Abstract
Quantum calorimetry, the thermal measurement of quanta, is a method of choice for ultrasensitive radiation detection ranging from microwaves to gamma rays. The fundamental temperature fluctuations of the calorimeter, dictated by the coupling of it to the heat bath, set the ultimate lower bound of its energy resolution. Here we reach this limit of fundamental equilibrium fluctuations of temperature in a nanoscale electron calorimeter, exchanging energy with the phonon bath at very low temperatures. The approach allows noninvasive measurement of energy transport in superconducting quantum circuits in the microwave regime with high efficiency, opening the way, for instance, to observe quantum jumps, detecting their energy to tackle central questions in quantum thermodynamics.
Highlights
Quantum calorimetry, the thermal measurement of quanta, is a method of choice for ultrasensitive radiation detection ranging from microwaves to gamma rays
There are two origins of noise in this heat current: (1) the standard randomness of transport known for particle current noise, and (2) random energies exchanged, leading to enhancement of fluctuations on top of those known for particle current only
We observe that the equilibrium fluctuations follow the fluctuation dissipation theorem (FDT) for temperature
Summary
The thermal measurement of quanta, is a method of choice for ultrasensitive radiation detection ranging from microwaves to gamma rays. The fundamental temperature fluctuations of the calorimeter, dictated by the coupling of it to the heat bath, set the ultimate lower bound of its energy resolution. ST ð0Þ 1⁄4 2kBT2=Gth; ð2Þ and the spectrum has Lorentzian cutoff at ωc 1⁄4 Gth=C These results hold for a system coupled to several equilibrium baths, if one takes Gth to represent the sum of all the individual thermal conductances to these baths. The noise of this calorimeter permits measurements of microwave photons in GHz regime at the lowest temperatures that we achieve. This method is a way to observe calorimetrically, e.g., the quantum trajectories with superconducting circuits[8,9,10]
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