arXiv subjects

Brendan J. Mahoney

Publications and source records attributed to Brendan J. Mahoney.

3 recordsLinked to original sources

arXiv2025

The Lieb-Robinson correlation function for long disordered transverse-field Ising chains

The transverse-field Ising model is useful for studying interacting qubit arrays. The Lieb--Robinson correlation function can be used to characterize the propagation of quantum information in Ising chains. Considerable work has been done to establish bounds on this correlation function in various circumstances. To actually calculate the value of the correlation function directly typically requires a state space which grows exponentially with system size, and so is intractable for all but relatively small systems. We employ a recently-developed method that enables direct calculation of the value of the Lieb--Robinson correlation function and which scales linearly with system size. This enables the computation for systems with many hundreds of qubits, revealing the propagation of quantum information down the chain. We extend this technique to the problem of Ising chains with randomly disordered coupling strengths. Increasing disorder causes localization of the quantum correlations and halts propagation of quantum information.

quant-ph↗

arXiv2024

Lieb-Robinson correlation function for the quantum transverse field Ising model

The Lieb-Robinson correlation function is the norm of a commutator between local operators acting on separate subsystems at different times. This provides a useful state-independent measure for characterizing the specifically quantum interaction between spatially separated qubits. The finite propagation velocity for this correlator defines a "light-cone" of quantum influence. We calculate the Lieb-Robinson correlation function for one-dimensional qubit arrays described by the transverse field Ising model. Direct calculations of this correlation function have been limited by the exponential increase in the size of the state space with the number of qubits. We introduce a new technique that avoids this barrier by transforming the calculation to a sum over Pauli walks which results in linear scaling with system size. We can then explore propagation in arrays of hundreds of qubits and observe the effects of the quantum phase transition in the system. We observe the emergence of two distinct velocities of propagation: a correlation front velocity, which is affected by the phase transition, and the Lieb-Robinson velocity which is not. The correlation front velocity is equal to the maximum group velocity of single quasiparticle excitations. The Lieb-Robinson velocity describes the extreme leading edge of correlations when the value of the correlation function itself is still very small. For the semi-infinite chain of qubits at the quantum critical point, we derive an analytical result for the correlation function.

quant-ph↗

arXiv2022

The early-time Lieb-Robinson correlation function for qubit arrays

The Lieb-Robinson correlation function captures propagation of quantum correlations in a many-body system. We calculate the value of the leading order of the correlation function, not its bound, for a system of interacting qubits at early times. The general analytical result is compared to numerical calculations and is applied to regular qubit lattices in one, two, and three dimensions. The Lieb-Robinson velocity and the approximately exponential leading edge of correlations emerge in the limit of large arrays.

quant-ph↗