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arXiv:2502.08067 · npj Quantum Information

Reducing thermal noise by quantum refrigerators

Han-Jia Bi, Sheng-Wen Li

Published Oct 21, 2025• 0 citations
arXivHTMLPDF

Abstract

Reducing the thermal noises in microwave (MW) resonators can bring about significant progress in many research fields. In this study, we consider using three-level or four-level systems as “quantum refrigerators” to cool down MW resonators so as to reduce the thermal noises, and investigate their possible cooling limits. In such a quantum refrigerator system, the MW resonator is resonantly coupled with many three-level or four-level systems via two of their energy levels. A driving laser is applied on the multilevel systems, then together with the help of proper optical emissions, the populations of the multilevel systems can be concentrated into their ground states. Effectively that creates a system with zero temperature, which could continuously absorb the thermal photons in the MW resonator. By adiabatic elimination, we give a more precise description for this cooling process. For three-level systems, it turns out a too strong driving laser significantly perturbs the energy levels, which impedes the energy exchange between the multilevel systems and the resonator, and weakens the cooling effect. For four-level systems, by adopting an indirect pump, such an upper constraint for the driving strength can be released. In both cases, we obtain analytical results for the cooling limit of the MW resonator. Based on practical experimental parameters, our estimation shows the cooling limit could reach lower than the liquid helium temperature, without resorting to the traditional cryogenic systems.

