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Advanced Photon Counting Techniques

Trap-integrated superconducting nanowire single-photon detectors for trapped-ion qubit state readout

Benedikt Hampel, D. H. Slichter, Dietrich G. Leibfried, Richard P. Mirin, Sae Woo Nam, Varun Boehm Verma

Published Jun 7, 2024• 0 citations
Publisher

Abstract

State readout of trapped-ion qubits is usually achieved by observing qubit-state-dependent fluorescence from the ion while driving an optical cycling transition with laser light. The integration of photon detectors for fluorescence detection into the ion trap itself may benefit the development of many-qubit setups for applications like quantum computing. Superconducting nanowire single-photon detectors (SNSPDs) are promising candidates for trap-integrated detectors for high-fidelity trapped-ion qubit state readout. However, the strong oscillating electromagnetic fields that are typically used to trap and manipulate ions can affect the function of the SNSPDs significantly. In this work, we demonstrate an improved design to integrate SNSPDs into linear rf ion traps that reduces the susceptibility of the SNSPDs to applied rf trapping potentials. Our measurement results represent an improvement in rf tolerance by an order of magnitude with an increase in operation temperature from 3.5K to 6K compared to previous work.

DOI
10.1117/12.3014455

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State readout of trapped-ion qubits is usually achieved by observing qubit-state-dependent fluorescence from the ion while driving an optical cycling transition with laser light. The integration of photon detectors for fluorescence detection into the ion trap itself may benefit the development of many-qubit setups for applications like quantum computing. Superconducting nanowire single-photon detectors (SNSPDs) are promising candidates for trap-integrated detectors for high-fidelity trapped-ion qubit state readout. However, the strong oscillating electromagnetic fields that are typically used to trap and manipulate ions can affect the function of the SNSPDs significantly. In this work, we demonstrate an improved design to integrate SNSPDs into linear rf ion traps that reduces the susceptibility of the SNSPDs to applied rf trapping potentials. Our measurement results represent an improvement in rf tolerance by an order of magnitude with an increase in operation temperature from 3.5K to 6K compared to previous work.
Atomic ions can be trapped in vacuum with dc and rf electric potentials generated by surface electrode ion traps, and used as qubits in quantum computing applications. Qubit state preparation and control is usually accomplished with laser light while the qubit state readout is achieved by observing qubit-state-dependent fluorescence from the ion while driving an optical cycling transition. The monolithic integration of optical waveguides and single-photon detectors into the ion trap itself provides both light delivery and fluorescence photon counting capability, offering benefits for scaling in trapped ion quantum computing by eliminating the need for free-space optical access to the trap. One promising candidate for integrated high-fidelity trapped-ion qubit state readout are superconducting nanowire single-photon detectors (SNSPDs). However, the joint integration of photonics and superconducting detectors into ion trap chips presents challenges for their design, fabrication, and operation. In this work, we present our progress on the integration of SNSPDs into linear ion traps with integrated photonics designed for Ca+ qubits.
State readout of trapped-ion qubits with trap-integrated detectors can address important challenges for scalable quantum computing, but the strong radio frequency (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 Vpeak 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 a typical cryogenic apparatus.
We report high-fidelity state readout of a trapped ion qubit using a trap-integrated photon detector. We determine the hyperfine qubit state of a single ^{9}Be^{+} ion held in a surface-electrode rf ion trap by counting state-dependent ion fluorescence photons with a superconducting nanowire single-photon detector fabricated into the trap structure. The average readout fidelity is 0.9991(1), with a mean readout duration of 46 μs, and is limited by the polarization impurity of the readout laser beam and by off-resonant optical pumping. Because there are no intervening optical elements between the ion and the detector, we can use the ion fluorescence as a self-calibrated photon source to determine the detector quantum efficiency and its dependence on photon incidence angle and polarization.
We demonstrate superconducting nanowire single photon detectors with 76 ± 4% system detection efficiency at a wavelength of 315 nm and an operating temperature of 3.2 K, with a background count rate below 1 count per second at saturated detection efficiency. We propose integrating these detectors into planar surface electrode radio-frequency Paul traps for use in trapped ion quantum information processing. We operate detectors integrated into test ion trap structures at 3.8 K both with and without typical radio-frequency trapping electric fields. The trapping fields reduce system detection efficiency by 9%, but do not increase background count rates.
Advancements in integrated photonics have enabled the miniaturization and scaling of complex photonic circuits, providing stability and reliability beyond the capabilities of conventional bulk setups. A central component of quantum photonic circuits is the single-photon detector, responsible for quantum state measurement. Due to the stringent performance requirements of quantum applications, superconducting nanowire single-photon detectors (SNSPDs) have emerged as the preferred detection technology, offering superior efficiency, low noise, and high timing resolution. While tabletop SNSPDs are now mature, commercially available and contributed to significant progress in quantum computing and communication, their integration into photonic circuits is still in its infancy. Integrated SNSPDs have shown the potential to not only match but surpass tabletop detectors in several performance metrics. Nevertheless, demonstrations have largely focused on individual integrated detectors, with limited investigation into more complex integrated circuits and advanced functionalities. In this thesis, we present a novel integration platform based on photonic bound states in the continuum (BICs) to address a critical limitation of conventional SNSPD designs, where nanowires placed directly atop the waveguide core are constrained by the core geometry. The etchless nature of our BIC waveguides provides a flat surface that decouples the design of the electronic readout circuitry below from the optical circuit layer above, enabling innovative integrated-nanowire geometries and facilitating large-scale integration of SNSPDs with CMOS-compatible circuits. We established a complete fabrication and characterization workflow for integrating NbTiN-based superconducting nanowires with BIC waveguides. Our integrated detectors achieved state-of-the-art performance, including detection efficiencies exceeding 70%, intrinsic jitter below 70 ps, and ultrafast recovery times (~1.7 ns). These results pave the way toward GHz-rate quantum state measurements, essential for scalable quantum photonic processors. Leveraging the design flexibility of BIC waveguides, we introduce the concept of inline detection with SNSPDs which exploits their unique ability to precisely engineer absorption through nanowire length control. By cascading multiple partially absorbing SNSPDs, it is possible to significantly reduce the number of required optical components and miniaturize complex quantum measurement setups within an extremely compact footprint. Traditional integration geometries prevent such implementations due to significant scattering losses at nanowire terminations, which limit the feasibility of cascaded designs. On the other hand, the design flexibility provided by our BIC platform makes it ideal for development of inline detection schemes. As a proof-of-concept, we use inline detection to miniaturize two fundamental quantum optics tools: an integrated Hanbury Brown and Twiss (HBT) interferometer and a photon-number-resolving (PNR) detector. We replicated the functionalities of these setups using only two nanowires integrated along a single waveguide. These devices show extremely compact footprint, highlighting the scalability potential of our approach. We validated these devices through characterization of a single-photon source and the discrimination of collinear photon pairs. The experimental results aligned well with previous literature and theoretical predictions, confirming the reliability of our integration method. Overall, this work presents a novel approach to quantum measurements within integrated quantum photonic circuits. Inline detection represents a step forward in quantum integrated photonics, moving from individual device demonstrations toward functional quantum circuits. By introducing our BIC-based integration platform, we simplify detector design and fabrication, lowering barriers to the development of inline detection circuits and opening new opportunities in multiphoton quantum state measurements, single-photon spectroscopy, and multiphoton subtraction.