Welcome by organisers and Mikael Fogelström (Nordita director)

• Sofia Qvarfort (Stockholm University)

Room: Albano 3: 4203 - SU Conference Lunch Room (48 seats)

Magdalena Zych (Stockholm University): Quantum and gravity interface - foundations and QS connections

• Magdalena Zych (Stockholm University)

Vaishali Adya (KTH): All Fibre Squeezed Light Sources For Quantum Enhanced Sensing And Communication

• Vaishali Adya

Squeezed light technology has improved the sensitivity of precision measurement experiments ranging from gravitational wave detection to microscopy. Right from its conception in 1950’s to its implementation in the Laser Interferometer Gravitational-wave Observatory (LIGO), squeezed light generation has come a long way. In this talk today, I will discuss the principles of squeezed light generation in using nonlinear waveguides and present some of the progress we have made on experiments that use this fibre based squeezed light source.

Zoltán Zimborás (University of Helsinki): Will it glue? On short-depth designs beyond the unitary group

• Zoltán Zimborás (University of Helsinki)

In this talk we present a range of results on several groups of broad interest in quantum information science: the Clifford group, the orthogonal group, the unitary symplectic groups, and the matchgate group. For all of these groups, we prove that analogues of unitary designs cannot be generated by any circuit ensemble with light-cones that are smaller than the system size. This implies linear lower bounds on the circuit depth in one-dimensional systems. For the Clifford and orthogonal group, we moreover show that a broad class of circuits cannot generate designs in sub-linear depth on any circuit architecture. We show this by exploiting observables in the higher-order commutants of each group, which allow one to distinguish any short-depth circuit from truly random. While these no-go results rule out short-depth unitary designs, we prove that slightly weaker forms of randomness -- including additive-error state designs and anti-concentration in sampling distributions -- nevertheless emerge at logarithmic depths in many cases. Our results reveal that the onset of randomness in shallow quantum circuits is a widespread yet subtle phenomenon, dependent on the interplay between the group itself and the context of its application.

Jose Lado (Aalto University): Solving single-particle matter above one trillion sites with quantum many-body tensor network algorithms

• Jose Lado (University of Aalto)

Moiré and super-moiré materials provide exceptional platforms to engineer exotic correlated quantum matter. The vast number of sites required to model moiré systems in real space remains a formidable challenge due to the immense computational resources required. Super-moiré materials push this requirement to the limit, where millions or even billions of sites need to be considered, a requirement beyond the capabilities of conventional methods. Here, we establish methodologies [1,2,3] to solve exponentially large single particle problems using many-body tensor networks. This enables us to solve exponentially large lattice models, including those whose single-particle Hamiltonian is too large to be stored explicitly. First [1], we show that tensor network algorithms allow performing time-evolution of non-linear dynamics, where we reach system sizes of one trillion sites. Second [2], we establish a methodology that allows solving correlated states in systems reaching a billion sites, that exploits tensor-network representations of real-space Hamiltonians and self-consistent real-space mean-field equations. We demonstrate our methodology with super-moiré systems featuring spatially modulated hoppings, many-body interactions, and domain walls, showing that it allows access to self-consistent symmetry broken states and spectral functions of real-space models reaching a billion sites. Finally [3], we demonstrate a method to compute local topological invariants of exceptionally large systems using tensor networks, enabling the computation of invariants for Hamiltonians with hundreds of millions of sites. Our approach leverages a tensor-network representation of the density matrix using a Chebyshev tensor network algorithm, enabling large-scale calculations of topological markers in quasicrystalline and moire systems. Our methodology establishes strategies to solve exceptionally large single-particle problems with quantum many-body algorithms, providing a widely applicable strategy to compute topological and correlated super-moiré quantum matter. [1] Marcel Niedermeier, Adrien Moulinas, Thibaud Louvet, Jose L. Lado, Xavier Waintal, Solving the Gross-Pitaevskii equation on multiple different scales using the quantics tensor train representation, arXiv:2507.04262 (2025) [2] Yitao Sun, Marcel Niedermeier, Tiago V. C. Antão, Adolfo O. Fumega, Jose L. Lado, Self-consistent tensor network method for correlated super-moiré matter beyond one billion sites, arXiv:2503.04373 (2025) [3] Tiago V. C. Antão, Yitao Sun, Adolfo O. Fumega, Jose L. Lado, Tensor network method for real-space topology in quasicrystal Chern mosaics, arXiv:2506.05230 (2025)

