A masing ladder

doi:10.1126/science.abg1081

GSTDTAP > 气候变化

DOI	10.1126/science.abg1081
	A masing ladder
	Ren-Bao Liu
	2021-02-19
发表期刊	Science
出版年	2021
英文摘要	Masers, the precedent and radio-frequency (rf) counterpart of lasers, have many applications in metrology, communication, and spectroscopy. Jiang et al. ([ 1 ][1]) demonstrate a new type of maser in which rf radiation is amplified by stimulated emission from transitions between periodically modulated quantum states, called Floquet states ([ 2 ][2]), unlike a conventional maser, which uses transitions between stationary quantum states. The Floquet maser presents a phase-locked frequency comb equally spaced by the modulation frequency. With a modulating signal at a low frequency (1 to 100 mHz) converted to maser emission at high transition frequencies (∼10 Hz) of ultranarrow spectral linewidth (<0.3 mHz), the authors overcome the low-frequency noise problem in metrology and show that subpicotesla sensitivity of magnetometry is achievable. Conceivable applications of this work include precision clocks and detection of ultralight dark-matter particles such as axions. The quantum emitters used are the nuclear spins associated with xenon-129 (129Xe) nuclei, whose quantum states have long lifetimes (∼14 s). A static magnetic field splits the quantum states of the nuclear spins into the upper and lower energy levels (see the figure). The periodic modulation of the magnetic field can be viewed as what would result from a magnet mounted onto an oscillator. The spins in the upper or lower levels, which point upward or downward, would be attracted or repulsed by the magnet and in turn offset the oscillator in the opposite direction. In quantum mechanics, the offset oscillator has a ladder of equally spaced energy levels, corresponding to the upper and the lower ladders of Floquet states. Jiang et al. optically pump the 129Xe spins to achieve population inversion (the upper ladder becomes more populated than the lower one) so that the radiation by the 129Xe spins is amplified by stimulated emission. The 129Xe nuclear spins emit photons coherently and collectively, by the so-called superradiance process ([ 3 ][3]), and in turn undergo coherent precession. In the experiment, the spin precession is monitored through spin-exchange collisions with optically pumped rubidium-87 atoms, and then the signal is amplified and fed back to the spins by a pair of coils. The gain by the stimulated emission and the feedback amplification enables masing between the Floquet states, which results in equally spaced, phase-locked sharp spectral lines. ![Figure][4] Floquet-state ladders Jiang et al. created a maser that amplifies emission of periodically modulated quantum states (Floquet states). They converted low-frequency (1 to 100 mHz) signals into maser emission at ∼10 Hz with ultranarrow spectral linewidth (<0.3 mHz) to create a phase-locked frequency comb. GRAPHIC: C. BICKEL/ SCIENCE Transitions between Floquet states are also the basis of two other important and extremely nonlinear optical phenomena. High-harmonic generation (HHG) occurs when a system driven by a strong laser radiates at large integer multiples of the driving frequency ([ 4 ][5], [ 5 ][6]). HHG has applications in coherent x-ray spectroscopy as well as attosecond and even zeptosecond ultrafast optics ([ 5 ][6]). High-order sideband generation (HSG) occurs when a system driven by a strong low-frequency field and excited by a weak high-frequency laser emits sidebands of the weak laser separated by integer multiples of the driving frequency ([ 6 ][7], [ 7 ][8]). HSG may be used for high-speed optical communication ([ 6 ][7]). Both HHG and HSG are platforms that can be used to study extreme strong-field physics ([ 5 ][6], [ 7 ][8]). The Floquet maser resembles the HSG but has two new features, collective coherence (superradiance) ([ 3 ][3]) and gain. New physics and applications are expected from the study of HHG and HSG in gain media that exhibit collective quantum coherence. The Floquet maser is closely related to another innovation in lasers, quantum amplification of superradiant emission of radiation, or QASER ([ 8 ][9]). The idea is to drive atoms by a strong low-frequency laser so that the atoms are in Floquet states that contain a small, periodically oscillating mixture between the upper and lower levels. When the population oscillation in the Floquet states is resonant with the superradiance-induced oscillation between the emission and absorption of photons, radiation at the atomic transition frequency (which is much larger than the driving frequency) can be amplified without population inversion. If realized, QASER will be useful for achieving extreme high-frequency lasers, such as gamma-ray lasers. The Floquet maser demonstrated by Jiang et al. , although still at the rf waveband, already has a few key elements of QASER, in particular, the superradiance and the Floquet states. Nonetheless, some important modifications are needed to demonstrate the idea of QASER. It will be relatively straightforward to reconfigure the geometry and polarization of the driving field to achieve coherent population oscillation needed for QASER. The nontrivial missing piece is that the feedback amplification in the Floquet maser is not quantum and is unfeasible for QASER in the gamma-ray frequency band. Further theoretical and experimental investigations can show whether the Floquet maser could be sustained with only quantum amplification in place. The Floquet maser demonstrated by Jiang et al. provides a new platform for studying exotic temporal phenomena in quantum many-body systems ([ 9 ][10]–[ 12 ][11]). An outstanding example is time crystals ([ 11 ][12], [ 12 ][11]), wherein the Floquet states of a periodically driven quantum system undergo a spontaneous symmetry breaking so that the system oscillates at a fractional frequency of the driving field. It would be interesting to study time crystals in the presence of quantum amplification, the interplay with a Floquet maser, or even mutual excitation of a time crystal and a Floquet maser. The Floquet maser and the versions of it yet to come provide a ladder to high-frequency, high-precision metrology applications and a platform for studying quantum phenomena. 1. [↵][13]1. M. Jiang, 2. H. Su, 3. Z. Wu, 4. X. Peng, 5. D. Budker , Sci. Adv. 7, eabe0719 (2021). [OpenUrl][14][FREE Full Text][15] 2. [↵][16]1. J. H. Shirley , Phys. Rev. 138 (4B), B979 (1965). [OpenUrl][17][CrossRef][18] 3. [↵][19]1. R. H. Dicke , Phys. Rev. 93, 99 (1954). [OpenUrl][20][CrossRef][21][PubMed][22][Web of Science][23] 4. [↵][24]1. M. Ferray et al ., J. Phys. 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领域	气候变化 ; 资源环境
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引用统计
文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/315713
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Ren-Bao Liu. A masing ladder[J]. Science,2021.
APA	Ren-Bao Liu.(2021).A masing ladder.Science.
MLA	Ren-Bao Liu."A masing ladder".Science (2021).