High-speed control technique enables record fidelity in superconducting qubits

In the artist’s impression of a recent MIT experiment, the sphere in the center represents a qubit, illuminated by two control signals: electric charge (blue) and magnetic flux (purple). These control signals are designed to combine to produce circularly polarized microwaves that are not affected by counter-rotation effects. The signals consist of repeating waveforms and represent the similarity of control pulses resulting from the authors’ corresponding drive techniques. Credit: Sampson Wilcox / Research Laboratory of Electronics
Quantum computing promises to solve complex problems exponentially faster than traditional computers by using principles of quantum mechanics to encode and manipulate information in quantum bits (qubits). I am.
Qubits are the building blocks of quantum computers. However, one challenge to scaling is that qubits are highly sensitive to background noise and control imperfections, which introduce errors in quantum operations and ultimately limit the complexity and duration of quantum algorithms. That’s it. To improve the situation, researchers at MIT and around the world have continued to focus on improving the performance of qubits.
In a new study, researchers at MIT’s Department of Physics, Laboratory of Electronics and Computer Science (RLE), and Department of Electrical Engineering and Computer Science (EECS) use superconducting qubits called fluxonium to make the world a reality. We have developed two new control technologies. Recorded single qubit fidelity of 99.998%. This result complemented the two-qubit gate fidelity of 99.92% demonstrated last year by then-MIT researcher Leon Ding.
The results of this research are published in the journal “PRX Quantum”.
The paper’s senior author is Dr. David Rower, a recent physics postdoctoral fellow in MIT’s Engineering Quantum Systems (EQuS) group and currently a research associate in the Google Quantum AI Institute. Leon Ding Ph.D. (from EQuS) currently leads the proofreading team at Atlantic Quantum. William D. Oliver is the Henry Ellis Warren Professor of Physics at EECS, leader of EQuS, director of the Center for Quantum Engineering, and deputy director of RLE.
Decoherence and back-rotation errors
A major challenge in quantum computing is decoherence, the process by which qubits lose their quantum information. For platforms such as superconducting qubits, decoherence hinders the realization of higher-fidelity quantum gates.
Quantum computers need to achieve high gate fidelity to implement persistent computations through protocols such as quantum error correction. The higher the gate fidelity, the easier it is to achieve practical quantum computing.
MIT researchers are developing techniques to make quantum gates, the basic operation of quantum computers, as fast as possible to reduce the effects of decoherence. However, as gates get faster, another type of error can arise due to the way electromagnetic waves are used to control the qubits, resulting from counter-rotating dynamics.
Single-qubit gates are typically implemented using resonant pulses that induce Rabi oscillations between qubit states. However, if the pulses are too fast, the “Ravi gate” is not very stable, as it introduces undesirable errors due to back-rotation effects. The faster the gate, the more pronounced the back-rotation error will be. For low-frequency qubits such as fluxonium, back-rotation errors limit the fidelity of fast gates.
“It was a fun challenge for us to eliminate these errors,” says Rower. “Originally, Leon had the idea of utilizing a circularly polarized microwave drive, which is similar to circularly polarized light, but achieved this by controlling the relative phase of the charge and flux drives in a superconducting qubit. A circularly polarized drive like this would ideally be immune to back-rotation errors.
Ding’s idea worked immediately, but the fidelity achieved with circularly polarized drives was not as high as expected from coherence measurements.
“In the end, we came up with a beautiful and simple idea,” says Rower. “If you apply the pulses at exactly the right time, you should be able to consistently generate a back-rotation error between pulses. This makes the back-rotation error correctable. Even better, the back-rotation error is correctable. Errors are automatically taken into account.” This is the usual Rabbigate proofreading. ”
They called this idea the “coincidence pulse.” This is because pulses need to be applied from time to time, depending on the interval determined by the qubit frequency and its inverse, the time period. The corresponding pulses are easily defined by timing constraints and can be applied to drive a single linear qubit. In contrast, circularly polarized microwaves require two drives and additional calibration.
“It was a lot of fun developing the appropriate techniques,” says Lower. “It’s simple, we understand why it works so well, and it should be portable to any qubit that suffers from back-rotation errors.
“This project reveals that back-rotation errors can be easily dealt with, which is great for low-frequency qubits like fluxonium, which are increasingly showing promise in quantum computing.”
Fluxonium Promise
Fluxonium is a type of superconducting qubit that consists of a capacitor and a Josephson junction. However, unlike transmon qubits, fluxonium also contains a large “superinductor,” which by design helps protect the qubit from environmental noise. This allows logical operations (gates) to be performed more accurately.
However, despite its high coherence, fluxonium has a low qubit frequency and typically has proportionally longer gates.
“Here, we have demonstrated the fastest and highest-fidelity gate of all superconducting qubits,” Ding says. “Our experiments really show that fluxonium is a qubit that supports interesting physical exploration and also delivers absolute gains in terms of engineering performance.”
With further research, they hope to uncover new limitations and yield even faster, higher-fidelity gates.
“Counter-rotational dynamics are understudied in the context of superconducting quantum computing because the rotating wave approximation holds well in general scenarios,” Ding says. “Our paper shows how to accurately calibrate fast low-frequency gates where the rotating wave approximation does not hold.”
Physics and engineering work together
“This is a great example of the type of work we want to do at EQuS, as it leverages fundamental concepts from both physics and electrical engineering to achieve better results,” Oliver says. “This builds on previous work using non-adiabatic qubit control and applies it to a new qubit, fluxonium, making a beautiful connection with counter-rotating dynamics.”
The science and engineering teams achieved high fidelity in two ways. First, the research team demonstrated a “commensurate” (synchronous) nonadiabatic control that goes beyond the standard “rotating wave approximation” of the standard Rabi approach. It leverages the idea that won the 2023 Nobel Prize in Physics for ultrafast “attosecond” light pulses.
We then demonstrated it using an analog of circularly polarized light. Rather than a physical electromagnetic field with rotating polarization vectors in real xy space, we used the xy space of the qubit to achieve a synthetic version of circularly polarized light. In this case, it corresponds to the magnetic flux and charge of the qubit.
This result was made possible by a combination of a new interpretation of an existing qubit design (Fluxonium) and the application of advanced control techniques applied to an understanding of the underlying physics.
Because this work is platform-independent and does not require additional calibration overhead, it establishes a simple strategy for mitigating back-rotation effects due to strong drives in circuit quantum electrodynamics and other platforms. They hope this will aid efforts to achieve high-fidelity control of faults. -Resilient quantum computing.
“While Google’s Willow quantum chip was recently announced and demonstrated suprathreshold quantum error correction for the first time, this is a timely result as we continue to improve performance even further,” Oliver said. “Higher performance qubits lead to lower data overhead requirements.” Implemented error correction. ”
Further information: 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
Provided by Massachusetts Institute of Technology
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