Seminars

In ETH Zurich, we have assembled a primitive two-node superconducting circuit QED network consisting of two dilution cryostats connected into a 30m-long cryogenic quantum link. Performing a loophole-free Bell test with this system allowed us to gain access to the elusive resource of non-locality, and enabled implementing {\it device-independent} routines. In the first part of the talk, I will present device-independent self-testing and randonmess amplification protocols, which perform two tasks not attainable in classical information processing without extra assumptions: verifying correct operation of the communication network without trusting the constituent devices, and generating certified high-quality randomness without having prior access to perfect randomness. In the second part of the talk, I will show new experimental results based on the ongoing joint work with CSIC. We perform a bidirectional photon transfer protocol, where a microwave photon emitted by Alice’s communication qubit acquires a conditional phase based on the state of Bob’s qubit. We vary the communication photon frequency, hence tuning the conditional phase acquired during the reflection off the remote node, effectively implementing an arbitrary-phase CZ gate. We apply it to perform remote single-shot readout of a quantum bit in a network node located 30~m away, achieving single-shot readout fidelity exceeding 83%. An interesting feature of the scheme is that it requires neither precise synchronization between active node and the remote node, nor any active participation by the remote node.

Scaling up quantum computers remains a significant challenge due to their extreme sensitivity to environmental interactions. While early theoretical proposals have led to proof-of-concept quantum computers, the problem of achieving large-scale, fault-tolerant computing is still unresolved. Distributed Quantum Computing (DQC) offers a promising approach by distributing computation and memory across multiple medium-sized nodes, reducing cross-talk and control overhead. However, this introduces the need for fast, reliable, and synchronized quantum state transfer between nodes. This thesis focuses on quantum state transfer networks, where quantum information is transmitted by mapping stationary qubits onto propagating ones. Superconducting circuits, with their strong and tunable light-matter interactions, provide an ideal platform for implementing these networks. The research has two main objectives: (1) developing a theoretical quantum optical model for state transfer networks based on microwave quantum links, and (2) designing novel protocols and mitigation strategies to address critical challenges in current implementations. Through detailed modeling, this work identifies often-overlooked error sources in superconducting circuits, such as wavepacket distortion from nonlinear dispersion relations and non-uniform couplings, which compound known issues like slow measurements, limited coherence times, and cryogenic requirements. Additionally, this thesis addresses the challenges of multiplexing quantum information and distributing entanglement across the network, both of which are crucial for scalability. The results demonstrate that state transfer networks based on microwave quantum links are a viable and scalable solution for quantum information processing, provided that appropriate design protocols and error mitigation strategies are implemented.

The generation of high Fock states of light with provable quantum non-Gaussian features is still very challenging, although the power of conditional methods to herald the approximate state from the available Gaussian states is growing. The atom-light interaction in the high-Q cavity has been considered a viable alternative to the heralded Fock states from nonlinear optics limited to three-photon Fock states for the last decade. In the presentation, combining atom-light interaction with available optical delay elements, we conclusively predict filtering of the Fock states of up to ten photons with a high success rate of 20% using a hierarchy of quantum non-Gaussian criteria. To demonstrate their quality for applications, we evaluate the robustness of such features, the bunching capability in a linear network, and the sensing capability to estimate the magnitude of unknown force and noise.

The talk will report recent theoretical and experimental achievements opening the door to highly non-Gaussian quantum states at optical and mechanical platforms. This territory is challenging for investigation, both theoretically and experimentally. We will present recent achievements, mainly the experimental tests of climbing the hierarchy of quantum non-Gaussian photonic and phononic states suitable for applications. Particular focus will be on new quantum non-Gaussian coherences and their experimental verification. The talk will conclude with related results and the following challenges in theory and experiments with light, atoms, mechanical oscillators and superconducting circuits to stimulate discussion and further development of this advancing and prospective field.

Hyperbolic lattices, characterized by negative curvature and non-commutative translations, offer a rich playground for exploring exotic electronic states. This talk explores these systems through a multifaceted approach, bridging theory and experiment. We begin with a concise introduction to hyperbolic lattices and then move to the results to present curvature-dependent Hofstadter butterfly spectrum in the presence of a magnetic field; we acknowledge the experimental challenges in directly realizing these structures. We introduce an indirect approach that decomposes the problem with hyperbolic lattices into two parts: curvature and non-commutative geometry. This method breaks down hyperbolic lattices into curved Euclidean lattices (amenable to strain engineering in graphene) and simpler non-Abelian Z_2​ lattices generated by non-commuting translations (Cayley crystals). In the first case, we investigate topological states in curved graphene, leveraging Kitaev’s real-space index to characterize their behavior. Finally, in the end, I will present our very recent findings (still under progress), revealing two distinct classes of states within these Z_2​ lattices: Abelian states exhibiting conventional behavior and non-Abelian states experiencing a surprising Hall drift motion under an electric field. This intriguing result suggests the presence of an effective internal magnetic field in the non-Abelian sector, opening exciting avenues for investigating novel physical phenomena.

