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Quantum Computing, Communication, and Simulation

A scalable PIC-based addressing unit for segmented quadrupole ion traps

Erik Beckert, Marcus Babin, Michael Reibe, S. Trautmann, P. Dannberg, T. Gemeinhardt, J.-Ch. Mueller, R. Strohmaier, Ferdinand Schmidt‐Kaler

Published Mar 5, 2026• 0 citations
Publisher

Abstract

Scalability is crucial for quantum computing to achieve high computing performance and ultimately true quantum advantage. Such a quantum computing platform requires elements to be replicated reliably within an extendable system framework. Photonic integrated circuits (PICs) allow the conversion of inputs in an accurate process due to the employment of lithographic production techniques. We present an addressing unit for ion-based quantum computing employing PICs that are arranged for multiple addressing zones inside a segmented ion-trap chip. The addressing unit is designed for individual and parallel addressing of single ions inside a chain of ten ions held by one trap segment. At PIC level, each addressing beam is realized with one waveguide channel and its mode field adapted to be optimal for imaging onto a trap segment. An additional imaging system in the same formfactor as the PIC, couples optically in free space from the waveguide channel to the individual ions. It is based on wafer-level microlenses combined in one stack and forms one unit with the PIC. Such a hardware-based capability eventually allows precise gate operations that are necessary for a universal quantum computer.

