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Journal of Statistical Mechanics Theory and Experiment

Quantum exploration of high-dimensional canyon landscapes

Pierfrancesco Urbani

Published Aug 12, 2024• 4 citations
Publisher

Abstract

Abstract Canyon landscapes in high dimension can be described as manifolds of small, but extensive dimension, immersed in a higher dimensional ambient space and characterized by a zero potential energy on the manifold. Here we consider the problem of a quantum particle exploring a prototype of a high-dimensional random canyon landscape. We characterize the thermal partitionfunction and show that around the point where the classical phase space has a satisfiability transition so that zero potential energy canyons disappear, moderate quantum fluctuations have a deleterious effect: they induce glassy phasesat temperature where classical thermal fluctuations alone would thermalize the system. Surprisingly we show that even when, classically, diffusion is expected to be unbounded in space, the interplay between quantum fluctuations and the randomness of the canyon landscape conspire to have a confining effect.

DOI
10.1088/1742-5468/ad0635

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Abstract Canyon landscapes in high dimension can be described as manifolds of small, but extensive dimension, immersed in a higher dimensional ambient space and characterized by a zero potential energy on the manifold. Here we consider the problem of a quantum particle exploring a prototype of a high-dimensional random canyon landscape. We characterize the thermal partitionfunction and show that around the point where the classical phase space has a satisfiability transition so that zero potential energy canyons disappear, moderate quantum fluctuations have a deleterious effect: they induce glassy phasesat temperature where classical thermal fluctuations alone would thermalize the system. Surprisingly we show that even when, classically, diffusion is expected to be unbounded in space, the interplay between quantum fluctuations and the randomness of the canyon landscape conspire to have a confining effect.
In this paper we expand our previous investigation of a quantum particle subject to the action of a random potential plus a fixed harmonic potential at a finite temperature T. In the classical limit the system reduces to a well-known ``toy'' model for an interface in a random medium. It also applies to a single quantum particle such as an electron subject to random interactions, where the harmonic potential can be tuned to mimic the effect of a finite box. Using the variational approximation, or alternatively, the limit of large spatial dimensions, together with the use of the replica method, we are able to solve the model and obtain its phase diagram in the T-(${\mathrm{\ensuremath{\Elzxh}}}^{2}$/m) plane, where m is the particle's mass. The phase diagram is similar to that of a quantum spin glass in a transverse field, where the variable ${\mathrm{\ensuremath{\Elzxh}}}^{2}$/m plays the role of the transverse field. The glassy phase is characterized by replica symmetry breaking. The quantum transition at zero temperature is also discussed. \textcopyright{} 1996 The American Physical Society.
When a quantum many-particle system exists on a randomly diluted lattice, its intrinsic thermal and quantum fluctuations coexist with geometric fluctuations due to percolation. In this paper, we explore how the interplay of these fluctuations influences the phase transition at the percolation threshold. While it is well known that thermal fluctuations generically destroy long-range order on the critical percolation cluster, the effects of quantum fluctuations are more subtle. In diluted quantum magnets with and without dissipation, this leads to novel universality classes for the zero-temperature percolation quantum phase transition. Observables involving dynamical correlations display nonclassical scaling behavior that can nonetheless be determined exactly in two dimensions.
Phase transitions are prevalent throughout physics, spanning thermal phenomena like water boiling to magnetic transitions in solids. They encompass cosmological phase transitions in the early universe and the transition into a quark-gluon plasma in high-energy collisions. Quantum phase transitions, particularly intriguing, occur at temperatures near absolute zero and are driven by quantum fluctuations rather than thermal ones. The strength of the fluctuations is very sensitive to the dimensionality of the physical systems, which determines the existence and nature of phase transitions. Low-dimensional systems often exhibit suppression of phase transitions, while high-dimensional systems tend to exhibit mean-field-like behavior. The localization-delocalization Anderson transition stands out among quantum phase transitions, as it is thought to retain its non-mean-field character across all dimensions. This work marks the first observation and characterization of the Anderson transition in four dimensions using ultracold atoms as a quantum simulator with synthetic dimensions. We characterize the universal dynamics in the vicinity of the phase transition. We measure the critical exponents describing the scale-invariant properties of the critical dynamics, which are shown to obey Wegner's scaling law. Our work is the first experimental demonstration that the Anderson transition is not mean-field in dimension four.
Dimensionality is a fundamental concept in physics, which plays a hidden but crucial role in various domains, including condensed matter physics, relativity and string theory, statistical physics, etc. In quantum physics, reducing dimensionality usually enhances fluctuations and leads to novel properties. Owing to these effects, quantum simulators in which dimensionality can be controlled have emerged as a new area of interest. However, such a platform has only been studied in specific regimes and a universal phase diagram is lacking. Here, we produce an interacting atomic quantum simulator with continuous tunability of anisotropy and temperature, and probe the universal phase diagram of dimensional crossover. At low temperatures, we identify the regimes from quantum three to zero dimensions. By increasing temperature, we observe the non-trivial emergence of a thermal regime situated between the quantum zero and integer dimensions. We show that the quantum-to-thermal transition falls into four different universality classes depending on the dimensionality. Surprisingly, we also detect a fifth type where the high-dimensional quantum system can reach the thermal phase by crossing a low-dimensional quantum regime. Our results provide a crucial foundation for understanding the projective condensed matter structures in unconventional dimensions. The universal properties of dimensional crossovers in quantum simulation are not yet fully characterized. To address this gap, the authors develop a quantum simulator with ultracold bosons in triangular lattices unveiling new regimes and universality classes of quantum-tothermal transitions.
Quantum systems subjected to a continuous weak measurement process evolve according to stochastic differential equations. Depending on the outcomes of these stochastic measurements, the quantum state may diffuse in various directions across the state space. This note points out that, in many scenarios relevant to quantum engineering, this diffusion is effectively constrained to a low-dimensional space. Specifically, the quantum state remains confined to a low-dimensional, nonlinear manifold---often time-dependent, yet independent of the specific measurement outcomes. This note derives the corresponding low-dimensional formulations for expressing the stochastically evolving state in several prototypical cases: quantum nondemolition measurements in arbitrary dimensions; quadrature measurements of a harmonic oscillator (linear quantum system); and measurements of subsystems within multipartite quantum systems. Additionally, it introduces an algebraic criterion to determine whether such low-dimensional manifolds exist or persist when additional dynamics are present.