Diagram of a device consisting of two superconducting circuits: a cold high-frequency circuit (blue) and a hot low-frequency circuit (red). Here, the current flowing through the red circuit creates an oscillating magnetic field, resulting in a pressure photon coupling. By sending a strong signal to the blue high frequency circuit, this high frequency circuit is converted into an amplifier capable of detecting RF photons flowing in the red circuit with greater sensitivity.Credit: Researcher
Physicists have built a quantum heat pump from light particles. The device brings scientists closer to the quantum limit of measuring radio frequency signals useful in the search for dark matter.
If two objects with different temperatures are placed together, such as a bottle of warm white wine in a freezer bag, the heat usually flows in the same direction, from hot (wine) to cold (cooler bag). If you wait long enough, both will reach the same temperature. This process is known in physics as reaching equilibrium: an equilibrium between heat flows in one direction and in the other.
If you’re willing to do some work, you can upset that balance and cause heat to flow the “wrong” way. This principle is used in refrigerators to keep food cold and in an efficient heat pump that steals heat from the cold air outside to heat your home. In their publication, Gary Steele and his colleagues demonstrate a quantum simulation of a heat pump that causes quantum particles of light called photons to “counter-current” from hot to cold objects.
The study was carried out by physicists from the Delft University of Technology, ETH Zurich and the University of Tübingen. Their work will be published today (August 26, 2022) as an open access article in the journal Science Advances.
Although physicists have used their device as a cooling bath for thermal RF photons in previous studies, they have now managed to turn it into an amplifier at the same time. Thanks to the built-in amplifier, the device is more sensitive to RF signals. This is similar to what happens to amplified microwave signals from superconducting quantum processors. “This is very exciting because we can get closer to the quantum limit of measuring RF signals, where frequencies are difficult to measure in other ways. This new measurement tool could have many applications, one of which is the search for dark matter.” Steele said.
The device, called a photon pressure circuit, consists of superconducting inductors and capacitors on a silicon chip cooled to just a few millidegrees above absolute zero. Although it may seem cold, this temperature is very high for some of the photons in the circuit, which are excited by thermal energy. Using photon pressure, scientists can couple these excited photons with higher frequency cold photons, which in previous experiments allowed them to cool hot photons back to their ground quantum state.
In the new work, physicists have added a new twist: By sending additional signals into the cold loop, they have been able to create an engine that amplifies cold photons and heats them up. At the same time, the additional signal preferentially “pumps” photons in one direction between the two chains. By pushing photons harder in one direction than the other, the researchers were able to cool photons in one part of the circuit to a lower temperature than another, creating a quantum version of a heat pump for photons in superconducting circuits.
Reference: “Enhanced Parameter Interactions and Nonreciprocal Bathtub Dynamics in Photonic Pressure Kerr Amplifiers”, August 26, 2022, Science Advances.DOI: 10.1126/sciadv.abq1690
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