DOI
10.1038/s41534-025-01105-x

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Reducing the thermal noises in microwave (MW) resonators can bring about significant progress in many research fields. In this study, we consider using three-level or four-level systems as “quantum refrigerators” to cool down MW resonators so as to reduce the thermal noises, and investigate their possible cooling limits. In such a quantum refrigerator system, the MW resonator is resonantly coupled with many three-level or four-level systems via two of their energy levels. A driving laser is applied on the multilevel systems, then together with the help of proper optical emissions, the populations of the multilevel systems can be concentrated into their ground states. Effectively that creates a system with zero temperature, which could continuously absorb the thermal photons in the MW resonator. By adiabatic elimination, we give a more precise description for this cooling process. For three-level systems, it turns out a too strong driving laser significantly perturbs the energy levels, which impedes the energy exchange between the multilevel systems and the resonator, and weakens the cooling effect. For four-level systems, by adopting an indirect pump, such an upper constraint for the driving strength can be released. In both cases, we obtain analytical results for the cooling limit of the MW resonator. Based on practical experimental parameters, our estimation shows the cooling limit could reach lower than the liquid helium temperature, without resorting to the traditional cryogenic systems.
The maser, a microwave (MW) analog of the laser, is a well-established method for generating and amplifying coherent MW irradiation with ultra-low noise. This is accomplished by creating a state of population inversion between two energy levels separated by MW frequency. Thermodynamically, such a state corresponds to a small but negative temperature. The reverse condition, where only the lower energy level is highly populated, corresponds to a very low positive temperature. In this work, we experimentally demonstrate how to generate such a state in condensed matter at moderate cryogenic temperatures. This state is then used to efficiently remove microwave photons from a cavity, continuously cooling it to the quantum limit, well below its ambient temperature. Such an "anti-maser" device could be extremely beneficial for applications that would normally require cooling to millikelvin temperatures to eliminate any MW photons. For instance, superconducting MW quantum circuits (such as qubits and amplifiers) could, with the use of this device, operate efficiently at liquid helium temperatures.
The maser, a microwave (MW) analog of the laser, is a well-established method for generating and amplifying coherent MW irradiation with ultralow noise. This is usually accomplished by creating a state of population inversion between two energy levels separated by MW frequency. Thermodynamically, such a state corresponds to a small but negative temperature. The reverse condition, where only the lower energy level is highly populated, corresponds to a very low positive temperature. In this work, we experimentally demonstrate how to generate such a state in condensed matter at moderate cryogenic temperatures. This state is then used to efficiently remove microwave photons from a cavity, continuously cooling it, well below its ambient temperature. Such an “anti-maser” device could be extremely beneficial for applications that would normally require cooling to millikelvin temperatures to eliminate any MW photons. For instance, superconducting MW quantum circuits (such as qubits and amplifiers) could, with the use of this device, operate efficiently at liquid helium temperatures.
We use a transmon qubit and its dispersively coupled readout resonator to measure the Fock state populations of another microwave resonator, to which we have attached a quantum-circuit refrigerator (QCR). First, we apply noise generated at room temperature to the resonator and show that such noise drive leads to a thermal distribution of the resonator Fock states. Subsequently, we detune the noise frequency band far away from the resonance condition and vary the power of the noise applied on the QCR. We observe that such artificial thermal noise may lead to major damping of a coherent state of the resonator. Importantly, we also demonstrate that the effective temperature of a thermal resonator state can be reduced from roughly 300 to 130 mK by the introduction of the artificial thermal noise. These observations pave the way for a purely thermally powered quantum-circuit refrigerator that may unlock the use of waste heat in resetting superconducting qubits in a quantum processor and in building autonomous quantum heat engines.
For a standard two-level atom coupled to the quantized field of a resonant cavity, finite-temperature effects lead to thermal occupation of the cavity modes that obfuscates measurement of the quantum nature of the atom-light interaction. In this paper we demonstrate that using a hybrid system of a superconducting cavity coupled to a multilevel Rydberg atom, it is possible to observe the quantum nature of strong coupling even at finite temperatures and to exploit this coupling to permit cooling of the thermal microwave mode towards the ground state, enabling observation of coherent vacuum Rabi oscillations even at 4 K for realistic experimental parameters. Cooling for multiple atoms is also explored, showing maximal cooling for small samples, making this a viable approach to cavity cooling with potential applications in long-range coupling of superconducting qubits via thermal waveguides.
The quantum bits (qubits) on which superconducting quantum computers are based have energy scales corresponding to photons with GHz frequencies. The energy of photons in the gigahertz domain is too low to allow transmission through the noisy room-temperature environment, where the signal would be lost in thermal noise. Optical photons, on the other hand, have much higher energies, and signals can be detected using highly efficient single-photon detectors. Transduction from microwave to optical frequencies is therefore a potential enabling technology for quantum devices. High conversion efficiency and high fidelity (the similarity of input microwave state to the output optical state.) are important characteristics of these devices. In such devices, the efficiency of the up-conversion process is typically improved by increasing the power in the pump. In addition to this, the optical pump is dissipated as thermal photons via absorption-induced heating into the system. Since the frequency of thermal photons and qubits are in the same range, this can affect the quality of the transferred quantum state which is characterized by fidelity. Thus, the optical pump can be a source of thermal noise and degrade the fidelity. Realizing an optimum pump power level while investigating its effect on fidelity via temperature are important tasks in the evolution of these upconversion devices. In this work, we explore two kinds of hybrid quantum devices that can be used as frequency upconverts; the first one is a resonant electro-optic modulator consisting of a microwave cavity and a WGM resonator (made of LiNbO3) and the next one is a magneto-optic modulator consisting of a microwave cavity and the rare earth crystal gadolinium vanadate (GdVO4) in the cuboid shape. We model the sub-Kelvin thermal behavior of an electro-optic transducer based on a lithium niobate whispering gallery mode resonator. We compute thermal photon numbers and find that there is an optimum power level for a continuous pump, whilst pulsed operation of the pump increases the fidelity of the conversion. In the magneto-optic modulator via photon-magnon coupling, we investigate the effect of GdVO4 temperature on the linewidth of magnons and the strength of photon-magnon coupling inside a dilution refrigerator. In the next step, we investigate the effect of absorption-induced heating of GdVO4 on the linewidth of magnons and the strength of photon-magnon coupling inside a dilution refrigerator, and the temperature of GdVO4 is estimated via comparison of two methods. The effect of heating of the GdVO4 via the mentioned methods on the transmission spectrum and the resonance frequency of the microwave cavity is studied. The transmission spectrum is used to comprehend the time evolution of the thermal equilibrium of the sample and estimate its temperature.
While dephasing noise often hinders quantum devices, it can become an asset for quantum thermal machines. Here we demonstrate a three-level thermal machine that leverages noise-assisted quantum transport to enable steady-state cooling of microwave modes. The device exploits symmetry-selective couplings between a superconducting artificial molecule and two physical heat baths. Each bath consists of a microwave waveguide populated with synthesized quasithermal radiation. Energy transport is enabled by injecting dephasing noise through a third channel longitudinally coupled to one artificial atom of the molecule. By varying the effective temperatures of the reservoirs and measuring photonic heat currents with sub-attowatt resolution, we demonstrate energy flow dynamics characteristic of a quantum heat engine, thermal accelerator, and refrigerator. Our work constitutes an experimental demonstration of the key operating principles of a noise-assisted three-level quantum refrigerator and opens new avenues for experiments in quantum thermodynamics using superconducting circuits coupled to physical heat baths.