An illustration of two entangled molecules which are individually trapped in magic-wavelength optical tweezers. [Image: Durham University]
Physicists in the UK have reportedly shown they can entangle pairs of molecules for more than a second at a time by trapping them with coherence-preserving optical tweezers (Nature, doi: 10.1038/s41586-024-08365-1). They say that the research could be exploited in areas such as ultracold chemistry, quantum metrology and quantum computing.
The complexity of molecules
When two particles are entangled, a measurement on one of them instantaneously fixes the state of the other—no matter how far apart they are. This strange phenomenon is now routinely observed between simple particles such as atoms, but realizing it with molecules is complicated by the latters' greater complexity. Their more numerous degrees of freedom raise the chances of interaction with the outside world, causing their wave functions to collapse.
Researchers have become increasingly expert at manipulating molecules quantum mechanically, having learned how to prepare individual ultracold molecules using lasers known as optical tweezers. They have entangled pairs of molecules and shown how to read out multiple molecular states as well as create logic gates and erase qubit errors. But entanglement between molecules had remained a very fleeting effect, with particles prepared in superpositions of rotational states decohering in less than a tenth of a second if not subject to phase-correcting pulses.
Setting a magic trap
In the latest work, Simon Cornish and colleagues at Durham University improve on this timescale by more than an order of magnitude thanks to the very careful way in which they prepared individual molecules. They used one set of optical tweezers to prepare arrays of ultracold molecules made from rubidium-87 and cesium-133 atoms, and then transferred the molecules to another set of tweezers at a "magic" wavelength to postpone the molecules' decoherence. At the wavelength in question (1145.31 nm), they were able to negate the effect of fluctuations in the lasers' intensity, which would otherwise quickly destroy the quantum superpositions of rotational states that enable pairs of molecules to interact with one another.
The team trap molecules in a vacuum chamber (left) using magic-wavelength tweezers and other laser beams (right). [Image: Durham University]
The researchers determined the magic wavelength by subjecting the trapped molecules to a pair of microwave pulses separated by a given delay—the first pulse to couple the molecules' ground state to a rotationally excited state and the second to read out the phase accrued by the molecules due to the trap light shifting the molecular transition. Repeating this measurement for slightly different trapping wavelengths, they concluded that a very precisely tuned magic tweezer should maintain coherent states for several minutes—a lifetime in the quantum world and far greater than the 93 milliseconds that had previously been achieved using magic polarization (rather than wavelength).
They then generated entanglement, which involved two molecules, each in a rotational superposition, interacting with one another to yield two entangled eigenstates separated by a tiny amount of energy. The team did this both indirectly—simply creating the superpositions and then allowing the resulting oscillating dipoles to evolve into an entangled state—and directly, by specifically targeting one of the entangled eigenstates with microwaves (at about 1 GHz). As Cornish points out, this second approach involves a very finely tuned frequency, which requires a long microwave pulse to reduce the Fourier width that in turn relies on long coherence. "The remarkable thing about our work is our interaction shift is only 2.6 Hz," he says. "Yet we see it clearly because of the magic trap."
Longer entanglement
To establish the longevity of entanglement, Cornish and colleagues varied the time between microwave pulses before applying additional pulses to read out the molecules' states. They found no significant changes in these states for pulse separations of up to 500 milliseconds, concluding that coherence times were around 1.6 s for both entangled states. Regarding the fidelity of the entangled states, they arrived at figures of 0.98 and 0.93 (after correcting for Raman scattering losses) for the indirect and direct approaches, respectively.
Despite these impressive figures, there is room for improvement. By transferring the molecules into an optical lattice made from lasers operating at the magic wavelength, the researchers say it should be possible to bring the molecules closer together. This, they explain, should boost both the speed and fidelity of entanglement, allowing them to create high-fidelity two-molecule gates operating on millisecond timescales (while still preserving coherence for seconds or more).
Potential applications are numerous, they reckon. One of these involves new ways of studying quantum interference effects in chemistry at ultralow temperatures. Other areas that stand to gain are quantum-enhanced metrology and the memories used in quantum repeater schemes, as well as the possibility of building computers from qudits—units of quantum information containing more than the two states of digital bits.