SISSA Finds Boundary Quenches Show Entanglement Jump via Logarithm of Ratios

Researchers at SISSA and collaborating institutions Coventry University, Ben-Gurion University of the Negev and Tel Aviv University, have discovered a distinct leap in quantum entanglement following a specific type of disturbance known as a boundary quench. The work demonstrates that when a subsystem becomes fully connected to this quench event, its entanglement entropy does not increase gradually, but undergoes a sharp finite jump. Remarkably, the magnitude of this jump is universally determined by the logarithm of the ratio of the boundary factors associated with the initial and final conditions of the quantum system. Validating these analytical predictions, the team performed Matrix Product State simulations on the critical Ising spin chain, confirming the theory’s applicability to a concrete physical model and revealing universal dynamical signatures of these boundary quenches.

Quantum Quenches: Global vs. Local Protocols

A change in entanglement entropy signals a fundamental shift in quantum connectedness following a specific type of disturbance, a phenomenon detailed by Fossati and colleagues in their recent research on conformal field theory. Their work distinguishes itself from previous studies of quantum disruptions by examining how changes at the edge of a quantum system propagate inwards. Unlike global quenches affecting the entire system at once, or localized disturbances originating within, boundary quenches initiate change by altering the conditions at a system’s boundary. This alteration doesn’t produce a gradual increase in entanglement, a measure of quantum correlation, but rather a sharp finite jump, which occurs once a subsystem becomes fully causally connected to the quench event. The research combines features of both previously discussed protocols, referencing similarities to both ‘cut and glue’ methods and local operator insertions.

The numerical analysis allowed for investigation of the time evolution of the spin-flip entanglement asymmetry, revealing how the symmetry-breaking perturbation emitted from the boundary propagates through the system. The researchers found excellent agreement with the predicted behavior of both the one-point functions and the entanglement entropy on length scales smaller than the system size, solidifying the connection between theoretical predictions and observable phenomena. Their results uncover universal dynamical signatures of boundary quenches and establish a direct connection between nonequilibrium entanglement dynamics and boundary critical phenomena.

Investigations into boundary quenches within (1+1)-dimensional conformal field theory are revealing increasingly precise details about how localized disturbances propagate and influence quantum entanglement. Researchers have derived a remarkably simple and universal expression for the time evolution of one-point functions on the half-line, providing a direct description of the propagation of the disturbance generated by the quench. This analytical approach builds upon existing work utilizing conformal field theories to understand quantum quenches, extending insights beyond strictly conformal systems. To rigorously test these analytical predictions, the researchers turned to numerical simulations, employing Matrix Product State techniques to model the critical Ising spin chain, a well-established physical system whose low-energy behavior aligns with the Ising conformal field theory.

Entanglement Entropy Jump via Boundary Factors

Researchers have discovered a precise relationship between how quickly quantum entanglement changes during a specific type of system disturbance and a mathematical property of the system’s boundaries. This finding, detailed in a research paper on boundary quenches in conformal field theory published on July 21, 2026, has implications for understanding how information spreads in quantum systems. The work demonstrates that once a subsystem becomes fully causally connected to the quench event, the entanglement entropy undergoes a sharp finite jump whose magnitude is universally given by the logarithm of the ratio of the boundary factors associated with the initial and final boundary conditions. Researchers have derived this jump and benchmarked these analytical predictions against Matrix Product State simulations of the critical Ising spin chain, finding excellent agreement.

The numerical analysis also allowed them to investigate the time evolution of the spin-flip entanglement asymmetry, revealing how the symmetry-breaking perturbation emitted from the boundary propagates through the system. Michele Fossati, affiliated with SISSA and INFN in Italy and Southern Denmark University, led research with colleagues into how sudden changes at the edges of quantum systems propagate disturbances. This work diverges from traditional quantum quench studies by focusing not on alterations throughout a system, but specifically at its boundaries. While prior work explored global changes affecting entire systems or localized excitations like single-point disturbances, this study focused on a boundary quench, a protocol that exhibits characteristics of both ‘cut and glue’ methods and local operator insertions. The analytical framework predicted this jump, but validation required a computational approach capable of modeling the complex interactions within the system.

The comparison between the analytical calculations and the numerical data reinforced the validity of the model, and the simulations allowed for examination of the system’s behavior on length scales where the infinite-chain approximation used in the analytical calculations is expected to hold. Researchers have demonstrated a surprising connection between seemingly disparate quantum phenomena: boundary quenches exhibit characteristics of both protocols and localized operator insertions. The analysis reveals that the time evolution of one-point functions follows a predictable light-cone behavior; points causally disconnected from the boundary change remain unaffected, while those within the light cone reflect the new boundary condition.

Researchers have demonstrated a surprising connection between seemingly disparate quantum phenomena: boundary quenches exhibit characteristics of both protocols and localized operator insertions, revealing a unified picture of quantum disturbance propagation. This work, focused on (1+1)-dimensional conformal field theory, details how abruptly altering a system’s boundary conditions generates a unique dynamic, distinct from simply modifying the Hamiltonian across the entire system or inserting a point-like excitation. The analysis reveals that the time evolution of one-point functions follows a predictable light-cone behavior; points causally disconnected from the boundary change remain unaffected, while those within the light cone reflect the new boundary condition.