Researchers have demonstrated a 97-nanosecond CZ gate using an energy-level engineering technique, potentially overcoming limitations in superconducting quantum computing. The work, led by Wan-Peng Gao and colleagues at the Shanghai Institute of Microsystem and Information Technology, uses the Autler-Townes effect to manipulate qubits without inducing decoherence, a common problem with existing frequency-tuning methods. This approach achieves 99.40(9)% fidelity, with error analysis indicating static decoherence as the primary source of errors, and facilitates fast, high-quality all-microwave gate operations applicable across various quantum platforms.

Autler-Townes Effect Enables All-Microwave Qubit Control

A 97-nanosecond CZ gate operation was experimentally demonstrated using a novel energy-level engineering technique, potentially accelerating the development of scalable superconducting quantum computers. This speed was achieved by manipulating qubits with the Autler-Townes effect, a physics principle that alters energy levels without disturbing the quantum information stored within them. The approach avoids the decoherence typically induced by frequency-tuning methods commonly used in quantum computing, a significant obstacle to maintaining qubit stability.

The team’s technique specifically addresses limitations inherent in all-microwave-driven schemes, which traditionally face a trade-off between gate speed and unwanted leakage errors. By employing the Autler-Townes effect, they engineered qubit nonlinearity, allowing for faster gate operations without compromising fidelity. This manipulation creates what the researchers describe as AT doublets, split energy levels, that facilitate precise control over qubit interactions. Detailed discussion of the Autler-Townes effect is provided in an appendix to their published work.

The resulting CZ gate achieved 99.40(9)% fidelity, a measure of how accurately the gate performs its intended operation. Error analysis revealed that static decoherence, rather than operational errors, was the primary source of inaccuracies in the gate. Static decoherence refers to the natural loss of quantum information over time, a fundamental challenge in quantum computing. The ability to isolate static decoherence as the dominant error source is significant, as it suggests that further improvements in materials and fabrication could lead to even higher fidelity gates.

This scheme also simplifies the topology required for qubit connections, potentially reducing the complexity of building large-scale quantum processors. The researchers indicate that this method is not limited to a single quantum platform, suggesting broad applicability across various qubit technologies.

They demonstrated the technique using transmon qubits, a leading architecture in superconducting quantum computing, but believe the underlying principles can be adapted to other systems. The team’s work uses fewer two-qubit gates than a standard Toffoli gate, optimizing hardware mapping for complex quantum algorithms. “This approach can adjust the nonlinearity of the qubit while protecting the computational subspace and coherence time,” the researchers conclude, highlighting the potential for building more robust and scalable quantum computers.

Transmon Qubit Coupling in Dispersive Regime

High-fidelity entangling gates are essential for realizing large-scale fault-tolerant superconducting quantum computing, yet existing approaches are limited. Here, the researchers introduce an energy-level engineering technique that employs the Autler-Townes effect to manipulate the nonlinearity of qubits without perturbing the computational subspace or degrading coherence time. The scheme facilitates fast, high-quality all-microwave gate operations with a simple topology and can be applied across various quantum platforms.

Superconducting two-qubit gates function by energy-level engineering to manipulate interactions involving computational states. Yet prevailing approaches are limited: flux-tuning methods, which employ frequency modulation for entanglement generation, compromise system coherence and flux noise immunity; on the other hand, all-microwave-driven schemes, which activate indirect interactions via adjustable virtual energy states, suffer a fundamental trade-off between gate speed and leakage rate. An energy shift induced by the Autler-Townes effect provides a potential solution to the aforementioned challenges.

The Autler-Townes effect employs microwave drive to produce selective energy-level splitting via resonance with the target transition frequency, thereby performing a drive-amplitude-dependent adjustment of the aiming state, while the system’s large anharmonicity ensures that irrelevant states remain unaffected. This technique is general and can be integrated into other quantum platforms. In this letter, the researchers propose a novel energy-level engineering technique based on the Autler-Townes effect for building a high-quality all-microwave superconducting two-qubit gate.

Nanosecond CZ Gate via AT Energy-Level Engineering

Researchers demonstrated a method for manipulating qubits that avoids this compromise by using the Autler-Townes effect, a physics principle used to selectively split energy levels with microwave drives. This precise energy-level engineering allows for faster gate operations without increasing the risk of errors stemming from unintended qubit excitation. This approach builds on the established use of energy-level engineering to control qubit interactions, but diverges from methods like flux-tuning, which can introduce decoherence and susceptibility to noise.

