Two-species atomic lattices show promise for quantum error correction

A study in Natural physics realized a two-species Rydberg lattice combining rubidium (Rb) and cesium (Cs) atoms to improve quantum computing and its applications.

The use of neutral atoms as quantum bits or qubits is not new in the quantum world. Neutral atoms are trapped using optical tweezers, which are focused laser beams, to manipulate the atoms for storage and calculations.

Neutral atoms are used for analog quantum simulations and digital quantum calculations. They constitute a promising avenue for fundamental research as well as for applications of quantum computing due to their scalability (up to thousands of qubits), their long coherence times (beyond seconds) and the availability of high-fidelity quantum operations.

However, using a single neutral atom species for quantum applications presents challenges, such as implementing mid-circuit readout (measurements performed on a subset of auxiliary qubits in the middle of a quantum circuit) and crosstalk between auxiliary and data qubits. , causing decoherence.

The research team led by Dr. Hannes Bernien of the University of Chicago got around this problem by using two different species of neutral atoms.

Phys.org spoke with Dr. Bernien, asking about his interest in exploring this particular area of ​​research. He said: “I am fascinated by the role of measurements in quantum mechanics. Of course, the measurements can be used to gather information about the quantum system, but they can also be used to prepare and manipulate the system.

The atoms used in these types of systems are called Rydberg atoms and have a unique property.

Rydberg atoms

Rydberg atoms, like those used in the study, are atoms in which electrons are excited to a highly excited state. This means they are far from the core.

These atoms are enormous in size, due to the great distance between the electrons and the nucleus. They also exhibit very large dipole-dipole interactions between them, with the electron being the negative end and the nucleus the positive end.

Highly excited states also enable large coherence times, an important requirement for qubits. Additionally, they interact well with other Rydberg atoms and can be easily manipulated using laser fields.

In the context of Natural physics In this study, the researchers used Rb and Cs Rydberg atoms to create their array of neutral atoms. The choice to use two different species of atoms arises from the limits presented by single-species systems.

A two-species Rydberg network

To achieve their two-species system, the researchers used optical tweezers to trap Rb and Cs atoms individually. The two atomic species can be manipulated individually and without crosstalk due to their large frequency separation. Yet when excited by particular Rydberg states, certain energy levels between atoms can be brought into resonance.

Simply put, each species of atom was tuned such that the energy levels of the two species were similar. The researchers chose to do this because they discovered a novel interspecies resonance between the energy states Rb and Cs, called the Förster resonance.

Using their two-species Rydberg network, the researchers implemented independent control systems for each species, allowing for more precise control. They also developed various techniques to exploit the unique properties of the facility.

Förster resonance, blocking effects and entanglement

Interspecies resonance increases the strength of interaction and is useful for many reasons. Stronger interspecific interaction means stronger dipole-dipole interactions, improving their ability to interact and entangle, allowing for efficient transfer of energy (or information) between the two species.

Another advantage is linked to the improvement of quantum entanglement, the cornerstone of quantum computing. Resonance can lead to strong blocking effects, in which the excitation of one atom prevents the excitation of another nearby atom or even several nearby atoms.

The Förster resonance leads to stronger blocking and therefore more efficient quantum entanglement between qubits.

Dr. Bernien further explained: “Using so-called Foerster resonances, we can obtain a regime in which the Rb-Cs interaction is much stronger than the Rb-Rb and Cs-Cs interactions. These asymmetric interactions could prove very useful for quantum information. processing and open new directions for quantum simulation.

“Additionally, we then used the interactions to realize a two-qubit gate between Rb and Cs as well as non-demolition quantum measurements that motivated this research.”

Using interspecies interactions, the researchers were able to generate a Bell state between the Rb and Cs qubits. This state of maximum entanglement demonstrates a high degree of quantum correlation that is essential for quantum information processing tasks such as teleportation and superdense coding.

The non-demolition quantum measurements mentioned by Dr. Bernien are crucial for improving the precision of quantum information processes, particularly quantum error correction, enabling repeated measurements without loss of coherence across data qubits.

A new paradigm for quantum computing

Although the experiment successfully demonstrated a new paradigm for qubit operations, it was not without its challenges.

Dr Bernien said: “As with most atom lattice experiments, the biggest problems are the lasers. For these Rydberg excitations, the lasers must be very narrow and precisely locked. Additionally, electronic transitions do not occur at the most practical wavelengths, so there is a lack of reliable laser technology.

Despite these challenges, the realization of a two-species Rydberg network is particularly interesting for quantum error correction (QEC). QEC is a method used to detect and correct errors in quantum calculations without disrupting the quantum information.

This method can be used effectively to extract errors from data. Dr. Bernien discussed this application and those of quantum simulations.

“I anticipate that multiple cycles of error correction will be possible as well as feedback operations to correct errors if desired. For quantum simulation, I am very intrigued by the possibilities that new interaction regimes open up. We are already collaborating to explore the phase diagram in these regimes and study non-ergodic dynamics in two-species networks,” he said.

The research team is also eager to explore the application of such systems for many-body quantum physics, measurement-induced phase transitions, and measurement-modified dynamics.

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