Trap-Integrated Superconducting Nanowire Single-Photon Detectors with Improved RF Tolerance for Trapped-Ion Qubit State Readout

Abstract

State readout of trapped-ion qubits with trap-integrated detectors can address important challenges for scalable quantum computing, but the strong rf electric fields used for trapping can impact detector performance. Here, we report on NbTiN superconducting nanowire single-photon detectors (SNSPDs) employing grounded aluminum mirrors as electrical shielding that are integrated into linear surface-electrode rf ion traps. The shielded SNSPDs can be operated at applied rf trapping potentials of up to 54 V_peak at 70 MHz and temperatures of up to 6 K, with a maximum system detection efficiency of 68 %. This performance should be sufficient to enable parallel high-fidelity state readout of a wide range of trapped ion species in typical cryogenic apparatus.

FIG. 1:

Photomicrograph of the linear ion trap with integrated SNSPD. The rf trapping potentials are applied to the labeled and false-colored rf electrodes. All other non-SNSPD electrodes are used for dc trapping potentials. The magnified inset (b) shows a trap design where the SNSPD is on top of a small aluminum mirror, while a larger aluminum mirror spanning the region beneath the leads of the SNSPD can be seen in a different trap design shown in panel (a). Both mirrors appear light blue in the gaps between gold electrodes and are surrounded by a light blue dotted line, while the black gaps show the substrate. The green dotted line in (b) borders the active area of the SNSPD. The white dashed lines indicate the plane of the 2D simulations in Fig. 2. The bias current I_b is applied to the leads of the SNSPD. The electrodes labeled “gnd” are electrically connected to the mirror and can be connected to ground.

FIG. 2:

Numerically simulated electric field amplitude from an rf potential of 100 V_peak at 70 MHz applied to the rf electrodes of the ion trap, viewed for a small cross-section of the SNSPD and for different shielding configurations. The thicknesses of the various material layers are shown on the figure. (a) SNSPD nanowire embedded in SiO₂ without shielding. (b) Grounded aluminum mirror under the nanowire. (c) Grounded gold mesh on top of the stack in addition to the grounded aluminum mirror. (d) Grounded ITO layer instead of the gold mesh.

FIG. 3:

Bright count rate (BCR) and dark count rate (DCR) in counts per second (cps) as a function of bias current for different rf voltage peak amplitudes on the ion trap at a temperature of 4.5 K for devices with a large mirror that is (a) not connected to ground and (b) connected to ground. Comparison to device with (c) a smaller grounded mirror at 4.5 K and (d) the same device as in (b) operated at 6 K. The switching currents for each value of applied rf are indicated with correspondingly colored vertical markers on the horizontal axes. BCR values shown are corrected for DCR. DCR is plotted on logarithmic vertical axes.

FIG. 4:

Bright count rate (BCR) and dark count rate (DCR) in counts per second (cps) as a function of bias current. (a) Curve fits for an induced rf current model to the data in Fig. 3(a). (b) BCR and DCR vs. I_b without rf drive at different temperatures for a trap-integrated SNSPD. The switching currents are indicated with markers on the horizontal axis.