Scaling trapped-ion quantum computers has been hindered by bulky tabletop lasers and complex optical systems. Demonstrating coherent control of strontium ions using a fully integrated photonic laser stabilised with an on-chip device is the achievement of researchers at the University of Massachusetts Amherst and the University of California Santa Barbara. Coherent control of strontium ions is key technology for quantum computing enabled by this fully integrated photonic laser system.

Current systems require bulky lasers and optics, but this work utilises a chip-based device stabilising the laser directly, paving the way for miniaturisation. High precision single- and two-qubit operations were achieved with fidelities exceeding ninety percent, extending coherence times vital for complex calculations. These results represent progress towards building more compact and strong trapped-ion quantum computers. Trapped ions function as tiny electrically charged atoms held in place by electromagnetic fields; these act as basic units of information, known as qubits, within a quantum computer.

Current systems rely on large lasers and optics, yet this team’s approach uses an on-chip device stabilising the laser directly, enabling potential miniaturisation. Precisely timed pulses of light to manipulate qubit states and coherent qubit gates were demonstrated, achieving fidelities exceeding ninety percent alongside extended coherence times akin to maintaining balance on a spinning top for longer periods. These results represent progress toward building more compact and strong trapped-ion computers, though questions remain regarding monolithic integration with ion traps and long-term stability under real-world conditions.

On-chip resonators deliver high-fidelity quantum control of trapped ion qubits

A core component enabling these results was an on-chip coil resonator used to stabilise a narrow-linewidth visible light laser. The device acts as a miniature tuning fork for light, maintaining remarkably consistent frequency over time. Stabilising lasers is important because even tiny fluctuations in their colour can disrupt delicate quantum states within trapped ions, making pulse control akin to hitting a moving target. Integrating this resonator directly alongside the Brillouin laser created a compact system; magnetic fields precisely adjust its properties and refine emitted light stability by reducing temperature variations.

Strontium-88 ions facilitated coherent single- and two-qubit gate operations using an on-chip Brillouin laser stabilised by the integrated resonator. Randomised benchmarking determined that single qubit fidelity averaged 99.61% ±0.03% per Clifford gate, demonstrating high precision individual qubit control. The bare Ramsey coherence time for the qubit measured at 660 ±9 microseconds but extended to 1.750 ±0.033 milliseconds when employing a spin echo technique to mitigate decoherence effects. Beyond this single-qubit precision, they generated an entangled Bell state, a fundamental resource for quantum communication and computation, with a fidelity of 92.35% ±1.50%, indicating successful two-qubit operations.

High fidelity single qubit gates via integrated photonic stabilisation of strontium ions

Single-qubit gate fidelities exceeded 99.61% ±0.03%, representing substantial improvement over previous integrated systems and surpassing the vital threshold needed for practical quantum error correction. Achieving such precision previously demanded bulky laboratory setups unsuitable for scalable devices; this advancement overcomes long-standing limitations imposed by complex optics typically required to manipulate trapped ion qubits. The team focused on coherent control of strontium-88 ions, a leading platform in efforts to build functional qubits, paving the way towards miniaturisation and more powerful processors with increased qubit counts.

Integrated photonics enable compact coherent control of trapped ions

Current systems are hampered by bulky, delicate external optics, presenting a fundamental bottleneck in scaling trapped ion quantum computers. Maintaining precise control over qubit phases is notoriously difficult when using chip-based lasers, despite the drive for miniaturisation; achieving high-fidelity operations requires both factors. While bare Ramsey coherence time remains relatively short at just under seven hundred microseconds, limiting complex calculations even with improvements from spin echo techniques, this advance represents a step toward building practical quantum computers. The integrated photonic laser device actively manipulates their states with high precision and ensures consistent frequency over time, resolving a key challenge traditionally faced by those reliant on bulky external optics which limit deployment.

The researchers demonstrated coherent single- and two-qubit gates utilising strontium-88 ions controlled by an integrated silicon nitride laser system. This achievement means that the precise phase control needed for quantum logic is now possible using compact, chip-based lasers instead of large optical setups. The authors suggest this work provides a pathway towards scalable optical systems fully integrated within trapped ion quantum processors.

👉 More information
🗞 Single- and Two-Qubit Gates Driven by an Integrated Photonic Laser
✍️ Chris Caron, Zhenyu Wei, Andrei Isichenko, David Heim, Meiting Song, Nick Montifiore, Kaikai Liu, Josiah Dill, Daniel J. Blumenthal and Robert J. Niffenegger
🧠 ArXiv: https://arxiv.org/abs/2609.07708

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