Kimmo Luoma (University of Turku): Open quantum system dynamics and strong light-matter interaction

• Kimmo Luoma (University of Turku)

In this talk i will present one possible approach to open quantum system dynamics which is based on solving the dynamics of both the system and the environment. I will present applications of this method to organic microcavity polaritons, quantum information and quantum measurement theory.

Robert Fickler (Tampere University): Quantum Photonics with Structured Light

• Robert Fickler (University of Tampere)

Structured light, i.e. light fields with a non-trivial shape in time, space, and polarization, has become a versatile approach to explore fundamental optics effects and develop novel applications in fields such as microscopy, imaging, optical communications, and quantum technologies, to name a few. In this talk, I will first introduce the field of structured light focusing on its applications in quantum photonics. Following this brief introduction, I will present a few of our recent works on structured light in the quantum domain, which include the advanced modulation of structured photons to perform complex unitary operation on high-dimensional quantum state as well as a so-called quantum frequency conversion process which we control using structured light. In the former, we leverage the spatial modulation ability when multiple consecutive phase-modulation planes are used and show some of its applications to quantum state engineering and sensing. I will also present initial results on implementing these modulations in a scalable, integrated manner through laser-written nanogratings in glass. In the second experiment, we explore the frequency conversion of a single photon from one wavelength to another while simultaneously changing its polarization state to a spatial structure and keeping the entanglement with a partner photon intact. We further show that the preservation of entanglement (non-local) during this process is conditioned upon the classical non-separability (local) of the structured field that drives the process.

Nils Johan Engelsen (Chalmers): Ultralow dissipation nanomechanical resonators for sensing and quantum optomechanics

• Nils Johan Engelsen (Chalmers)

Nanomechanical oscillators are among the most sensitive force and acceleration sensors and show promise as a quantum technology. However, their performance is fundamentally limited by mechanical dissipation, which admits thermomechanical noise from the environment, limiting force sensitivity, and cuts down the coherence time of mechanical quantum states. Over the last decade, the phenomenon ‘dissipation dilution’ has been exploited to reduce the dissipation of nanomechanical resonators by three orders of magnitude; thereby allowing nanomechanical oscillators to surpass the quality factors of the best macroscopic oscillators. I will explain how dissipation dilution works and how it is greatly enhanced by engineering the resonator geometry. I will then show how we use an ultrahigh-Q membrane in a Fabry-Pérot optical cavity to demonstrate quantum optomechanical effects at room temperature—a longstanding challenge in the field.

Juha Muhonen (University of Jyväskylä); Coupling Spins and Mechanical Systems in Silicon Donor spins in silicon have been shown to have good single qubit properties but their scalability is still limited by the lack of convenient coupling and readout methods. One possibility to solve these issues would be to couple the spins to mechanical modes, which can then be used as both a coupling pathway between the qubits and a readout bus to optical telecom range photons, taking advantage of the methods of cavity optomechanics. I’ll discuss our work in coupling donor spins in silicon to both MHz frequency and GHz frequency mechanical modes.

