Christopher Campbell
University of Oulu

Non-equilibrium collective dynamics in dissipatively coupled transmon pairs
Superconducting transmons arrays are increasing in popularity as viable platforms to study quantum computing and information processing with a vast amount of resources being invested  in this area of research in Finland. Transmons are unique qubits since a single transmon can be modelled as a multi-level system that considers higher level states and surpasses traditional two-level system in quantum information. When arrays of transmons are coupled to a waveguide, exotic states emerge where the dynamics from the interaction of these states can be studied in higher excitation manifolds (N>1). In this work we studied the dissipative dynamics and relaxation of a sub-class of these exotic states known as dark states, which are states that are decoupled from the system. Starting with a pair of transmons in the second excitation manifold, we investigate how anharmonicity affects the eigenspectra of higher manifolds, revealing an induced dissipative nature of these dark states. We also highlight the complexity of the dynamics due to Non-Hermitian properties of the system, such as the biorthogonal nature of the eigenstates, and show how these considerations contribute to the dissipation of photons in dark states, where superradiant bursts are observed in transmon pairs. Finally we extend these ideas past the second excitation manifold to show how even odd effects leads to relaxations into different states.

Maryam Darvishi
University of Jyväskylä

Josephson Effect at Arbitrary Disorder Strength in Systems with Generic Spin-Dependent Fields
We present a full microscopic theory of the Josephson current in superconductor–normal metal–superconductor (SNS) junctions with generic intrinsic SOC and Zeeman fields, valid at arbitrary disorder strength. We analyze systems with Rashba SOC, in-plane Zeeman field, and altermagnetism as specific cases. We provide exact expressions for the current and show how spin-dependent fields and disorder jointly influence the current-phase relation. Particular attention is given to the emergence of the anomalous Josephson effect in disordered systems with spin-dependent fields.

Behnaz Fazlpour
University of Eastern Finland

Relativistic polarization structure of blackbody radiation
We study the polarization properties of blackbody radiation under Lorentz transformations. We especially show that the degree of polarization of blackbody radiation changes in such relativistic transformations, contrary to the Lorentz invariance of the degree of polarization of transverse fields. Our work provides a deeper understanding of relativistic effects on blackbody radiation and may find use in astrophysics and cosmology.

Cristian A. Galvis-Florez
Aalto University

Provable Quantum Algorithm Advantage for Gaussian Process Quadrature
We developed novel quantum algorithms for Gaussian process quadrature methods. Gaussian process quadratures are numerical integration methods where Gaussian processes are used as functional priors for the integrands to capture the uncertainty arising from the sparse function evaluations. In this paper, we combine Gaussian process quadrature and quantum computing by proposing a quantum low-rank Gaussian process quadrature method based on a Hilbert space approximation of the Gaussian process kernel and enhancing the quadrature using a quantum circuit. We use numerical simulations of a quantum computer to demonstrate the effectiveness of the method. Furthermore, we provide a theoretical complexity analysis that shows a polynomial advantage over classical Gaussian process quadrature methods.

Toivo Hakanen
University of Jyväskylä

NbTiN-based superconducting refridgerators
Toivo Hakanen [1], Ilari Maasilta [1], Mika Prunnila [2]
[1] Nanoscience Center, Department of Physics, P.O. Box 35, FI-40014 University of
Jyväskylä, Finland
[2] VTT
Contact: toivo.a.hakanen@jyu.fi

As quantum computing and other applications of quantum physics become more prevalent, there is a growing need for alternatives to bulky and expensive cooling systems. On-chip solid-state coolers utilizing SINIS (Superconductor-Insulator-Normal conductor-Insulator-Superconductor) tunnel junctions could provide a complementary solution by providing local electronic cooling on-site, without the need to cool the entire substrate. These types of coolers can be easily controlled by adjusting the bias voltage across the junctions, and their operating range can be tuned by selecting different superconducting materials. The purpose of my research is to investigate methods for integrating NbTiN (highest superconducting critical temperature Tc of approximately 16 K) as a superconductor in SINIS coolers. The end goal is to engineer a high-Tc SINIS cooler, that could pave the way to cool down to sub-Kelvin temperatures from liquid He-4 bath.

