10 Feb (Mon)

Matt McEwen (Google)

TBA

---

Constance Laine (UCL)

Title: High fidelity single shot spin readout in a scalable array of quantum dots

Spin qubits in silicon are a promising platform for scalable quantum computers as they benefit from high density qubit arrays and compatibility with high-yield fabrication processes of the semiconductor industry. However, current qubit sensing technology limits their scalability. Here, we demonstrate a high-fidelity qubit sensor incorporated in a scalable unit cell of three silicon quantum dots fabricated industrially. The qubit sensor enables single shot spin parity measurement with fidelity of 99.95% measured in 290 us. This is made possible by experimentally tuning the quantum dot structure to (i) optimise the dispersive sensor signal-to-noise-ratio and (ii) increase the spin lifetime. Furthermore, we develop a new model to simulate single shot traces using a Hidden Markov Model, which allows for a more accurate definition of measurement fidelity as well as switching between measurement bases. Our results show a clear path to incorporating high fidelity and tunable qubit sensors in silicon spin qubit architectures with a higher degree of qubit connectivity.

---

Laura Caune (Riverlane)

Title: Demonstrating real-time and low latency quantum error correction with superconducting qubits

Quantum error correction (QEC) will be essential to achieve the accuracy needed for quantum computers to realise their full potential. The field has seen promising progress with demonstrations of early QEC and real-time decoded experiments. As quantum computers advance towards demonstrating a universal fault-tolerant logical gate set, implementing scalable and low-latency real-time decoding will be crucial to prevent the backlog problem, avoiding an exponential slowdown and maintaining a fast logical clock rate. Here, we demonstrate low-latency feedback with a scalable FPGA decoder integrated into the control system of a superconducting quantum processor. We perform an 8-qubit stability experiment with up to 25 decoding rounds and a mean decoding time per round below 1 μs, showing that we avoid the backlog problem even on superconducting hardware with the strictest speed requirements. We observe logical error suppression as the number of decoding rounds is increased. We also implement and time a fast-feedback experiment demonstrating a decoding response time of 9.6 μs for a total of 9 measurement rounds. The decoder throughput and latency developed in this work, combined with continued device improvements, unlock the next generation of experiments that go beyond purely keeping logical qubits alive and into demonstrating building blocks of fault-tolerant computation, such as lattice surgery and magic state teleportation.

---

Sara Bartolucci (PsiQuantum)

Title: Improvements to fault tolerant Fusion Based Quantum Computation: the power of adaptivity and beyond

Since the first Fusion-Based Quantum Computation (FBQC) architecture was introduced, schemes with significantly improved error tolerance have been proposed. In particular, the use of adaptivity has led to a substantial increase in the amount of erasure which can be tolerated. In the linear optical platform, this translates into the ability to cope with higher rates of photon loss. This talk reviews this progress and presents other opportunities for further improvements to the performance of the architecture beyond loss tolerance.

---

11 Feb (Tue)

Armanda Quintavalle (FU Berlin)

TBA

---

Adam Wills (MIT)

Title: Quantum Codes with Transversal and Addressable Non-Clifford Gates with Applications to Magic State Distillation

It is particularly difficult to design quantum codes with good error correction parameters for which one may fault-tolerantly address each logical qubit. Recently, the first quantum codes were constructed featuring asymptotically optimal error correction properties and transversal (but not addressable) non-Clifford gates. This, in particular, enabled the discovery of constant-overhead magic state distillation (arXiv:2408.07764). However, the question remained on how to act with these non-Clifford gates on particular logical qubits fault-tolerantly. In this talk, we will discuss the previous work on constant-overhead magic state distillation, and another work, soon to appear, in which we construct the first quantum codes, with good error correction parameters, for which each logical qubit may be transversally addressed with non-Clifford gates, opening up many possibilities in fault-tolerant quantum computation.

---

Kathleen Chang (Yale)

Title: Teleportation can decohere errors

While the performance of quantum error correction (QEC) codes under incoherent Pauli noise has been studied widely, the performance under coherent or unitary errors is less understood. For example, unlike its incoherent counterpart, a constant threshold theorem for coherent noise has not yet been established. Coherent errors are also much harder to simulate, making numerical estimation of thresholds difficult. It has been shown that physical coherent noise can lead to logical coherent errors after QEC, which are undesirable as they can coherently add in a logical circuit, leading to faster accumulation of faults compared to incoherent noise. One way to avoid these issues is to convert coherent errors into incoherent errors via randomized compiling, where the introduction of random single qubit Paulis decoheres the noise. This, however, may come at the cost of extra gates and additional noise. Thus, it is important to develop a better understanding of coherent errors in QEC and ways to directly deal with them. In this work, we extend the current theoretical understanding of coherent noise and show that, remarkably, they are automatically converted to incoherent Pauli errors during teleportation-based QEC. This is because the teleportation circuit teleports the state up to a random Pauli, decohering the noise in a process similar to randomized compiling. As a consequence, logical-level coherence will not constructively interfere over many rounds of error correction, leading to slower error growth with time. Furthermore, all analytical results for QEC of incoherent Pauli noise automatically apply to an experimentally-motivated model of circuit-level $e^{itheta Z}$ errors. Our work adds to the other known features of teleportation-based QEC, distinguishing it from conventional circuit-based QEC, such as inherent mitigation of leakage and qubit loss as well as higher thresholds and code symmetry preservation for certain noise models.

