arXiv subjects
Publications and source records attributed to I. Martin.
We propose a method for a weak continuous measurement of the energy eigenstates of a fast quantum system by means of a "slow" detector. Such a detector is only sensitive to slowly-changing variables, e. g. energy, while its back-action can be limited solely to decoherence of the eigenstate superpositions. We apply this scheme to the problem of detection of quantum jumps between energy eigenstates in a harmonic oscillator.
This paper reports on the first measurement of the ^16O(e,e'pn)^14N reaction. Data were measured in kinematics centred on a super-parallel geometry at energy and momentum transfers of 215 MeV and 316 MeV/c. The experimental resolution was sufficient to distinguish groups of states in the residual nucleus but not good enough to separate individual states. The data show a strong dependence on missing momentum and this dependence appears to be different for two groups of states in the residual nucleus. Theoretical calculations of the reaction using the Pavia code do not reproduce the shape or the magnitude of the data.
In a two-subband GaAs/AlGaAs two-dimensional electron system, the phase diagram of longitudinal resistivity ρ_xx in density and magnetic field plane exhibits an intriguing structure centered at filling factor ν= 4 which is strikingly different from the ring-like structures at lower magnetic fields. Thermal activation measurements reveal an anticrossing gap on each boundary of the structure where intersubband Landau Levels with parallel or antiparallel spin are brought into degeneracy. While the physics of the anticrossing can be ascribed to the pseudospin quantum Hall ferromagnetism, as reported earlier by Muraki et al, the mapping and modeling of the phase diagram topology allow us to establish a more complete picture of the consequences of real spin / pseudo-spin interactions for the two subband system.
We study a hole-doped two-leg ladder system including metal ions, oxygen, and electron-lattice interaction, as a model for Sr_{14-x}Ca_xCu_{24}O_{41-δ}. Single- and bi-polaronic states at 1/4-hole doping are modeled as functions of pressure by applying an unrestricted Hartree-Fock approximation to a multiband Peierls-Hubbard Hamiltonian. We find evidence for a pressure-induced phase transition between single-polaron and bi-polaron states. The electronic and phononic excitations in those states, including distinctive local lattice vibrational modes, are calculated by means of a direct-space Random Phase approximation. Finally, as a function of pressure, we identify a transition between site- and bond-centered bi-polarons, accompanied by a soft mode and a low-energy charge-sliding mode. We suggest comparisons with available experimented data.
Low-temperature dynamics of insulating glasses is dominated by a macroscopic concentration of tunneling two-level systems (TTLS). The distribution of the switching/relaxation rates of TTLS is exponentially broad, which results in non-equilibrium state of the glass at arbitrarily long time-scales. Due to the electric dipolar nature, the switching TTLS generate fluctuating electromagnetic fields. We study the effect of the non-thermal slow fluctuators on the dephasing of a solid state qubit. We find that at low enough temperatures, non-stationary contribution can dominate the stationary (thermal) one, and discuss how this effect can be minimized.
Recent experiments on quantum behavior in microfabricated solid-state systems suggest tantalizing connections to quantum optics. Several of these experiments address the prototypical problem of cavity quantum electrodynamics: a two-level system coupled to a quantum harmonic oscillator. Such devices may allow the exploration of parameter regimes outside the near-resonance and weak-coupling assumptions of the ubiquitous rotating-wave approximation (RWA), necessitating other theoretical approaches. One such approach is an adiabatic approximation in the limit that the oscillator frequency is much larger than the characteristic frequency of the two-level system. A derivation of the approximation is presented and the time evolution of the two-level-system occupation probability is calculated using both thermal- and coherent-state initial conditions for the oscillator. Closed-form evaluation of the time evolution in the weak-coupling limit provides insight into the differences between the thermal- and coherent-state models. Finally, potential experimental observations in solid-state systems, particularly the Cooper-pair box--nanomechanical resonator system, are discussed and found to be promising.
We consider a two-level (TL) system with energy level separation Omega_0 inside a Josephson junction. The junction is shunted by a resistor R and is current I (or voltage V = RI) biased. If the TL system modulates the Josephson energy and/or is optically active, it is Rabi driven by the Josephson oscillations in the running phase regime near the resonance 2eV = Omega_0. The Rabi oscillations, in turn, translate into oscillations of current and voltage which can be detected in noise measurements. This effect provides an option to fully characterize the TL systems and to find the TL's contribution to the decoherence when the junction is used as a qubit.
By applying an unrestricted Hartree-Fock and a Random Phase approximations to a multiband Peierls-Hubbard Hamiltonian, we study the phonon mode structure in models of transition metal oxides in the presence of intrinsic nanoscale inhomogeneities induced by hole doping. We identify low frequency $local$ vibrational modes pinned to the sharp interfaces between regions of distinct electronic structure (doped and undoped) and separated in frequency from the band of extended phonons. A characteristic of these ``edge'' modes is that their energy is essentially insensitive to the doping level. We discuss the experimental manifestations of these modes in inelastic neutron scattering, and also in spin and charge excitation spectra.
We consider the non-equilibrium dynamics of a molecular level interacting with local phonon modes in the case of a strong polaronic shift which prevents a perturbative treatment of the problem. Instead, we find that in an adiabatic regime when the electronic states react faster than the phonon modes it is possible to provide a fully non-perturbative treatment of the phonon dynamics including random noise and dissipation. The result shows intermittent switching between bistable states of the oscillator with an effective random telegraph noise.