Emily Haughton
Aalto University

Entanglement witnesses in a double resonant polariton optomechanical system
Our work explores entanglement witnesses in multipartite exciton-polariton and phonon-polariton systems with strong optomechanical interaction. In particular, we introduce entanglement criteria for a linearized Hamiltonian of exciton-polaritons and phonon-polaritons in both continuous and discrete variables, below the threshold of optomechanical instability. Our study demonstrates bi-partite entanglement between exciton and phonon polariton states, evidenced by crossing of the separability boundary in logarithmic negativity and strong violation of the Cauchy-Schwartz inequality in the second order cross correlation function. We propose practical setups to measure this bi-partite entanglement through the quadrature squeezing of exciton-polaritons emitted in the visible range.

Erik Hieta-aho
VTT

Current status of Post-Quantum Crytography
The acceleration of quantum computing technologies impacts various aspects of society. In particular cybersecurity is at risk due to the ability of implementing Shor’s algorithm. Our poster provides a status update of the current research and development of Post-Quantum Cryptography.

Youqiang Huang
Aalto University

Spin chirality in van der Waals antiferromagnet
Chirality-driven spin configurations hold great potential for advancing spintronics by enabling compact, energy-efficient memory devices and high-density data storage solutions. Here, we will present our experimental results of spin structures in 2D van der Waals magnet. These spin configurations exhibit distinct optical characteristics, arising from spin interactions influenced by external magnetic fields and thermal variations. The observed chiral optical responses serve as a highly sensitive probe for detecting non-collinear spin arrangements. Our findings highlight 2D magnetic materials and their heterostructures as promising candidates for reconfigurable spin-photonics and spintronic applications.

Hanna Jääskö
University of Jyväskylä

Tuning Electrical Properties through Metal Ion-Mediated Assembly in Au25 Nanocluster-Based Frameworks
Incorporating well-known monolayer-protected gold nanoclusters (NCs) into metal-organic frameworks has led to the creation of novel quantum materials with tunable characteristics [1]. By varying the connecting metal ions that link the gold NCs within the framework, it is possible to precisely control inter-cluster distances, which in turn significantly affect the electronic and optical characteristics of the materials. Here, we study the electronic properties of two cluster assembled materials, Au25-Mg and Au25-Cu [2] using density functional theory (DFT) calculations with the GPAW code [3].  Our results indicate that these structures exhibit semiconductor-like electronic band structures. Furthermore, the Au25 cluster retains its well-known eight-electron closed-shell superatom configuration within these frameworks. We also investigate the role of magnetic coordination ions in Au₂₅-Co and Au₂₅-Ni frameworks.

References

[1] Kim, Sinhyeop et al., Au25 Cluster-Based Atomically Precise Coordination Frameworks and Emission Engineering through Lattice Symmetry. ACS Nano, 18, 29036-29044 (2024)

[2] Kim, Sinhyeop et al., Tuning Electrical Properties through Metal Ion-Mediated Assembly in Au25 Nanocluster-Based Frameworks. [Manuscript submitted for publication].

[3] Mortensen, J.J et al. GPAW : An open Python package for electronic structure calculations. J.,Chem. Phys. 160, 092503 (2024).

Heidi Kivijärvi
Aalto University

Noise-induced quantum-circuit refrigeration
The on-going major efforts of building scalable superconducting quantum computers and quantum thermal machines have highlighted the need for efficient refrigerators that may be powered by waste heat, i.e., thermal noise. However, no versatile device applicable in these fields has been demonstrated. To this end, we show for the first time that a quantum-circuit refrigerator can be powered by noise. We use pure noise to damp a coherent resonator state and cool down the resonator from a high-temperature thermal state. These observations establish opportunities for engineering dissipative environments for qubits and quantum heat engines with no other power used in their operation except for thermal noise obtained for free from high-temperature stages of the cryostat.

