Researchers at the Max-Planck-Institut für Quantenoptik and Ludwig-Maximilians-Universität München have demonstrated vacuum-limited trapping of single strontium atoms in an optical tweezer array for over two hours. This achievement overcomes a longstanding trade-off between atom storage time and optical access in neutral-atom quantum processors, a technology that can scale toward systems exceeding ten thousand qubits. The work preserves full optical access, enabling complex manipulation of the atoms without the need for elaborate cryogenic enclosures. This architecture offers a viable path toward building larger, more powerful quantum computers using neutral atoms.

Cryogenic System Enables 2-Hour Strontium Atom Trapping

A cryogenic neutral-atom platform with full optical access and 2-hour trap lifetime allows for the realization of a new class of quantum computing algorithms and quantum simulation experiments. Current neutral-atom platforms generally have to find a compromise between the optical accessibility and the storage time of atoms in optical potentials, limiting the available qubit numbers.

The cooling process for this new platform reaches 4 Kelvin approximately three hours after activation of the helium compressor circuits, as demonstrated in accompanying data. This cryogenic environment is important for maintaining the extended trapping times achieved with strontium atoms, a key advancement for scaling neutral-atom quantum processors. Unlike previous systems, this apparatus demonstrates minimal degradation of vacuum conditions even with increased optical power delivered to the array, addressing a known scaling bottleneck in cryogenic tweezer architectures.

The platform’s ability to sustain trapping lifetimes without compromising vacuum levels stems from a design that directly investigates the relationship between optical power and atom retention. Researchers lowered tweezer power between imaging and cooling pulses to determine its impact on trapped-atom lifetime, revealing that the system can accommodate the increased power demands of larger arrays.

Measurements show the trapped-atom lifetime decreases predictably with increasing oven temperature, indicating background gas pressure is a limiting factor, but remains unaffected by MOT loading time, confirming strontium flux is not the primary cause of atom loss. This finding suggests that the primary limitation lies in maintaining a sufficiently low background gas pressure within the trapping region.

Optical Access Preserved in Cryogenic Neutral-Atom Platform

The apparatus achieved vacuum-limited trapping lifetimes exceeding 7000 seconds, over two hours, for single strontium-88 atoms held in an optical tweezer array, a duration previously difficult to attain without sacrificing optical control. This extended trapping time relies on a specific selection of materials for the vacuum apparatus and a design allowing for thorough baking of the ultra-high vacuum chamber, critical steps in minimizing background gas interference.

A dedicated 4 Kelvin cold tip positioned within the vacuum chamber actively removes residual hydrogen gas emanating from chamber walls, further enhancing the stability of the trapped atoms. The lifetime remained unaffected by variations in the magneto-optical trap (MOT) loading time, indicating that the flux of strontium atoms is not the primary factor limiting the duration of trapping.

The ability to maintain both long trapping times and full optical access is enabled by the apparatus’s ability to accommodate the increased optical power demands of large-scale tweezer arrays. This is a key advantage, as manipulating a greater number of qubits requires more light, which can introduce unwanted heating and instability in conventional systems.

Vacuum Pressure Mitigation for Extended Atom Lifetimes

The apparatus achieves a vacuum pressure of 1×10^-11 mbar even without cryogenic cooling, establishing a strong foundation for extended atom trapping times. This vacuum level already supports trapping lifetimes of several hundred seconds, suggesting that further improvements through cryogenic techniques could yield substantial gains in atom retention. Activating the cryostat, however, demonstrably reduces the vacuum pressure further, even with a 30 centimeter separation between the cold tip and the atom trapping region.

This reduction in pressure isn’t simply a localized effect; measurements indicate the vacuum improves deep within the molecular-flow regime of the ultra-high-vacuum chamber. This finding challenges the assumption that line-of-sight exposure to the strontium oven, a primary source of outgassing, is an insurmountable limitation to achieving exceptionally long atomic lifetimes. The system’s ability to maintain a low pressure despite this exposure highlights the efficiency of the cryopumping mechanism.

The cold tip functions as an efficient cryopump for hydrogen, the dominant residual gas limiting the system’s performance at room temperature. Deliberately heating the cold tip to 30 Kelvin induces a predictable pressure spike, consistent with the release of adsorbed hydrogen, and subsequent cooling restores the vacuum conditions.

This cycle confirms the cold tip’s role in actively removing hydrogen from the chamber, effectively extending the trapping time of strontium atoms. Long-term measurements reveal a characteristic 1/e decay time of approximately 10 days for the lifetime, indicating a gradual increase in background pressure over time due to hydrogen adsorption.