We calculate the noise spectrum of the electrical current in a quantum point contact which is used for continuous measurements of a two-level system (qubit). We generalize the previous results obtained for the regime of high transport voltages (when $V$ is much larger than the qubit's energy level splitting $B$ (we put $e=\hbar=1$)) to the case of arbitrary voltages and temperatures. When $V \sim B$ the background output spectrum is essentially asymmetric in frequency, i.e., it is no longer classical. Yet, the spectrum of the amplified signal, i.e., the two coherent peaks at $ω=\pm B$ is still symmetric. In the emission (negative frequency) part of the spectrum the coherent peak can be 8 times higher than the background pedestal. Alternatively, this ratio can be seen in the directly measureable {\it excess} noise. For $V < B$ and T=0 the coherent peaks do not appear at all. We relate these results to the properties of linear amplifiers.
We study magneto-electric properties of single electron traps in metal-oxide-semiconductor field effect transistors. Using a microscopic description of the system based on the single-site Anderson-Holstein model, we derive an effective low energy action for the system. The behavior of the system is characterized by simultaneous polaron tunneling (corresponding to the charging and discharging of the trap) and Kondo screening of the trap spin in the singly occupied state. Hence, the obtained state of the system is a hybrid between the Kondo regime, typically associated with single electron occupancy, and the mixed valence regime, associated with large charge fluctuations. In the presence of a strong magnetic field, we demonstrate that the system is equivalent to a two level-level system coupled to an Ohmic bath, with a bias controlled by the applied magnetic field. Due to the Kondo screening, the effect of the magnetic field is significantly suppressed in the singly occupied state. We claim that this suppression can be responsible for the experimentally observed anomalous magnetic field dependence of the average trap occupancy in ${\rm Si-Si0_2}$ field effect transistors.
We study tunneling between a localized defect state and a conduction band in the presence of strong electron-electron and electron-phonons interactions. We derive the tunneling rate as a function of the position of the defect energy level relative to the Fermi energy of conduction electrons. We argue that our results can explain the large tunneling timescales observed in experiments on random telegraph signals in ${\rm Si}$ metal-oxide-semiconductor field effect transistors.
By applying an unrestriced Hartree-Fock approximation and a Random Phase approximation to multiband Peierls-Hubbard Hamiltonians, we determine the phonon mode structure in models of transition metal oxides in the presence of intrinsic nanoscale inhomogeneities induced by hole doping. We identify low frequency $local$ vibrational modes pinned to the interface between regions of distinct electronic structure and separated in frequency from the band of extended phonons. A major characteristic of these ``edge'' modes is that their energy is essentially insensitive to the doping level, while their intensity increases with the density of interfaces (and thus doping level). We argue that the presence of such modes is a typical feature of systems with phase separation, including cuprates, nickelates, manganites, and bismuthates. We specifically address the experimental signatures of these modes in lattice inelastic neutron scattering.
We consider a model of a quantum-mechanical resonator capacitively coupled to a single electron transistor (SET). The tunnel current in the SET is modulated by the vibrations of the resonator, and thus the system operates as a displacement detector. We analyze the effect of the back-action noise of charge fluctuations in the SET onto the dynamics of the resonator and evaluate the displacement sensitivity of the system. The relation between the "classical" and "quantum" parts of the SET charge noise and their effect on the measured system are also discussed.
We introduce a Nambu-Poisson bracket in the geometrical description of the D=11 M5-brane. This procedure allows us, under some assumptions, to eliminate the local degrees of freedom of the antisymmetric field in the M5-brane Hamiltonian and to express it as a D=11 p-brane theory invariant under symplectomorphisms. A regularization of the M5-brane in terms of a multi D1-brane theory invariant under the $SU(N)\times SU(N)$ group in the limit when $N\to\infty$ is proposed. Also, a regularization for the D=10 D4-brane in terms of a multi D0-brane theory is suggested.
We calculate the noise spectrum of the output signal of a quantum detector during continuous measurement of a two-level system (qubit). We generalize the previous results obtained for the regime of high voltages (when $eV$ is much larger than the qubit's energy level splitting $Δ$) to the case of arbitrary voltages and temperatures. When $V \sim Δ$ the output spectrum is essentially asymmetric in frequency, i.e., the output signal is no longer classical. In the emission (negative frequency) part of the spectrum the peak due to the qubit's coherent oscillations can be 8 times higher that the background pedestal. For $V < Δ$ and T=0 the coherent peaks do not appear at all.
Two-dimensional systems in which there is a competition between long-range repulsion and short range attraction exhibit a remarkable variety of patterns such as stripes, bubbles, and labyrinths. Such systems include magnetic films, Langmuir monolayers, polymers, gels, water-oil mixtures, and two-dimensional electron systems. In many of these systems quenched disorder from the underlying substrate may be present. We examine the dynamics and stripe formation in the presence of both an applied dc drive and quenched disorder. When the disorder strength exceeds a critical value, an applied dc drive can induce a dynamical stripe ordering transition to a state that is more ordered than the originating undriven, unpinned pattern.
We study relaxation of a spin in magnetic resonance force microscopy (MRFM) experiments. We evaluate the relaxation rate for the spin caused by high-frequency mechanical noise of the cantilever under the conditions of adiabatic spin inversion. We find qualitative agreement between the obtained relaxation time and the experimental results of Stipe {\it et al.} [Phys. Rev. Lett. 87, 277602 (2001)]. Based on our analysis, we propose a method for improving the MRFM sensitivity by engineering cantilevers with reduced tip positional fluctuations.