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The Okinawa Institute of Science and Technology Graduate University (OIST), University College Dublin, and Trinity College Dublin have developed a method to use a single atom to measure temperatures of quantum systems approaching absolute zero.
The technique used by the team relies heavily on the quantum phenomenon of superposition to use a single atom as a thermometer to sensitively measure the temperature of an ultra-cold gas. Their findings are detailed in a paper published in the journal Physical Review Letters.
While taking temperatures is not something that we generally consider too challenging, when the temperatures to be investigated approach absolute zero (-273.15 ⁰C) the simple measurement task becomes exponentially more difficult. The precise measurement of such low temperatures has been a fundamental task for quantum physics almost since its inception in the early 20th century. But, to measure such systems, you have to create one first.
The creation and control of such low-temperature systems is not just an interesting experiment and an exercise in gathering knowledge for knowledge’s sake. There are significant practical applications that stem from bringing a system down to near absolute zero, specifically in, but not limited to, quantum computing.
Absolute Zero Where Nothing Moves
Even when you are standing still, you are not still at all. Ignoring the blood pumping around your body and other biological phenomena, your atoms are vibrating at incredible speeds. Absolute zero is the point at which all atomic movement ceases. That means it is the lowest possible achievable temperature — a full stop for all atomic activity.
This implies that a measure of temperature is just a measure of how quickly atoms of a system are moving. But, when you consider that at room temperature in an empty room with dimensions of 8 x 10 x 3 meters, there are about 6 x 10²⁷ molecules whizzing around at speeds of 300–400 meters per second. When we measure the temperature of a room, we use a thermometer rather than attempt to measure the speed of all these molecules.
With a quantum system comprised of far fewer atoms, however, one could, in principle, take the velocity measurements of each atom. But, that does not mean such a measurement would be a simple task. The first challenge is getting a ‘thermometer’ of the right size and with the right temperature itself to measure a quantum system.
Finding a suitable ‘Goldilocks thermometer’ is critical as quantum systems such as those the team aimed to measure are extremely cold — a temperature so low it is not found elsewhere in the universe — and significantly small, consisting of no more than 100,000 atoms. This means that, if a thermometer is too warm or too large, it will disturb the system being measured, in this case, heating the super-cooled gas and destroying its quantum properties.
That means the collaboration of researchers needed to find something tiny that they could keep extremely cold— deciding that a single super-cooled atom would be ideal for the task.
Find out more: Temperature Measurement Equipment
A Single Atom Thermometer
As it is introduced to the quantum system — a super-cooled gas — the quantum thermometer initially exists in a superposition of two distinct energy states. But, as the atom interacts with the system, this quantum quality of superposition — something unique to quantum objects — begins to decay.
The more rapidly the atoms of the gas are moving, the more the quantum thermometer atom is forced to interact with them. This means the faster the atoms are moving, the more rapidly the quantum behavior of the thermometer decays. That is how the single atom can give the researchers a measurement of the quantum system being probed, allowing them to infer the temperature.
The team was able to determine the optimal timings for when measurements should be taken and found that to measure temperatures at their lowest, they had to wait longer to collect their ‘readings.’ They also discovered the intensity of the interactions between the atom thermometer and the gas needed to get the best sensitivity and the least amount of noise.
This is critical for the functioning of quantum computers, which require extremely cold temperatures to reduce noise and function at maximum efficiency.
The researchers say the next step in their research is exploring methods that improve sensitivity, as well as looking for ways to introduce more quantum thermometers into a single quantum system, allowing more complex interactions to occur.
This method of pushing the boundary of thermometry should be available to use in experiments very shortly, providing a vital boost to quantum technology.
References and Further Reading
Mitchison. M. T., Fogarty. T., Guarnieri. G., et al, [2020] In Situ Thermometry of a Cold Fermi Gas via Dephasing Impurities. Physical Review Letters. https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.125.080402
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