Ashutosh Kumar
University of Jyvaskyla

RNN-Inspired Decoder For Heavy Hex Quantum Error Correction Code
The performance of quantum error correction codes depends significantly on the efficiency of syndrome decoding algorithms. While graph-based decoders, such as minimum-weight perfect matching (MWPM), have been widely explored, most existing algorithms focus on the surface code with a 2D qubit layout. IBM’s heavy hex qubit layout presents a unique challenge for decoding. It requires approaches that can handle the non-trivial connectivity under a circuit-level noise model. In this ongoing work, we explore a recurrent neural network (RNN)-inspired decoder that incorporates memory effects to capture correlations across consecutive error syndrome measurements. Preliminary numerical results indicate that leveraging memory improves both decoding accuracy and speed compared to traditional machine learning-based decoders. We also analyse the scalability of the RNN decoder and evaluate its performance in relation to other state-of-the-art decoding methods

Jichao Li
University of Helsinki

Holographic Phase Transitions in QCD
I will discuss my ongoing research on constructing effective theories that describe the dynamics of AdS black holes. These models, grounded in the gauge/gravity duality framework, allow us to probe the phase structure of strongly coupled quantum field theories. In particular, I focus on thermal QCD at finite baryon density—a regime that remains challenging for traditional computational approaches due to the sign problem. By developing large-D effective descriptions of black hole dynamics, we aim to uncover novel insights into phase transitions relevant for the QCD phase diagram and the physics of dense matter in neutron star cores

Clemens Lindner
University of Jyväskylä

Similarity based quantum state learning
I provide an introduction to a quantum machine learning algorithm based on similarities between quantum states. I present a use case with results where the algorithm is used to learn the noise free version of a given noisy quantum state.

Milla Männikkö
University of Jyväskylä

Silicon optomechanical devices for spin coupling
Quantum computation still lags behind classical computation in calculation speed due to the lack of scalable and noiseless architecture for multiple qubits. One avenue to achieve a quantum computer with minimal noise and a scalable number of qubits would be to use donor atom spins in silicon, as these have been shown to have a long coherence time and silicon offers several advantages for scalability. In this project, we aim to accomplish a readout of such qubits implemented via optomechanics, which has multiple advantages, such as the fact that light is a lossless way to transport information and that it causes minimal disturbance on the measured object. The mechanics are a necessary bridge between the optics and the donor spin, but it can also be used to couple multiple of these qubits to each other. The spin-mechanical coupling can be achieved either via strain or magnetic field gradient and the optomechanical coupling is achieved via the resonator, which is designed to be an optical cavity. As this spin mechanical coupling is yet to be observed, this poster will discuss a mechanical resonator with optimized design, its fabrication process and the coupling strength that this design achieves.

Natale Matranga
University of Jyväskylä

Vertex Corrections in Flat-Band Migdal-Eliashberg Theory
Flat-band systems, with their nearly dispersionless electronic bands and high density of states, amplify interaction effects and challenge conventional theories of superconductivity. Migdal-Eliashberg theory of superconductivity is based on the idea that electrons move much faster than the phonons, quantized vibrations of the atomic lattice, that mediate the pairing between them. Due to this separation of energy scales, certain interaction effects, called vertex corrections, can be neglected. This approximation, known as Migdal’s theorem, has worked remarkably well in conventional superconductors. However, in flat-band systems the conditions underpinning Migdal’s argument no longer apply, and interaction effects that are usually negligible may start to matter. In this work, we investigate how vertex corrections influence superconductivity in flat-band systems, computing their leading contributions numerically, providing analytical estimates of their size, and examining how they affect the equation that governs the formation of superconducting pairs. Preliminary findings suggest that these corrections may play a significant role, highlighting the possible limitations of the standard Migdal-Eliashberg approach in flat-band superconductors and pointing toward the need for a more refined theoretical treatment in this unusual regime.