Anton Zasedatelev (Aalto University): Optical dark trapping of sub-micron particles and entanglement generation in their We develop an experimental platform for trapping and controlling the motion of sub-micron solid particles at the quantum level in ultra-high vacuum (UHV). By integrating a stable dark optical trap within the large RF potential of a Paul trap, we combine the strengths of both approaches and overcome key limitations of conventional bright optical trapping of large masses. We demonstrate trapping conditions for resonant particles made of high-density transition-metal dichalcogenides (up to 10 g/cm³) in a blue-detuned bottle-beam trap, and our theory results show more than two orders of magnitude suppression of recoil-induced decoherence compared to standard bright-trap configurations. Since recoil heating is the dominant decoherence mechanism for optically trapped particles, dark trapping provides a clear pathway toward accessing the quantum regime of their motion. Building on this, I will also present a protocol for generating unconditional entanglement between two levitated masses interacting through a 1/r^n potential [1]. The protocol combines optimal quantum control of continuously measured particles [2] with their driven non-equilibrium dynamics, enabling entanglement generation at the fundamental conditional-state limit and requiring an order of magnitude weaker interaction strength compared to existing steady-state approaches. [1] Poddubny et al., Nonequilibrium entanglement between levitated masses under optimal control, arXiv:2408.06251 [2] Winkler et al., Steady-state entanglement of interacting masses in free space through optimal feedback control, arXiv:2408.07492

Fabio Costa (Nordita): Quantum causal structures and quantum memory

• Fabio Costa (Nordita)

The study of quantum causal structures brings insights into both foundational and applied aspect of quantum theory. Foundationally, it has been found that quantum theory can be formulated without assigning causal relations a priori. This provides the possibility to discover unknown causal relations and to model scenarios with indefinite causal order, which can arise in quantum-gravity scenarios but are also accessible in table-top experiments. When applied to ordinary, time ordered quantum processes, the framework clarifies the notion of Markovianity (absence of memory) and provides tools to fully characterize stochastic quantum processes through multi-time correlations and to discriminate quantum vs classical memory. I will give an overview of the approach and of some significant results, with an outlook to prominent research directions.

Daniel Franca (University of Copenhagen): Hamiltonian learning and certification of quantum simulators

• Daniel Franca

Analog quantum simulators have scaled rapidly to sizes that challenge classical simulation, offering a pathway to solving complex many-body physics problems. However, a paradox remains: how can we verify the quantitative accuracy of a device built to surpass our own computational capabilities? Without error correction, these NISQ devices are prone to calibration errors and environmental noise, often limiting their utility to qualitative observation rather than quantitative prediction. In this talk, I will present a framework for Bounded-Error Quantum Simulation (BEQS), a protocol that transforms analog devices into verifiable computational tools. By combining efficient Hamiltonian and Lindbladian learning techniques with rigorous uncertainty propagation, we can derive certified confidence intervals for quantum observables directly from experimental data. I will demonstrate the experimental implementation of this framework on a trapped-ion quantum simulator. By leveraging the method’s polylogarithmic sample complexity in system size and efficient postprocessing, we scale the approach to a 51-qubit system. We successfully reconstruct over 14,000 model parameters, revealing structural features of the simulator such as the exponential decay of long-range interactions and local versus collective dephasing. Finally, I will show how these learned models are used to generate rigorous error bounds for non-equilibrium dynamics, which are experimentally cross-validated. These results establish a blueprint for trusting the output of large-scale quantum simulators in the regime of quantum advantage.

Poster Session Room: Albano 3: 6203 - Floor 6 Large Lunch Room (44 seats)

Giulia Ferrini (Chalmers): Classical simulation, quantum resources and error correction of continuous- and discrete-variables quantum computers In this talk, I will present an overview of recent research developments from my group, covering three main directions: classical simulation of quantum computers, the resource theory of quantum computation, and the protection of quantum information using bosonic codes. A key challenge in the development of quantum computing architectures is the design of classical algorithms capable of reliably simulating quantum computations. Although such simulations are generally inefficient, they provide essential benchmarks for validating experimental platforms. I will discuss our latest work on classical simulation techniques for both discrete- and continuous-variable quantum systems, highlighting how the computational complexity of these simulations correlates with the resource content of the underlying quantum circuits, as quantified by suitable resource monotones. In the last part of the talk, I will shift focus to quantum error correction in continuous-variable systems. Specifically, I will describe our recent advances in encoding quantum information using bosonic codes. I will compare the performance of codes with rotational and translational symmetries under realistic conditions, and analyse their behaviour in the presence of relevant noise models such as photon losses and dephasing, and possible non-Markovian effects. Finally, I will outline a new construction of multimode bosonic codes with rotational symmetry, demonstrating how they can achieve enhanced protection of quantum information.