This distinction is important, as it isolates the vacuum environment as the key factor influencing atom retention. A chamber modification eliminating direct line-of-sight exposure between the oven and the glass cell could further improve performance at higher oven temperatures. The current design, however, already allows for operation with increased optical power requirements associated with large-scale tweezer arrays without compromising vacuum conditions or atom retention.

⁸⁸Sr Optical Tweezer Array Architecture & Components

This performance circumvents a common constraint in neutral-atom quantum computing, where extended storage times often necessitate reduced optical control, limiting the complexity of quantum operations. The architecture uses titanium, stainless steel and copper surfaces, colored blue, green and orange respectively, within a fused silica glass cell to house and manipulate the atoms. Optical access is preserved through the design of the cryogenic system, which positions a 4 Kelvin cold tip approximately 30 centimeters from the tweezer array without obstructing light paths.

This large distance ensures the cold tip remains unaffected by direct or stray exposure to the tweezer light, a critical feature for high-fidelity manipulation. The system employs an 8-watt laser at 813 nm, coupled with a Hamamatsu X15213-02R LCOS-SLM, to form a 16×16 square optical tweezer array with a spacing of 5.4 micrometers between neighboring tweezers.

Initial atom loading relies on a two-stage cooling process, beginning with an 800 millisecond blue magneto-optical trap, followed by a 150 millisecond SWAP MOT stage, and concluding with a 20 millisecond single-frequency red MOT for further compression. The experimental sequence includes a parity-projection pulse designed to deterministically remove atom pairs via light-assisted collisions, leaving each tweezer occupied by either zero or one atom.

Following this initial preparation, atoms are held in the optical tweezers for a variable duration, termed the “hold time,” before a second fluorescence image is acquired to measure the number of remaining trapped atoms. These measurements allow for the extraction of survival probability as a function of hold time, revealing the extended trapping lifetimes. The observed increase in lifetime with tweezer power suggests atoms overcome a higher potential barrier to escape the trap and benefit from lower relative intensity noise at higher powers.

The team conducted two bakeout procedures to minimize outgassing, first a high-temperature bakeout of the titanium chamber without the glass cell at 320°C for three weeks, and then a low-temperature bakeout of the full chamber at 150°C for two weeks. These efforts address a potential scaling bottleneck in cryogenic tweezer architectures, where increasing array sizes often demand higher total optical power, potentially compromising vacuum conditions.

Scaling Neutral-Atom Quantum Processors Beyond 10,000 Qubits

This extended trapping time represents an advance in cryogenic neutral-atom platform technology, directly addressing a key obstacle to scaling quantum processors beyond 10,000 qubits. The ability to hold atoms for such prolonged periods allows for more complex quantum operations and algorithms to be executed without significant signal degradation from atom loss. Unlike many existing systems that trade one for the other, this architecture preserves full optical access, meaning researchers can direct light to individual atoms from any direction without obstruction.

The team’s approach to maintaining this access while simultaneously extending trapping time relies on a carefully optimized cryogenic environment and vacuum system. The system’s performance is particularly notable given the challenges of balancing optical power with cryogenic cooling. Delivering light to manipulate the atoms also introduces heat, potentially warming the surrounding environment and reducing trapping times.

The apparatus achieves this balance through a combination of high vacuum and a robust cryostat design, minimizing thermal load and maximizing atom confinement. The experimental setup includes a two-stage bakeout procedure to minimize outgassing from the vacuum chamber materials. Maintaining ultra-high vacuum is critical for extending atom lifetimes, as collisions with background gas molecules are a primary source of atom loss. The team’s meticulous attention to vacuum quality contributes significantly to the observed two-hour trapping duration.

The researchers directly investigated the relationship between trapped-atom lifetime and total optical power by lowering the power delivered to the atoms. Measurements revealed that lifetimes are limited by vacuum pressure, not by optical power dissipation, indicating the system’s potential for further scaling.

This finding suggests that improvements in vacuum technology could lead to even longer trapping times and larger, more stable quantum processors. The architecture’s adaptability extends beyond strontium. The team indicates it can be straightforwardly ported to other atomic species, broadening its applicability to diverse quantum computing approaches. This combination of features positions the system as a viable path toward realizing large-scale, fault-tolerant quantum computation using neutral atoms.

👉 More information
🗞 Cryogenic neutral-atom platform with full optical access and 2-hour trap lifetime
✍️ Akhil Kumar et al.
🧠 DOI: http://link.aps.org/doi/10.1103/f674-hr1d

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