QUANT26: Quantum Dynamics - Fundamentals and Realizations

Collective behaviour such as flocking (the collective motion of a spontaneously formed group along a common direction) or aster formation (the binding of opposing flocks, inhibiting each others motion) are intriguing emergent phenomena in active systems with local alignment rules. Until recently, their occurrence was mainly studied for classical systems, a prime example being the active Ising model (AIM), which translates the main ingredients of flocking and aster formation (i.e., alignment and self-propulsion) to a lattice framework. Here we introduce and study a one-dimensional (1D) quantum lattice variant of the AIM, based on ideal bosons with a spin degree of freedom. We find that both the collective behaviours of the 1D classical model, flocking and aster formation, are markedly enhanced by the bosonic quantum statistics. This contrasts with a recent quantum generalization of the AIM based onto hard-core bosons, where flocking, but neither its quantum-statistical stabilization nor aster states were observed as a consequence of interactions. Moreover, we investigate the competition of this quantum statistical stabilization of collective phases with their suppression by the quantum fluctuations induced by a transverse external magnetic field.

We present a new approach to high-fidelity multistate Stimulated Raman adiabatic passage (STIRAP). Techniques that optimize multistate STIRAP have been proposed before and they require using additional (shortcut) fields. Here we propose an optimization which does not require additional fields but pulse shaping only. The optimization is based upon the concept of quasi-parallel eigenenergies, which are known to suppress nonadiabatic transitions between two states. It is shown analytically how the parallelization criterion imposes certain time-dependent pulse shapes of the driving fields. Similar to parallel three-level STIRAP, proposed earlier, the parallel multistate STIRAP is robust to errors in the driving fields and the detunings while leaving the intermediate states unpopulated in the adiabatic limit. Moreover, this improvement of fidelity does not require prohibitively large pulse areas. We manage to enhance the STIRAP fidelity by up to 4 orders of magnitude thereby making multistate STIRAP suitable for quantum information processing. We anticipate applications in atomic clocks and atom optics.

We study the ultrafast dynamics of a two-component unconventional superconductor with order parameter $\Delta = (\Delta_x,\Delta_y)$, driven by optically excited phonons. The system is described by a Ginzburg–Landau free energy $F = a|\Delta|^2 + b(\Delta_x^4 + \Delta_y^4 + 2\eta \Delta_x^2 \Delta_y^2)$, coupled to lattice displacements $\mathbf{u}$ induced by an external field. The driven phonons generate an effective interaction $\propto |\Delta|^2|\mathbf{u}|^2$, leading to a renormalized coefficient $a_{\mathrm{eff}} = a + \alpha \langle u^2 \rangle$. As a result, the superconducting state acquires a strong angular dependence, governed by the competition between lattice anisotropy and laser polarization. We show that the phonon amplitude scales as $\langle u^2 \rangle \propto E^2$, and induces locking of the order parameter orientation relative to the drive. This mechanism leads to a crossover from lattice dominated to laser- ontrolled superconductivity, and can produce abrupt angular switching consistent with a first-order transition. Our results demonstrate that driven phonons provide a controllable route to manipulate unconventional superconducting states on ultrafast timescales.

Josephson junctions in quantum gases are typically realized using externally imposed optical barriers. We show that quantized vortices can act as self-induced weak links in immiscible two-component Bose-Einstein condensates with contact interactions. A quantized vortex in the central component forms a hollow channel that enables atomic transport between otherwise isolated regions of the other component, realizing a vortex-mediated Josephson junction. This system introduces a novel weak-link mechanism that emerges from the intrinsic structure of the condensate: for the flowing component, the vortex core acts as an effective barrier set by quantum pressure arising from the radial confinement within the channel. Crucially, tuning the interspecies scattering length directly controls the vortex-core width and therefore the barrier height, enabling a continuous crossover between hydrodynamic transport and Josephson tunneling regimes. Using numerical Gross–Pitaevskii simulations, we investigate AC Josephson dynamics, observing both plasma oscillations and macroscopic self-trapping, and find good agreement between the full numerical dynamics and an effective two-mode description.