Quantum energy science is rapidly emerging as an influential domain interested in the generation, transfer and storage of energy at the quantum level. In particular, quantum batteries have the scope to exploit the wonders of quantum mechanics in order to boost their performance as compared to their electrochemical counterparts [1, 2, 3]. In this introductory talk, we will discuss the history, theoretical progress and experimental state of the art in the field of energy storage using quantum objects, finishing with some interesting open research questions. [1] F. Campaioli et al., Colloquium: Quantum batteries, Rev. Mod. Phys. 96, 031001 (2024) https://doi.org/10.1103/RevModPhys.96.031001 [2] B. Moura Gomes et al., A perspective on the building blocks of a solid-state battery: from solid electrolytes to quantum power harvesting and storage, J. Mater. Chem. A 12, 690 (2024) https://doi.org/10.1039/D3TA04228F [3] J.Q. Quach et al., Quantum batteries: The future of energy storage?, Joule 7, 2195 (2023) https://doi.org/10.1016/j.joule.2023.09.003

Partial differential equations (PDEs) are relevant for solving real-world problems across many areas. However, their solution may be challenging, especially for large-dimensional or high-resolution problems with high memory demands. This thesis develops new quantum and quantum-inspired numerical analysis methods for solving PDEs with potential memory and time savings while maintaining high accuracy. First, we resort to quantum computing, which benefits from exponential encoding advantages and speedups in key operations. Due to the lack of error correction of existing quantum computers, we propose a variational quantum algorithm to solve Hamiltonian PDEs, combining a classical and a quantum computer to exploit the properties of the quantum register. However, the noise sources and limited number of measurements of current quantum devices restrict the scalability of this approach. The high efficiency of the quantum register function encoding motivates its use in developing quantum-inspired algorithms. The second part of the thesis focuses on creating a matrix product state (MPS) finite precision algebra and applying it to quantum-inspired numerical analysis. More concretely, we develop MPS methods to solve static and time-dependent PDEs, motivated by the solution of problems of physical interest: the study of superconducting circuits and the expansion of a particle’s wavefunction in the context of levitodynamics. Using a two-dimensional squeezed harmonic oscillator of up to $2^{30}$ points as a benchmark, MPS methods for Hamiltonian PDEs show exponential memory advantage compared to vector implementations and asymptotic advantage in time while achieving a low error in the solution approximation. Similarly, the time evolution MPS techniques demonstrate exponential memory compression and comparable accuracy and cost to standard vector methods. We conclude that the MPS framework constitutes a memory-efficient and accurate tool for solving PDEs. These findings present new opportunities for applying quantum-inspired algorithms to a wider range of PDEs and numerical analysis problems, opening exciting avenues for future research and applications.

Selecting the architecture of a superconducting quantum processor requires making many design choices, sometimes trying to meet conflicting demands. In this talk, I will discuss the architecture we have developed for our processors in the Wallenberg Centre for Quantum Technology. I will show how we use tunable couplers between fixed-frequency transmon qubits to realize various two- and three-qubit gates based on parametric modulation of the couplers [1,2,3], and how the frequencies of the qubits are chosen to avoid crosstalk between such gates in a square lattice containing several tens, or more, qubits [4]. I will also discuss how we can both mitigate and use ZZ coupling between the qubits in this setup [5], as well as quantify the impact of decoherence for all gates in a simple way [6,7]. [1] Kosen et al., Quantum Sci. Technol. 7, 035018 (2022) [2] Gu et al., PRX Quantum 2, 040348 (2021) [3] Warren et al., npj Quantum Inf. 9, 44 (2023) [4] Osman et al., Phys. Rev. Res. 5, 043001 (2023) [5] Pettersson Fors et al., arXiv:2408.15402 [6] Abad et al., Phys. Rev. Lett. 129, 150504 (2022) [7] Abad et al., arXiv:2302.13885

Inspired by the discoveries of topological phenomena in solid state systems, the study of topology in the propagation of light in photonic crystals has been the subject of much recent attention. Among all topological states of matter, time-reversal symmetry (TRS) broken topological materials, such as Chern insulators have been a particular focus due to their topologically protected unidirectional edge states with non-reciprocal propagation properties. In these systems, scattering processes from one boundary state into another are strongly suppressed, due to decoupling of counter-propagating 1D chiral edge channels.In this contribution, we will firstly introduce a general strategy to design 3D Chern insulating (3D CI)cubic photonic crystals in a weakly TRS broken environment with orientable and arbitrarily large Chern vectors. The resulting 3D Chern insulator, is a photonic topological phase which is forbidden in solid state materials.Secondly, will show how these designs can be used to build photonic axion insulators. Such a 3Dtopological photonic crystal exhibits extraordinary properties such as half-quantized surface effects and unidirectional chiral hinge states that propagate exclusively along the edges. These chiral hinge states form intricate networks that enable light to travel without loss or interference, even when obstacles are present. This breakthrough offers a deeper understanding of exotic particles and paves the way for practical applications in quantum computing, advanced sensors or in the development of axion-like particle detectors such as axion dark matter particles.