---

Timo Hillmann (Chalmers)

Title: Single-shot and measurement-based quantum error correction via fault complexes

Measurement-based quantum computation is an alternative paradigm to circuit-based quantum computation well-suited to platforms such as photonics, but also has advantages for matter-based platforms such as superconducting circuits and neutral atoms, where it can be used for, e.g., leakage reduction. In this paradigm, in contrast to circuit-based quantum computation, the central object is not a quantum error-correcting code but a fault-tolerant graph state. Various methods for constructing fault-tolerant graph states exist, with the concept of foliation offering a prescription for any CSS code. Here, we introduce the fault complex, a representation of faults in a dynamic quantum error correction protocol rather than a static quantum error-correcting code. We focus on fault complexes obtained via foliation, which we recast in the language of homology as a tensor product between a CSS code and a repetition code. These fault complexes also have an interpretation as repeated rounds of error correction over time in gate-based quantum computation. Analysis of fault complexes enables us to understand the decoding of single-shot codes in measurement-based quantum computation and achieve record error thresholds for the 3D and 4D toric codes. Through the explicit calculation of the homology groups of the fault complex, we generalize the notion of stability experiments, which allows us to conclude that single-shot lattice surgery is possible in higher dimensional codes. This implies that a fault-tolerant quantum computing architecture based on the 4D toric code has an asymptotic spacetime overhead reduction compared to the standard 2D toric code architecture, warranting serious consideration of this approach to building a scalable fault-tolerant quantum computer.

---

12 Feb (Wed)

Hui Khoon Ng (NUS)

Title: Fault-tolerant quantum circuits on connectivity-constrained hardware with swap gates

In near-term quantum computing devices, connectivity between qubits remain limited by architectural constraints. A computational circuit with given connectivity requirements necessary for multi-qubit gates have to be embedded within physical hardware with fixed connectivity. Long-distance gates have to be done by first routing the relevant qubits together. The simplest routing strategy involves the use of swap gates to swap the information carried by two unconnected qubits to connected ones. Ideal swap gates just permute the qubits; real swap gates, however, have the added possibilities of causing simultaneous errors on the qubits involved and spreading errors across the circuit. A general swap scheme thus changes the error-propagation properties of a circuit, including those necessary for fault-tolerant functioning of a circuit. Here, we present a simple strategy to design the swap scheme needed to embed an abstract circuit onto a physical hardware with constrained connectivity, in a manner that preserves the fault-tolerant properties of the abstract circuit. The embedded circuit will, of course, be noisier, compared to a native implementation of the abstract circuit, but we show in the examples of embedding surface codes on heavy-hexagonal and hexagonal lattices that the deterioration is not severe. This then offers a straightforward solution to implementing circuits with fault-tolerance properties on current hardware.

---

Arne Grimsmo (AWS)

TBA

---

Josh Combes (Melbourne)

Title: Two-qubit gate for the soft 0-π qubit

The 0− π qubit is an attractive candidate to replace the Transmon as a higher quality qubit. Single qubit gates have been experimentally demonstrated on this qubit, in the “soft” regime. However, to date there are no proposals for a practical two-qubit gate for 0–π qubits. In this talk we propose a fast microwave-activated controlled-Z gate for the capacitively coupled soft 0− π qubit. We show gate fidelities over 99.99% for gate times around 100ns.

---

13 Feb (Thu)

Markus Frembs (LU Hannover) – Online

Title: A new perspective on Kochen-Specker contextuality

Contextuality – roughly, the impossibility to assign observables definite outcomes prior to experiment – is a key distinguishing feature between classical and quantum physics. Consequently, when understood as a resource for quantum computation, contextuality is expected to hold the key to quantum advantage. Yet, despite its long recognised importance in quantum foundations and, more recently, in quantum computation, the essence of contextuality and its mathematical structure remain poorly understood.
I will give a brief introduction to the subject and present a new framework for contextuality that allows to characterise it in algebraic form. I will also discuss how it subsumes a number of other frameworks, thereby connecting various results that had previously seemed unrelated and even contradictory.

---

David Stephen (Quantinuum)

Title: Fantastic universal gate sets and where to find them

In this talk, I will begin by describing a new universal scheme of measurement-based quantum computation. This scheme has some practical advantages, but it comes at the cost of having an unusual gate set consisting of rotations generated by long Pauli strings. I will then pivot into discussing such gate sets, including how to identify when they are universal and how to compile quantum algorithms with them. I will also show how certain universal gate sets generate all unitaries faster than others, explain this in terms of the fraction of anticommuting pairs of generators, and discuss potential implications to fault-tolerant quantum computation. Overall, these results reveal the necessity and intrigue of considering universal gate sets with distinct structure and properties compared to standard gate sets.

---

Mackenzie Shaw (TU Delft)

Title: Lowering Connectivity Requirements For Bivariate Bicycle Codes Using Morphing Circuits, and More

In Ref. [1], Bravyi et al. found examples of Bivariate Bicycle (BB) codes with similar logical performance to the surface code but with an improved encoding rate. In this work, we generalize a novel parity-check circuit design principle called morphing circuits and apply it to BB codes. We define a new family of BB codes whose parity check circuits require a qubit connectivity of degree five instead of six while maintaining their numerical performance. Logical input/output to an ancillary surface code is also possible in a biplanar layout. We develop a general framework for designing morphing circuits and present a sufficient condition for its applicability to two-block group algebra codes. Finally, we will discuss future directions and other applications of morphing circuits.

[1] S. Bravyi, A. W. Cross, J. M. Gambetta, D. Maslov, P. Rall, and T. J. Yoder, Nature 627, 778 (2024).