Polarization-entangled photon pairs can be generated using spontaneous parametric down-conversion (SPDC) in nonlinear crystals, where one pump photon of higher energy is converted into two lower-energy photons. The process is governed by energy and momentum conservation (phase matching). The non-degenerate case poses distinct experimental challenges due to the need for optical elements to exhibit the desired performance over wide wavelength ranges. The dispersion-induced walk-off of photons must be considered as well to improve the quality of the generated state. In this work, we discuss the experimental implementation of a source of non-degenerate polarization-entangled photon pairs using the SPDC process in a periodically-poled potassium titanyl phosphate crystal. The crystal is integrated into a Sagnac-type interferometer and pumped at a wavelength of 405 nm. The entangled photon pairs are generated at wavelengths of 550 and 1550 nm. The generated state is characterized by quantum state tomography.

Starting from the Keldysh path integral, a field-theoretic framework for the computation of the quantum Fisher information (QFI) has been developed for driven-dissipative systems. In this framework, the degrees of freedom are organised into "quantum" and "classical" fields and truncating the action to leading order in quantum fields yields a compact semiclassical expression for the QFI. For closed-system dynamics, this simplifies to the semiclassical expression found in recent work (Phys. Rev. Lett. 135, 190202). Concretely, the expression is the variance of the derivative of the classical action with respect to the parameter being estimated, such as the frequency of an external drive or the rate of a dissipative process, directly computable along trajectories generated by truncated Wigner approximation (TWA) simulations. The semiclassical character of the result enters through the sampling of initial conditions from the Wigner distribution of the initial state. In fact, the derivation of this closely resembles that of the standard TWA route. As a first benchmark, this framework has been applied to the driven-dissipative harmonic oscillator estimating the pump rate, recovering exact results of the QFI. In the ongoing Master's thesis, the truncation in the quantum fields will be extended to incorporate higher-order corrections, providing systematic access to the genuinely quantum contributions to the QFI beyond the semiclassical limit, which could also be used to improve upon TWA simulations. Importantly, we will explore the application of this framework to spatially extended systems in one and two dimensions like the dissipative Bose condensates such as exciton-polariton systems, where collective modes can carry parameter information in qualitatively new ways not seen before in the effectively zero-dimensional models.

Lattice spin models featuring kinetic constraints constitute a paradigmatic setting for the investigation of glassiness and localization phenomena. The intricate dynamical behavior of these systems is a result of the dramatically reduced connectivity between many-body configurations. This truncation of transition pathways often leads to a fragmentation of the Hilbert space, yielding highly collective and therefore often slow dynamics. Moreover, this mechanism supports the formation of characteristic elementary excitations, which we investigate here theoretically in a two-dimensional Rydberg lattice gas. We explore their properties as a function of interaction strength and range, and illustrate how they can be experimentally probed with a spectroscopic scheme. Here, we show that the transition rate to certain delocalized superposition states of elementary excitations displays collective many-body enhancement.

Analytical field theory, finite group theory, and numerics are used to study lifetime and structure of bound states of polarons, with the goal of seeing a Feshbach pairing mechanism of the 3D Fermi-hubbard model.

Floquet engineering enables the realization of effective Hamiltonians that are difficult to access in static systems by periodically driving highly controllable quantum platforms. However, a major limitation is the gradual absorption of energy from the drive, which eventually pushes the system towards a featureless infinite-temperature state – a process known as Floquet heating. This becomes particularly challenging for interacting systems, where the interactions open up additional energy absorption pathways. One promising strategy to mitigate heating is the design of driving protocols that suppress absorption channels through destructive interference. Recent cold-atom experiments in optical lattices have demonstrated the effectiveness of such an approach using two-tone driving. Motivated by these results, we present a theoretical and numerical study of interference-based heating suppression in a Hubbard lattice via multi-tone periodic drives. Our results provide a microscopic understanding of Floquet heating in this interacting system and determine under which conditions the multi-tone driving protocols considered can extend the timescales of Floquet-engineered dynamics.