Instead, the Autler-Townes effect is employed to adjust qubit nonlinearity while actively protecting the computational subspace, a critical area for maintaining quantum information. The system’s large anharmonicity, a property of superconducting qubits, ensures that irrelevant energy states remain unaffected during the gate operation, further enhancing stability. This compatibility with commonly used coupler structures suggests potential for scalability in larger quantum processors.

The experimental validation of this technique resulted in a 97-nanosecond CZ gate, a significant reduction in operation time. The gate’s implementation involves a three-step process, beginning with a square-shaped Autler-Townes drive applied to a coupler, followed by amplitude modulation to induce conditional phase accumulation, and concluding with a return to the initial drive amplitude. The scheme’s efficiency extends beyond speed and fidelity. It also requires fewer two-qubit gates compared to a standard Toffoli gate.

This simplification optimizes hardware mapping for complex quantum algorithms, potentially reducing the resources needed for computation. The use of a fixed-frequency topology also simplifies control and reduces the need for precise calibration.

Experimental Validation of Xmon Qubit Device

Superconducting quantum chips featuring two coupled Xmon qubits were fabricated and experimentally validated, demonstrating a device with frequencies of 5.272 GHz and 5.124 GHz for the qubits, and anharmonicities of -178 MHz and -205 MHz respectively. These Xmon qubits, a modification of the standard transmon, incorporate a cross-type electrode intended to provide larger shunt capacitance, a key feature for achieving the reported gate speeds and fidelities.

Detailed device information is available in appendix A2, but the core of the validation lies in demonstrating control over qubit properties using an energy-level engineering technique. The experimental setup, housed within a dilution refrigerator detailed in appendix A3, allowed researchers to probe the system’s response to varying drive amplitudes. Measurements of two-tone spectroscopy revealed a clear dependence of qubit frequency on the Autler-Townes drive amplitude, confirming the tunability of the qubit’s anharmonicity.

Specifically, the frequency difference between Autler-Townes doublets of one qubit increased predictably with the applied drive, a result presented in the source material and demonstrating precise control over the qubit’s energy levels. While one qubit was held at a fixed frequency of 5.124 GHz, an avoided crossing observed between its frequency and the upper branch of the driven qubit indicated transverse coupling between the two, an essential step for entangling operations.

This controlled coupling enabled the implementation of a CZ gate, a fundamental operation in quantum computing, by rapidly adjusting the Autler-Townes drive amplitude to bring specific energy levels into resonance. Measured energy relaxation times, T1, were and for the two qubits, while echo dephasing times, T2e, reached and respectively, indicating a level of coherence necessary for high-fidelity gate operations.

The researchers demonstrated that the |10>|11> transition frequency could be predictably pushed away by adjusting the Autler-Townes drive amplitude, further refining the control over qubit interactions. The ability to manipulate qubit anharmonicity without disturbing the computational subspace is a key advantage of this approach, as it avoids the decoherence often induced by frequency-tuning techniques.

“This driven system acts as a qubit with tunable anharmonicity,” the researchers note, allowing for precise control over the interaction between qubits. The resulting CZ gate operation achieved a duration of 97 nanoseconds, a speed that positions this technique as a potential solution to scaling up superconducting quantum computing.

Static Decoherence Limits High-Fidelity Gate Performance

Error analysis of a newly demonstrated 97-nanosecond CZ gate reveals that static decoherence currently poses the largest obstacle to further improvements in fidelity. While the gate itself achieves 99.40(9)% fidelity, the research team determined that approximately 78% of the total error budget stems from decoherence effects during the gate operation. Calculations showed that relaxation contributed errors of 2/5τgate/T1, and pure dephasing added 2/5τgate/Tφ to the total error rate, where τgate represents the duration of the gate operation.

This detailed breakdown highlights the specific mechanisms limiting performance and guides future optimization efforts. Optimizing pulse shape and enhancing coherence times are identified as the most promising avenues for further refining gate performance. The researchers also discussed an optimized Autler-Townes coupler scheme designed for compatibility with existing quantum processor architectures.

This focus on integration suggests a practical pathway toward scaling up the technology for use in larger, more complex quantum systems. “Its performance can be improved by enhancing the coherence time and optimizing the pulse shape,” the team states, emphasizing the immediate priorities for future work.

👉 More information
🗞 Autler-townes energy-level engineering for high-quality all-microwave qubit gate
✍️ Wan-Peng Gao et al.
🧠 ArXiv: http://arxiv.org/abs/2611.07967v1

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