Cavity Cooling a Single Charged Levitated Nanosphere

Optomechanical cavity cooling of levitated objects offers the possibility for laboratory investigation of the macroscopic quantum behavior of systems that are largely decoupled from their environment. However, experimental progress has been hindered by particle loss mechanisms, which have prevented levitation and cavity cooling in a vacuum. We overcome this problem with a new type of hybrid electro-optical trap formed from a Paul trap within a single-mode optical cavity. We demonstrate a factor of 100 cavity cooling of 400 nm diameter silica spheres trapped in vacuum. This paves the way for ground-state cooling in a smaller, higher finesse cavity, as we show that a novel feature of the hybrid trap is that the optomechanical cooling becomes actively driven by the Paul trap, even for singly charged nanospheres.

Figure 1

(a) A schematic of the hybrid electro-optical trap, consisting of a Paul trap and standing-wave optical cavity potential, used to levitate and cool a charged silica nanosphere in vacuum. (b) The sphere is trapped by the Paul trap and driven across the standing wave formed by the light in the cavity, scattering light. (c) When cooled, the sphere is trapped by the optical field and confined to one fringe of the standing wave.

Figure 2

(a) The experimental setup for trapping and cooling nanospheres. A Paul trap sits in the middle of an optical cavity. (b) Optical layout for cooling. A weak beam resonant with the optical cavity is used to stabilize the cavity through Pound-Drever-Hall locking. A much stronger beam which can be detuned from the cavity resonance is used for trapping and cooling. The light which the nanosphere scatters as it moves through the optical cavity field is collected and is used to characterize its motion.

Figure 3

Free motion in the Paul trap. (a) The scattered signal $S$ (green trace), and a simulation (black trace, offset for clarity), as a nanosphere undergoes free motion. The nanosphere is driven across the optical standing wave, and the turning points of the motion are marked with arrows. (b) The power spectral density (PSD) of the nanosphere’s motion during free motion shows the Paul trap secular frequency ${\omega }_S=2\pi \times f_S$ and micromotion ${\omega }_d=2\pi \times f_d$. The measurement of $f_S$ allows readout of the charge, in this case just a single charge $q/e=1$.

Figure 4

The effect of detuning $\mathrm{\Delta }$ from the cavity resonance on the nanospheres motion. Scattered light signal $S$ from the nanosphere at a pressure of $7.4\times {10}^{-3}\mathrm{mbar}$ with $\mathrm{\Delta }\simeq \pm \kappa$, where $\kappa$ is the cavity linewidth. When the light is red detuned from the cavity resonance (upper red trace, signal offset for clarity), there are periods of free motion in the Paul trap and regions where the nanosphere is captured in the optical potential (marked with arrows), with a cooling rate depending upon the location within the optical standing wave. The nanosphere is not captured by the optical potential when the light is blue detuned from the cavity resonance (lower blue trace).

Figure 5

Capture and optomechanical cooling of a nanosphere. (a) The scattered signal $S$ from the captured nanosphere, at a pressure of $4.6\times {10}^{-4}\mathrm{mbar}$, and a simulation of the process (offset for clarity). The mechanical motion is cooled, and the motion due to the Paul trap is still present (marked by arrows). (b) The PSD of the nanosphere’s motion while captured by the optical field. At a low pressure of $4.6\times {10}^{-4}\mathrm{mbar}$ (orange line) the split mechanical frequency $f_M\pm f_d$ is visible, but the doubled frequency $2f_M$ is heavily suppressed, indicating cooling. At a higher pressure of $7.4\times {10}^{-3}\mathrm{mbar}$ (gray line), mechanical damping from the background gas dominates and the nanoparticle is not cooled, as shown by the unsuppressed peak at $2f_M$.