Boson sampler uses atoms rather than photons

28 May 2024 Tim Wogan

Collected at: https://physicsworld.com/a/boson-sampler-uses-atoms-rather-than-photons/

A boson sampler that uses atoms rather than photons has been developed by researchers in the US. The team used its system to determine a complex quantum state more accurately than would be practicable using a conventional (classical) computer. Atoms interact much more strongly than photons, so the researchers believe their system is a promising platform for simulating condensed-matter systems. Looking further in the future, it could also be used for quantum computing.

A defining property of bosons is that an unlimited number can occupy the same state at any one time. This leads to strange behaviour such as the Hong-Ou-Mandel effect, in which two indistinguishable photons striking a 50:50 beamsplitter at the same instance will always come out of the same port. Similar effects involving multiple photons and beamsplitters are extremely difficult for classical computers to model. The best classical algorithms can only manage around 50 bosons.

Boson sampling machines are proto-quantum computers, or quantum simulators, that utilize the properties of bosons themselves. A specific quantum state is imprinted into the system at the input and the state is measured after a given time. However, photons are easily lost in a system, making it very challenging to achieve a reliable measurement.

Square optical lattice

In the new work, researchers in Adam Kaufman’s group at JILA in Boulder, Colorado implemented a boson sampler using atom optics. They placed 180 strontium-88 atoms into a 48×48 site square optical lattice potential. Key to their success, the researchers used an optical tweezer array to move atoms around. Aaron Young, then Kaufman’s PhD student, explains: “We worked hard in this experiment to engineer the tweezers to really address single lattice sites. Our tweezers are smaller than typical tweezers.”

Once the researchers had placed the atoms appropriately, they turned off the tweezers. They then laser cooled the atoms and imaged the initial quantum state. Next, they reduced the depth of the lattice potential, allowing atoms to tunnel between sites. After allowing the state to evolve for a fixed time, they increased the potential depth again, tightly confining the atoms and thereby allowing them to image the positions of the atoms using photons.

The next step was to test the system, explains Young. “Certification of a boson sampler is believed to be as hard as simulating a boson sampler in the first place”. As it is not possible to simulate a boson sampler of 180 indistinguishable atoms classically in a reasonable amount of time, nor is it possible to check whether or not such a boson sampler produced the correct result. The researchers therefore turned to indirect certification by looking at cases in which the bosons were not indistinguishable. Such a state might arise in an experiment as a result of imperfect cooling, for example.

Distinguishable atoms

“As we make the atoms more and more distinguishable, we go from this problem that’s really hard to simulate closer to the case where you’re doing the one atom problem 180 times,” says Young. “And somewhere in the middle we cross the threshold where it’s once again possible to simulate our problem on a normal computer. We check two things: first, that as we turn this knob, things look well behaved and nothing dramatically goes wrong; second, that as things become sufficiently distinguishable to simulate, the experiment is in agreement with theory.”  The results suggested that the atoms were around 99.5% indistinguishable.

The team now intends to investigate how the system could be used as a platform for reprogrammable quantum logic. “In our system, we’re at this fine-tuned point where the atoms are, to a very good approximation, not interacting with each other, but it’s very easy to turn interactions back on.” This could allow the simulation of problems in condensed matter physics, for example. Beyond this, it could even provide a route to universal quantum computation. He points out that the optical tweezers can be used to shift the energies of lattice sites. “It turns out that the ability to just shift sites up and down like that gives you access to a universal set of controls,” says Young.

Atomic, molecular and optical physicist Cheng Chin at the University of Chicago is impressed with the research. He says that, thanks to the low loss observed compared with photons, Kaufman’s group has shown that atoms provide “much higher fidelity to the ideal boson sampling the algorithm would require”. He adds, “As far as this specific problem is concerned I think the application of cold atoms is a very remarkable step to demonstrate the advantage of quantum information processing. Perhaps now with Adam’s approach he can control which way the bosons are going and introduce interactions between atoms, which is much, much easier than introducing interactions between photons. It does open a lot of new opportunities beyond what photons can do.”

The research is described in Nature.