DOI
10.1117/12.3079334

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Scalability is crucial for quantum computing to achieve high computing performance and ultimately true quantum advantage. Such a quantum computing platform requires elements to be replicated reliably within an extendable system framework. Photonic integrated circuits (PICs) allow the conversion of inputs in an accurate process due to the employment of lithographic production techniques. We present an addressing unit for ion-based quantum computing employing PICs that are arranged for multiple addressing zones inside a segmented ion-trap chip. The addressing unit is designed for individual and parallel addressing of single ions inside a chain of ten ions held by one trap segment. At PIC level, each addressing beam is realized with one waveguide channel and its mode field adapted to be optimal for imaging onto a trap segment. An additional imaging system in the same formfactor as the PIC, couples optically in free space from the waveguide channel to the individual ions. It is based on wafer-level microlenses combined in one stack and forms one unit with the PIC. Such a hardware-based capability eventually allows precise gate operations that are necessary for a universal quantum computer.
In trapped-ion-based quantum computing, logic gates are performed by laser addressing ions with a specific wavelength to manipulate its quantum state. We present a trap-integrated addressing unit that is ultra-high vacuum compatible and quasi-monolithically bonded onto the trap substrate. Our setup is based on a quadrupole trap for containing a chain of ten calcium ion. A fiber-coupled aluminum oxide waveguide chip emits ten addressing beams at 405 nm, that are imaged by a micro-optical lens stack onto the ion trap axis. The setup allows to address each ion individually and in parallel. Imaging transmission losses are below 2 dB with an ion-addressing spot diameter of 1 µm at 1/e^2 intensity and a Strehl ratio above 0.9 for all ten beams. With an inner minimum ion spacing of 5 µm and a total chain length of approximately 60 µm, the addressing beam crosstalk ratio is less than one percent.
Trapped ions provide a high-fidelity platform for quantum information processing, yet delivery of multiple, distinct wavelengths across large networks of interaction zones remains a bottleneck. Conventional free-space light delivery lacks scalability, while on-chip grating couplers suffer from narrow operational bandwidth that increases circuit footprint and optical interfacing complexity. Here we show a broadband photonic integrated circuit capable of addressing individual ions. The circuit combines a planar waveguide lens with a micromirror fabricated using two-photon polymerization at wafer scale. This implementation can address three individual ions from λλλ = 405 - 880 nm with -27 dB average intensity crosstalk at 5 μm5\,μ\mathrm{m}5μm pitch. We trap 40Ca+^{40}\mathrm{Ca}^{+}40Ca+ and 138Ba+^{138}\mathrm{Ba}^{+}138Ba+ ions above such devices, characterize optical crosstalk with barium ions, and demonstrate individual repumping of calcium ions. This monolithic photonic architecture brings broadband addressing in an on-chip modality to trapped-ion technology. More generally, integrating additive manufacturing into quantum devices is poised to unlock expanded design space for implementing novel quantum architectures.
With the emergence of different quantum computers approaches, ion-trapped based quantum processors offer a promising concept with coherence times of over 2 s and gate times of 10 to 80 µs with fidelities of over 99.9%. In such systems, ions are being held inside a linear quadrupole trap in defined position zones. The laser induced qubit rotation, initialization and readout operations require an ion individual addressing unit. Our hardware is able to create individual laser foci for 10 captured ions with beam waists of 1.1 µm. Qubit manipulations are realized with a laser wavelength of 396 nm employing 40Ca+ ions.
Laser-based addressing units are core components for trapped-ion quantum computers, in which ions are held in defined, separate, micron-scale position fields and are brought into different qubit states via an addressing beam with an ion-specific wavelength. The spin of the valence electrons of the atomic ions represents the qubits states. An addressing action causes a stimulated Raman transition, which in turn causes a change in electron spin and thus a qubit rotation. Our unit is designed for 10 40Ca+ ions kept in a linear microchip ion trap. The hardware platform utilizes waveguides with integrated collimating micro lenses, followed by a cascaded 4f optical setup, consisting of a focusing and a beam expander unit. An aspherical lens realizes a lateral shift tolerant focus inside the trap. The addressing unit achieves minimal foci beam waists of 1.1 μm, with a minimal ion separation of 5.31 μm. The operating addressing wavelength ranges from 395 nm to 405 nm. Fault relevant ion crosstalk ratio is less than 10−4. Addressing operations can be conducted in parallel on all captured ions. The specified laser-based addressing unit provides a stable and low-error solution for qubit operations with coherence times of several seconds. Further adaptions of the waveguide structures and the implementation of micro optical lenses offer the potential of fully integrating the system into the microchip ion trap. Such a quasi-monolithic trap integration constitutes a compact, and highly scalable addressing and detection system with the ability to address as many qubit registers as the ion trap capacity allows.
Trapped-ion quantum computing requires precise optical control for individual qubit manipulation. However, conventional free-space optics face challenges in alignment stability and scalability as the number of qubits increases. Integrated photonics offers a promising alternative, providing miniaturized optical systems on a chip. Here, we propose a design for a multimode photonic circuit integrated with a surface-electrode ion trap capable of targeted and reconfigurable light delivery. Three closely positioned ions can be addressed using a focusing grating coupler that emits multimode light through electrode openings to ions trapped 80 μμμm above the chip. Simulations show that the couplers achieve diffraction-limited spot with a 4.3 μμμm beam waist along the trap axis and 2.2 μμμm perpendicular to the trap axis. Controlled interference of the TE10_{\text{10}}10​ and TE20_{\text{20}}20​ modes results in crosstalk of -20 dB to -30 dB at ion separations of 5-8 μμμm when addressing ions individually, and down to -60 dB when two of the three ions are addressed simultaneously. Additionally, the higher-order TE modes can offer a novel mechanism for driving spin-motion coupling transitions, potentially enabling alternative approaches to quantum gates and simulations. The proposed integrated platform offers a viable path for constructing large-scale trapped-ion systems, leveraging the benefits of nanophotonic design for precise and reliable ion manipulation.
Trapped-ion quantum computing requires precise optical control for individual qubit manipulation. However, conventional free-space optics face challenges in alignment stability and scalability as the number of qubits increases. Integrated photonics offers a promising alternative, providing miniaturized optical systems on a chip. Here, we propose a design for a multimode photonic circuit integrated with a surface-electrode ion trap capable of targeted and reconfigurable light delivery. Three closely positioned ions can be addressed using a focusing grating coupler that emits multimode light through electrode openings to ions trapped 68 μm above the chip. Simulations show that the couplers achieve a diffraction-limited spot with a 4.3 μm beam waist along the trap axis and 2.2 μm perpendicular to the trap axis. Controlled interference of the TE 00 and TE 10 modes results in crosstalk of –20 dB to –30 dB at ion separations of 5–8 μm when addressing ions individually, and down to –60 dB when two of the three ions are addressed simultaneously. Additionally, the higher-order TE modes can offer a novel mechanism for driving spin-motion coupling transitions, potentially enabling alternative approaches to quantum gates and simulations. The proposed integrated platform offers a viable path for constructing large-scale trapped-ion systems, leveraging the benefits of nanophotonic design for precise and reliable ion manipulation.