Controlling the net charge on a nanoparticle optically levitated in vacuum

Optically levitated nanoparticles in vacuum are a promising model system to test physics beyond our current understanding of quantum mechanics. Such experimental tests require extreme control over the dephasing of the levitated particle's motion. If the nanoparticle carries a finite net charge, it experiences a random Coulomb force due to fluctuating electric fields. This dephasing mechanism can be fully excluded by discharging the levitated particle. Here, we present a simple and reliable technique to control the charge on an optically levitated nanoparticle in vacuum. Our method is based on the generation of charges in an electric discharge and does not require additional optics or mechanics close to the optical trap.

Figure 1

(a) Experimental setup. A silica nanoparticle is trapped in a focused laser beam (1064 nm). A second laser (780 nm) is used to measure the position of the particle. The metal housings of the microscope objective and of the lens collecting the scattered light form a capacitor to generate a low-frequency electric field at the particle position. The signal of the function generator driving the capacitor serves as a reference to a lock-in amplifier (LI) used to demodulate the photodetector (PD) signal. The LI output is recorded by a data-acquisition card (DAQ). A wire reaching into the vacuum chamber is connected to a high-voltage source to ionize residual gas molecules and provide charges to the particle. (b) Power spectral density of particle position along the optical axis at a pressure of 10 mbar.

Figure 2

(a) False color plot of the simulated electric field strength $E_z$ generated by a potential of 10 V applied to the metallic collection lens holder with the metallic objective housing grounded. The refractive index of the optics is $n=1.4$. (b) Cross section along dashed line in (a), showing field strength $E_z$ along the optical axis. The focal plane is at $z=0$.

Figure 3

(a) Power spectral density $S_z$ of the motion along the optical axis of a charge-carrying particle at a pressure of 1.9 mbar in the presence of a drive tone at $f_d={\omega }_d/\left(2\pi \right)$ applied to the capacitor. The solid line is a Lorentzian function fit to the data. (b) Quadrature component of particle oscillation in response to a driving voltage $U_0=10\mathrm{V}$, demodulated in a bandwidth of 7 Hz. The high-voltage (HV) discharge is turned on at $t=0$. The oscillation amplitude changes in discrete steps while the high voltage is on. (c) Preparation of charge state. The high voltage is turned off at $t=0$, while the particle carries a net charge of $1e$. The charge stays constant over the remainder of the measurement.