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Bulky and hard to wrangle, molecules have long defied physicists' attempts to lure them into a state of controlled quantum entanglement, whereby the molecules are intimately linked even at a distance.
Now, for the first time, two separate teams have succeeded in entangling pairs of ultra-cold molecules using the same method: microscopically precise optical 'tweezer traps'.
Quantum entanglement is a bizarre yet fundamental phenomenon of the quantum realm that physicists are trying to tap into to create the first, commercial quantum computers.
All objects – from electrons to atoms to molecules to even whole galaxies – can theoretically be described as a spectrum of possibility before they're observed. It's only by measuring a property that the wheel of chance settles on a clear description.
If two objects are entangled, knowing something about the properties of one object – its spin, position, or momentum – instantly acts as a measurement on the other, bringing both of their spinning wheels of possibility to a complete stop.
So far, researchers have managed to entangle trapped ions, photons, atoms, and superconducting circuits in lab experiments. Three years ago, for example, a team entangled trillions of atoms in 'hot and messy' gas. Impressive, but not very practical.
Physicists have also entangled an atom and a molecule before, and even biological complexes found in plant cells. But controlling and manipulating pairs of individual molecules – with enough precision for quantum computing purposes – has been a harder task.
Molecules are difficult to cool down and readily interact with their surroundings, which means they easily fall out of fragile quantum entangled states (what's known as decoherence).
One example of those interactions are dipole-dipole interactions: the way the positive end of a polar molecule can be yanked towards the negative end of another molecule.
But those same properties also make molecules promising candidates for qubits in quantum computing because they offer up new possibilities for computation.
"Their long-lived molecular rotational states form robust qubits while the long-range dipolar interaction between molecules provides quantum entanglement," explains Harvard University physicist Yicheng Bao and colleagues, in their paper.
Qubits are the quantum version of classical computing bits, which can assume a value of 0 or 1. Qubits, on the other hand, can represent numerous possible combinations of 1 and 0 at the same time.
By entangling qubits, their combined quantum blur of 1s and 0s can operate as speedy calculators in specially designed algorithms.
Molecules, being more complex entities than atoms or particles, have more inherent properties, or states, that could be coaxed into coupling together to make a qubit.
"What this means, in practical terms, is that there are new ways of storing and processing quantum information," says Yukai Lu, a graduate student in electrical and computer engineering at Princeton University, who co-authored the second study.
"For example, a molecule can vibrate and rotate in multiple modes. So, you can use two of these modes to encode a qubit. If the molecular species is polar, two molecules can interact even when spatially separated."
Both teams generated ultra-cold calcium monofluoride (CaF) molecules and then trapped them, one by one, in optical tweezers.
Using these tightly focused beams of laser light, the molecules were positioned in pairs, close enough that one CaF molecule could sense the long-range electric dipolar interaction of its partner. This led to each pair of molecules becoming linked in an entangled quantum state when not long before they had been strangers.
The method, with its precise manipulation of individual molecules, "paves the way for developing new versatile platforms for quantum technologies," writes Augusto Smerzi, a physicist at the National Research Council of Italy, in an accompanying perspective.
Smerzi was not involved in the research, but sees its potential. By leveraging the dipole interactions of molecules, he says the system might one day be used to develop super-sensitive quantum sensors capable of detecting ultraweak electric fields.
"Applications span from electroencephalography
to measure electrical activity in the brain to monitoring changes in electric
fields in the Earth's crust for earthquake predictions," he speculates.
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