Information transfer along the causal lightcone of a brickwork quantum circuit
Understanding how local information propagates through many-body quantum systems is a central problem in nonequilibrium dynamics, with important implications for quantum communication, state transfer, and remote sensing. In this work, we investigate information transfer along a one-dimensional open chain of qudits, focusing on the task of recovering information initially encoded at one end via measurements performed at the opposite end. By restricting the dynamics to brickwork quantum circuits, and considering small $M$-qudit subsystems on the causal ``lightcone'' of the circuit, we obtain several results valid even for large system sizes $N$ or for nonintegrable global dynamics. Within this framework, lossless information transfer is linked to the existence of peripheral eigenvalues of a quantum channel $Φ_M$, which governs the evolution of the $M$-qudit local subsystem along the lightcone. We investigate conditions under which brickwork circuits admit such peripheral eigenvalues. For qubit chains and $M=1$, we show that the dual-unitary property is necessary, whereas for larger local subsystems ($M \geq 2$) or higher-dimensional qudits, this requirement can be relaxed. Perhaps surprisingly, we can use the peripheral eigenvalue condition to construct examples exhibiting lossless information transfer through chains of arbitrary system size $N$, even when the underlying circuit dynamics is nonintegrable and thermalising at long times.