• Physics 18, 103
By housing an optical tweezer array inside a cryogenic vacuum chamber, researchers have trapped rubidium Rydberg atoms for up to 50 minutes.
L. R. B. Picard/Caltech; APS/C. Cain
Arrays of neutral atoms in optical tweezers have emerged as one of the most promising platforms for scaling up quantum computers and simulators. These arrays are produced by laser-cooling clouds of atoms to a few microkelvins and loading them into optical tweezers, which are deep optical traps formed from tightly focused laser beams. Researchers have produced large 2D arrays of optical tweezers, making it possible to trap and control thousands of individual atoms at once [1]. Such experiments are performed in ultrahigh vacuum chambers, which protect the ultracold atoms from interacting with the room-temperature outside world. Even so, the atoms still encounter thermal radiation from the walls of the chamber, which can potentially lead them to transition to unwanted states. Now Zhenpu Zhang, Ting-Wei Hsu, and their collaborators at the University of Colorado and the National Institute of Standards and Technology—both in Boulder, Colorado—have introduced a new apparatus that enables large atom arrays to be produced in a cryogenic environment, protecting the atoms from thermal radiation and further reducing the vacuum pressure in their chamber [2].
For optical tweezers to be individually resolvable, they are typically separated by distances of a few micrometers. Two neutral atoms in their ground states separated by such a distance would not normally interact. To engineer useful quantum interactions between the atoms, the atoms are transferred to highly excited Rydberg levels, in which the wave function of the atom’s outermost electron extends orders of magnitude further than the ground-state wave function does [3].
Two atoms in Rydberg states will couple via electric dipole interactions, which can lead to interesting many-body physics and the generation of entanglement in atom arrays. The use of Rydberg states forms the basis of quantum computing with neutral atoms, an approach that boasts some of the largest qubit numbers of any quantum computing platform [4]. Unfortunately, Rydberg states can decay through spontaneous emission of photons or by absorption of thermal, or blackbody, photons from the environment, processes that shorten the lifetimes of Rydberg states and limit the fidelity of quantum operations that they could perform.
The spectrum of a room-temperature blackbody peaks at a wavelength of around 10 µm, with a long tail stretching to the microwave spectral range. If photons in this microwave spectral range reach the Rydberg atoms, they can drive transitions to other states, effectively destroying any quantum information stored in the atoms. In some cases, this process provides the dominant limit on the lifetime of these Rydberg states [3]. So-called circular Rydberg states, which have the maximum possible orbital angular momentum in each Rydberg level, are especially vulnerable. These states have very long spontaneous emission lifetimes and could offer a pathway for orders of magnitude improvements in the fidelity of Rydberg gates, provided they can be shielded from blackbody radiation [5].
Zhang, Hsu, and their collaborators have addressed these challenges by creating an atom array in a cryogenic environment. Importantly, they did so without compromising either the quality of the vacuum or the optical access needed for the high-resolution trapping and imaging of atoms. Only one other group, based in Paris, has previously created a neutral-atom array in a cryogenic environment. However, the Paris researchers discovered that the vacuum-limited lifetime of their atoms shortened significantly when they incorporated an in-vacuum objective lens into their design [6]. The Boulder group’s chamber design overcomes this problem. Here the atoms are trapped within a custom cold box, which is cooled to 45 K using a helium cryostat. Thanks to the cold box’s multiple cold windows, the researchers could place a room-temperature objective lens outside the box. Laser light that passed through the lens and then through a window trapped rubidium atoms in a 5-by-5 optical tweezer array. The whole system of cold box and warm objective lens is contained within a room-temperature vacuum chamber.
