Stuttgart Team Traps Alkaline-Earth Atoms, Enhancing Quantum Computing Potential

Researchers from Universität Stuttgart have successfully trapped alkaline-earth circular Rydberg atoms in optical tweezers, a significant step towards overcoming limitations in coherence time and gate fidelities in Rydberg atom quantum simulators and computers. The team demonstrated the creation of high circular states of 88Sr, a type of strontium, with lifetimes as long as 255 ms at room temperature. This was achieved through cavity-assisted suppression of blackbody radiation. The study paves the way for quantum simulations with circular Rydberg states of divalent atoms, potentially leading to significant advancements in quantum computing and simulations.

What are Long-Lived Circular Rydberg Qubits of Alkaline-Earth Atoms in Optical Tweezers?

The article discusses the first realization of alkaline-earth circular Rydberg atoms trapped in optical tweezers. These atoms are highly promising candidates for overcoming limitations in coherence time and gate fidelities in Rydberg atom quantum simulators and computers. The Rydberg state lifetime fundamentally limits these parameters. However, circular Rydberg states can be effectively protected from decay due to their maximum angular momentum.

The researchers demonstrate the creation of very high circular states of 88Sr, a type of strontium. They measure lifetimes as long as 255 ms at room temperature, achieved via cavity-assisted suppression of blackbody radiation. They also show coherent control of a microwave qubit encoded in circular states of nearby manifolds and characterize the qubit coherence time via Ramsey and spinecho spectroscopy.

The study opens routes for quantum simulations with circular Rydberg states of divalent atoms, exploiting the emergent toolbox associated with the optically active core ion. The research was conducted by C Hölzl, A Götzelmann, E Pultinevicius, M Wirth, and F Meinert from the 5 Physikalisches Institut and Center for Integrated Quantum Science and Technology at Universität Stuttgart in Germany.

How are Rydberg Atoms Used in Quantum Simulators and Computers?

Rydberg atoms based on optical tweezer technology have recently enabled rapid advances in the development of neutral-atom quantum simulators and computers. Examples of their applications range from large-scale simulation of quantum spin models, the implementation of optimization problems, high-fidelity gate operations in quantum circuits, and even demonstrations of key steps toward quantum error correction.

The lifetime of the highly excited Rydberg levels sets a fundamental limit for achievable coherence times or gate fidelities. Circular Rydberg states have recently attracted increasing attention to overcome this constraint for both analog quantum simulators and gate-based quantum computers. These states have maximum allowed angular momentum, which inhibits optical decay to low-lying orbitals by selection rules. This opens up exciting prospects to increase the coherence time of Rydberg atom arrays by orders of magnitude in both cryogenic or room-temperature setups.

The researchers demonstrated the first tweezer-trapped circular states of alkaline-earth atoms, which provide a second optically active electron. This combination of the richer low-lying electronic structure of divalent atoms with atom arrays has given rise to powerful new tools for optical clock metrology or neutral-atom quantum computing.

What are the Unique Possibilities of Alkaline-Earth Atoms?

Alkaline-earth atoms, in contrast to alkali atoms, allow for ionic-core excitation in the absence of rapid autoionization, providing a plethora of unique possibilities. First, the ion core enables conservative trapping in standard Gaussian-beam tweezers, which brings scaling advantages in view of power requirements when compared to the bottle traps needed for alkali atoms.

Second, photon scattering at the broad and narrow core transitions can be exploited for direct laser cooling and imaging of the trapped Rydberg atoms. This makes use of central manipulation techniques developed for trapped ions. Third, combining microwave control of the Rydberg electron with narrow-line optical core spectroscopy involving the ion’s D-level enables local control and readout of the circular Rydberg qubit via the quadrupole interaction between the two electrons.

The researchers created very high circular Rydberg states of 88Sr atoms from an array of optical tweezers and demonstrated coherent control of a qubit encoded in circular states separated by two principal quantum numbers. This is driven by a two-photon microwave transition.

How is the Long Lifetime of Circular Rydberg States Achieved?

The researchers observed lifetimes of the circular Rydberg states as long as 255 ms, which is about an order of magnitude longer than the free-space blackbody decay at room temperature. This long lifetime is achieved by placing the atoms inside a pair of optically transparent capacitor plates, which suppress the blackbody field at microwave frequencies.

The experiments start with an array of ten optical tweezers at a wavelength of λ=53.991nm and a waist of 56.45 nm, which are stochastically loaded with single 88Sr atoms cooled close to the motional ground state. The atoms are prepared inside a structure consisting of six electrodes. Four of them form a ring structure and allow the researchers to apply electric fields in the xy plane of the tweezer array. The remaining two plate electrodes are placed below and above this ring structure.

The researchers demonstrate trapping of the circular Rydberg atom in the optical tweezer, exploiting the dominating Sr+ core polarizability, and analyze the effect of the trapping light on the qubit coherence.

What is the Significance of this Research?

This research is significant as it demonstrates the first realization of alkaline-earth circular Rydberg atoms trapped in optical tweezers. These atoms are highly promising candidates for overcoming limitations in coherence time and gate fidelities in Rydberg atom quantum simulators and computers.

The researchers’ work opens routes for quantum simulations with circular Rydberg states of divalent atoms, exploiting the emergent toolbox associated with the optically active core ion. This could potentially lead to significant advancements in the field of quantum computing and simulations.

The research also demonstrates the creation of very high circular states of 88Sr, a type of strontium. They measure lifetimes as long as 255 ms at room temperature, achieved via cavity-assisted suppression of blackbody radiation. This long lifetime is a significant achievement, as it is about an order of magnitude longer than the free-space blackbody decay at room temperature.

The research was conducted by C Hölzl, A Götzelmann, E Pultinevicius, M Wirth, and F Meinert from the 5 Physikalisches Institut and Center for Integrated Quantum Science and Technology at Universität Stuttgart in Germany. Their work was published on 3 May 2024, after being received on 19 January 2024 and accepted on 28 March 2024. The research was published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license.