A suite of laser cooling techniques has successfully pushed the temperature of atoms and gases of atoms down to near absolute zero, where quantum mechanics dictates the particles’ behavior. But extending those techniques to molecules and solids has proven tricky for many reasons, among them the absence of well-defined electronic transitions (see Physics Today, January 2010, page 9). Now Markus Aspelmeyer of the University of Vienna in Austria and his colleagues have cooled a nanoparticle to its ground state in an optical harmonic trap. To do so, they combined optical tweezers with a laser-cooling technique known as cavity cooling.
In typical cavity cooling, a particle is placed in an optical cavity that is irradiated by a laser. The light in the cavity scatters off the particle and leaves it in either a higher motional energy state, so-called Stokes scattering, or a lower motional energy state, anti-Stokes scattering. The driving laser is tuned such that the cavity enhances anti-Stokes scattering, and each photon scattered into the cavity lowers the energy of the particle one quantum of motion at a time.
In theory, that tactic should cool a nanoparticle to its motional ground state. But reducing the center-of-mass temperature below tens of millikelvin has faced two hurdles: First, fluctuations in the laser’s phase lead to cavity-field fluctuations that jostle the particle and introduce additional heating. Second, cooling the nanoparticle further than tens of millikelvin requires more photons—that is, the laser intensity needs to be increased. But a strong cavity field pulls the particle into one of its nodes, where it no longer interacts with photons and thus isn’t cooled.
Aspelmeyer and his colleagues removed those obstacles in their cooling scheme (see the figure) by incorporating an optical tweezer that is slightly detuned from the cavity’s resonant frequency. The optical trap (purple) both holds the particle in a stable position in the cavity and provides position-independent photons for the cooling process; no driving laser is necessary. Otherwise, the process is similar to the usual cavity cooling: Photons from the tweezer scatter off the particle, and the cavity enhances anti-Stokes scattering (larger blue arrows), which gradually lowers the particle’s energy, rather than Stokes scattering (smaller red arrows).
The team cooled a silica nanoparticle about 143 nm in diameter down to its ground state, a temperature of about 10 microkelvin. In the current experiment, the nanoparticle’s wavepacket is localized to a few picometers, but in the future, Aspelmeyer plans to expand the wavepacket to the size of the particle itself and examine the properties in that macroscopic quantum regime. Expansion of the wavepacket is possible by turning off the tweezers and letting the particle free fall. Alternatively, changing the tweezer’s beam profile would swap the harmonic potential for a double well or any number of nonlinear potentials that would expand the wavepacket. (U. Delić et al., Science 367, 892, 2020.)
