Nuclear Spins Offer a New Route to Longer-Lasting Quantum Data Storage

A new encoding, the spin Kerr-cat encoding, extends the coherence of quantum information within qubits. Z. M. McIntyre and Daniel Loss at University of Basel utilise clock transitions in quadrupolar nuclei to suppress qubit dephasing. Calculations based on antimony donors in silicon indicate this encoding could achieve a coherence time of 100 seconds, a key advance in hardware-level qubit lifetime extension. Moreover, they propose a two-qubit gate design potentially reaching 99% fidelity with a moderate enhancement of existing quadrupolar splittings. Spin Kerr-cat encoding extends antimony-123 qubit coherence to near 100 seconds A 100-second coherence time is now predicted for antimony-123 nuclear spin qubits, surpassing previous benchmarks by several orders of magnitude. This substantial extension of quantum information lifetime stems from the new spin Kerr-cat encoding, which actively suppresses noise through a carefully designed nuclear-spin basis. Previous methods struggled to maintain coherence for more than milliseconds, hindering progress in quantum computation. The encoding leverages naturally occurring “clock transitions” within quadrupolar nuclei to shield qubits from disruptive magnetic field fluctuations, a key source of decoherence. The significance of this improvement lies in its potential to reduce the overhead associated with quantum error correction, a crucial component of building scalable quantum computers. This approach provides a pathway towards more stable and reliable quantum computation by passively enhancing qubit longevity, reducing the need for complex error correction protocols. Calculations indicate that antimony-123 nuclear spin qubits, utilising this encoding, could maintain coherence for up to 100 seconds. These transitions, representing specific energy levels highly resistant to external disturbances, effectively shield the qubits from disruptive magnetic field fluctuations. The underlying principle relies on the fact that the qubit’s energy splitting is precisely tuned to match the frequency of these clock transitions, minimising sensitivity to off-resonant noise. This is a departure from traditional qubit designs where coherence is often limited by the susceptibility to a broad spectrum of noise frequencies. The use of “clock transitions” within the nuclei is central to achieving this extended coherence. These transitions arise from the interaction between the nuclear quadrupole moment, a measure of the nucleus’s deviation from spherical symmetry, and electric field gradients present in the silicon lattice. Calculations also suggest a two-qubit gate fidelity of 99% is achievable with a quadrupolar splitting enhancement of approximately 4, enabling more complex quantum operations. The encoding method exploits the natural nonlinearities present in nuclei with spins greater than or equal to one, interacting with electric field gradients created by lattice strain in silicon. This nonlinearity is crucial for creating the desired qubit behaviour and suppressing decoherence. While these simulations demonstrate a major leap in potential qubit longevity, they currently do not account for errors introduced during the electron shuttling and readout processes, representing a vital hurdle before practical implementation. Addressing these practical challenges will require further research into optimised device architectures and control techniques. Antimony-123 Qubit Encoding via Nuclear Spin Clock Transitions The spin Kerr-cat encoding represents a strong refinement in qubit design, actively mitigating decoherence through a carefully chosen nuclear-spin basis. This technique centres on using clock transitions to suppress noise that leads to qubit dephasing. By defining the qubit’s energy splitting to coincide with these transitions, the system becomes inherently less sensitive to disruptive magnetic field fluctuations, much like tuning a delicate instrument to avoid specific frequencies of interference. Researchers in Wales are developing a new qubit encoding technique utilising antimony-123 nuclei within silicon to extend the lifetime of quantum information. Simulations suggest a coherence time of 100 seconds is achievable, differing from alternatives like phosphorous donors which have a spin of only 1/2. Phosphorous donors, possessing a spin of 1/2, lack the necessary properties to leverage clock transitions and achieve comparable coherence times. This encoding uses the unique properties of quadrupolar nuclei, a nucleus with an uneven distribution of electric charge, to achieve this extended coherence. Quadrupolar nuclei interact with electric field gradients, creating the clock transitions essential for noise suppression. The choice of antimony-123 is particularly advantageous due to its relatively strong quadrupolar moment and favourable nuclear spin properties. The spin Kerr-cat encoding defines the qubit’s basis states using the two lowest energy levels of a $\mathbb{Z}_$2-symmetric nuclear-spin Hamiltonian. This specific Hamiltonian ensures that the qubit is protected from certain types of noise, further enhancing its coherence. The clock transitions, acting as a spectral filter, effectively suppress dephasing caused by magnetic field fluctuations. This is achieved by ensuring that the qubit’s energy splitting is precisely matched to the frequency of these transitions, minimising the impact of off-resonant noise. The theoretical framework underpinning this encoding builds upon established principles of nuclear magnetic resonance and quantum control. Antimony donors and spin Kerr-cat encoding extend qubit coherence towards practical timescales Scientists are continually seeking ways to build more durable quantum computers, battling the inherent fragility of quantum information. This work, potentially reaching up to 100 seconds in estimations and modelling, acknowledges this limitation. Despite depending on complex modelling, these coherence time estimations represent a significant step forward in materials-based qubit development. Extending coherence, the time quantum information remains stable, is vital for building practical quantum computers, allowing for more complex calculations. A longer coherence time translates directly to the ability to perform more quantum gate operations before the information is lost, enabling more sophisticated algorithms and computations.

The team’s use of antimony donors in silicon offers a promising avenue, exploiting nuclear spin to shield qubits from environmental noise, a major source of errors. A new qubit design utilising the properties of quadrupolar nuclei to suppress noise and extend coherence is introduced. By utilising naturally occurring clock transitions within these nuclei, the encoding creates a qubit basis inherently less susceptible to magnetic field fluctuations, a primary cause of information loss. Simulations utilising antimony-123 donors in silicon predict a coherence time of 100 seconds, substantially exceeding current benchmarks and opening possibilities for more complex quantum calculations. This represents a significant improvement over existing qubit technologies, potentially paving the way for the realisation of fault-tolerant quantum computation. The ability to maintain quantum information for extended periods is crucial for overcoming the challenges associated with building large-scale, reliable quantum computers. Researchers demonstrated a new qubit encoding, termed the spin Kerr-cat code, which utilises antimony-123 donors in silicon to suppress noise and extend qubit coherence. This encoding leverages the properties of nuclear spin to achieve a predicted coherence time of 100 seconds, representing a substantial improvement over current qubit technologies. Extending coherence is vital because it allows for more quantum gate operations to be performed before information is lost, enabling more complex computations. The authors suggest that enhancing quadrupolar splittings by a factor of approximately four could lead to a two-qubit gate fidelity of 99 percent. 👉 More information 🗞 Spin Kerr-cat qubits 🧠 ArXiv: https://arxiv.org/abs/2604.19687 Tags: