Researchers from Chalmers University of Technology in Sweden developed the first of its kind quantum refrigerator running on noise. Disturbances and fluctuations, which are normally considered harmful for quantum systems as they decrease their stability, in this device are used as the energy source for cooling.
The development is based on the noise principle of energy transfer against the temperature gradient – from cooler unit to a warmer one. The role of the working medium here is performed not by well-ordered cycles, but by chaotic fluctuations. To implement this idea, the scientists created an artificial molecule of superconducting qubits – tiny quantum objects, which can be subject to precise fine-tuning and control. Qubits were interconnected and also connected to two microwave guides performing as hot and cold thermal baths, i.e., vessels participating in energy exchange.
The third controlling line connected to one of the qubits became the key element of the circuit. A specially generated noise signal was supplied to this line resulting in dephasing – the loss of quantum coherence in the system. Normally, this process is undesirable, however, in this particular case it was the controlled dephasing that became the locomotive causing the energy to flow from the cold vessel to the hot one. To register such weak effects the researchers developed high-sensitivity measurement system allowing for fixing thermal flows across the microwave irradiation spectrum at the level of attowatts – parts per billion of pert per billion of watt.
The experiment showed: the device is capable of operating in three different modes depending on the ratio of the effective temperatures of the vessels. Under one mode the system performs as a quantum heat engine converting the temperature differential into a directed flow of energy.
Under the second mode – as a refrigerator transferring heat from the cold area to a hotter one contrary to the natural course of heat transfer.
Under the third mode the circuit performs as a thermal accelerator boosting the natural flow of energy between the vessels.
It was the refrigerator mode that became the most meaningful outcome of the experiment demonstrating sustainable cooling of microwave guides due to “noise injection”.
In future, the scientists plan to connect such quantum refrigerators not only to microwave guides, but also to real elements with terminal heat capacity – e.g., to micro resistors or other components of superconductive circuits. In this case, extraction of energy must result in measurable decrease of their physical temperature bringing the practical applications of quantum refrigerators closer in quantum electronics and computer systems.
