QuLunch is a biweekly QuSoft seminar on topics in quantum computation.

• It is chaired by **Māris Ozols,** co-organised by Sarah Meng Li and Jelena Mackeprang.
• In the winter of 2026, QuLunch will feature two formats:
  • Lightning Talks 💡, featuring two 20-minute talks where QuSofters share recent results or discuss problems they are currently working on.
  • QuLunch Seminar 💭, featuring a longer 45-minute talk that allows for a more in-depth exploration of the technical details of a recent work.

Winter 2026

🗓️ Starting from Feb 3rd, every second Tuesday

Jan 21st, 12:00 @L017 QuLunch Seminar 💭

<aside>

Ultra Low Overhead Syndrome Extraction for the Steane code

• Speaker: Boldizsár Poór (Oxford University)

• Abtstract: We establish a new performance benchmark for the fault-tolerant syndrome extraction of $\llbracket7, 1, 3\rrbracket$ Steane code with a dynamic protocol. Our method is built on two highly optimized circuits derived using fault-equivalent ZX-rewrites: a primary fault-tolerant circuit with 14 CNOTs and an efficient non-fault-tolerant recovery circuit with 11 CNOTs. The protocol uses an adaptive response to internal faults, discarding flagged measurements and falling back to the recovery circuit to correct potentially detrimental errors. Monte Carlo simulations confirm the efficiency of our protocol, reducing the logical error rate per cycle by an average of ∼14.3% relative to the optimized Steane method [1] and ∼ 17.7% compared to the Reichardt’s three-qubit method [2], the leading prior techniques.

  [1] Rodatz, B., Poór, B., & Kissinger, A. (2025). Fault Tolerance by Construction. arXiv preprint arXiv:2506.17181.

  [2] Reichardt, B. W. (2020). Fault-tolerant quantum error correction for Steane’s seven-qubit color code with few or no extra qubits. Quantum Science and Technology, 6(1), 015007.

• Joint-work with: Benjamin Rodatz and Aleks Kissinger </aside>

Feb 3rd, 12:00 @L017 QuLunch Seminar 💭

<aside>

QAC^0 and the Quest to Compute Parity

• Speaker: Francisca Vasconcelos (UC Berkeley)
• Abstract: In the NISQ era, where noise limits device coherence times, we are fundamentally constrained to shallow quantum computation. This talk will explore the power and limitations of the QAC^0 circuit class—i.e., the family of polynomial-sized, constant-depth quantum circuits composed of arbitrary single-qubit gates and unbounded CZ/Toffoli gates. I will center the discussion around a long-standing open problem in quantum circuit complexity: is Parity computable in QAC^0? This question is central to our understanding of the power of QAC^0, its relationship to classical AC^0, and entanglement propagation beyond light-cones. I will survey a growing literature of upper- and lower-bounds for this problem, highlighting how the techniques behind these results have led to a range of exciting and surprising applications, such as efficient learnability of QAC^0 as well as constant-depth unitary circuit constructions for pseudorandomness and Dicke state preparation.
• The talk is based on joint work with Ben Foxman, Hsin-Yuan Huang, Malvika Joshi, Shivam Nadimpalli, Natalie Parham, Avishay Tal, John Wright, and Henry Yuen, as well as results by several others in the field. </aside>

Feb 17th, 13:00 @L017: Lightning Talks 💡

Dmitry Grinko

<aside> ⚡

TBA

</aside>

Galina Pass

<aside>

A Quantum Time-Space Tradeoff for Directed st-Connectivity

• Abstract: Directed st-connectivity (dstcon) is the problem of deciding if there exists a directed path between a pair of distinguished vertices s and t in an input directed graph. This problem appears in many algorithmic applications, and is also a fundamental problem in complexity theory, due to its NL-completeness. We show that for any $S \geq \log^2(n)$, there is a quantum algorithm for dstcon using space $S$ and time $T \leq 2^{\frac{1}{2}\log(n)\log(n/S)+o(\log^2(n))}$, which is an (up to quadratic) improvement over the best classical algorithm for any $S = o(\sqrt{n})$. Of the S total space used by our algorithm, only $O(\log^2(n))$ is quantum space – the rest is classical. This effectively means that we can trade off classical space for quantum time.
• Joint-work with: Stacey Jeffery </aside>