Besides shielding the atoms from blackbody radiation, the cryogenic environment lowers the vacuum pressure in the chamber. The reduction occurs because when warm background gas particles collide with a cold surface, they tend to condense on it instead of bouncing off, a process known as cryopumping (Fig. 1). To improve their chamber’s cryopumping performance, Zhang, Hsu, and their colleagues incorporated a protruding cold finger in their cold box. They cooled this device to an even lower temperature of 4 K, allowing a greater proportion of background gas particles to condense on its surface. They then measured the lifetime of their trapped rubidium atoms. After correcting for some losses induced by the atom cooling and imaging process, they found a vacuum-limited lifetime of 3000 s, an order of magnitude longer than that which typical room-temperature atom array experiments can sustain. Some atoms inevitably escape their traps when they are imaged. The researchers predict that, given their current imaging losses, a 3000-s lifetime is sufficient to prepare defect-free arrays of close to 1000 atoms with the potential to scale to even larger sizes if the losses can be reduced.
Having established the lifetime-extending ability of their setup, the Boulder group went on to take several steps toward the goal of performing novel quantum experiments. The researchers encoded a qubit in two hyperfine levels of the electronic ground state of the atoms and drove transitions between the levels using a microwave antenna positioned inside the cold box. Using a sequence of microwave pulses, they measured a coherence time of a superposition of hyperfine qubit states of close to 1 ms, a value limited by fluctuations in the ambient magnetic field.
An important technique in neutral-atom qubits is the use of laser-driven Rabi oscillations to drive an atom between two states with resonant electromagnetic radiation. In another demonstration of their machine’s capabilities, the researchers showed that they could drive Rabi oscillations of atoms to the Rydberg state within the cold box. This feat was achieved using a two-photon transition, driven by one infrared and one ultraviolet laser that they aimed at the atoms through two of the cold box’s side windows. To transfer between states with high fidelity, the coherence time of the Rabi oscillations needs to be much longer than their period.
Zhang, Hsu, and their colleagues measured an oscillation period of 0.26 µs and a coherence time of 5.4 µs. At present, their coherence is limited by instrument noise from the lasers driving the oscillations. If this noise was eliminated, they would be able to approach the fundamental limit imposed by the Rydberg-atom lifetime, which is expected to increase from 147 µs at room temperature to 306 µs in the cryogenic environment.
The capabilities demonstrated by the Boulder group make up a complete toolbox for quantum computing and simulation with Rydberg atoms in a cryogenic environment. In addition to the state-of-the-art vacuum lifetimes afforded by cryopumping, the group’s setup might open the door for greater use of extremely long-lived circular Rydberg atoms. Excitation to these Rydberg states has been demonstrated recently in room-temperature optical tweezer arrays [7, 8]. If the use of circular Rydberg atoms in a cryogenic environment leads to improvements in the fidelity of quantum operations, the field of neutral-atom quantum computing and simulation will undergo a paradigm shift.
References
- H. J. Manetsch et al., “A tweezer array with 6100 highly coherent atomic qubits,” arXiv:2403.12021.
- Z. Zhang et al., “High optical access cryogenic system for Rydberg atom arrays with a 3000-second trap lifetime,” PRX Quantum 6, 020337 (2025).
- M. Saffman et al., “Quantum information with Rydberg atoms,” Rev. Mod. Phys. 82, 2313 (2010).
- M. A. Norcia et al., “Iterative assembly of 171Yb atom arrays with cavity-enhanced optical lattices,” PRX Quantum 5, 030316 (2024).
- S. R. Cohen and J. D. Thompson, “Quantum computing with circular Rydberg atoms,” PRX Quantum 2, 030322 (2021).
- G. Pichard et al., “Rearrangement of individual atoms in a 2000-site optical-tweezer array at cryogenic temperatures,” Phys. Rev. Appl. 22, 024073 (2024).
- C. Hölzl et al., “Long-lived circular Rydberg qubits of alkaline-earth atoms in optical tweezers,” Phys. Rev. X 14, 021024 (2024).
- R. G. Cortiñas et al., “Laser trapping of circular Rydberg atoms,” Phys. Rev. Lett. 124, 123201 (2020).
