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Quantum Computing, Communication, and Simulation

Monolithic ion trap integration of superconducting nanowire single-photon detectors and photonics for trapped-ion qubit state readout

Benedikt Hampel, D. H. Slichter, Dietrich G. Leibfried, Colin Bruzewicz, John Chiaverini, Ashton Hattori, Dave Kharas, Thomas Mahony, Robert McConnell, Richard P. Mirin, Sae Woo Nam, Varun B. Verma

Published Mar 5, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.3080210

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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 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.
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.
Although trapped ion technology is well-suited for quantum information science, scalability of the system remains one of the main challenges. One of the challenges associated with scaling the ion trap quantum computer is the ability to individually manipulate the increasing number of qubits. Using micro-mirrors fabricated with micro-electromechanical systems (MEMS) technology, laser beams are focused on individual ions in a linear chain and steer the focal point in two dimensions. Multiple single qubit gates are demonstrated on trapped 171Yb+ qubits and the gate performance is characterized using quantum state tomography. The system features negligible crosstalk to neighboring ions (< 3e-4), and switching speeds comparable to typical single qubit gate times (< 2 us). In a separate experiment, photons scattered from the 171Yb+ ion are coupled into an optical fiber with 63% efficiency using a high numerical aperture lens (0.6 NA). The coupled photons are directed to superconducting nanowire single photon detectors (SNSPD), which provide a higher detector efficiency (69%) compared to traditional photomultiplier tubes (35%). The total system photon collection efficiency is increased from 2.2% to 3.4%, which allows for fast state detection of the qubit. For a detection beam intensity of 11 mW/cm2, the average detection time is 23.7 us with 99.885(7)% detection fidelity. The technologies demonstrated in this thesis can be integrated to form a single quantum register with all of the necessary resources to perform local gates as well as high fidelity readout and provide a photon link to other systems.
Trapped-ion qubits have been advantageous in quantum information processing due to their long coherence times and high-fidelity state preparation, logic gate operation, and readout. In recent years, we have fabricated chips with electrodes – for generating the electric field for trapping ions – and photonic waveguides and grating out-couplers – for delivering light to the ions. The integrated photonics can potentially lower the laser power requirement and lead to faster gates; however, there are challenges that need to be overcome for the platform to be beneficial. Here, we demonstrate single and two qubit gates with Sr+ trapped ion qubits, driven by delivering light to the ions both via free space and integrated photonics. We show that robust quantum logic is within reach using the integrated photonic platform, and the approaches we are taking in mitigating current challenges.