Quantum computing has been hailed as a revolutionary technology, one that promises to solve complex computational problems far faster than classical computers. By exploiting the principles of quantum mechanics, quantum computers encode and process information in quantum bits, or qubits. These qubits leverage quantum phenomena such as superposition and entanglement to perform operations that would be unfeasible with classical systems. However, the challenge lies in achieving high-fidelity operations on these qubits, as imperfections and errors limit their computational power.
Superconducting qubits have emerged as one of the most promising platforms for quantum computing, but like other quantum systems, they are prone to errors from decoherence and control imperfections. These errors undermine the performance and reliability of quantum algorithms, making it essential for researchers to develop methods to improve qubit fidelity. In new groundbreaking work, researchers at the Massachusetts Institute of Technology (MIT) have achieved an unprecedented milestone in improving qubit performance. The team, composed of physicists and engineers from various departments at MIT, has implemented innovative control techniques to push the fidelity of superconducting qubits to record-breaking levels.
The Challenge of Quantum Errors: Decoherence and Counter-Rotating Dynamics
A key challenge in quantum computing is decoherence, the process by which qubits lose their quantum information due to interactions with their environment. Superconducting qubits are particularly susceptible to this phenomenon, which restricts their ability to maintain coherence over long periods. Quantum gates—operations that manipulate qubits—must be executed with high fidelity to maintain quantum information over time and enable effective quantum algorithms. Quantum error correction techniques, such as fault-tolerant quantum computing, require gates to be as precise as possible. This high gate fidelity is crucial for scaling quantum computers and achieving practical, real-world applications.
As quantum gate speeds increase in an attempt to reduce the time qubits are exposed to decoherence, a new type of error arises—counter-rotating errors. These errors occur when qubits are manipulated using electromagnetic pulses, and their effects become more prominent at higher gate speeds. In a typical implementation, single-qubit gates are executed through resonant microwave pulses that induce Rabi oscillations—a process by which qubits oscillate between their quantum states. However, when the pulses are too fast, counter-rotating errors can occur, resulting in less accurate operations. This is particularly problematic for low-frequency qubits like fluxonium, a type of superconducting qubit that is particularly susceptible to counter-rotating dynamics.
To overcome these issues, the researchers at MIT developed two innovative techniques to improve the fidelity of single-qubit gates while minimizing these errors.
Innovations in Control: Commensurate Pulses and Circularly Polarized Drives
The breakthrough stemmed from two distinct control strategies: commensurate pulses and circularly polarized drives. The former was an idea introduced by Leon Ding, a former MIT researcher and lead author of the study, and it provided a simple yet effective method for mitigating counter-rotating errors in fluxonium qubits. The idea behind commensurate pulses is to apply electromagnetic pulses at specific intervals that are commensurate with the qubit’s natural oscillation frequency. This synchronization ensures that counter-rotating errors remain consistent across pulses, making them easier to correct through standard calibration methods.
“In developing the commensurate pulse technique, we realized that the counter-rotating errors could be understood and dealt with in a straightforward way,” says David Rower, a senior author on the paper and a former MIT postdoc. “This technique is not only effective, but also portable—meaning it can be applied to a wide range of qubits suffering from similar errors.”
The second technique, circularly polarized drives, was proposed by Ding as a way to neutralize counter-rotating errors in a superconducting qubit. This approach is analogous to the concept of circularly polarized light, where the polarization direction of light rotates in a continuous fashion. By controlling the relative phase of the electric and magnetic fields that drive the fluxonium qubit, the team was able to create a synthetic version of circularly polarized light in the qubit’s two-dimensional space, which is defined by its electric charge and magnetic flux. This strategy minimizes unwanted interactions, leading to improved gate fidelity.
While circularly polarized drives showed promise, the team found that the fidelities achieved with this method were not as high as anticipated, based on the results of coherence measurements. This led the researchers to explore further refinements, which ultimately led to the discovery of commensurate pulses as the more reliable approach for mitigating counter-rotating dynamics.
Fluxonium Qubits: A Promising Superconducting Platform
One of the key factors that allowed the MIT team to achieve this impressive result was the use of fluxonium qubits, a type of superconducting qubit that incorporates a large “superinductor” to enhance its resilience against environmental noise. Fluxonium is constructed with a capacitor and a Josephson junction, which together form a nonlinear inductive circuit. The inclusion of a superinductor helps to stabilize the qubit against unwanted external interference, making it a promising candidate for scalable quantum computing.
However, fluxonium qubits come with their own set of challenges. Specifically, their lower operating frequency, compared to other superconducting qubits like transmons, results in longer gate times. While this provides improved coherence times, it also means that the qubits require additional control to execute logical operations at high fidelity.
In their experiments, the MIT researchers demonstrated that fluxonium qubits could achieve one of the fastest and highest-fidelity single-qubit gates in superconducting qubit platforms. “Our experiments show that fluxonium is not only an interesting system for basic physical explorations but also a qubit platform that delivers exceptional engineering performance,” says Ding. “With further research, we aim to push this technology to even higher performance levels.”
By combining the fluxonium qubit’s high coherence with the new control techniques, the team was able to significantly reduce errors, enabling high-performance operations that bring us closer to fault-tolerant quantum computing.
A Collaborative Effort: The Intersection of Physics and Engineering
The success of this work highlights the importance of collaboration between physics and engineering to solve complex challenges in quantum computing. William D. Oliver, the senior author and director of MIT’s Center for Quantum Engineering, emphasized the significance of combining theoretical physics with engineering insights to develop practical solutions. “This work exemplifies the type of interdisciplinary collaboration that is crucial for advancing quantum technologies,” says Oliver. “We’ve applied fundamental concepts from both physics and electrical engineering to achieve these high-fidelity results.”
The research also draws on advances in ultrafast attosecond pulses, which are crucial for the control of quantum systems. The team leveraged these concepts, building on techniques that were awarded the 2023 Nobel Prize in Physics. Their approach uses non-adiabatic qubit control and synchronized pulses to execute faster and more accurate gate operations on fluxonium qubits.
Future Prospects: Scaling and Quantum Error Correction
The ultimate goal of quantum computing is to scale systems to a level where they can perform meaningful computations, such as simulating molecular interactions for drug discovery or solving optimization problems for supply chain management. To achieve this, quantum computers need to be able to perform error correction efficiently and without excessive overhead. The recent work at MIT advances this goal by demonstrating high-fidelity qubit operations, which are essential for implementing error correction protocols.
With the success of their new techniques, the researchers believe that further improvements in qubit fidelity will lead to even more powerful quantum computers, capable of performing computations with minimal error rates. This work also comes at a time when other research groups, such as Google’s Quantum AI team, have made significant strides in quantum error correction, with their Willow quantum chip demonstrating quantum error correction beyond threshold for the first time.
As quantum computing moves from theory to practice, these breakthroughs will play a crucial role in building fault-tolerant quantum computers. Higher-performance qubits, like those developed using fluxonium, will reduce the overhead required for quantum error correction, making large-scale quantum computers more feasible. As the MIT team’s work shows, the path to practical quantum computing is built on overcoming fundamental challenges in qubit control, and their work is a significant step forward in that journey.
Reference: David A. Rower et al, Suppressing Counter-Rotating Errors for Fast Single-Qubit Gates with Fluxonium, PRX Quantum (2024). DOI: 10.1103/PRXQuantum.5.040342