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John O | March 2019

Quantum model for achieving ground-state cooling demonstrated using nitrogen vacancy centers

By Josh Perry, Editor


Scientists from the Center for Nanoscale Materials at Argonne National Laboratory (Lermont, Ill.) presented a quantum model for ground-state cooling in low-frequency mechanical resonators and demonstrated that cooperativity and entanglement are critical to enhancing the cooling figure of merit.


(left) Model of cooling cycle: an external laser pumps atoms into a two-level subspace coupled directly to a mechanical resonator; phonon absorption results in cooling of the mechanical system. (right) Schematic of a mechanical resonator interacting with an atomic ensemble. (Argonne National Lab)


Cryogenic cooling techniques, according to a report from Argonne Lab, have previously been successful in cooling high-frequency resonators, but have been ineffective for low-frequency systems.


“This method parametrically couples a mechanical resonator to a driven optical cavity, and, through careful tuning of the drive frequency, achieves the desired cooling effect,” the report explained. “The optomechanical effect is expanded to an alternative approach for ground-state cooling based on embedded solid-state defects. Engineering the atom-resonator coupling parameters is proposed, using the strain profile of the mechanical resonator allowing cooling to proceed through the dark entangled states of the two-level system ensemble.”


Researchers believe that this method provides ground-state cooling even with weak interactions. They also claim that this system will work with silicon and nitrogen vacancy centers in diamond and quantum dots to advance miniaturization and room-temperature operation.


“The approach, accessible for experimental demonstrations and universal to a variety of systems, overcomes the main obstacles that have blocked realization of ground-state cooling using embedded solid-state defects,” the article continued.


The research was recently published in Physical Review B. The abstract read:


“We analyze the cooling of a mechanical resonator coupled to an ensemble of interacting two-level systems via an open quantum systems approach. Using an exact analytical result, we find optimal cooling occurs when the phonon mode is critically coupled (γ∼g) to the two-level system ensemble.


“Typical systems operate in suboptimal cooling regimes due to the intrinsic parameter mismatch (γ?g) between the dissipative decay rate γ and the coupling factor g. To overcome this obstacle, we show that carefully engineering the coupling parameters through the strain profile of the mechanical resonator allows phonon cooling to proceed through the dark (subradiant) entangled states of an interacting ensemble, thereby resulting in optimal phonon cooling.


“Our results provide an avenue for ground-state cooling and should be accessible for experimental demonstrations.”

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