Researchers at Fraunhofer ILT have devised a complex laser-optical system capable of controlling 2,000 trapped strontium atoms with high precision, a key step toward building a functional Rydberg quantum computer. The setup utilizes an array of 20 rows containing 100 individually controllable laser foci, enabling the precise positioning of Rydberg atoms 3.5 µm apart. To achieve this, the team employed a segmented mirror with steps as small as a few hundred µm, reducing the spacing of 2,000 laser spots to less than 200 µm, although a further 50-fold reduction is still needed. This alignment was accomplished using a hexapod system with six actuators, achieving accuracy of less than 100 nanometers in positioning the atoms within the vacuum chamber, the processing unit of the quantum computer.
Rydberg Atom Control via 2,000 Optical Tweezers
Achieving control over a substantial number of qubits remains a central challenge in the development of practical quantum computers, and researchers have now demonstrated the ability to precisely trap and position 2,000 Rydberg atoms using an intricate optical system. The complexity of the system stems from the need to split just four initial laser beams into the 2,000 individually addressable beams necessary for trapping the atoms. This initial reduction to under 200 µm proved insufficient; a further 50-fold reduction was required to achieve optimal atom trapping. “We achieved this by directing the intermediate image via a periscope mirror onto a second plane, where a two-stage telecentric relay unit reduces it and projects it into the vacuum chamber,” explains Dr. Martin Traub, group leader of Optical Design and Diode Lasers at Fraunhofer ILT.
The vacuum chamber serves as the processing unit of the Rydberg quantum computer, where laser excitation brings adjacent atoms into a state allowing for controlled interactions and two-qubit logic gates, the fundamental building blocks of quantum computation. This level of precision is essential for positioning strontium atoms within the vacuum chamber and ensuring the qubits function correctly. The team successfully split four incoming laser beams, totaling 20 W of power, into 2,000 individually controllable beams of equal power using beam-splitter cubes and acousto-optic deflectors, a process described as splitting the four incoming, collimated laser beams, with a total power of 20 W, into 2,000 individually controllable beams of equal power.
We were only able to design and successfully implement the system thanks to the extensive expertise that Fraunhofer ILT has built up over its 40-year history.
Cascading Laser Optics for Beam Splitting
The pursuit of scalable quantum computing has increasingly focused on Rydberg atom systems, demanding precise control over individual qubits. Current approaches rely on trapping and manipulating these atoms with laser light, a process now refined by a system developed at Fraunhofer ILT in Aachen, Germany, capable of independently addressing 2,000 Rydberg atoms within a tightly constrained space. This is not simply about generating many laser beams; it’s about sculpting those beams with extreme accuracy to create the conditions for quantum interactions. The core of this advancement lies in a cascading optical system designed to split four initial laser beams into 2,000 individually controllable foci, each positioned 3.5 µm apart. This intricate process begins with beam-splitter cubes, diverting 20 percent of the light at a 90-degree angle with each pass, repeated five times per beam to generate 20 parallel beams.
These then enter acousto-optic deflectors (AODs), which utilize sound waves to modulate refractive index and create controllable optical gratings. “The sound waves cause a periodic modulation of the refractive index in the crystal,” explains Dr. This initial stage reduces the beam spacing, but a significant challenge remained. The team then employed a custom-made, segmented mirror with progressively smaller steps, ultimately reaching dimensions of just a few hundred micrometers. This cascade further reduced the spacing to under 200 µm, yet a 50-fold reduction was still necessary to achieve the optimal 3.5 µm separation for trapping strontium atoms. Maintaining this level of precision required a hexapod system, allowing for free adjustment of the mirrors in three spatial dimensions and angles, ensuring alignment within less than 100 nanometers. The entire system, designed to fit within a one-square-meter footprint and incorporating over 150 optical components, demonstrates a sophisticated integration of laser technology and optical design, critical for scaling quantum computing architectures.
At this wavelength, both states of the qubit and the Rydberg state are held equally strongly in the optical tweezers, which makes the system particularly robust.
Fine-Structure Qubit at 592 nm Enables Robustness
Researchers at the University of Stuttgart are pioneering a novel approach to quantum computing centered around a patented fine-structure qubit operating at a wavelength of 592 nm. This specific wavelength, according to the team led by Dr. Florian Meinert and Prof. Tilman Pfau, ensures both qubit states and the Rydberg state are equally held within optical tweezers, significantly enhancing system robustness. The core innovation lies in leveraging the unique properties of Rydberg atoms, atoms with an outer electron excited to a high energy level, for quantum operations. These excited atoms, exceeding one micrometer in size, exhibit heightened sensitivity to electric fields, a characteristic the scientists are harnessing for precise electromagnetic control. Central to scaling this quantum computer is an intricate optical system developed by the Fraunhofer Institute for Laser Technology ILT in Aachen.
The challenge, as Dr. This was achieved through a cascade of beam-splitter cubes and acousto-optic deflectors, ultimately creating 100 sub-beams from each initial laser. The resulting beams are then focused onto a custom-made, segmented mirror featuring progressively smaller steps, reducing the spacing between laser spots to under 200 µm. However, even this reduction proved insufficient; a further 50-fold decrease was required. Precise alignment of the mirrors, facilitated by a hexapod system with six actuators, was paramount. The team’s expertise proved crucial to the project’s success.
The distances between the laser focal points are precisely defined to within < 100 nm to ensure fast, reliably switchable interactions between the excited atoms.
Fraunhofer Institute for Laser Technology ILT
The system, developed by Fraunhofer ILT in collaboration with the University of Stuttgart, relies on a complex array of laser beams focused onto individual atoms, each held in place by optical tweezers. This arrangement allows for the creation of a highly controlled environment for quantum operations, where interactions between atoms can be manipulated to perform calculations. Central to this achievement is a segmented mirror designed to drastically reduce the spacing of the 2,000 laser spots. Initially, the beams were spread across a larger area, but the team employed a cascade of beam splitters and lenses to compress the array. “The final steps measure only a few hundred µm,” explains Dr. 3.5 µm spacing required for effective atom interaction. Achieving this level of precision demanded exceptional control over the alignment of the optical components. “Even minimal deviations in the alignment of the optical components would result in incorrect spacing within the array,” notes Traub, emphasizing the sensitivity of the system. The ability to precisely position these atoms is paramount, as misaligned qubits would compromise the computer’s ability to perform logical functions.
With every logical qubit, the potential performance of quantum computers grows exponentially.
