Extremely cold atoms have been connected into a state of quantum “hyperentanglement” for the first time. This demonstrates a new level of control over their quantum properties, which could prove useful for quantum computing.
When cooled to temperatures very close to absolute zero – the lowest possible temperature – atoms don’t fully freeze in place, because quantum effects that don’t exist at warmer temperatures enable them to keep making tiny motions. A research team at the California Institute of Technology has now used laser light to take control of those motions in an unprecedented way.
Since the 1990s, researchers have been making atoms ultracold by hitting them with laser light and electromagnetic forces, but some atoms in these experiments typically still remained relatively warm, or picked up tiny bits of heat through experimental imperfections. This leads to errors when the ultracold atoms are used to simulate quantum materials or store quantum information, says team member , now at Stanford University in California.
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To address this, Shaw and his colleagues cooled a collection of strontium atoms and then used 39 specially tuned laser beams, or optical tweezers, to grab atoms one by one and arrange them into a neat array. They then illuminated them with extra laser light, which changed only the quantum properties of atoms that were at the desired ultracold temperature. This approach meant the researchers could identify any atoms that were still undesirably warm. They either re-cooled these or simply removed them from the experiment. In this way, the team assembled atomic arrays where as many as 99 per cent of the atoms were in the coldest possible quantum state, corresponding to temperatures only several trillionth of a degree above absolute zero.
With this very controlled array as a starting point, the researchers could then use more lasers to encode information into the atoms, and connect – or entangle – them to each other. However, the complexity of entanglement they could achieve goes beyond what has been possible in previous work. Typically, researchers entangle atoms by manipulating the energies of electrons within each one – changing their “electronic states”. But Shaw and his team had another control knob to tweak: they could manipulate each atom’s precise movement, or its “motional state”. The researchers leveraged this to put two atoms into a state of “hyperentanglement”, which has never been done with any type of matter.
These hyperentangled atoms had several quantum properties that would all stay correlated even if the atoms were pulled extremely far apart. If entanglement is like ensuring that you and your friend on the other side of the world always wear blue socks on the same day without having to call each other about it, hyperentanglement is like also ensuring that when your blue socks are wool your friend’s will be polyester, says Shaw. This could be useful for building very information-dense quantum memory devices, or ultracold hard drives.
“The motion of individual atoms in optical tweezers is an untapped resource for quantum science,” says at the University of Illinois Urbana-Champaign. He says the new experiment showcases ways in which this resource could be put to use – for instance, in correcting computational errors or adding new operations in quantum computers made from ultracold atoms.
at Princeton University says that identifying which atoms introduce warmth – and errors – and then re-cooling them echoes error-correction procedures already in use in quantum computing. In this way, the new experiment is part of a continued expansion of quantum computing technologies. It may eventually be used as a simulator for exploring the properties of poorly understood quantum matter, he says.
For Shaw, properties like hyperentanglement are just the first step in working out everything that precise control over the motion of ultracold atoms could be used for. “We have only scratched the surface,” he says.
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