⚛️ quantum physics

Ultracoherent superconducting cavity-based multiqudit platform with error-resilient control

This paper demonstrates a two-mode superconducting radio-frequency cavity platform coupled to an ancillary transmon that achieves ultracoherent storage (with photon lifetimes exceeding 15 ms) and high-fidelity error-resilient control, enabling the preparation of high-dimensional Fock states and near-perfect entanglement to advance cavity-based systems beyond memory-only applications toward scalable modular quantum processing.

Original authors: Taeyoon Kim, Tanay Roy, Xinyuan You, Andy C. Y. Li, Henry Lamm, Oleg Pronitchev, Mustafa Bal, Sabrina Garattoni, Francesco Crisa, Daniel Bafia, Doga Kurkcuoglu, Roman Pilipenko, Paul Heidler, Nicholas

Published 2026-07-22

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CC BY 4.0

Original authors: Taeyoon Kim, Tanay Roy, Xinyuan You, Andy C. Y. Li, Henry Lamm, Oleg Pronitchev, Mustafa Bal, Sabrina Garattoni, Francesco Crisa, Daniel Bafia, Doga Kurkcuoglu, Roman Pilipenko, Paul Heidler, Nicholas Bornman, David van Zanten, Silvia Zorzetti, Roni Harnik, Akshay Murthy, Andrei Lunin, Sergey Belomestnykh, Shaojiang Zhu, Changqing Wang, Andre Vallieres, Ziwen Huang, Jens Koch, Anna Grassellino, Srivatsan Chakram, Alexander Romanenko, Yao Lu

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the world of quantum computing as a massive, high-stakes library where information isn't stored on paper, but in the delicate vibrations of tiny, invisible particles. For years, scientists have been trying to build this library using "qubits," which are like tiny light switches that can be either on or off. But there's a problem: these switches are incredibly fragile. A tiny bump, a whisper of heat, or a stray magnetic field can flip them by accident, ruining the story they are trying to tell. To fix this, researchers have been trying to make the switches last longer, but it's like trying to keep a soap bubble from popping while you try to paint a masterpiece on it.

However, there is a different way to think about this library. Instead of just using on/off switches, imagine using musical instruments that can play many different notes at once. In physics, these are called "qudits." While a qubit is like a coin that is heads or tails, a qudit is like a piano key that can be pressed to play a low note, a high note, or anything in between. This allows you to pack much more information into a single spot. The challenge has always been that these "instruments" are hard to control without breaking them. If you try to play a note too loudly or too quickly, the instrument might shatter. The big question for scientists has been: Can we build a musical instrument that is so perfect it can hold a note for a long time, yet still be easy enough to play complex songs without breaking it?

This is exactly what a team of researchers at Fermi National Accelerator Laboratory and Northwestern University set out to solve. They built a special "quantum instrument" using a superconducting radio-frequency (SRF) cavity, which is essentially a hollow, ultra-clean metal box that traps light (in the form of microwave photons) inside. Think of this box as a perfectly smooth, silent bowling alley where a ball can roll forever without losing speed. But to do anything useful, you need to be able to hit the ball and change its path. Usually, the tools used to hit the ball (called "control circuits") are so messy that they introduce noise and cause the ball to stop rolling much faster than it should.

The team's breakthrough was designing a system where the "hitter" (a small circuit called a transmon) is so gently connected to the "bowling alley" (the cavity) that it doesn't disturb the ball's perfect roll, yet is still strong enough to guide it. They managed to keep a single photon (a particle of light) alive inside this box for an astonishing 20.6 milliseconds in one mode and 15.6 milliseconds in another. In the world of quantum mechanics, where things usually vanish in microseconds, this is like keeping a soap bubble floating for an entire afternoon.

But keeping the ball rolling is only half the battle; you also need to be able to play a song. The researchers developed a clever "sideband" technique, which is like using a specific rhythm to nudge the ball up a ladder of energy levels. They wanted to create states with up to 20 photons (like stacking 20 balls in a row). The problem was that every time they nudged the ball, there was a chance the "hitter" circuit would make a mistake, causing the ball to slip down a rung. To fix this, they invented a "sideband feedforward protocol." Imagine a referee watching every single step of the ball's climb. If the referee sees the ball slip, they immediately shout a correction to push it back up before it falls too far. If the referee sees a bigger problem, they can even choose to ignore that specific attempt and try again.

Using this error-checking system, the team successfully prepared states with up to 20 photons with a success rate (fidelity) exceeding 95%. They also managed to link the two modes of their cavity together, creating an entangled state where the two "bowling alleys" shared a secret connection. When they applied their error-checking and post-selection tricks, they achieved an entanglement fidelity approaching 99.9%. This means they could swap information between the two modes with almost perfect accuracy, limited only by the natural, unavoidable decay of the system.

The paper explicitly rules out the idea that you need to sacrifice coherence (the "long life" of the state) to get good control. They showed that by using weak coupling and smart error correction, you can have both. They also demonstrated that while heating of the control circuit was a major source of noise, it could be detected and filtered out, rather than being an unsolvable problem. The results are not just simulations; they are measured experimental data showing that high-dimensional quantum states can be created and manipulated with high precision. This work suggests a practical path forward for building quantum computers that use these "multi-note" qudits, potentially making them more compact and powerful for simulating complex chemistry and physics, all while keeping the delicate quantum information safe from the chaos of the outside world.

