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SPIE Quantum Information Science, Sensing, and Computation

Integrated and fiber-coupled imaging system for the addressing of single ions

Marcus Babin, Michael Reibe, Steffen Trautmann, Simone Fabian, Andre Steinbach, Erik Beckert

Published May 30, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.3053335

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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.
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.
Ion-trapped-based quantum computers offer long coherence times of over 2 seconds with high fidelity and hence constitute a promising architecture for realizing quantum computer systems beyond the NISQ classification. Utilizing a fiber-coupled laser source at a wavelength of 396 nm our setup is able to manipulate any combination of 40Ca+ ions inside a coherent chain of 10 ions simultaneously, while also being able to collect the readout of each individual ion. Our setup achieves an ion-sided mode field diameter of less than 2 μm and a pointing accuracy of 250 nm for one chain of ions. A micro-lens integrated waveguide carrier chip with linear optics is used to create 10 separate laser beams at the actual ion dimensions. The modularity of the systems allows the system to be arranged onto additional linked ion trap chains, pathing the way for a scalable quantum computer.
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.
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.
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.
Fraunhofer IOF presents a laser-optical setup that enables the parallel manipulation of ions in an ion-trap for quantum computer. The unit creates focus spots less than 2 microns wide that allow for addressing ions in a chain, having a variable pitch around 5 micron. The implemented and individual movement of each focus creates versatile qubit manipulation on the single level, is demonstrated for 10 ions in a row and can be scaled up to ca. 50 ions easily. Also a roadmap towards compact designs by micro-optics based system integration is derived.