@article {lavasani_measurement-induced_2021,
title = {Measurement-induced topological entanglement transitions in symmetric random quantum circuits},
journal = {Nat. Phys.},
volume = {17},
number = {3},
year = {2021},
note = {Place: HEIDELBERGER PLATZ 3, BERLIN, 14197, GERMANY Publisher: NATURE RESEARCH Type: Article},
month = {mar},
pages = {342+},
abstract = {Random quantum circuits, in which an array of qubits is subjected to a series of randomly chosen unitary operations, have provided key insights into the dynamics of many-body quantum entanglement. Recent work has shown that interleaving the unitary operations with single-qubit measurements can drive a transition between high- and low-entanglement phases. Here we study a class of symmetric random quantum circuits with two competing types of measurement in addition to unitary dynamics. We find a rich phase diagram involving robust symmetry-protected topological, trivial and volume law entangled phases, where the transitions are hidden to expectation values of any operator and are only apparent by averaging the entanglement entropy over quantum trajectories. In the absence of unitary dynamics, we find a purely measurement-induced critical point, which maps exactly to two copies of a classical two-dimensional percolation problem. Numerical simulations indicate that this transition is a tricritical point that splits into two critical lines in the presence of arbitrarily sparse unitary dynamics with an intervening volume law entangled phase. Our results show that measurements alone are sufficient to induce criticality and logarithmic entanglement scaling, and arbitrarily sparse unitary dynamics can be sufficient to stabilize volume law entangled phases in the presence of rapid, yet competing, measurements.},
issn = {1745-2473},
doi = {10.1038/s41567-020-01112-z},
author = {Lavasani, Ali and Alavirad, Yahya and Barkeshli, Maissam}
}
@article {16956,
title = {Instantaneous braids and Dehn twists in topologically ordered states},
journal = {Phys. Rev. B},
volume = {102},
year = {2020},
month = {Aug},
pages = {075105},
doi = {10.1103/PhysRevB.102.075105},
url = {https://link.aps.org/doi/10.1103/PhysRevB.102.075105},
author = {Zhu, Guanyu and Lavasani, Ali and Barkeshli, Maissam}
}
@article {16951,
title = {Universal Logical Gates on Topologically Encoded Qubits via Constant-Depth Unitary Circuits},
journal = {Phys. Rev. Lett.},
volume = {125},
year = {2020},
month = {Jul},
pages = {050502},
doi = {10.1103/PhysRevLett.125.050502},
url = {https://link.aps.org/doi/10.1103/PhysRevLett.125.050502},
author = {Zhu, Guanyu and Lavasani, Ali and Barkeshli, Maissam}
}
@article {ISI:000462882400010,
title = {Scrambling in the Dicke model},
journal = {Phys. Rev. A},
volume = {99},
number = {4},
year = {2019},
month = {APR 1},
pages = {043602},
publisher = {AMER PHYSICAL SOC},
type = {Article},
abstract = {The scrambling rate lambda(L) associated with the exponential growth of out-of-time-ordered correlators can be used to characterize quantum chaos. Here we use a particular Majorana fermion representation of spin-1/2 systems to study quantum chaos in the Dicke model. We take the system to be in thermal equilibrium and compute lambda(L) throughout the phase diagram to leading order in 1/N. We find that the chaotic behavior is strongest close to the critical point. At high temperatures lambda(L) is nonzero over an extended region that includes both the normal and superradiant phases. At low temperatures lambda(L) is nonzero in (a) close vicinity of the critical point and (b) a region within the superradiant phase. In the process we also derive an effective theory for the superradiant phase at finite temperatures Our formalism does not rely on the assumption of total spin conservation.},
issn = {2469-9926},
doi = {10.1103/PhysRevA.99.043602},
author = {Alavirad, Yahya and Lavasani, Ali}
}
@article {ISI:000482956700001,
title = {Universal logical gates with constant overhead: instantaneous Dehn twists for hyperbolic quantum codes},
journal = {Quantum},
volume = {3},
year = {2019},
month = {JUL 26},
publisher = {VEREIN FORDERUNG OPEN ACCESS PUBLIZIERENS QUANTENWISSENSCHAF},
type = {Article},
