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Quantum Technology: Driving Commercialisation of an Enabling Science

UV laser-based addressing unit for trapped-ion quantum computers

Marcus Babin, Erik Beckert, Peter Schreiber, Simone Fabian, Felix Kraze

Published Jan 11, 2023• 1 citation

Abstract

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.

DOI: 10.1117/12.2647249

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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.

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.

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.

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.

Ion traps are one approach to represent Qubits in future quantum computer architectures. Nevertheless, progress has to be made to scale ion trap setups for multiple ions and qubit scaling. Next to the ion trap architecture itself the manipulation of the ion’s state and their readout by means of a laser and fluorescence detection optics are key architectural building blocks. A setup dedicated to the optical addressing of each individual ion in the trap will be described, consisting of micro-optics for diffraction limited focusing of the laser beam onto the ion and a miniaturized actuation mechanism for the micron scale movement of the focus with the ion, where the electrical feedback signal stems from the detection unit. The setup’s design with an initial layout for up to 10 ions in parallel can further be scaled towards ca. 50 ions in parallel at least. Publisher’s Note: The manuscript PDF, originally published on 5 March 2021, was withdrawn on 12 March 2021 per author request.

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.