Horacio M. Pastawski

Environment-Independent Decoherence Rate in Classically Chaotic Systems

We study the decoherence of a one-particle system, whose classical correspondent is chaotic, when it evolves coupled to a weak quenched environment. This is done by analytical evaluation of the Loschmidt echo (i.e., the revival of a localized density excitation upon reversal of its time evolution), in the presence of the perturbation. We predict an exponential decay for the Loschmidt echo with a (decoherence) rate which is asymptotically given by the mean Lyapunov exponent of the classical system, and therefore independent of the perturbation strength, within a given range of strengths. Our results are consistent with recent experiments of polarization echoes in nuclear magnetic resonance and numerical simulations.

Tuning laser-induced band gaps in graphene

Could a laser field lead to the much sought-after tunable band gaps in graphene? By using Floquet theory combined with Greens functions techniques, we predict that a laser field in the mid-infrared range can produce observable band gaps in the electronic structure of graphene. Furthermore, we show how they can be tuned by using the laser polarization. Our results could serve as a guidance to design optoelectronic nanodevices.

Attenuation of polarization echoes in nuclear magnetic resonance: A study of the emergence of dynamical irreversibility in many-body quantum systems

The reversal of the time evolution of the local polarization in an interacting spin system involves a sign change of the effective dipolar Hamiltonian which refocuses the “spin diffusion” process generating a polarization echo. Here, the attenuation of these echo amplitudes as a function of evolution time is presented for cymantrene and ferrocene polycrystalline samples, involving one and two five spin rings per molecule, respectively. We calculate the fraction of polarization which is not refocused because only the secular part of the dipolar Hamiltonian is inverted. The results indicate that, as long as the spin dynamics is restricted to a single ring, the non-inverted part of the Hamiltonian is not able by itself to explain the whole decay of the polarization echoes. A crossover from exponential (cymantrene) to Gaussian (ferrocene) attenuation is experimentally observed. This is attributed to an increase of the relative importance of the spin dynamics, as compared with irreversible interactions, which ...

A nuclear magnetic resonance answer to the Boltzmann–Loschmidt controversy?

A unique experimental tool to deepen into the Boltzmann–Loschmidt controversy is provided by the NMR polarization echoes (PE). These appear when a local spin excitation, evolving with a many-body “diffusive” spin dynamics, is reversed. The attenuation of the PEs represents a progresive failure of the quantum interferences to rebuild the local excitation. Our results indicate that, in the absence of detectable environmental interactions, the characteristic time of this attenuation is determined by the reversible dynamics itself, i.e., spin–spin interaction time. This supports the Boltzmanns hypothesis of molecular “chaos”.

Quantum Dynamical Echoes in the Spin Diffusion in Mesoscopic Systems

The evolution of local spin polarization in finite systems involves interference phenomena that give rise to quantum dynamical echoes and nonergodic behavior. We predict the conditions to observe these echoes by exploiting the NMR sequences devised by Zhang et al. [Phys. Rev. Lett. 69, 2149 (1992)], which uses a rare

Environmentally induced quantum dynamical phase transition in the spin swapping operation.

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Quantum interference phenomena in the local polarization dynamics of mesoscopic systems: an NMR observation

as the local probe for a dipolar coupled

Measuring the Lyapunov exponent using quantum mechanics.

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Non-Markovian decay beyond the Fermi Golden Rule: Survival collapse of the polarization in spin chains

spin system. The nonideality of this probe when testing mesoscopic systems is carefully analyzed revealing the origin of various striking experimental features.

Decoherence as decay of the Loschmidt echo in a Lorentz gas.

Quantum information processing relies on coherent quantum dynamics for a precise control of its basic operations. A swapping gate in a two-spin system exchanges the degenerate states |(up arrow, down arrow)> and |(down arrow, up arrow)>. In NMR, this is achieved turning on and off the spin-spin interaction b=DeltaE that splits the energy levels and induces an oscillation with a natural frequency DeltaE/Plancks. Interaction of strength Plancks/tau(SE), with an environment of neighboring spins, degrades this oscillation within a decoherence time scale tau(phi). While the experimental frequency omega and decoherence time tau(phi) were expected to be roughly proportional to b/Plancks and tau(SE), respectively, we present here experiments that show drastic deviations in both omega and tau(phi). By solving the many spin dynamics, we prove that the swapping regime is restricted to DeltaEtau(SE) similar or greater than Plancks. Beyond a critical interaction with the environment the swapping freezes and the decoherence rate drops as 1/tau(phi) proportional to (b/Plancks)2tau(SE). The transition between quantum dynamical phases occurs when omega proportional to sqrt (b/Plancks)2-(k/tau(SE)2 becomes imaginary, resembling an overdamped classical oscillator. Here, 0< or =k2< or =1 depends only on the anisotropy of the system-environment interaction, being 0 for isotropic and 1 for XY interactions. This critical onset of a phase dominated by the quantum Zeno effect opens up new opportunities for controlling quantum dynamics.