Quantum Glassiness and Topological Overprotection. Claudio Chamon. Collaborators: Claudio Castelnovo (BU), Christopher Mudry (PSI), Pierre Pujol (ENS-Lyon). PRL 05, cond-mat/0404182 PRB 04, cond-mat/0310710 PRB 05, cond-mat/0410562 Annal of Phys. 05, /0502068. DMR 0305482.
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Collaborators: Claudio Castelnovo (BU), Christopher Mudry (PSI), Pierre Pujol (ENS-Lyon)
PRL 05, cond-mat/0404182
PRB 04, cond-mat/0310710
PRB 05, cond-mat/0410562
Annal of Phys. 05, /0502068
Source: JOM, 52 (7) (2000)
eg. quantum spin glasses
Bray & Moore, J. Phys. C (1980)
Read, Sachdev, and Ye, PRB (1995)
eg. 1 frustrated Josephson junctions
with long-range interactions
eg. II self-generated mean-field glasses
Kagan, Feigel'man, and Ioffe, ZETF/JETP (1999)
Westfahl, Schmalian, and Wolynes, PRB (2003)
Does one need an order parameter?
Does one need a thermodynamic or quantum phase transition?
Why not simply remain in a mixed state and not reach the ground state instead!?
Does the classical glassy state need to be a phase?
Does one need a phase transition?
Kinetic constraints can lead to slow relaxation even in classical paramagnets!
Ritort & Sollich - review of kinetic constrained classical models
What about quantum systems?
Not free to toy with the dynamics - it is given.
Where to look: systems with hard constraints: ice models, dimer models, loop models,...
Some clean strongly correlated systems with
topological order and fractionalization
Strong correlations that can lead to these exotic quantum spectral properties can in some instances also impose kinetic constraints, similar to those studied in the context of classical glass formers.
A solvable toy model
Classical glasses can be efficiently simulated in a computer; but real time simulation of a quantum system is doomed by oscillating phases (as bad as, if not worse, than the fermion sign problem)!
Even a quantum computer does not help; quantum computers are good for unitary evolution. One needs a “quantum supercomputer”, with many qubits dedicated to simulate the bath.
Solvable toy model can show unambiguously and without arbitrary or questionable approximations that there are quantum many body systems without disorder and with only local interactions that are incapable of reaching their quantum ground states.
(not glassy yet)
Kitaev, Ann. Phys. (2003) - quant-phys/97
Wen, PRL (2003)
topological order for quantum computing
Same spectrum as free spins in a magnetic field
on a torus:
4 ground states
bath of quantum oscillators;
acts on physical degrees of freedom
Caldeira & Leggett, Ann. Phys. (1983)
defects cannot simply be annihilated; plaquettes are flipped in multiplets
defects must go away
defects cannot be annihilated;
must be recombined
simple defect diffusion (escapes glassiness)
Garrahan & Chandler, PNAS (2003)
Buhot & Garrahan, PRL (2002)
equivalent to classical glass model by
always flip 4 octahedra: never simple defect diffusion
topological quantum protection
quantum OVER protection
Beyond the toy model...
of T-breaking superconductors
Constrained Ising model
Moore & Lee, cond-mat/0309717
Castelnovo, Pujol, and Chamon, PRB (2004)
Snyder et al, Nature (2001)
Source: Snyder et al, Nature (2001)
Presented solvable examples of quantum many-body Hamiltonians of systems with exotic spectral properties (topological order and fractionalization) that are unable to reach their ground states as the environment temperature is lowered to absolute zero - systems remain in a mixed state down to T=0.
New constraint for topological quantum computing: that the ground state degeneracy is protected while the system is still able to reach the ground states.
Out-of-equilibrium strongly correlated quantum systems is an open frontier!