Suppression of Heating Rates in Cryogenic Surface-Electrode Ion Traps

Dense arrays of trapped ions provide one way of scaling up ion trap quantum information processing. However, miniaturization of ion traps is currently limited by sharply increasing motional state decoherence at sub-$100\mu \mathrm{m}$ ion-electrode distances. We characterize heating rates in cryogenically cooled surface-electrode traps, with characteristic sizes in the 75 to $150\mu \mathrm{m}$ range. Upon cooling to 6 K, the measured rates are suppressed by 7 orders of magnitude, 2 orders of magnitude below previously published data of similarly sized traps operated at room temperature. The observed noise depends strongly on the fabrication process, which suggests further improvements are possible.

Figure 1

(a) Computed pseudopotential in the $x\mathrm{\text{−}}z$ plane at the trap center for $V_1=V_2=16\mathrm{V}$, $V_3=-16\mathrm{V}$, $V_4=-13\mathrm{V}$, ${\mathrm{V}}_{\mathrm{rf}}=240{\mathrm{V}}_{\mathrm{amp}}$. Isosurfaces are separated by 50 mV. The trap electrodes are outlined above $x$ axis. (b) Microscope image of the electrode geometry for the $150\mu \mathrm{m}$ trap. The dc electrodes are numbered from 1 to 4. The notch in the central electrode tilts the principal axes of the trap by 6° to 20°, depending on dc voltages, and defines a point where the rf field is zero. In this trap, the width of the central electrode at the notch is $136\mu \mathrm{m}$, with electrode spacing of $21\mu \mathrm{m}$. The potential minimum is $150\mu \mathrm{m}$ above the surface. (c) SEM image of one of the trap electrodes before annealing and (d) after annealing at $760°\mathrm{C}$ for 1 h.

Figure 2

Sample data taken in the $150\mu \mathrm{m}$ trap at 6 K. (top) Sideband spectra after cooling (solid circles) and after a 40 ms delay (crosses). Note that the scale for red (left) sideband is 10 times smaller than the scale for blue (right) sideband. (bottom) Average number of quanta versus delay time, with a linear fit. The slope of the fit line is $2.1\pm 0.2\mathrm{\text{quanta}}/\mathrm{s}$.

Figure 3

Measured noise density in 7 different traps, with secular frequencies in 0.85–1.25 MHz range. Each data point is normalized to 1 MHz, assuming the spectrum scales as ${\omega }^{-1}$. Circles correspond to traps annealed at $760°\mathrm{C}$, and squares correspond to traps annealed at $720°\mathrm{C}$, while the triangle corresponds to a trap fabricated on sapphire.

Figure 4

Measured field noise density in a $75\mu \mathrm{m}$ trap at different frequencies of the ion motion. The dependence is consistent with ${\omega }^{-1}$ scaling.