Brij Mohan
University Of Oulu

Enhancing the Performances of Autonomous Quantum Refrigerators via Two-Photon Transitions
Conventional autonomous quantum refrigerators rely on uncorrelated heat exchange between the working system and baths via two-body interactions enabled by single-photon transitions and positive-temperature work baths, inherently limiting their cooling performance. Here, we introduce distinct qutrit refrigerators that exploit correlated heat transfer via two-photon transitions with the hot and cold baths, yielding a genuine enhancement in performance over conventional qutrit refrigerators that employ uncorrelated heat transfer. These refrigerators achieve at least a twofold enhancement in cooling power and reliability compared to conventional counterparts. Moreover, we show that cooling power and reliability can be further enhanced simultaneously by several folds, even surpassing existing cooling limits, by utilizing a synthetic negative-temperature work bath. Such refrigerators can be realized by combining correlated heat transfer and synthetic work baths, which consist of a four-level system coupled to hot and cold baths and two conventional work baths via two independent two-photon transitions. Here, the composition of two work baths effectively creates a synthetic negative-temperature work bath under suitable parameter choices. Our results demonstrate that correlated heat transfers and baths with negative temperatures can yield thermodynamic advantages in quantum devices. Finally, we discuss the experimental feasibility of the proposed refrigerators across various existing platforms.

Jake Muff
VTT

Improving the performance of GHZ fidelity through pulse schedule rearranging
We present work on a novel technique to improve the performance of GHZ circuits on superconducting qubit hardware by analyzing the pulse schedule and reducing qubit
idling time through rearranging specific sequences of pulses after compiling our quantum circuit to its pulse schedule representation. Our method focuses on
rearranging pulses to activate the qubits as late as possible in the execution sequences, thereby minimizing the decoherence effect that degrades the fidelity (measured via
multiple quantum coherences[1]) of our GHZ states. In this technique, we target specific sequences of gates that can be scheduled as late as possible whilst scheduling the rest of the schedule as soon as possible.

We demonstrate this using VTT Q50 superconducting QPU using a tree-based algorithm to generate GHZ state circuits [2] with the aim to minimize parallel CNOT depth in addition to optimizing the fidelity of the subset of qubits chosen. We demonstrate two diKerent methods for constructing GHZ circuits in a tree fashion and formulate an analytical equation for the parallel CNOT depth scaling in an infinite square lattice case to demonstrate the scalability of our methods. The analytical formulism and scaling is compared directly to superconducting QPUs which employ the heavy-hex topology[3]. Our results show that hardware-aware compilation techniques applied at the pulse level can significantly enhance quantum algorithm performance on NISQ era devices.

This work establishes the the importance of compiler-level optimizations that account for hardware-specific characteristics and timing constraints in quantum circuit execution, articularly at the pulse level which we see as an area that has been underexplored.

[1] Wei, Ken X et al 2020 Verifying multipartite entangled Greenberger-Horne-Zeilinger
states via multiple quantum coherences American Physical Society
[2] K. -C. Chen 2023 Short-Depth Circuits and Error Mitigation for Large-Scale GHZ-
State Preparation, and Benchmarking on IBM’s 127-Qubit System 2023 IEEE
International Conference on Quantum Computing and Engineering
[3] Kam, John F et al 2024 Characterization of entanglement on superconducting
quantum computers of up to 414 qubits American Physical Society

Nima Nematimansur
University of Jyväskylä

Towards spin-photon interfaces in silicon
One of the most promising candidates for harnessing the power of quantum information processing is to use the benefits of spins and photons in a single hybrid platform. The coupling of spins and photons unites the computational strength of spin qubits with the communication efficiency of photons. Here, we study photon sources in silicon – a highly studied, developed and matured material – and we aim to characterize the photon emission characteristics from transitions involving also spins, and eventually integrate them with silicon photonics and spin control systems on-chip. Our candidate emitters are phosphorus and bismuth donors, and color centers such as T-centers. In first stage of our work, which is being presented in the poster, we demonstrate the photoluminescence setup and the emission characteristic of bound excitons in cryogenic temperature.