Technical Summary: Ultracoherent Superconducting Cavity-Based Multiqudit Platform with Error-Resilient Control

Problem Statement
Realizing the full potential of quantum computing requires simultaneously expanding the Hilbert space and extending coherence times. While qubit-based architectures are dominant, qudits (multi-level systems) offer more compact circuits and natural representations for quantum simulation in chemistry, condensed matter, and high-energy physics. Superconducting radio-frequency (SRF) cavities are ideal candidates for qudit-based technologies due to their exceptionally low dissipation and large bosonic Hilbert spaces. However, a critical bottleneck exists: the nonlinear circuitry required for control and measurement typically introduces loss and noise that erode the cavity's intrinsic memory advantage. The central challenge is to achieve universal control and high-fidelity operations in high-coherence cavities without reintroducing the errors the cavity is designed to suppress, particularly when targeting high-dimensional manifolds requiring many sequential operations.

Methodology and System Design
The authors demonstrate a two-mode bosonic module implemented in a two-cell niobium SRF cavity based on the TESLA geometry. The system features:

Hardware Architecture: Two elliptically shaped cells connected by an iris, supporting symmetric ("Alice") and antisymmetric ("Bob") normal modes with frequencies of 5.779 GHz and 6.872 GHz. The cavity is fabricated from high-purity niobium with e-beam welding and chemical/heat treatments to minimize surface loss.
Ancilla Coupling: A transmon qubit (frequency 6.402 GHz) is weakly coupled to both cavity modes. The geometry is engineered to minimize chip participation and dispersive shifts (–71 kHz for Alice, –96 kHz for Bob) while maintaining controllable coupling. This weak-coupling regime preserves the cavity's long coherence times.
Control Protocols:
- Sideband (SB) Interactions: Used to implement fast cavity control on timescales shorter than the dispersive shift limit.
- Sideband Feedforward Protocol (SFP): To mitigate ancilla-induced errors, the system employs high-fidelity single-shot measurements of the transmon in $|g\rangle$ , $|e\rangle$ , and $|f\rangle$ states after each sideband pulse. Detected errors trigger corrective pulses.
- Parity Filter (PF): A post-selection technique applied at the end of state preparation sequences to filter out off-target populations (specifically $|N-1\rangle$ states caused by relaxation).
- Virtual Raman-Assisted Beamsplitter (VRBS): A two-mode entangling operation mediated by the transmon's $|f\rangle$ level. By driving the transmon at a detuning $\Delta$ from the $|f0\rangle \leftrightarrow |g1\rangle$ sideband transitions of both modes, a coherent photon exchange is activated. Mid-circuit measurements detect transmon heating errors to further improve fidelity.

Key Results

Record Coherence: The system achieves single-photon lifetimes ( $T_1$ ) of 20.6 ms (Alice) and 15.6 ms (Bob), with dephasing times ( $T_2$ ) exceeding 40 ms. These represent record multimode quantum memory lifetimes. The decay rate of Fock states follows the theoretical $\gamma_n = n\gamma_1$ relationship, with the $|20\rangle$ state maintaining a lifetime above 1 ms.
High-Fidelity State Preparation: Using the SB scheme combined with SFP and PF, the authors prepare Fock states up to $N=20$ with fidelities exceeding 95% (95.3% for Alice, 94.6% for Bob). The SFP significantly suppresses leakage to lower levels, and PF further refines the target population.
Two-Mode Entanglement: The VRBS interaction enables coherent photon exchange between modes. The coherence-limited beamsplitter fidelity reaches 99.686%. After post-selecting to remove erasure errors (photon loss) and heating errors (via mid-circuit checks), the estimated fidelity approaches 99.9%.
Error Characterization: The study identifies that at large detunings, decoherence is dominated by drive-induced transmon heating rather than Purcell decay. Mid-circuit detection of these heating events allows for a twofold fidelity enhancement in SWAP operations.

Significance and Claims
This work establishes a practical route toward high-dimensional encodings and scalable modular quantum information processing by combining ultracoherent storage with high-fidelity, error-resilient control. The authors claim that their results move cavity-based hardware beyond "memory-only" operation. Key contributions include:

Validation of Primitives: Providing experimentally validated primitives for high-dimensional bosonic encodings, specifically demonstrating reliable access to large Fock manifolds ( $N=20$ ) and verified two-mode exchange.
Error Resilience: Demonstrating that control strategies can remain fast and reliable even in the high-coherence limit by making key operations leading-order immune to ancilla errors through measurement-based correction and post-selection.
Scalability: Showing that the two-cell elliptical architecture is extensible. Increasing the number of TESLA cells could yield larger multimode systems for random-access quantum memories, while two-cell modules can serve as networkable building blocks for modular quantum systems.
Qudit Potential: The demonstrated capabilities support a scalable route toward qudit-based quantum computation and simulation, leveraging the natural multi-level structure of SRF cavities for applications in chemistry, condensed matter, and high-energy physics.

The paper concludes that while gate speeds are currently limited by engineering constraints (sideband rates), the fundamental approach of combining SRF cavities with error-detecting ancilla control offers a viable path to high-dimensional quantum processing.

Technical Summary: Ultracoherent Superconducting Cavity-Based Multiqudit Platform with Error-Resilient Control

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