Atoms and Molecules Combined Unlock Faster Quantum Entanglement Generation

Scientists are exploring novel methods to harness the potential of polar molecules for advanced quantum technologies, but current limitations in state detection and weak interactions hinder progress. Chi Zhang from the Centre for Cold Matter, Blackett Laboratory, Imperial College London, Sara Murciano working with colleagues at Universit e Paris-Saclay, CNRS, LPTMS, and Nathanan Tantivasadakarn from the C. N. Yang Institute for Theoretical Physics, Stony Brook University, in collaboration with Ran Finkelstein from School of Physics and Astronomy, Tel Aviv University, present a groundbreaking scheme for logic control and state preparation utilising a hybrid system of polar molecules and neutral atoms. This research demonstrates a pathway to overcome existing challenges by employing fast, high-fidelity gates and measurements, potentially enabling the creation of large-scale entangled molecular states and significantly advancing precision measurements, topological quantum states, and measurement-based criticality. The proposed approach represents a paradigm shift in logic control and highlights the benefits of optimally utilising hybrid quantum systems for near-term device development.

Polar molecules possess a complex internal structure ideally suited for quantum information storage and manipulation, but their practical use has been restricted by slow and imperfect state detection alongside weak intermolecular interactions.

This work introduces a scheme leveraging the strengths of both molecule and atom qubits, achieving significantly faster entanglement than previously possible with molecule-only systems. The core innovation lies in a resonant dipole-dipole exchange mechanism between molecular rotational transitions and atomic Rydberg transitions, creating a controlled-phase gate operating three orders of magnitude faster than existing molecular entangling gates.

This hybrid approach circumvents the challenges inherent in solely utilising molecules by employing fast, high-fidelity atom-molecule gates and high-fidelity atomic measurements. By integrating these capabilities, researchers can efficiently prepare and manipulate complex molecular states, including GHZ states, a type of entangled state useful for enhanced precision measurements, and exotic qudit states exhibiting topological order.

The system is not limited to specific molecular species, offering broad applicability across diverse molecular platforms and expanding the possibilities for quantum logic control. Specifically, the ability to rapidly and accurately measure atomic ancilla qubits, auxiliary qubits used to assist in computations, enables mid-circuit operations difficult to implement in purely molecular systems.

This capability is crucial for applications like quantum error correction and the generation of scalable multipartite entanglement. The research demonstrates a concrete system where each qubit type is optimally utilised, and measurement-based techniques provide a substantial advantage for near-term quantum devices. Gate errors in the proposed hybrid atom-molecule system are predicted to reach as low as 1x 10−3, determined by careful analysis of several contributing factors.

Decay of the Rydberg state contributes 7x 10−4 to the overall error, scaling with the rotational frequency to the power of 3/2 and inversely with the dipole moment of the molecule. Adiabaticity breakdown introduces an error of 2.5x 10−4, while leakage from the qubit subspace is minimised at 5x 10−8, dependent on the rotational frequency to the power of -10/3 and the square of the dipole moment.

Electrical field fluctuations contribute 8x 10−5 to the total error, scaling with the 10/3 power of the rotational frequency and inversely with the dipole moment. The interaction strength between the molecule and atom, denoted VMA, is directly proportional to the product of the atomic and molecular dipole moments and the square of the principal quantum number, highlighting the importance of selecting molecular species with large dipole moments and utilising higher Rydberg states to enhance gate fidelity.

Resonant dipole-dipole exchange between molecular rotational transitions and atomic Rydberg transitions underpins the creation of a controlled-phase gate. This gate leverages the strong interaction achievable when aligning a polar molecule with a highly excited Rydberg atom, facilitating rapid entanglement at approximately 1MHz, a three orders of magnitude improvement compared to direct molecule-molecule entangling gates.

To implement this hybrid quantum system, individual polar molecules and neutral atoms are trapped and controlled using tightly focused optical tweezers. Each molecule serves as a qudit, exploiting its rich internal structure to encode multiple levels of quantum information, while the atoms function as ancilla qubits for fast, high-fidelity measurements.

The atoms are prepared in a superposition state, allowing for projective measurements that determine the state of the molecule without directly interacting with it, circumventing the limitations of direct molecule state detection. This measurement-based approach achieves non-destructive, high-fidelity readout of molecular states and allows for entanglement between molecules that are not nearest neighbours, a significant advantage over molecule-only platforms.

Beyond GHZ state preparation, the research demonstrates the potential for encoding quantum information using qutrits, three-level quantum systems. Implementing a qutrit entangling gate requires an additional stable level in both the atom and molecule, alongside the ability to perform single qutrit rotations, achievable with additional coupling fields.

Utilising qutrit encoding with atomic ancilla mid-circuit measurement enables efficient preparation of the ground state of the qutrit GHZ state and the Z3 Toric code, a crucial step towards universal topologically protected quantum computation. The Z3 Toric code, the smallest non-abelian group, is generated through measurement and feedforward on the qutrit cluster state.

Scientists have long recognised the potential of polar molecules as qubits, but realising that potential has proven remarkably difficult due to the inherent complexity of these molecules. This work offers a compelling solution, a hybrid quantum system pairing polar molecules with neutral atoms, effectively outsourcing the most challenging aspects of qubit control to a more mature technology.