Andras Norrman (University of Eastern Finland): Quantum light scattering at an electromagnetic time interface Light-matter interactions in time-varying materials have attracted significant interest recently, uncovering novel electromagnetic phenomena and offering enhanced functionalities of photonic devices. However, compared to the research efforts and progresses that have taken place in the classical context, the quantum aspects of this emerging subject have been less explored. Here, we study quantum light scattering at an isotropic and nondispersive material with a suddenly changing refractive index, creating a time interface. By considering the case in which a forward and a backward propagating mode exist before the temporal discontinuity, we first show that the time interface transforms the bosonic mode operators and corresponding quantum states in terms of the two-mode squeeze operator. Our analysis then focuses on quantum state engineering and photon statistics of the scattered light, which reveals and connects various quantum optical phenomena: photon-pair production and destruction, photon bunching and antibunching, vacuum generation, quantum state removal, and quantum state preservation. In general, our work provides new fundamental insights about quantized light in time-varying media and supports further investigations on more sophisticated time-interface systems, including dispersive materials and photonic time crystals, with potential applications in future quantum photonic technologies

Adam Kinos (Lund University): Rare-Earth Crystals for Computing, Communication and Medical Imaging My research as an Assistant Professor focuses on using rare-earth crystals for quantum technologies. These crystals exhibit unique properties, such as hour-long coherence times, optical integration, and high qubit densities, making them promising candidates for quantum computing and quantum communication. My vision is to develop a small-scale quantum processor that in the future can store quantum information that is physically transported on satellites, paving a path to a global quantum internet. Additionally, I will briefly explain the usage of these crystals in Ultrasound Optical Tomography (UOT), a noninvasive medical technique that combines optical contrast and ultrasound resolution for deep tissue imaging.

Andrea Maiani (Nordita): Hybrid superconducting devices for quantum information

• Andrea Maiani (Nordita)

David Busto (Lund University): Ultrafast quantum photoelectronics The interaction of high energy light with matter leads to the emission of electrons in a process known as photoionization. This process underpins numerous measurement techniques in atomic and molecular physics, and material science to study the structure and properties of matter. Despite the inherently quantum nature of the photoionization process, existing photoelectron-based measurement techniques mostly rely on measuring the classical momentum of the emitted electrons, overlooking fundamental quantum aspects of the photoionization process. The emergence of a new research field at the interface of attosecond physics and quantum information offers the opportunity to revisit the photoionization process to develop photoelectron-based quantum metrology. In this this talk I will first present advances in photoelectron quantum state tomography before discussing recent experiments illustrating how ultrafast photoionization could serve as a novel platform for testing quantum mechanics.

Teiko Heinosaari (University of Jyväskylä): Metainformation in Quantum Information Processing

• Teiko Heinosaari (University of Jyväskylä)

Quantum guessing games provide a framework for analyzing how information encoded in quantum states can be optimally extracted through measurement. Beyond the standard role of side information, we introduce the concept of metainformation: knowledge that further side information of a certain type will later become available, even if it is not yet revealed. This distinction uncovers a finer structure in the interplay between timing, information, and strategy, and shows that metainformation can, in some scenarios, raise success probabilities to match those achievable with prior side information. Metainformation connects directly to two applications: the detection of quantum incompatibility and parallel hybrid computation. This talk outlines the basic framework of metainformation and reviews these applications.

