Title: Efficient benchmarking of logical magic state Abstract: High-fidelity logical magic states are a critical resource for fault-tolerant quantum computation, enabling non-Clifford logical operations through state injection. However, benchmarking these states presents significant challenges: one must estimate the infidelity $\epsilon$ with multiplicative precision, while many quantum error-correcting codes only permit Clifford operations to be implemented fault-tolerantly. Consequently, conventional state tomography requires $\sim1/\epsilon^2$ samples, making benchmarking impractical for high-fidelity states. In this work, we show that any benchmarking scheme measuring one copy of the magic state per round necessarily requires $\Omega(1/\epsilon^2)$ samples for single-qubit magic states. We then propose two approaches to overcome this limitation: (i) Bell measurements on two copies of the twirled state and (ii) single-copy schemes leveraging twirled multi-qubit magic states. Both benchmarking schemes utilize measurements with stabilizer states orthogonal to the ideal magic state and we show that $O(1/\epsilon)$ sample complexity is achieved, which we prove to be optimal. Finally, we demonstrate the robustness of our protocols through numerical simulations under realistic noise models, confirming that their advantage persists even at moderate error rates currently achievable in state-of-the-art experiments.

  Speaker: Dr. Kimin Park (Department of Optics, Palacky University, Olomouc, Czech Republic)

Title: Quantum Sensing with Unbalanced Cat States: A Practical Pathway Beyond the Gaussian Bound Abstract Quantum metrology aims to enhance measurement precision beyond classical limits. A critical milestone in this field is to surpass the fundamental Gaussian bound. However, the non-Gaussian states required to achieve this are often fragile and experimentally demanding to create. This talk presents a practical and robust pathway to overcome this challenge using asymmetric superpositions of coherent states (SCS), or "unbalanced cat states." We explain how asymmetry is the key resource: for a fixed energy budget, a larger vacuum component allows for maximizing the state's phase-space separation, which dramatically boosts measurement precision. Our protocol, implemented on a circuit QED platform, is experimentally feasible and efficient. We demonstrate a 7.5 dB metrological gain over the classical limit, confirming the quantum advantage. This work establishes asymmetric SCS as a high-performance and scalable resource for next-generation quantum sensors. [1] Park et al, arXiv:2508.13046 [2] Pan et al, PRX Quantum 6, 010304 (2025)

- Venue: Seminar Room 56-219 - Time: 2 p.m

- Title : Quantum computing without quantum computers by active learning.

Venue: 56-521

Speaker: Soumyakanti Bose

Title: Distant communication with Quantum hybrid-light

Abstract: Cavity-quantized optical states possessing non-regular probability distributions, formally known as quantum light, play a crucial role in modern information processing tasks. We bring forth the efficacy of optical-hybrid states, which adjoin the best of both DV and CV worlds. Alongside sharing entanglement over large distances, such states outperform DV-only and CV-only endeavors in nonlocal teleportation and loophole-free Bell experiments. Besides, it is also intriguing to understand what quantum is. Here, we develop a resource-theoretic formulation of optical nonclassicality that provides an entropic quantification of additional sampling required to simulate a quantum state. Moreover, the generic structure enables an extension of other similar theories and systems with discrete observables.