Causes of Quantum System Instability

I’ve definitely done this before: placing wirebonds across the resonator to connect ground planes. Back then, it seemed harmless—maybe even necessary. But it turns out, a single wirebond can form a parasitic Josephson junction with the oxidized aluminum pad beneath. And if that junction happens to be enclosed in a superconducting loop—formed by other bond wires or traces—it becomes a parasitic RF-SQUID. And then things start to break. This parasitic SQUID can cause: • 𝗦𝘁𝗿𝗼𝗻𝗴 𝗗𝗖 𝗺𝗮𝗴𝗻𝗲𝘁𝗶𝗰 𝗰𝗼𝘂𝗽𝗹𝗶𝗻𝗴 to nearby flux-tunable transmons, modulating the qubit frequency in a hysteretic, sawtooth-like pattern. • 𝗗𝗶𝘀𝗽𝗲𝗿𝘀𝗶𝘃𝗲 𝗔𝗖 𝗰𝗼𝘂𝗽𝗹𝗶𝗻𝗴 to the readout resonator, producing sharp, asymmetric dips in frequency at regular intervals. • 𝗖𝗼𝗺𝗽𝗹𝗲𝘁𝗲 𝘀𝘂𝗽𝗽𝗿𝗲𝘀𝘀𝗶𝗼𝗻 𝗼𝗳 𝗾𝘂𝗯𝗶𝘁 𝗳𝘂𝗻𝗰𝘁𝗶𝗼𝗻𝗮𝗹𝗶𝘁𝘆 in some cases.    All of this—from a wirebond! And what’s worse: the entire effect can vanish the moment the wirebond is removed. It’s the kind of issue that’s easy to miss, especially in early-stage experiments where manual bonding is common and attention is focused on the qubits. But it’s a crucial reminder: in superconducting quantum circuits, the entire assembly 𝘪𝘴 the device. Wirebonds, airbridges, packaging—none of it is outside the quantum system. We spend enormous effort optimizing gates, fidelities, and calibration routines. But sometimes, the root cause of instability isn’t in the software—or even in the circuit design. It’s in the loop you didn’t mean to make. 📸 Image Credits: B. Berlitz et al. (2025, arXiv:2505.20458)

New Approach Reduces Decoherence in Qudit-Based Quantum Processors A team of physicists from the University of Southern California (USC) and UC Berkeley has developed a new method to reduce decoherence in qudit-based quantum computers, potentially improving their stability and computational power. The research, published in Physical Review Letters, introduces dynamical decoupling (DD) protocols tailored for qudits, which could significantly enhance the performance of multi-level quantum computing systems. Why Qudits Matter • Traditional quantum computers store and process information using qubits, which exist in a superposition of two states (0 and 1). • Qudits, on the other hand, can exist in more than two states, allowing them to store more information per unit and perform computations more efficiently. • The challenge? Qudits are more prone to decoherence, a process where quantum states degrade due to environmental interference, leading to errors and data loss. How the New Protocol Works • The researchers developed a novel dynamical decoupling (DD) technique specifically designed to counteract environmental noise in qudit-based systems. • By applying precisely timed quantum operations, the system cancels out decoherence effects, allowing for longer coherence times and more stable quantum operations. • This approach could enable more practical and scalable quantum processors, as qudits have the potential to perform complex calculations more efficiently than qubit-based systems. Implications for Quantum Computing • Enhanced Quantum Performance – More stable qudit-based quantum computers could outperform qubit systems in optimization, simulation, and cryptography. • Lower Hardware Requirements – Because each qudit can store more information, future quantum processors could require fewer physical qubits, reducing hardware complexity. • A Step Closer to Practical Quantum Computing – Solving decoherence issues is one of the biggest challenges in making large-scale quantum computers viable for real-world applications. The Bigger Picture While qubit-based quantum computers dominate current research, this breakthrough highlights the growing interest in qudits as a more powerful alternative. If further developed, qudit-based quantum systems could revolutionize computing, unlocking greater efficiency and computational power while overcoming some of the biggest limitations of current quantum technology.

TL;DR The McDermott group (and Qolab) identified the main cause of decoherence plaguing the scalability superconducting qubits - defects within 500 nm of the Josephson Junction. 💥 To provide more context - superconducting qubits have good coherence in the 10-100 qubit range, but suddenly drop off at the 1000+ range. This is because as the array size of the qubits grows, there us a higher probability of picking up defects that killing the overall performance of the system. What makes this paper great is the excruciating amount of data that was taken to draw these conclusions. Whenever dealing with distributions📊 of performance, one need to look a large number of experiments over time and over a wide range of frequencies = ridiculous amount of time in the lab.🧪 Congrats to Spencer Weeden for the hard work.😓 This paper further strengthens the argument that we need to move away from liftoff (a fabrication process that has long been abandoned by the semiconductor industry) to a semiconductor fabrication compatible process such as the window junction or overlap junction. https://lnkd.in/eZ6rEXis

One of the most important problems in quantum computing is… stability!! And a recent research update highlights why that might be changing. Scientists are making progress in understanding and controlling Majorana-based quantum states, a key step toward building more stable quantum systems. Could you tell me why this matters?? Most quantum systems today struggle with: - noise - decoherence - fragile qubits This is why scaling quantum computers is so difficult. Enter Majorana states: Majorana-based systems are exciting because they are: - topologically protected - more resistant to environmental noise - potentially better suited for fault-tolerant quantum computing In simple terms: Instead of constantly correcting errors… We design systems that are naturally more stable. The bigger picture, this isn’t just a physics milestone. It’s part of a broader shift toward: - hardware that is easier to scale - reduced error correction overhead - more practical quantum architectures Quantum progress doesn’t come from one big breakthrough. It comes from many small steps like this that reduce friction in the system. Curious to hear your view: What will matter more for practical quantum systems? - Better hardware stability - Better error correction - Better algorithms - Better integration with classical systems Comment below 🔗 Source: https://lnkd.in/gDxj5CNT #QuantumComputing #DeepTech #QuantumHardware #Innovation #Physics

The stability of quantum computations using trapped ions faces a significant challenge from the loss of individual ions, an event that can cascade and destroy the entire quantum state. Nolan J. Coble from the University of Maryland, College Park, Min Ye, and Nicolas Delfosse from IonQ Inc, now demonstrate a method to correct for these chain losses in long sequences of trapped ions. Their work addresses a critical problem, as even rare ion loss events destabilise the entire chain, effectively erasing all quantum information. The team proposes a distributed error correction code, incorporating ‘beacon’ qubits to detect chain loss and a decoder to convert these losses into correctable errors, thereby safeguarding quantum computations against this pervasive source of instability. https://lnkd.in/ettbCF6i