Matti Silveri (University of Oulu): Gate teleportation-assisted routing for quantum algorithms Limited qubit connectivity challenges practical deployment of quantum algorithms on quantum processors. When a gate between not-neighboring qubits is implemented, the qubit state must be moved to a nearby connected qubit for local execution, achieved typically through a series of SWAP operations and leading to increased circuit depth, gate count, and reduced total error performance. This work explores teleported gates to improve qubit routing efficiency. Our practical routing method balances between SWAP paths and teleported gates to reduce circuit depths and error accumulation. We implement the teleported-assisted routing method by modifying the Qiskit-native routing routines and benchmark it with various algorithms, e.g. Deutsch–Jozsa, quantum Fourier transform and the quantum approximation optimization algorithms on heavy-hexagon qubit topology. Our results show a 10%–25% depth reduction in the routing of selected algorithms compared to regular SWAP-only routing. Furthermore, for certain cases, we found that directly teleporting controlled unitaries is highly beneficial over teleporting CNOT gates.

Simone Gasparinetti (Chalmers)

Jonatan Bohr Brask (DTU): Private continuous-variable distributed sensing In private distributed sensing, multiple parties aim to collectively estimate a global function of local parameters while retaining privacy of the individual parameters. We introduce a new protocol for distributed phase sensing using continuous-variable quantum states and measurements. We consider a multipartite network in which each node encodes a local phase into a shared entangled Gaussian state. We show that the average phase can be estimated with high precision, exhibiting Heisenberg scaling in the total photon number, while individual phases are inaccessible. Complete privacy is unattainable for finite squeezing but emerges in the large-squeezing limit. We investigate the impact of displacements and optical losses and study trade-offs between estimation accuracy and privacy.

Rafael Chaves (IIP-UFRN): Witnessing and Certifying Magic with Bell-like inequalities Non-stabilizerness, or magic, is a key resource enabling quantum computers to outperform classical devices. Yet, characterizing and certifying this resource remains challenging due to the complex geometry of stabilizer polytopes and the lack of direct experimental witnesses. In this talk, we will discuss two complementary approaches connecting the resource theory of magic to operational frameworks rooted in quantum foundations. First, we will see how carefully constructed Bell inequalities can serve as witnesses of magic. Second, we will introduce a semi-device-independent framework for certifying nonstabilizer states in prepare-and-measure scenarios, relying only on assumptions about system dimension. Together, these results provide both conceptual and operational tools for probing the quantum resources that underlie computational advantages.

Anton Frisk Kockum (Chalmers): Theory tools for designing a superconducting quantum computer 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 architectures we have developed and are exploring for our processors in the Wallenberg Centre for Quantum Technology, and the theory tools we use in that work. 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 calculate, 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]. Finally, I will say something about how we can characterize our quantum computers through state and process tomography [8,9,10]. [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., Quantum 9, 1684 (2025) [8] Gaikwad et al., Quantum Sci. Technol. 10, 045055 (2025) [9] Patel et al., arXiv:2503.20979 (2025) [10] Huang et al., arXiv:2505.07725, Phys. Rev. Lett. (accepted, 2025)

Jing Yang (Nordita): Precision Limits in Many-body Quantum Sensing Many-body interactions can introduce entanglement between particles and hence are valuable resources for quantum information processing. After a brief introduction of quantum metrology, I will discuss a variational principle for controlling many-body quantum systems with restricted operations in the context of quantum sensing. We show that in a spin chain model containing three-body interactions, the Heisenberg scaling can be still achieved even if the control operations are restricted to one-body and two-body interactions, given an initial GHZ state can be prepared. When the GHZ state cannot be efficiently prepared in experiments, one may consider many-body sensing with separable initial states. We find that using separable initial states cannot beat the shot noise limit in locally interacting systems, unless long-range non-local interactions are utilized. These findings identify two important ingredients in many-body sensing: initial entanglement and long-range interactions.