Temperature measurements are crucial for many experiments using ultracold atomic gases, such as when calibrating quantum simulators or when determining state equations. Unfortunately, standard thermometry techniques such as time-of-flight or in situ absorption imaging are inherently destructive and involve integration over the line of sight.
Physicists from Trinity College Dublin have developed a thermometer based on quantum entanglement to accurately measure temperatures a billion times colder than those in outer space. Instead of mercury, this thermometer measures the state of single atoms that are entangled (or correlated) with a quantum gas.
To study how matter behaves in extreme quantum states, scientists usually create Fermi gasses, which has clouds of atoms with ultra-cold temperatures.
Professor John Goold, a postdoctoral fellow, working at OIST, said, “Ultra-cold gasses like these are now routinely created in labs worldwide, and they have many uses, ranging from testing fundamental physics theories to detecting gravitational waves. But their temperatures are mind-bogglingly low at nanokelvin and below! To give you an idea, one kelvin is -272.15 degrees Celsius. These gasses are a billion times colder than that—the coldest places in the universe, and they are created right here on Earth.”
What exactly is a Fermi gas?
Goold said, “All particles in the universe, including atoms, come in one of two types called ‘bosons’ and ‘fermions.’ A Fermi gas comprises fermions, named after the physicist Enrico Fermi. At shallow temperatures, bosons and fermions behave entirely differently. While bosons like to clump together, fermions do the opposite. They are the ultimate social distancers! This property makes their temperature tricky to measure.”
Dr. Mark Mitchison, the first author of the paper, explains: “Traditionally, the temperature of an ultra-cold gas is inferred from its density: at lower temperatures the atoms do not have enough energy to spread far apart, making the gas denser. But fermions always keep far apart, even at ultra-low temperatures, so at some point, the density of a Fermi gas tells you nothing about temperature. Instead, we proposed using a different kind of atom as a probe.”
“Let’s say that you have an ultra-cold gas made of lithium atoms. You now take a different atom, say potassium, and dunk it into the gas. Collisions with the surrounding atoms change your potassium probe’s state, which allows you to infer temperature. Technically speaking, our proposal involves creating a quantum superposition: a weird state where the probe atom simultaneously does and doesn’t interact with the gas. We showed that this superposition changes over time in a way that is very sensitive to temperature.”
Dr. Giacomo Guarnieri gives the following analogy: “A thermometer is just a system whose physical properties change with temperature predictably. For example, you can take your body’s temperature by measuring the expansion of mercury in a glass tube. Our thermometer works analogously, but instead of mercury, we measure the state of single atoms that are entangled (or correlated) with a quantum gas.”
Professor Steve Campbell, UCD, remarks: “This isn’t just a far-flung idea—what we are proposing here can be implemented using the technology available in modern atomic physics labs. That such fundamental physics can be tested is amazing. Among the various emerging quantum technologies, quantum sensors like our thermometer are likely to make the most immediate impact, so it is a timely work highlighted by the editors of Physical Review Letters for that reason.”
Professor Goold adds: “In fact, one of the reasons that this paper was highlighted was precise because we performed calculations and numerical simulations with a particular focus on an experiment that was performed in Austria and published a few years ago in Science.”
“Here the Fermi gas is a dilute gas of trapped Lithium atoms which were in contact with Potassium impurities. The experimentalicane to control the quantum state radio frequency pulses and measure out information on the gas. These are operations that are routinely used in other quantum technologies. The timescales that are accessible ply amazing and would be unprecedented in traditional condensed matter physics experiments. We are excited that our of using these impurities as a quantum thermometer with exquisite precision could be implemented and tested with existing technology.”
- Mark T. Mitchison et al. In Situ Thermometry of a Cold Fermi Gas via Dephasing Impurities, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.125.080402