Risto Ojajärvi
University of Jyväskylä

Superconductivity due to fluctuating loop currents
Orbital magnetism and the loop currents (LCs) that accompany it have been proposed to emerge in many systems, including cuprates, iridates, and kagome superconductors. In the case of cuprates, LCs have been put forward as the driving force behind the pseudogap, strange-metal behavior, and dx2−y2-wave superconductivity. Here, we investigate whether fluctuating intra-unit-cell LCs can cause unconventional superconductivity. For odd-parity LCs, we find that they are repulsive in all pairing channels near the underlying quantum-critical point (QCP). For even-parity LCs, their fluctuations give rise to unconventional pairing, which is not amplified in the vicinity of the QCP, in sharp contrast to pairing mediated by spin-magnetic, nematic, or ferroelectric fluctuations. Applying our formalism to the cuprates, we conclude that fluctuating intra-unit-cell LCs are unlikely to yield dx2−y2-wave superconductivity. If LCs are to be relevant for the cuprates, they must break translation symmetry.

Daniel Paz Ramos
Department of Electronics and Nanoengineering, Aalto University

Incremental Structural Learning for Simulation of Tavis-Cummings and Heisenberg Models in NISQ hardware.
Fault-tolerant quantum computers promise unprecedented simulations of complex quantum systems, but current noisy-intermediate scale quantum (NISQ) devices require elaborate noise reduction protocols. We compare two of such methods: zero noise extrapolation (ZNE), employing circuit-folding noise amplification, and incremental structural learning (ISL), a variational recompilation technique. These methods are used for simulating Trotterized dynamics of the Tavis-Cummings model and the Heisenberg spin chain (HSC). Under simulated noise conditions, ISL outperforms ZNE for smaller three-qubit systems in both models, and for larger systems in the HSC, suggesting the ansatz construction of ISL provides better approximation for certain dynamics. In such cases, we demonstrate scenarios where ZNE degrades with increased system size and  noise, yet ISL demonstrates robustness in reconstructing the desired quantum dynamics.

Sofia Pöntys
Aalto University

Superconductivity and pair density waves from nearestneighbor interactions in frustrated lattice geometries
We consider superconductivity and pair density waves (PDWs) arising from off-site pairing in frustrated lattice geometries. We express the pair susceptibility in a generic form that highlights the importance of both the density of states, and the quantum geometry of the eigenstates and calculate the superfluid weight (stiffness). In the Lieb lattice flat band, we find a PDW at a finite interaction and show that its pair wave vector is determined by the quantum geometry of the band. At the kagome van Hove singularity, the pair susceptibility predicts a PDW due to sublattice interference, however, we find that its stiffness is zero due to the shape of the Fermi surface.

Matti Raasakka
Aalto University

Classical approximations to quantum computing
The source for the quantum speedup for some computational tasks is still not well understood. There exist various different classes of quantum circuits that can be efficiently simulated on a classical computer, indicating that perhaps there does not exist a single quantum resource for computing but several different ones. In this poster we present a coherent picture of the different proposed quantum computing resources, the relations between them, their applications to classical simulation of quantum algorithms as well as to improving the performance of quantum computing.

Vladislav Shishkov
Aalto University

Entangled Polariton States in the Visible and Mid-Infrared Spectral Ranges
Entanglement generation in polariton systems has long been limited by high losses and decoherence, which typically outweigh polariton nonlinearities.
Here, we propose a fundamentally new approach that uses optomechanical interactions, rather than polariton–polariton interactions, to generate entangled polaritons.
Our double-resonant scheme relies on strong exciton–phonon coupling, found in both inorganic and molecular semiconductors, enabling room-temperature generation of spectrally disparate photon pairs.

