At Trinity College Dublin, physicists have come up with the concept of a thermometer that works based on quantum entanglement and can accurately measure temperatures a billion times colder compared to those in outer space.
Such ultra-cold temperatures appear in clouds of atoms, called Fermi gases, which are made by researchers to analyze the behavior of matter under extreme quantum states.
The study was headed by the QuSys group at Trinity with postdoctoral fellows, Dr Mark Mitchison, Dr Giacomo Guarnieri, and Professor John Goold, together with Professor Steve Campbell (UCD) and Dr Thomas Fogarty and Professor Thomas Busch working at OIST, Okinawa, Japan.
The findings of the study were published recently as an Editor’s Suggestion in Physical Review Letters, a prestigious journal. Readers can obtain a PDF copy of the article on request.
Professor Goold, head of Trinity’s QuSys group, discussed the proposal by explaining what an ultra-cold gas is. He stated that “The standard way in which a physicist thinks about a gas is to use a theory known as statistical mechanics. This theory was invented by giants of physics such as Maxwell and Boltzmann in the 19th century.”
“These guys revived an old idea from the Greek philosophers that macroscopic phenomena, such as pressure and temperature, could be understood in terms of the microscopic motion of atoms. We need to remember that at the time, the idea that matter was made of atoms was revolutionary,” added Professor Goold.
At the dawn of the 20th century, another theory came to fruition. This is quantum mechanics and it may be the most important and accurate theory we have in physics. A famous prediction of quantum mechanics is that single atoms acquire wave-like features, which means that below a critical temperature they can combine with other atoms into a single macroscopic wave with exotic properties.
John Goold, Professor and Head, QuSys Group, Trinity College Dublin
Professor Goold continued, “This prediction led to a century-long experimental quest to reach the critical temperature. Success was finally achieved in the 90s with the creation of the first ultra-cold gases, cooled with lasers (Nobel Prize 1997) and trapped with strong magnetic fields—a feat which won the Nobel Prize in 2001.”
“Ultra-cold gases 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! Just to give you an idea, one kelvin is -271.15 degrees Celsius. These gases are a billion times colder than that—the coldest places in the universe and they are created right here on Earth,” noted Professor Goold.
So What Exactly is a Fermi Gas?
“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 very low temperatures, bosons and fermions behave completely differently. While bosons like to clump together, fermions do the opposite. They are the ultimate social distancers! This property actually makes their temperature tricky to measure.”
According to Dr Mark Mitchison, the first author of the paper, “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 the state of your potassium probe and this allows you to infer temperature.
Dr Mark Mitchison, Study First Author, QuSys Group, Trinity College Dublin
“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,” added Dr. Mitchison.
The analogy offered by Dr. Giacomo Guarnieri is that “A thermometer is just a system whose physical properties change with temperature in a predictable way. For example, you can take the temperature of your body by measuring the expansion of mercury in a glass tube. Our thermometer works in an analogous way, but instead of mercury we measure the state of single atoms that are entangled (or correlated) with a quantum gas.”
This isn’t just a far-flung idea—what we are proposing here can actually be implemented using technology available in modern atomic physics labs. That such fundamental physics can be tested is really 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 and it was highlighted by the editors of Physical Review Letters for that reason.
Steve Campbell, Professor, University College Dublin
“In fact one of the reasons that this paper was highlighted was precisely 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,” added Professor Goold.
Professor Goold continued, “The experimentalists are able to control the quantum state with 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 are simply amazing and would be unprecedented in traditional condensed matter physics experiments. We are excited that our idea to use these impurities as a quantum thermometer with exquisite precision could be implemented and tested with existing technology,” Professor Goold concluded.
Professor Goold and his QuSys research group are financially supported by Science Foundation Ireland. He has been awarded a European Research Council Starting Grant and a Royal Society University Research Fellowship. Recently, he was elected as Fellow of the Young Academy of Europe.
Mitchison, M. T., et al. (2020) In Situ Thermometry of a Cold Fermi Gas via Dephasing Impurities. Physical Review Letters. doi.org/10.1103/PhysRevLett.125.080402.