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Measurement-induced cooling of magnons via acting on the outside

Jia‐shun Yan, Jun Jing

Published Jun 15, 2021• 0 citations
OpenAlex

Abstract

A quantum resonator in a thermal-equilibrium state with a high temperature has a large average exciton number and has a significant occupation over the high Fock-states. The resonator could be cooled down through constant measurements on the ground state of a coupled two-level system (qubit). We find, however, that the measurement-induced cooling might become inefficient in the high-temperature regime. Beyond the conventional strategy, we propose a measurement-induced-cooling protocol via acting a strong interaction outside the space of the resonator-qubit system. In particular, the excited state of the two-level system is coupled to an outside state through a driving field. The strong interaction between these two states can greatly enlarge the cooling range in regard to the non-vanishing populated Fock-states of the resonator. Without any pre-cooling procedure, acting outside allows a significant reduction on the populations over the high Fock-states and then the ground-state cooling after a sequence of measurements. The fast transition induced by a strong acting-outside constrains the ancillary system at its ground state and then simulates a quantum Zeno effect, that consistently provides a possible realization for measurements required for cooling. Our protocol is applied to cool down a high-temperature magnetic resonator. It is then generalized to a hybrid cooling protocol by interpolating the methods with and without acting on the outside, which can accelerate the cooling process and promote the overlap between the final state of the resonator and its ground state.

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A quantum resonator in a thermal-equilibrium state with a high temperature has a large average exciton number and has a significant occupation over the high Fock-states. The resonator could be cooled down through constant measurements on the ground state of a coupled two-level system (qubit). We find, however, that the measurement-induced cooling might become inefficient in the high-temperature regime. Beyond the conventional strategy, we propose a measurement-induced-cooling protocol via acting a strong interaction outside the space of the resonator-qubit system. In particular, the excited state of the two-level system is coupled to an outside state through a driving field. The strong interaction between these two states can greatly enlarge the cooling range in regard to the non-vanishing populated Fock-states of the resonator. Without any pre-cooling procedure, acting outside allows a significant reduction on the populations over the high Fock-states and then the ground-state cooling after a sequence of measurements. The fast transition induced by a strong acting-outside constrains the ancillary system at its ground state and then simulates a quantum Zeno effect, that consistently provides a possible realization for measurements required for cooling. Our protocol is applied to cool down a high-temperature magnetic resonator. It is then generalized to a hybrid cooling protocol by interpolating the methods with and without acting on the outside, which can accelerate the cooling process and promote the overlap between the final state of the resonator and its ground state.
A quantum resonator in a thermal-equilibrium state with a high temperature has a large average population and is featured with significant occupation over Fock states with a high excitation number. The resonator could be cooled down via continuous measurements on the ground state of a coupled two-level system (qubit). We find, however, that the measurement-induced cooling might become inefficient in the high-temperature regime. Beyond the conventional strategy, we introduce strong driving between the excited state of the qubit and an external level. It can remarkably broaden the cooling range in regard to the nonvanishing populated Fock states of the resonator. Without any precooling procedure, our strategy allows a significant reduction of the populations over Fock states with a high excitation number, giving rise to nondeterministic ground-state cooling after a sequence of measurements. The driving-induced fast transition constrains the resonator and the ancillary qubit at their ground state upon measurement and then simulates the quantum Zeno effect. Our protocol is applied to cool down a high-temperature magnetic resonator. Additionally, it is generalized to a hybrid cooling protocol by interpolating the methods with and without strong driving, which can accelerate the cooling process and increase the overlap between the final state of the resonator and its ground state.
Ground state cooling of a nanomechanical resonator coupled to a superconducting flux qubit is discussed. By inducing quantum interference to cancel unwanted heating excitations, ground state cooling becomes possible in the nonresolved regime. The qubit is modeled as a three-level system in Lambda configuration, and the driving fluxes are applied such that the qubit absorption spectrum exhibits electromagnetically induced transparency, thereby canceling the unwanted excitations. As our scheme allows the application of strong cooling fields, fast and efficient cooling can be achieved.
Abstract Nondeterministic measurement‐based techniques are efficient in reshaping the population distribution of a quantum system but suffer from a limited success probability of holding the system in the target state. To save the experimental cost, a two‐step protocol is proposed to cool a resonator down to the ground state with a near‐unit probability by exploiting the state‐engineering mechanisms of both conditional and unconditional measurements on an ancillary qubit. In the first step, the unconditional measurements on the ancillary qubit are applied to reshape the target resonator from a thermal state to a reserved Fock state. The measurement sequence can be efficiently optimized by reinforcement learning for maximum fidelity. In the second step, the population on the reserved state can be faithfully transferred in a stepwise way to the resonator's ground state with a near‐unit fidelity by the conditional measurements on the qubit. Properly designing the projection operator and the measurement interval enables the Kraus operator to act as a lowering operator for neighboring Fock states. Through dozens of measurements in all, the initial thermal average population of the resonator can be reduced by five orders in magnitude with a success probability of over 95%.
We present a simultaneous-cooling protocol for a double-resonator system via projective measurements on an ancillary VVV-type qutrit. Through repeated measurements on the ground state of the ancillary system, the two resonators can be cooled down to their respective ground states from thermal states. With respect to the measurement-based cooling, an optimized measurement interval ${\ensuremath{\tau}}_{\mathrm{opt}}$ is analytically obtained, which is inversely proportional to the collective thermal Rabi frequency ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{th}}$ as a function of the resonators' average population of the last round. Under about only 20 optimized measurements, the average population of the target resonators can be reduced by six orders in magnitude. Our simultaneous or collective cooling protocol is scalable to the systems with more numbers of resonators and robust to the fluctuation in the resonator frequency. Also we discuss the robustness of our cooling protocol against the thermal environment and the measurement error.
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.
Cooling microwave resonators to near the quantum ground state, crucial for their operation in the quantum regime, is typically achieved by direct device refrigeration to a few tens of millikelvin. However, in quantum experiments that require high operation power such as microwave-to-optics quantum transduction, it is desirable to operate at higher temperatures with non-negligible environmental thermal excitations, where larger cooling power is available. In this Letter, we present a radiative cooling protocol to prepare a superconducting microwave mode near its quantum ground state in spite of warm environment temperatures for the resonator. In this proof-of-concept experiment, the mode occupancy of a 10 GHz superconducting resonator thermally anchored at 1.02 K is reduced to 0.44±0.05 from 1.56 by radiatively coupling to a 70 mK cold load. This radiative cooling scheme allows high-operation-power microwave experiments to work in the quantum regime, and opens possibilities for routing microwave quantum states to elevated temperatures.