A quantum thermometer designed to measure the coldest temperatures in the universe

Physicists at Trinity College Dublin have proposed a thermometer based on quantum entanglement. It can accurately measure temperatures a billion times colder than in outer space. The work was led by the QuSys team at Trinity in collaboration with an international team of scientists. Their results have been published as in Physical Review Letters.

Such ultra-low temperatures occur in clouds of atoms known as Fermi gases. They were created by scientists to study the behavior of matter in extreme quantum states.

Professor Gould, head of Trinity’s QuSys Group, explains what ultracold gas is.

The standard way physicists think about gas is by using a theory known as statistical mechanics. But with the advent of quantum mechanics, everything changed. A well-known prediction of quantum mechanics is that individual atoms below the critical temperature can combine with other atoms into a single macroscopic wave with exotic properties. This prediction led to a century-long experimental search for reaching critical temperatures. Success was finally achieved in the 1990s with the creation of the first ultracold gases cooled by lasers (1997 Nobel Prize) and contained by strong magnetic fields – a feat that won the 2001 Nobel Prize.

Professor Gould, Head of Trinity’s QuSys Group

Ultra-cold gases like these are now routinely created in laboratories around the world. They have many applications, ranging from testing fundamental physical theories to detecting gravitational waves. But their temperatures are incredibly low at the nanokelvin level and below (one kelvin equals -271.15°C). These gases are a billion times colder than the coldest places in the universe, and they are created right here on Earth.

So what is Fermi gas? All particles in the Universe, including atoms, belong to one of two types – “bosons” and “fermions”. Fermi gas is made up of fermions named after the physicist Enrico Fermi. At very low temperatures, bosons and fermions behave very differently. While bosons are grouped together, fermions are the other way around. This property actually makes it difficult to measure its temperature.

A thermometer is simply a system whose physical properties change with temperature in a predictable way. For example, you can measure your body temperature by measuring the expansion of mercury in a glass tube. The new thermometer works in a similar way, but instead of mercury, scientists measure the state of individual atoms that are associated (or correlated) with the quantum gas.

The new solution can be implemented using technologies available in modern atomic physics laboratories. The fact that such fundamental physics can be verified is truly amazing, scientists emphasize. Various emerging quantum technologies, quantum sensors such as the new thermometer, are likely to have the most direct impact on science.