Trapped Electrons and Ions as Particle Detectors

Electrons and ions trapped with electromagnetic fields have long served as important high-precision metrological instruments, and more recently have also been proposed as a platform for quantum information processing. Here we point out that these systems can also be used as highly sensitive detectors of passing charged particles, due to the combination of their extreme charge-to-mass ratio and low-noise quantum readout and control. In particular, these systems can be used to detect energy depositions many orders of magnitude below typical ionization scales. As illustrations, we suggest some applications in particle physics. We outline a nondestructive time-of-flight measurement capable of sub-eV energy resolution for slowly moving, collimated particles. We also show that current devices can be used to provide competitive sensitivity to models where ambient dark matter particles carry small electric millicharges $\ll e$. Our calculations may also be useful in the characterization of noise in quantum computers coming from backgrounds of charged particles.

Figure 1

Kinematics of a scattering event in a Paul trap. The charged particle $\chi$ impinges on the trapped electron with impact parameter $b$ and velocity $v$.

Figure 2

Detectable levels of flux of ambient charged particles (here taken to be electrons), as a function of the ambient particle’s kinetic energy. Each curve represents a trap with an electron or ion (here we use beryllium [41] to illustrate) with various frequencies. A background of electrons with the flux shown will produce impulses above threshold (1) at a rate of $1\mathrm{event}/\mathrm{day}$.

Figure 3

Projected millicharged dark matter sensitivity with various trap frequencies and integration times. We consider detection along only a single trap axis (4). Top plot: high-charge DM thermalized at room temperature [300 K] (or at liquid He temperature [4 K]). Bottom: low-charge DM virialized to the galaxy. Currently excluded parameter space is shaded: bounds in light blue come from stellar astrophysics [57, 58], dark brown from cosmology (big bang nucleosynthesis and cosmic microwave background $N_{\mathrm{eff}}$ [57, 59]), dark blue from direct detection experiments [45], and light brown from a variety of collider experiments [60, 61, 62, 63].