Mar 3rd, 12:00 @L016: Lightning Talks 💡

Freek Witteveen

<aside>

A Random Purification Channel for Arbitrary Symmetries with Applications to Fermions and Bosons

• Abstract: The random purification channel maps n copies of any mixed quantum state to n copies of a random purification of the state. We generalize this construction to arbitrary symmetries: for any group G of unitaries, we construct a quantum channel that maps states contained in the algebra generated by G to random purifications obtained by twirling over G. In addition to giving a surprisingly concise proof of the original random purification theorem, our result implies the existence of fermionic and bosonic Gaussian purification channels. As applications, we obtain the first tomography protocol for fermionic Gaussian states that scales optimally with the number of modes and the error, as well as an improved property test for this class of states.
• Joint-work with: Michael Walter </aside>

John van de Wetering

<aside>

Can You Derive Quantum Theory from Nice Physical Principles?

</aside>

Mar 17th, 12:00 @L016 QuLunch Seminar 💭

<aside>

QuantumBoost: A Lazy, yet Fast, Quantum Algorithm for Learning with Weak Hypotheses

• Speaker: Ronald de Wolf
• Abstract: The technique of combining multiple votes to enhance the quality of a decision is the core of boosting algorithms in machine learning. In particular, boosting provably increases decision quality by combining multiple weak learners-hypotheses that are only slightly better than random guessing-into a single strong learner that classifies data well. There exist various versions of boosting algorithms, which we improve upon through the introduction of QuantumBoost. Inspired by classical work by Barak, Hardt and Kale, our QuantumBoost algorithm achieves the best known runtime over other boosting methods through two innovations. First, it uses a quantum algorithm to compute approximate Bregman projections faster. Second, it combines this with a lazy projection strategy, a technique from convex optimization where projections are performed infrequently rather than every iteration. To our knowledge, QuantumBoost is the first algorithm, classical or quantum, to successfully adopt a lazy projection strategy in the context of boosting.
• Joint-work with: Amira Abbas, Yanlin Chen, and Tuyen Nguyen </aside>

Mar 31st, 12:00 @L120: Lightning Talks 💡

**Joppe Stokvis**

<aside>

Hidden Shift Problem for Complex Functions

• Abstract: We study quantum algorithms for the hidden shift problem of complex scalar- and vectorvalued functions on finite abelian groups. Given oracle access to a shifted function and the Fourier transform of the unshifted function, the goal is to find the hidden shift. We analyze the success probability of our algorithms when using a constant number of queries. For bent functions, they succeed with probability 1, while for arbitrary functions the success probability depends on the ‘bentness’ of the function.
• Joint-work with: Serge Adonsou, Peter Bruin, and Maris Ozols </aside>

Sarah Meng Li

<aside>

A Complete and Natural Rule Set for Multi-Qudit Clifford Circuits in All Odd Prime Dimensions

• Abstract: We present a complete set of rewrite rules for multi-qudit Clifford circuits, where $\emph{qudit}$ denotes a d-level quantum system with d an odd prime. Completeness means that any two Clifford circuits representing the same linear map can be transformed into each other using these rules. In total, there are 19 $\emph{Clifford relations}$, each involving at most three qudits and admitting an intuitive interpretation.