The quantum coherent and delocalized nature of polariton states inside optical cavity ensures efficient single-mode outcoupling and allows for nearly deterministic unconditional quantum state preparation – not relying on any projective or weak measurements.
When conditioned on exciton-polariton emission, single phonon–polariton states can be prepared with the on-demand emission of single mid-IR photons.
We demonstrate the double-resonant optomechanical system offers a scalable, room-temperature platform for quantum polaritonics without use of conventional excitonic nonlinearities.

Taneli Tolppanen
University of Oulu

Passive leakage removal unit based on a disordered transmon array
Leakage out from the qubit subspace causes correlated errors that accumulate and propagate among qubits, challenging the effectiveness of quantum error correction for fault-tolerant quantum computing. Even in mature technological platforms, such as superconducting transmon qubits many operations can produce leakage, such as single-qubit and entangling gates and measurements. Different protocols have been proposed to remove leakage. In general, the implementation of a quantum algorithm needs to be adapted for the specific leakage removal method used.

We propose a leakage removal unit based on an array of coupled disordered transmons and last-site reset by feedback-measurement or dissipation. The leakage removal unit is passive, meaning that it is capable of removing leakage in the background while the qubit is in use. We protect the qubit subspace from additional qubit errors by taking advantage of parametric disorder in the transmon frequencies, which helps keep the qubit subspace localized through energy level mismatch. For maximal leakage mobility we use parametric disorder in the transmon anharmonicity to tune the energy levels for the leakage excitations in resonance. Leakage excitations propagate through the idle transmons until reaching the last site where they are removed by feedback-measurement or dissipation.

We simulate the leakage removal unit numerically, and study the leakage population as a function of time and measurement/dissipation rates. We find two optimal rates for removing leakage excitations. The optimal rates are comprehensively understood through two distinct timescales between the coherent propagation and breaking of the leakage population as a boson stack. Further, we study how our leakage removal unit performs when we introduce native dissipation, dephasing and thermalization. We find optimal parameters in terms of transmon frequency disorder, physical coupling between the transmons and the number of transmons used in the leakage removal unit. Based only on an array of standard transmon devices, our approach is readily compatible with existing superconducting quantum processor designs, and the optimal parameters we find are experimentally realizable.

Leonie Wrathall
Aalto University

Probing macroscopic quantum states of phonons via Raman scattering in a light–matter Bose–Einstein condensate
We propose a mechanism for achieving phonon Bose–Einstein condensation through strong optomechanical coupling between high-energy vibrational modes and an exciton–polariton condensate. In this regime, nonequilibrium polariton condensation coexists with macroscopically occupied vibrational states, enabling vibrational amplification in a resonant blue-detuned configuration. To probe such a phonon condensate for the first time, we design a nonresonant Raman scattering experiment that exploits a specific “magic angle” geometry, where scattering signatures emerge only in the presence of the polariton BEC. Being carried out in the single shot configuration this experiment provides a direct means to detect and characterize macroscopic quantum states of vibrations.

Kaan Yurtseven
University of Oulu

Developing Efficient and Scalable Ansatz for Quantum Chemistry and Material Science
NISQ (Noisy Intermediate-Scale Quantum) devices offer the potential for scalable quantum computing, with quantum chemistry simulation as a key application. However, simulating large molecules remains challenging due to the exponential growth of computational costs.
Variational Quantum Algorithms (VQAs), a hybrid quantum-classical framework, are among the most promising methods for addressing these challenges for near term quantum computing. Despite their success, optimization issues like Barren Plateaus  and the dependence on problem-specific ansatz design remain open problems.

We explore the performance of Hamiltonian Variational Ansatz(HVA), which has recently been shown to have reasonable convergence under some strict assumptions. We tried to ease these strong assumptions. Then, we explore the compression and factorization techniques such as Compressed Factorized Hamiltonian(CDF) and Explicit Factorized Hamiltonian(XDF) and tried to reduce the cost of simulation and decreasing the depth of the ansatz in HVA. These issues are particularly relevant in the context of Finland’s advancing quantum hardware infrastructure, such as the Helmi quantum computer with 50+ qubits.