Laser-induced cooling of broadband heat reservoirs

We explore, theoretically and experimentally, a method for cooling a broadband heat reservoir, via its laser-assisted collisions with two-level atoms followed by their fluorescence. This method is shown to be advantageous compared to existing laser-cooling methods in terms of its cooling efficiency, the lowest attainable temperature for broadband baths, and its versatility: it can cool down any heat reservoir, provided the laser is red detuned from the atomic resonance. It is applicable to cooling down both dense gaseous and condensed media.

Figure 1

(a) Schematic representation of cooling by laser-induced collisional redistribution (LICORE). Left: The level scheme along with the corresponding laser detuning $\Delta$, collisional pumping rate ${\Gamma }_P$, and atomic spontaneous decay rate $\gamma$. Right: Heat-flow chart (TLA, two-level atom; EM, electromagnetic vacuum; H bath, hot bath; C bath, cold bath). (b) Sideband cooling compared to LICORE. Top: Efficiency as a function of the bandwidth ${\gamma }_{\text{coll}}$ scaled by $\Omega$. While sideband cooling (solid red line) is highly efficient for resolved sidebands ($\Omega \gg {\gamma }_{\text{coll}}$), LICORE (dashed blue line) is much more efficient for unresolved sidebands ($\Omega \ll {\gamma }_{\text{coll}}$). In sideband cooling $\Omega$ is the trapping frequency, while in LICORE $\Omega$ is the coupling Rabi frequency, taken to have the same value. The resolved sidebands spectrum conforms to the Mollow triplet [5, 6, 7].

Figure 2

Experimental results (red dashed line) compared to theoretical prediction (blue solid line) of the total cooling power as a function of the laser detuning $\Delta$. The $y$ axis indicates the cooling power. For red detuning, $J_H$ corresponds to buffer gas cooling (purple, dark area), while blue detuning results in negative $J_H$, namely, buffer gas heating (orange, light area).

Figure 3

Experimental setup for confocal spectroscopy on high-pressure helium-rubidium gas mixtures.

Figure 4

Fluorescence spectra for rubidium vapor at 530 K with 200 bar helium buffer gas for excitation frequency 360 THz (red) and 401 THz (blue). Arrows indicate the direction of the mean frequency shift of the scattered photons by the redistribution process toward the $D$-lines center.