As quantum algorithms advance, the realization of fault-tolerant quantum computing becomes imperative. Quantum Error Correction (QEC) provides the essential framework for scalability by shielding fragile quantum states from noise and decoherence. However, efficient decoding remains a bottleneck for real-time applications. The primary objective of this work is to test and understand the latest advancements in the area of QEC, with a strong focus on quantum Low-Density Parity-Check (qLDPC) codes and Belief Propagation (BP) based decoders. We evaluate four recent BP decoding strategies designed to overcome standard limitations in quantum settings: Order Statistic Decoding (BP-OSD), Guided Decimation (BP-GD), Relay-BP, and Syndrome Flip (BP-SF). Furthermore, we outline a benchmarking pipeline utilizing High-Performance Computing (HPC) and predecoding techniques to mitigate classical overhead. We compare their performance in terms of error suppression and computational complexity, providing insights into their viability for near real-time quantum error correction.

We study recurrence times and non-equilibrium steady states in noisy, stroboscopically monitored qubit systems. In the noiseless limit, complete measurements lead to integer-quantized mean recurrence times, with sharp dips near revival sampling times. Using IBM quantum processors, we show that this ideal picture is robust far from revivals but breaks down dramatically near resonance, where weak asymmetric noise can transform ideal dips into pronounced peaks. We explain this behavior using an effective Markov description in which repeated measurements favor an infinite-temperature-like steady state, while relaxation toward the physical qubit ground state produces a competing low-temperature-like regime.

Considering a simple all-qubit collision model with system-ancilla interaction hamiltoninan $H_{SA} = J_x \sigma_x \otimes \sigma_x + J_y \sigma_y \otimes \sigma_y$, it has been proven that with a simmetric interaction ($J_x = J_y$) the system reaches the ancilla's thermal state. However, simulations suggest the presence of infinite thermalization conditions, that we can define analitically.

Flat band systems provide a unique platform for studying strongly correlated quantum matter. In these systems, the kinetic energy is quenched, and the then-dominant interactions can lead to the emergence of intricate many-body physics. Concretely, we investigate the ground state of the Bose-Hubbard model on the Kagome lattice for a negative tunnelling coefficient. These frustrated tunnelling kinetics result in the lowest single-particle band being flat and formed by states, so-called Aharonov-Bohm cages, localized on hexagons. We first present the known theoretical predictions for the interacting many-body ground state at low filling and analyse them numerically via exact diagonalization. We then discuss how these predictions are challenged when the filling is increased beyond a known critical value, in particular how this influences the properties of the resulting ground state(s)

Spontaneous symmetry breaking is one of the central organizing principles in physics. Time crystals have emerged as an exotic phase of matter, spontaneously breaking the time translational symmetry, and are mainly categorized as discrete or continuous. While these distinct types of time crystals have been extensively explored as standalone systems, intriguing effects can arise from their mutual interaction. Here, we demonstrate that a time-independent coupled system of discrete and continuous time crystals induces a simultaneous two-fold temporal symmetry breaking, resulting in a hierarchical time crystal phase. Interestingly, one of the subsystems breaks an emergent discrete temporal symmetry that does not exist in the dynamical generator but rather emerges dynamically, leading to a convoluted non-equilibrium phase. We demonstrate that hierarchical time crystals are robust, emerging for fundamentally different coupling schemes and persisting across wide ranges of system parameters.

Collective behaviour such as flocking (the collective motion of a spontaneously formed group along a common direction) or aster formation (the binding of opposing flocks, inhibiting each others motion) are intriguing emergent phenomena in active systems with local alignment rules. Until recently, their occurrence was mainly studied for classical systems, a prime example being the active Ising model (AIM), which translates the main ingredients of flocking and aster formation (i.e., alignment and self-propulsion) to a lattice framework. Here we introduce and study a one-dimensional (1D) quantum lattice variant of the AIM, based on ideal bosons with a spin degree of freedom. We find that both the collective behaviours of the 1D classical model, flocking and aster formation, are markedly enhanced by the bosonic quantum statistics. This contrasts with a recent quantum generalization of the AIM based onto hard-core bosons [Khasseh et al., Phys.~Rev.~Lett.\ {\bf135}, 248302 (2025)], where flocking, but neither its quantum-statistical stabilization nor aster states were observed as a consequence of interactions. Moreover, we investigate the competition of this quantum statistical stabilization of collective phases with their suppression by the quantum fluctuations induced by a transverse external magnetic field.

