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Bulletin of the Lebedev Physics Institute

Optical System for Implementing Accurate Two-Qubit Gates on Neutral Atoms

P. I. Betleni, Ilya I. Beterov

Published May 31, 2025• 1 citation
Publisher

Abstract

Abstract— Schemes for laser excitation of ultracold Rydberg rubidium atoms in optical dipole traps are considered for implementing two-qubit gates on neutral atoms. We discuss advantages and disadvantages of the schemes comprising objectives arranged outside a glass cell and utilizing aspheric lenses placed inside a vacuum chamber. A scheme is suggested of the objective with a numerical aperture of 0.39 and flange focal length of 19 mm, which can be placed in a vacuum chamber and combined with a system of electrodes for controlling an electric field in the domain of atom interaction with laser radiation.

DOI
10.3103/s1068335625600226

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Abstract— Schemes for laser excitation of ultracold Rydberg rubidium atoms in optical dipole traps are considered for implementing two-qubit gates on neutral atoms. We discuss advantages and disadvantages of the schemes comprising objectives arranged outside a glass cell and utilizing aspheric lenses placed inside a vacuum chamber. A scheme is suggested of the objective with a numerical aperture of 0.39 and flange focal length of 19 mm, which can be placed in a vacuum chamber and combined with a system of electrodes for controlling an electric field in the domain of atom interaction with laser radiation.
Abstract We considered a promising platform for the implementation of quantum computing – an array of ultracold neutral atoms with individual addressing to individual atoms during laser excitation. A comparison of laser excitation schemes for atoms in arrays is carried out from the point of view of individual addressing. An optical scheme of an experimental setup for conducting experiments on three-photon laser excitation of atoms in arrays of optical dipole traps in a high-vacuum glass cell is described. The results of implementing an array of focal spots with individually controlled intensity are presented.
Abstract Single alkali-metal atoms in arrays of optical dipole traps represent a quantum register that can be used for quantum computation and simulation based on short-term Rydberg excitations, which switch the interactions between qubits. To load single atoms into optical dipole traps and then detect them by resonance fluorescence, lenses with a large numerical aperture (NA > 0.5) inside a vacuum chamber and expensive EMCCD cameras are commonly used. We present our recent experimental results on demonstrating the trapping of single 87Rb atoms using a long-focus objective lens with a low numerical aperture (NA = 0.172) placed outside the vacuum chamber, and detecting single atoms with a low-cost sCMOS camera. We also present our current results on implementing a single-qubit gate based on optical pumping and subsequent microwave transition between two hyperfine sublevels of a single 87Rb atom with fidelity near 95%.
In this thesis, the development of an experimental system for microscopic dipole trapping of ultracold neutral rubidium atoms is presented. The purpose of this system is to advance towards the experimental realisation of a quantum computational protocol utilising neutral atoms as qubits. It is intended that the quantum gate operations between qubits will be implemented by a scheme using Rydberg blockade, imposing a restriction on the maximum size of the dipole-trapped atom cloud; the spatial extent of the atomic ensemble contained in this trap must be smaller than the blockade radius to ensure that one single collective Rydberg state per qubit can be achieved. Therefore the experiment was designed with the intent of fulfilling these challenging requirements. This project involved the design and construction of an improved ultra-high vacuum chamber containing the optical setup for the experiment, successfully achieving pressures below 5 x 10-10 mbar. A magneto-optical trap was produced to act as a background reservoir of atoms from which to load the dipole trap. Numerous experimental measurements were done to characterise the physical properties of the trapped atoms, including the number, density and temperature of atoms, as well as the lifetime of the trap. The results of these measurements led to the conclusion that a suitable reservoir for loading the dipole trap had been produced. Significant work was carried out to set up and obtain the dipole trap in the laboratory. Measurements of the characteristic properties of the trap and the atoms confined in the trap were carried out to investigate the behaviour of the atoms and to validate our design. Ultimately a trap containing tens of atoms was achieved, with an atom cloud diameter of ~1.2 µm in two dimensions, being well within the estimated Rydberg blockade radius of ~4.4 µm for n ~ 60 as intended. The two-photon excitation laser system for the probing of Rydberg states, for future applications in Rydberg blockade-based quantum gate operations, was also developed during the course of this work. Different Rydberg states were detected experimentally by the observation of Autler-Townes splitting in a three-level atom scheme. Overall, the work presented in this thesis provides a strong groundwork for the advancement towards neutral atom-based quantum gates, including the development of the experimental system and the production of standard procedures to carry out characterisation measurements of the traps efficiently in the future. The main achievements of this work are the establishment of the experimental apparatus, the achievement of a microscopic dipole trap which conforms to the requirements of an atomic qubit, and the significant growth in the knowledge of atom trapping specific to our system.
We present a brief overview of the current state of the experimental research on the development of the element base of quantum computers with qubits based on single neutral atoms trapped in optical traps. The discussion focuses on the requirements for qubits, peculiarities of single neutral atoms as qubits, methods for the quantum register development and for the implementation of single-qubit quantum logic operations in the laser and microwave fields, and two-qubit operations through the dipole–dipole interaction after a short laser excitation of atoms to the Rydberg states. The results of the experiments on the observation of the interaction of two and three Rydberg atoms by a Förster resonance controlled by dc and radio-frequency electric field are presented.
The properties of Rydberg atoms are utilized for the implementation of quantum computers. Their large dipole moments provide strong interaction between qubits, as well as high sensitivity to external electric fields. Short-term laser excitation of atoms into Rydberg states allows for switching the interactions between qubits on and off, which is necessary for performing quantum operations and simulations. Within a high-vacuum cell, atoms are captured in a dipole trap. Ring electrodes, located inside the high-vacuum cell, are used to apply an electric field. In this work, the electric field distribution was obtained through numerical simulation using QuickField.