at it, knocking a nitrogen molecule off the precursor and turning it into the carbene. The beam can be steered with pinpoint accuracy, so the researchers controlled where the qubits formed and how crowded they were, thinning them out until single molecules could be studied.

Light Tests Confirm Single Molecules Stayed Steady for Over an Hour

To prove the glow came from one molecule and not a small cluster, the researchers sent the light into a detector that tracks whether photons arrive alone or in bunches. A cluster would produce frequent pairs, but this molecule produced a clear shortage of them, the signature of a single emitter.

Steady color matters because quantum networks need light from different sources to match, like instruments tuned to the same note. In semiconductor crystals like diamond, stray electrical charges can make an emitter’s color wander. Molecular crystals lack those charge traps, the authors say. Over more than an hour of repeated scans, the color of two tracked molecules barely budged, drifting by less than a millionth of a percent. The authors describe the result as “remarkable spectral stability.”

Microwave Pulses Keep Quantum Memory Alive for 2.2 Milliseconds

Qubit information lives in the molecule’s spin, and pulses of microwaves, cousins of the energy inside a kitchen microwave oven, flipped that spin between states. Each spin state responds to a slightly different color of laser light, so shining one color and watching for a glow showed which way the spin pointed.

Left unprotected, the spin signal faded in a few millionths of a second. A repeating sequence of microwave pulses designed to shield the spin from its surroundings stretched that to roughly 2.2 thousandths of a second, hundreds of times longer. That is the molecule’s memory time, the point where the signal has faded to about a third of its strength. For near-perfect storage, 99% accuracy, the authors calculate about 40 millionths of a second. A separate measure, how long the spin takes to drift back to rest, came in at about 21 thousandths of a second. By the authors’ account, those single-molecule numbers top earlier results from large groups of molecular qubits by more than tenfold, and they call them “a substantial leap for molecular qubits.”

Molecular Qubits Could Sidestep Tricky Chip Etching, Researchers Say

Measured against the checklist for quantum network parts, which includes long-lasting memory, usable light, and steady color, the molecule “already combines many of the required properties,” the paper reports.

Chemistry could also ease manufacturing. Molecules can be spread into thin films and placed directly onto prebuilt light-based chips, the authors write, which would avoid that etching step. None of that has been built yet, and linking photons to a single molecule’s spin, the move a working quantum network requires, remains a future goal. If molecules can be tweaked the way chemists adjust dyes or drugs, quantum parts might someday be designed to order.

A compound type known for being too reactive to last has become an individually controlled memory, which makes molecular design a credible second route to quantum hardware alongside diamond defects. A molecule-powered quantum internet is not around the corner. Still, quantum parts designed in a flask rather than carved from exotic crystals now have a first proof of concept.

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