  Our approach leverages the isomorphism between the symplectic group $\mathrm{Sp}(2n, \mathbb{Z}_d)$ and the quotient of the Clifford group by the Pauli group. We first derive a complete set of $\emph{symplectic relations}$ for $\mathrm{Sp}(2n, \mathbb{Z}_d)$, and then lift them to Clifford relations by incorporating Pauli corrections. To do this, we introduce a $\emph{symplectic normal form}$ that captures the stabiliser tableau of a Clifford operator and is unique up to Pauli correction. This simplification enables a streamlined derivation of a complete set of 66 relations, which we further compress to 18 symplectic relations. Our computations in $\mathrm{Sp}(2n, \mathbb{Z}_d)$ are formalised in the Agda proof assistant, providing a machine-verified proof of correctness.

• Joint-work with: Xiaoning Bian, Neil J. Ross, John van de Wetering, and Yuming Zhao </aside>

Apr 14th, 12:00 @M290: QuLunch Seminar 💭

<aside>

Quantum Search With Generalized Wildcards

• Speaker: Nithish Raja

• Abstract: In the “search with wildcards” problem [Ambainis, Montanaro, Quantum Inf. Comput.’14], one’s goal is to learn an unknown bit-string $x \in \{−1, 1\}^n$. An algorithm may, at unit cost, test equality of any subset of the hidden string with a string of its choice. Ambainis and Montanaro showed a quantum algorithm of cost $O(\sqrt{n} \log n)$ and a near-matching lower bound of $\Omega(\sqrt{n})$. Belovs [Comput. Comp.’15] subsequently showed a tight $O(\sqrt{n})$) upper bound.

  We consider a natural generalization of this problem, parametrized by a subset $Q ⊆ 2^{[n]}$, where an algorithm may test whether xS = b for an arbitrary $S ∈ Q$ and $b ∈ \{−1, 1\}^S$ of its choice, at unit cost. We show the following:

  • For all $k \in [n]$, when $Q$ is the collection of all sets of size at most k, the quantum query complexity is $\Theta(n/\sqrt{k})$. In particular when $k=n$, this corresponds to the standard search with wildcards setting. This recovers and generalizes the tight characterization of Belovs, and Ambainis and Montanaro, using completely different techniques.
  • When $Q$ is the collection of contiguous blocks, the quantum query complexity is $\widetilde{\Theta}(n)$.
  • When $Q$ is the collection of prefixes, the quantum query complexity is $\Theta(n)$.

  All of these results are derived using a framework that we develop. We apply a symmetry reduction to the primal version of the negative-weight adversary bound, and show that the quantum query complexity of learning x is characterized, up to a constant factor, by a particular optimization program, which can be succinctly described as follows: ‘maximize over all odd functions $f : \{−1, 1\}^n \rightarrow \mathbb{R}$ the ratio of the maximum value of $f$ to the maximum (over $T ∈ Q$) standard deviation of f on a subcube whose free variables are exactly T.’

  To the best of our knowledge, ours is the first work to use the primal version of the negative-weight adversary bound (which is a maximization program typically used to show lower bounds) to show new quantum query upper bounds without explicitly resorting to SDP duality.

• Joint-work with: Arjan Cornelissen, Nikhil S. Mande, Subhasree Patro, and Swagato Sanyal </aside>

Apr 28th, 12:00 @L016: Lightning Talks 💡

Jonas Helsen

<aside>

Anti-Concentration is (almost) All You Need

• Abstract: Until very recently, it was generally believed that the (approximate) 2-design property is strictly stronger than anti-concentration of random quantum circuits, mainly because it was shown that the latter anti-concentrate in logarithmic depth, while the former generally need linear depth circuits. This belief was disproven by recent results which show that so-called relative-error approximate unitary designs can in fact be generated in logarithmic depth, implying anti-concentration. Their result does however not apply to ordinary local random circuits, a gap which we close in this letter, at least for 2-designs. More precisely, we show that anti-concentration of local random quantum circuits already implies that they form relative-error approximate state 2-designs, making them equivalent properties for these ensembles. Our result holds more generally for any random circuit which is invariant under local (single-qubit) unitaries, independent of the architecture.
• Joint-work with: Markus Heinrich, Jonas Haferkamp, and Ingo Roth, </aside>