Oregon Team Cools Ion Crystal to Ground State

A new method cools a global motional mode of a mixed metastable-ground state Coulomb crystal to the motional ground state via dissipative operations on the ground qubit without disturbing coherence of the metastable qubit. Sean Brudney of the University of Maryland and colleagues demonstrate that this enables quantum logic spectroscopy to non-destructively read out the state of the metastable qubit using fluorescence detection of the ground qubit. Expanding these demonstrations to larger system sizes should enable the mitigation of motional heating after ion shuttling and syndrome extraction for quantum error correction, both key primitives for future fault-tolerant quantum computers based on trapped ions. Dissipative cooling preserves metastable qubit coherence for improved readout fidelity Error rates for metastable qubit readout dropped to 0.02 ±0.01 in motional quanta, representing a key improvement over previous sympathetic cooling methods. Traditionally, sympathetic cooling relied on co-trapping different ion species, where one species acts as a ‘coolant’ to reduce the motional energy of the other. However, this approach necessitates careful selection of ion species and can introduce complexities in managing the differing mass and charge characteristics. The current research circumvents these limitations by utilising a two-species approach within a single ion species, leveraging the distinct energy levels of the same ion to achieve cooling and readout. This ‘optical-metastable-ground’ (omg) architecture offers a significant advantage in hardware simplification and scalability. The ability to achieve near-ground state cooling without disturbing the metastable qubit’s delicate quantum state is crucial for high-fidelity quantum operations. Maintaining qubit coherence is paramount, as any loss of coherence introduces errors into calculations. Previous methods often compromised coherence during the cooling process, limiting the accuracy of subsequent quantum operations. This mid-circuit ground-state cooling and ancilla readout within an optical-metastable-ground architecture paves the way for mitigating motional heating during ion transport and enabling syndrome extraction essential for scalable, fault-tolerant quantum computation. Motional heating, caused by fluctuating electric fields, is a major source of decoherence in trapped-ion systems. Reducing this heating is vital for maintaining qubit fidelity over extended periods. Syndrome extraction, a key component of quantum error correction, involves measuring the errors that occur during computation without collapsing the quantum state. The ability to perform non-destructive ancilla readout, facilitated by this cooling method, is essential for effective syndrome extraction. A final average motional occupation number of 0.02 ±0.01, measured in motional quanta, was achieved by cooling a global motional mode of a mixed metastable-ground state Coulomb crystal. The Coulomb crystal, consisting of a chain of ions held in place by electromagnetic fields, exhibits collective motional modes. Cooling these modes to the ground state minimises unwanted quantum fluctuations and improves the stability of the qubits. Quantum logic spectroscopy now allows for non-destructive readout of the metastable qubit state using fluorescence detection of the ground qubit. Cooling to the motional ground state is typically accomplished using resolved-sideband cooling or measurement-based cooling, also termed erasure-conversion cooling. Resolved-sideband cooling selectively addresses and cools specific motional modes of the ions. Measurement-based cooling, on the other hand, utilises repeated measurements to extract energy from the motional modes. The process currently faces limitations due to unintended couplings to other energy levels within the ions, with strengths of 0.166 ±0.004 and 0.019 ±0.001 relative to the primary cooling drive, suggesting that spectral purification of the laser light is needed to reach even lower temperatures. These unintended couplings arise from the complex energy level structure of the ions. Laser light intended to cool the desired motional mode can also interact with other energy levels, introducing unwanted heating and reducing the efficiency of the cooling process. Improving the spectral purity of the laser light, by filtering out unwanted wavelengths, is crucial for minimising these couplings and achieving even lower temperatures. Laser pulse optimisation reveals limits to ion cooling for enhanced qubit stability Ground-state cooling offers a pathway towards more stable and reliable quantum computations, addressing the persistent challenge of motional heating that degrades qubit performance. The ability to maintain qubit coherence for longer durations is directly linked to the effectiveness of the cooling process. Reducing motional heating not only improves the accuracy of individual quantum operations but also extends the time available for performing complex calculations. Simulations indicate diminishing returns, with the optimal duration of each cooling pulse decreasing as the number of pulses increases. This suggests a fundamental limit imposed by the experimental setup and highlights a complex interplay between pulse characteristics and cooling efficiency. The experimental setup includes factors such as the trap geometry, laser power, and ion spacing, all of which influence the cooling process. The modelling reveals that increasing the number of cooling pulses does not guarantee lower temperatures, instead demonstrating a diminishing return governed by pulse duration scaling inversely with the square root of the pulse count. The 6μm Coulomb crystal was cooled to its motional ground state using operations on the ground qubit, enabling non-destructive readout via fluorescence detection. This cooling of a Coulomb crystal allows for non-destructive readout of a co-trapped metastable qubit, a vital step for complex calculations. The size of the Coulomb crystal, in this case 6μm, influences the strength of the inter-ion interactions and the efficiency of the cooling process. Optimising the pulse sequence, rather than simply increasing the number of pulses, is crucial for achieving maximum cooling efficiency. This approach offers a means of improving qubit stability by addressing the limitations of current cooling techniques and exploring the relationship between pulse duration and the number of pulses applied, potentially leading to more efficient and effective cooling strategies. Further research will focus on refining the pulse sequence and exploring alternative cooling techniques to overcome these limitations and achieve even lower temperatures. Researchers cooled a 6μm Coulomb crystal to its motional ground state using operations performed on a ground qubit. The authors intend to refine pulse sequences and explore alternative cooling techniques to further lower temperatures in future work. 👉 More information🗞 Mid-circuit ground-state cooling and ancilla readout in the architecture✍️ Sean Brudney, Connor Burns, Gabriel J. Gregory, Evan Ritchie, David J. Wineland, David T. C. Allcock and Jameson O'Reilly🧠 ArXiv: https://arxiv.org/abs/2608.13181 Stay currentSee today’s quantum computing news on Quantum Zeitgeist for the latest breakthroughs in qubits, hardware, algorithms, and industry deals. Tags: