Physicists from TU Delft, ETH Zürich and the University of Tübingen have built a quantum scale heat pump made from particles of light. This device brings scientists closer to the quantum limit of measuring radio frequency signals, useful in for example the hunt for dark matter.
Above-An illustration of the device, which consists of two superconducting circuits: a cold high frequency circuit (in blue) and a hot low frequency circuit (in red). Here, the current that flows in the red circuit generates an oscillating magnetic field which leads to the photon-pressure coupling. By sending in a strong signal to the blue high-frequency circuit, this one is transformed into an amplifier capable of detecting radio-frequency photons flowing in the red circuit with much higher sensitivity.
A quantum heat pump
The device, known as a photon pressure circuit, is made from superconducting inductors and capacitors on a silicon chip cooled to only a few millidegrees above absolute zero temperature. While this sounds very cold, for some of photons in the circuit, this temperature is very hot, and they are excited with thermal energy. Using photon pressure, the researchers can couple these excited photons to higher frequency cold photons, which in previous experiments allowed them to cool the hot photons into their quantum ground state.
In this new work, the authors add a new twist: by sending an extra signal into the cold circuit, they are able to create a motor which amplifies the cold photons and heats them up. At the same time, the extra signal “pumps” the photons preferentially in one direction between the two circuits. By pushing photons harder in one direction than the other, the researchers are able to cool the photons in one part of the circuit to a temperature that is colder than the other part, creating a quantum version of the heat pump for photons in a superconducting circuit.
Photon-pressure coupling between two superconducting circuits is a promising platform for investigating radiation-pressure coupling in distinct parameter regimes and for the development of radio-frequency (RF) quantum photonics and quantum-limited RF sensing. Here, we implement photon-pressure coupling between two superconducting circuits, one of which can be operated as a parametric amplifier. We demonstrate a Kerr-based enhancement of the photon-pressure single-photon coupling rate and an increase of the cooperativity by one order of magnitude in the amplifier regime. In addition, we observe that the intracavity amplification reduces the measurement imprecision of RF signal detection. Last, we demonstrate that RF mode sideband cooling is unexpectedly not limited to the effective amplifier mode temperature arising from quantum noise amplification, which we interpret in the context of nonreciprocal heat transfer between the two circuits. Our results demonstrate how Kerr amplification can be used as resource for enhanced photon-pressure systems and Kerr cavity optomechanics.
Photon-pressure and radiation-pressure coupled oscillators, where the amplitude of one oscillator modulates the resonance frequency of the second, have enabled a large variety of groundbreaking experiments in the recent decades. In cavity optomechanics , this type of coupling has been used for unprecedented precision in the detection and control of mechanical displacement, to generate entanglement between two mechanical oscillators and to realize nonreciprocal signal processing, parametric microwave amplification, frequency conversion, and the generation of entangled radiation, to name just a few of the highlights. More recently, the implementation of photon-pressure coupling between two superconducting circuits has attracted a lot of attention. Notably, within a short period of time, the strong-coupling regime, the quantum-coherent regime, and the sideband cooling of a hot radio-frequency (RF) circuit into its quantum ground state have been achieved. These recent results open the door for quantum-limited photon-pressure microwave technologies, RF quantum photonics, and quantum-enhanced dark matter axion detection at low-energy scales and for new approaches in circuit-based quantum information processing in terms of fault-tolerant bosonic codes.
Photon-pressure coupled circuits use a superconducting quantum interference device (SQUID) as a key coupling element, similar to flux-mediated optomechanics, and therefore, these platforms naturally come with Kerr cavities due to the Josephson nonlinearity of the SQUID inductance. Most experimental and theoretical works on optomechanical and photon-pressure systems have considered only the case of photon-pressure coupled linear oscillators, but lately, there has been growing interest in Kerr-like nonlinearities in photon-pressure interacting systems. Kerr nonlinearities in superconducting circuits are already extremely useful resources for cat-state quantum computation; for quantum-limited signal processing and detection by means of stand-alone Josephson parametric amplifiers (JPAs), circulators, and converters; and for Josephson metamaterials. Adding these exciting functionalities to photon-pressure coupled and optomechanical systems constitutes therefore a highly promising approach for enhanced quantum sensing devices and novel photon control schemes.
Here, we report photon-pressure coupling between a superconducting RF circuit and a strongly driven superconducting Kerr cavity, operated as a parametric amplifier. As well known from previous work, by strongly driving the high-frequency (HF) SQUID cavity of our system, we can activate a four-wave mixing process and obtain an effective signal-idler double-mode cavity, here reaching up to ∼12 dB of intracavity gain. Furthermore, by using an additional pump tone applied to the red sideband of the signal-mode resonance, we simultaneously switch on the photon-pressure coupling between this quasi-mode and the RF circuit.
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