VCQ Colloquium Winter Semester 25/26

Is Physics Timeless?

To find a common origin for different concepts used in the mathematical description of nature has been a lasting motivation to evolve theory in physics. Here, we argue that time and temperature originate from a stationary global entangled state of a system and its environment. Time evolution emerges in the relation of system and environment when separating them.  Imaginary relational time gives rise to temperature and the canonical ensemble for the system, if the global state is maximally entangled.

Beyond Bell: Testing the physical significance of non-locality

In the formulation of Jarrett and Shimony, Bell inequalities follow from three assumptions: measurement independence (MI), parameter independence (PI), and outcome independence (OI). We have recently demonstrated that the conflict between quantum theory and hidden variables extends far beyond Bell’s theorem. Quantum theory violates inequalities even when MI and PI are simultaneously and arbitrarily relaxed (except for a complete relaxation), as well as inequalities where OI is arbitrarily relaxed (again, except completely). We will discuss the implications, experimental tests, and potential applications of these results.

Nanoscale Quantum Sensing: From Water Structure to Electronic Correlations

Spin defects in wide-bandgap semiconductors have emerged as powerful platforms for quantum sensing, providing spatial resolution at the scale of just a few nanometers. This capability unlocks a broad spectrum of applications across both material science and the life sciences. These quantum sensor spins offer intrinsic sensitivity to a variety of environmental parameters—including temperature, magnetic and electric fields, and even nanoscale forces. When combined into multiqubit architectures, they deliver exceptional sensitivity while enabling on-chip signal processing.

In this talk, I will highlight recent advances in nanoscale quantum sensing and imaging, spanning diverse research areas. Examples include probing the structure and dynamics of ultrathin water layers, discovering previously unknown magnetic phases in two-dimensional materials, and revealing lithium-ion transport and localization mechanisms inside solid-state batteries. Together, these developments demonstrate how quantum sensing can provide unprecedented insights into fundamental physical processes and guide the design of next-generation quantum-enabled technologies.

Certified Randomness in Quantum Physics

It is usually said that quantum physics contains an intrinsic form of randomness with no classical analogue, but what does this exactly mean? And how can this intrinsically quantum randomness be detected and quantified? The talk first explains that there are two inequivalent notions of randomness: stochastic and private. We then present a framework for the certification of private randomness and use it to show that quantum physics allows for the generation of private randomness, while this is classically impossible. We finally conclude with recent results quantifying the private randomness of quantum states and measurements.

Classical and Qantum Simulation at Infinite Temperature: Coherent Quantum Dynamics and Dynamical Phase Transitions

Is infinite temperature matter classical and trivial? In contrast to naive expectations, quantum systems at infinite temperature display truly fascinating dynamics.

Infinite temperature states are much more common than they appear; subsystems of infinite Haar-random pure states are typically close to maximal entanglement, and correspond to an infinite temperature state.

Using quantum computers, it is now possible to explore the structure of infinite temperature states and study full counting statistics and quantum correlations in great detail. Recent Google experiments [1] simulated the dynamics of infinite temperature one-dimensional spin chains and demonstrated anomalous quantum diffusion and a dynamical phase transition. The recently developed quantum generating function (QGF) method [2] has allowed us to access time scales hitherto inaccessible to state-of-the-art classical and quantum simulations and shed light on the structure of the critical anomalous diffusion [3].

Infinite temperature interacting fermions also host intriguing coherent dynamics: in the interacting Hubbard chain, for example, anomalous diffusion appears in both spin and charge due to hidden non-Abelian symmetries [3]. At strong interactions, doublons are stabilized. They retain their coherence [4] and display Bloch oscillations. Furthermore, a curious spinless particle emerges at strong interactions and carries quantum information ballistically even at infinite temperature [5].

[1] E. Rosenberg et al., Dynamics of magnetization at infinite temperature in a Heisenberg spin chain, Science 384, 48–53 (2024).

[2]A. Valli, C.P. Moca, M.A. Werner, M. Kormos, Z. Krajnik, T. Prosen, G. Zaránd, Efficient computation of cumulant evolution and full counting statistics: application to infinite temperature quantum spin chains, Phys. Rev. Lett. 135, 100401 (2025).

[3] C.P. Moca, B. Dóra, D. Sticlet, A. Valli, T. Prosen, G. Zaránd, Dynamic scaling and Family-Vicsek universality in SU(N) quantum spin chains, arXiv:2503.21454 [Phys. Rev. B, in print].

[4] C.P. Moca, B. Dóra, G. Zaránd, Hot but Coherent: Doublons at Infinite Temperature in the Hubbard chain, arXiv:2509.20504.

[5] P. Penc, C.P. Moca, Ö. Legeza, T. Prosen, G. Zaránd, M.A. Werner, Loss-induced quantum information jet in an infinite temperature Hubbard chain, Phys. Rev. Lett. 133, 190403 (2024).

Quantum mechanics based on real numbers: A consistent description

Complex numbers play a crucial role in quantum mechanics. However, their necessity remains debated: whether they are fundamental or merely convenient. Recently, it was claimed that quantum mechanics based on real numbers can be experimentally falsified in the sense that any real-number formulation of quantum mechanics either becomes inconsistent with multipartite experiments or violates certain postulates. We show that a physically motivated postulate about composite quantum systems allows to construct quantum mechanics based on real numbers that reproduces predictions for all multipartite quantum experiments. Thus, we argue that real-valued quantum mechanics cannot be falsified, and therefore the use of complex numbers is a matter of convenience.