Spin systems naturally exhibit quantum squeezing [1, 2], which can be modified—and even enhanced—by dissipation [3, 4]. Understanding the interplay between intrinsic and dissipative squeezing is therefore essential for controlling quantum correlations in magnonic systems. While previous studies have focused on specific dissipative mechanisms, we develop a unified framework for arbitrary linear system-bath couplings. By mapping orthogonal spin fluctuations onto the position and momentum quadratures of a bosonic mode, we show that the Landau-Lifshitz-Gilbert equation is equivalent to an unconventionally damped harmonic oscillator. We derive a general formalism describing the competition between intrinsic and dissipative squeezing and demonstrate how this competition governs the generation of entanglement. Furthermore, we demonstrate dissipation-induced entanglement and provide a complete characterization of entanglement for arbitrary coupling channels. Our results identify engineered dissipation as a versatile resource for controlling both the squeezing direction and the generation of entanglement in magnonic systems, providing new perspectives for dissipative quantum-state engineering. [1] A. Kamra et al, Phys. Rev. B 100, 174407 (2019) [2] D. Wuhrer et al, Appl. Phys. Lett. 125, 022404 (2024) [3] G. Rastelli, New. J. Phys. 18, 053033 (2016) [4] H. Y. Yuan et al, Phys. Rev. B 106, 224422 (2022)

While thermalization in isolated quantum many-body systems is believed to be described by the eigenstate thermalization hypothesis [1, 2], the thermalization process itself can be nonmonotonic depending on an initial state. Here, we propose a numerical method to construct a low-entangled initial state that creates a “burst,” which refers to a transient deviation of an observable from its thermal equilibrium at a designated time [3]. Utilizing the density matrix renormalization group algorithm [4], our method identifies such an initial state within the matrix product state manifold. We apply this method to demonstrate that a burst of magnetization can be realized in a nonintegrable mixed-field Ising chain on a short timescale. Contrary to the typical information spreading observed in this regime, the created burst is accompanied by a negative entanglement growth. By employing a local random quantum circuit, we analytically show that a burst becomes probabilistically rare at late times. Our results suggest that a quantum state stays out of equilibrium starting from an appropriately chosen initial state until scrambling becomes dominant. Finally, we discuss how this burst phenomenon serves as a new analogue of the quantum Mpemba effect [5]. Due to the low-entangled nature of the initial state, our predictions can be tested by quench experiments using programmable quantum simulators. [1] J. M. Deutsch, Phys. Rev. A 43, 2046 (1991). [2] M. Srednicki, Phys. Rev. E 50, 888 (1994). [3] S. Yamada, A. Hokkyo, and M. Ueda, arXiv:2602.09665 [quant-ph] (2026). [4] S. R. White, Phys. Rev. Lett. 69, 2863 (1992). [5] L. K. Joshi et al., Phys. Rev. Lett. 133, 010402 (2024).

Non-reciprocity presented in the system often leads to a time-dependent state, which is well observed in a wide variety of physical systems across neuroscience to chemistry. A demonstrative example is the nonreciprocal Ising model with local interactions. Several studies regarding the induced global oscillation have been done in the last year. However, the conditions for such an oscillatory state to survive under long-range couplings and the critical behaviors of the corresponding phase transitions still remain to be fully understood. In this thesis, we approach these problems by first introducing the modification of a long-range inter–/intraspecies coupling to the existing nonreciprocal Ising model in 1 dimension. The analytical study with the mean-field approximation suggests three different phases occurring in the system: the static-ordered phase, the disordered phase, and the time-dependent oscillatory swap phase. The oscillation presented in the swap phase does not come from a periodic driven force from the Hamiltonian and has properties of a time crystal. With a Landau–Peierls-type argument, we aim to show the critical condition for the survival of the static-ordered phase on \alpha, which determines the range of the couplings. We then carry out a large-scale Monte-Carlo simulation to further investigate the transitions between the phases and the different novel characteristics of the swap phase. Based on the simulation data and our analytical calculation of the two-point spatial correlation function of the averaged magnetization, we aim to find another critical condition on alpha for the swap phase to survive in the thermodynamic limit. Our studies of finite-size scaling reveal critical exponents of the transition between the disordered and the swap phase to be $\nu = 2.1903 \pm 0.0092, \gamma =0.9434 \pm 0.0029, \beta =0.3866\pm0.0008$. We also provide details on the phase diagram, which presents a continuous second-order-like transition between the disordered and swap phase and a sharp first-order-like transition between the swap and static-ordered phase.