abstract = {A basic question in the theory of fault-tolerant quantum computation is to understand the fundamental resource costs for performing a universal logical set of gates on encoded qubits to arbitrary accuracy. Here we consider qubits encoded with constant space overhead (i.e. finite encoding rate) in the limit of arbitrarily large code distance d through the use of topological codes associated to triangulations of hyperbolic surfaces. We introduce explicit protocols to demonstrate how Dehn twists of the hyperbolic surface can be implemented on the code through constant depth unitary circuits, without increasing the space overhead. The circuit for a given Dehn twist consists of a permutation of physical qubits, followed by a constant depth local unitary circuit, where locality here is defined with respect to a hyperbolic metric that defines the code. Applying our results to the hyperbolic Fibonacci Turaev-Viro code implies the possibility of applying universal logical gate sets on encoded qubits through constant depth unitary circuits and with constant space overhead. Our circuits are inherently protected from errors as they map local operators to local operators while changing the size of their support by at most a constant factor; in the presence of noisy syndrome measurements, our results suggest the possibility of universal fault tolerant quantum computation with constant space overhead and time overhead of O(d/log d). For quantum circuits that allow parallel gate operations, this yields the optimal scaling of space-time overhead known to date.},
issn = {2521-327X},
author = {Lavasani, Ali and Zhu, Guanyu and Barkeshli, Maissam}
}
@article {ISI:000457732400002,
title = {Wiedemann-Franz law and Fermi liquids},
journal = {Phys. Rev. B},
volume = {99},
number = {8},
year = {2019},
month = {FEB 4},
pages = {085104},
publisher = {AMER PHYSICAL SOC},
type = {Article},
abstract = {We consider in depth the applicability of the Wiedemann-Franz (WF) law, namely that the electronic thermal conductivity (K) is proportional to the product of the absolute temperature (T) and the electrical conductivity (a) in a metal with the constant of proportionality, the so-called Lorenz number L-0, being a materials-independent universal constant in all systems obeying the Fermi liquid (FL) paradigm. It has been often stated that the validity (invalidity) of the WF law is the hallmark of an FL {[}non-Fermi liquid (NFL)]. We consider, both in two (2D) and three (3D) dimensions, a system of conduction electrons at a finite temperature T coupled to a bath of acoustic phonons and quenched impurities, ignoring effects of electron-electron interactions. We find that the WF law is violated arbitrarily strongly with the effective Lorenz number vanishing at low temperatures as long as phonon scattering is stronger than impurity scattering. This happens both in 2D and in 3D for T < T-BG, where T-BG is the Bloch-Griineisen temperature of the system. In the absence of phonon scattering (or equivalently, when impurity scattering is much stronger than the phonon scattering), however, the WF law is restored at low temperatures even if the impurity scattering is mostly small angle forward scattering. Thus, strictly at T = 0 the WF law is always valid in a FL in the presence of infinitesimal impurity scattering. For strong phonon scattering, the WF law is restored for T > T-BG (or the Debye temperature T-D, whichever is lower) as in usual metals. At very high temperatures, thermal smearing of the Fermi surface causes the effective Lorenz number to go below L-0, manifesting a quantitative deviation from the WF law. Our paper establishes definitively that the uncritical association of an NFL behavior with the failure of the WF law is incorrect.},
issn = {2469-9950},
doi = {10.1103/PhysRevB.99.085104},
author = {Lavasani, Ali and Bulmash, Daniel and S. Das Sarma}
}
@article { ISI:000450258700003,
title = {Low overhead Clifford gates from joint measurements in surface, color, and hyperbolic codes},
journal = {PHYSICAL REVIEW A},
volume = {98},
number = {5},
year = {2018},
month = {NOV 15},
pages = {052319},
issn = {2469-9926},
doi = {10.1103/PhysRevA.98.052319},
author = {Lavasani, Ali and Barkeshli, Maissam}
}