Trapped Ion Quantum Computation Advances with Individual Addressing Technique.

Quantum computation utilising trapped ions represents a promising avenue towards building scalable quantum processors, yet precise control over individual qubits remains a significant challenge. Researchers are continually refining methods for implementing quantum gates, the fundamental building blocks of quantum algorithms, with high fidelity and individual addressing capability. A team led by Jin-Ming Cui, Yan Chen, Yi-Fan Zhou, and colleagues at the Laboratory of Quantum Information, University of Science and Technology of China, detail a novel approach to achieving this control in their recent publication, “Transverse Polarization Gradient Entangling Gates for Trapped-Ion Quantum Computation”. The work demonstrates the successful implementation of Mølmer–Sørensen (MS) gates, a type of entangling gate, using a polarization gradient of light to manipulate the ions’ internal states and motional degrees of freedom, achieving fidelities exceeding 98.5% in two-ion chains and 97.2% in four-ion chains. This technique offers a potentially simpler pathway towards scalable trapped-ion quantum computation, particularly when integrated with optical tweezer architectures.

Trapped ion quantum computing continues to advance as a leading platform for realising practical quantum computation, and recent research details improvements in both the fidelity of quantum gates and the maintenance of qubit coherence within these systems. The work centres on a novel method utilising precisely shaped polarisation gradient fields, generated by tightly focused laser beams, to manipulate the motion of individual ions held within a linear trap. This approach facilitates individual addressing of ions, simplifying the implementation of optical tweezer gates, which are used to entangle qubits.

The researchers demonstrate single-qubit gate fidelities exceeding 98.5% in chains of two ions and 97.2% in four-ion chains. This represents a considerable improvement in the accuracy with which quantum operations can be performed. Maintaining qubit coherence, the ability of a qubit to exist in a superposition of states, is paramount for complex computations. A significant challenge arises from the AC Stark shift, an unwanted energy shift induced in the ion’s energy levels by the laser light used for manipulation. The team meticulously quantified and mitigated these shifts, thereby preserving coherence.

Theoretical modelling, utilising Debye-Wolf vector diffraction theory, accurately predicted the observed AC Stark shifts. Experimental results revealed a variation in the phonon frequency, a measure of the ion’s vibrational motion, of approximately 10 kHz due to the optical tweezer effects. Furthermore, a differential AC Stark shift, the difference in energy shift between different ions, of around 1 kHz was observed and accounted for. The system employs Raman operations, a technique utilising the interaction of light with matter to manipulate quantum states, on nuclear spin qubits. Specifically, the isotope ¹⁷¹Yb⁺ (ytterbium-171) is used, leveraging the spin of its nucleus as the qubit.

Performance of the Mølmer-Sørensen (MS) gate, a two-qubit gate commonly used for entanglement, also saw substantial improvement, achieving fidelities exceeding 98.5% and 97.2% in two and four-ion chains respectively. By carefully calibrating the system to minimise AC Stark shifts, the researchers successfully maintained qubit coherence for extended periods, enabling more complex quantum computations. The technique’s simplification of optical tweezer gate implementation suggests a pathway towards scaling up to larger, more complex quantum systems, a crucial step in realising the full potential of trapped ion quantum computing. This work represents a significant advancement in the field, offering a promising route towards building more powerful and scalable quantum computers.