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arXiv:2302.09615 · Physical Review Letters

Laser Cooling of Nuclear Magnons

Haowei Xu, Guoqing Wang, Changhao Li, Hua Wang, Hao Tang, Ariel Barr, Paola Cappellaro, Ju Li

Published Feb 9, 2023• 4 citations
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Abstract

The initialization of nuclear spin to its ground state is challenging due to its small energy scale compared with thermal energy, even at cryogenic temperature. In this Letter, we propose an optonuclear quadrupolar effect, whereby two-color optical photons can efficiently interact with nuclear spins. Leveraging such an optical interface, we demonstrate that nuclear magnons, the collective excitations of nuclear spin ensemble, can be cooled down optically. Under feasible experimental conditions, laser cooling can suppress the population and entropy of nuclear magnons by more than 2 orders of magnitude, which could facilitate the application of nuclear spins in quantum information science.

DOI
10.1103/physrevlett.130.063602

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We propose a method for laser cooling group-II-like atoms without changing the quantum state of their nuclear spins, thus preserving coherences that are usually destroyed by optical pumping in the cooling process. As group-II-like atoms have a (1)S(0) closed-shell ground state, nuclear spin and electronic angular momentum are decoupled, allowing for their independent manipulation. The hyperfine interaction that couples these degrees of freedom in excited states can be suppressed through the application of external magnetic fields. Our protocol employs resolved-sideband cooling on the forbidden clock transition, (1)S(0) --> (3)P(0), with quenching via coupling to the rapidly decaying (1)P(1) state, deep in the Paschen-Back regime. This makes it possible to laser cool neutral atomic qubits without destroying the quantum information stored in their nuclear spins, as shown in two examples, (171)Yb and (87)Sr.
The authors leverage the recently proposed optonuclear quadrupolar effect to develop a two-photon pumping scheme for the isomeric state of 229{}^{229}229Th, an exotic nucleus with an ultralow excitation energy of 8.3 eV. Additionally, the authors demonstrate that the lasing between nuclear states, which has been pursued for decades, may be realized based on the two-photon pumping scheme in ultrawide-band-gap compounds like ThF4{}_{4}4​, Na2{}_{2}2​ThF6{}_{6}6​, or K2{}_{2}2​ThF6{}_{6}6​.
We show that a quantum interference effect in optical absorption from two electronic spin states of a solid-state emitter can be used to prepare the surrounding environment of nuclear spins in well-defined states, thereby suppressing electronic spin dephasing. The coupled electron-nuclei system evolves into a coherent population trapping state by optical-excitation-induced nuclear-spin diffusion for a broad range of initial optical detunings. The spectroscopic signature of this evolution where the single-electron strongly modifies its environment is a drastic broadening of the dark resonance in optical absorption experiments. The large difference in electronic and nuclear time scales allows us to verify the preparation of nuclear spins in the desired state.
Nuclear-spin memories of divalent neutral atoms can allow spin-preserving resolved-sideband cooling in a strong magnetic field [I. Reichenbach and I. H. Deutsch, Phys. Rev. Lett. 99, 123001 (2007)]. We present a theory for cooling 87Sr^{87}\mathrm{Sr}87Sr nuclear-spin qubits in a weak magnetic field. The theory depends on laser excitation of 5s5p3P15s5p\phantom{\rule{0.16em}{0ex}}^{3}P_{1}5s5p3P1​ to a nearby state which results in mJ{m}_{J}mJ​-dependent AC Stark shifts large compared to the hyperfine interaction. This effectively suppresses the nuclear-spin mixing due to the hyperfine interaction. Sideband cooling via the clock state quenched by the AC Stark-shifted 3P1^{3}P_{1}3P1​ state leads to nuclear-spin-preserving spontaneous emission back to the ground state. More than being compatible with low magnetic fields, the theory is applicable when the nuclear-spin qubits are defined by the two lowest Zeeman substates.