Quantum mechanics has been around for more than 100 years, and it continues to be as counterintuitive as it was back then. Its challenges and mysteries are slowly being resolved though, and now scientists have measured the temperature of a quantum system.
Temperature is one of those quantities that it is both naturally familiar and incredibly complicated once we go beyond its basic definition. For an ideal gas, it is proportional to the kinetic energy of its constituent particles. In general, it is described as connected to the internal energy of the system. Measuring the temperature is done macroscopically by thermometers – two bodies in contact will reach over time the same temperature.
Doing that in the quantum world is not as straightforward. Energy cannot be known with arbitrary precision, due to the Heisenberg uncertainty principle. You either isolate the object completely to establish its energy or you use a thermometer that will influence your system.
There’s no winning if you want to measure the quantum temperature. A recent study in Nature Communications describes a generalized version of the energy-temperature uncertainty relation, which remains valid for both quantum and classical systems alike. The temperature has a certain uncertainty that can’t be reduced, but this allows us to take it into account.
One issue, in particular, is the superposition of the energy states. The concept of superposition has been made famous by physicist Erwin Schrödinger. In his thought experiment, a cat is trapped in a box with a vial of poison that can be activated by a quantum process. Since the scientist doesn’t know what’s happening in there, the cat is both alive and dead – it exists in two states at the same time. For the thermometer, this happens for temperature states.
"In the quantum case, a quantum thermometer... will be in a superposition of energy states simultaneously," author Harry Miller, from the University of Exeter, told Live Science. "What we find is that because the thermometer no longer has a well-defined energy and is actually in a combination of different states at once, that this actually contributes to the uncertainty in the temperature that we can measure."
The findings are important for the design of optimal nanoscale thermometers. These might not be useful in everyday life, but they will play a pivotal role in the successful functioning of many upcoming technologies.
[H/T: Live Science]