QUIONE, a quantum simulator capable of observing individual atoms in a strontium quantum gas
Quantum physics needs high-precision sensing techniques to delve deeper into the microscopic properties of materials. From the analog quantum processors that have emerged recently, the so-called quantum-gas microscopes have proven to be powerful tools for understanding quantum systems at the atomic level. These devices produce images of quantum gases with very high resolution: they allow individual atoms to be detected.
Now, ICFO researchers and DAALI team members Sandra Buob, Jonatan Höschele, Dr. Vasiliy Makhalov and Dr. Antonio Rubio-Abadal, led by ICREA Professor at ICFO Leticia Tarruell, explain in a new article in PRX Quantum how they built their own quantum-gas microscope, named QUIONE after the Greek goddess of snow. The group’s quantum-gas microscope is the only one imaging individual atoms of strontium quantum gases in the world, as well as the first of its kind in Spain.
Beyond producing impactful images of individual atoms, QUIONE’s primary goal is quantum simulation. ICREA Prof. Leticia Tarruell explains, “Quantum simulation simplifies complex systems to understand unanswered questions beyond current computers, like why certain materials conduct electricity without loss at high temperatures.” This research at ICFO receives support from national entities (Royal Spanish Society of Physics, BBVA Foundation, Ramón Areces Foundation, La Caixa Foundation, and Cellex Foundation) and European initiatives (including an ERC project).
This experiment’s uniqueness lies in achieving three key milestones: bringing strontium gas to the quantum regime, placing it in an optical lattice where atoms interact through collisions, and applying single atom imaging techniques. Combined, these make ICFO’s strontium quantum-gas microscope the only one of its kind.
Why strontium?
Until now, these microscope setups relied on alkaline atoms, like lithium and potassium, which have simpler properties in terms of their optical spectrum compared to alkaline-earth atoms such as strontium. This means that strontium offers more ingredients to play with in these experiments.
In fact, in recent years, the unique properties of strontium have made it a very popular element for applications in the fields of quantum computing and quantum simulation. For example, a cloud of strontium atoms can be used as an atomic quantum processor, which could solve problems beyond the capabilities of current classical computers.
All in all, ICFO researchers saw great potential for quantum simulation in strontium, and they set to work to build their own quantum-gas microscope. This is how QUIONE was born.
QUIONE, a quantum simulator of real crystals
To this end, they first lowered the temperature of the strontium gas. Using the force of several laser beams, the speed of atoms can be reduced to a point where they remain almost motionless, barely moving, reducing their temperature to almost absolute zero in just a few milliseconds. Then, the laws of quantum mechanics rule their behavior, and the atoms display new features like quantum superposition and entanglement.
After that, the researchers used special lasers to activate the optical lattice, arranging the atoms in a grid-like structure across space. Sandra Buob, the first author of the article, explained, “You can imagine it like an egg carton, where the individual sites are where you put the eggs. But instead of eggs, we have atoms, and instead of a carton, we have the optical lattice.”
The atoms within the optical lattice interacted, sometimes utilizing quantum tunneling to move between sites. This quantum dynamic mimics the behavior of electrons in certain materials. Therefore, studying these systems can illuminate the complex behavior of certain materials, the key idea behind quantum simulation.
As soon as the gas and optical lattice were ready, the researchers captured images with their microscope, finally observing their strontium quantum gas atom by atom. At this point, QUIONE was already a success, but its creators sought to push its capabilities further.
Observing Quantum Phenomena and Superfluidity
In addition to pictures, the researchers captured videos of the atoms, observing them occasionally jump to nearby lattice sites, despite expectations of stillness during imaging. This behavior, explained by quantum tunneling, delighted Buob: “We literally witnessed the atoms ‘hopping’ between sites, a direct manifestation of their inherent quantum nature.”
Finally, the team confirmed the strontium gas’s superfluidity, a quantum state of matter flowing without viscosity. They switched off the lattice laser, allowing the atoms to expand and interfere, creating an interference pattern indicative of the atoms’ wave-particle duality. Dr. Antonio Rubio-Abadal explained, “Our equipment captured this pattern, verifying the superfluidity in the sample.”
“It is a very exciting moment for quantum simulation”, shares ICREA professor Leticia Tarruell. “Now that we have added strontium to the list of available quantum-gas microscopes, we might be able to simulate more complex and exotic materials soon. Then new phases of matter are expected to arise. And we also expect to obtain much more computational power to use these machines as analog quantum computers”.
Reference article
Buob, S., Höschele, J., Makhalov, V., Rubio-Abadal, A., & Tarruell, L. (2024). A Strontium Quantum-Gas Microscope. PRX Quantum, 5(2), 20316. https://doi.org/10.1103/PRXQuantum.5.020316
