Nicolas Vogt

Ab initio calculation of energy levels for phosphorus donors in silicon

The s manifold energy levels for phosphorus donors in silicon are important input parameters for the design and modeling of electronic devices on the nanoscale. In this paper we calculate these energy levels from first principles using density functional theory. The wavefunction of the donor electron’s ground state is found to have a form that is similar to an atomic s orbital, with an effective Bohr radius of 1.8 nm. The corresponding binding energy of this state is found to be 41 meV, which is in good agreement with the currently accepted value of 45.59 meV. We also calculate the energies of the excited 1s(T2) and 1s(E) states, finding them to be 32 and 31 meV respectively.

One-dimensional Josephson junction arrays: Lifting the Coulomb blockade by depinning

Physikalisches Institut,Karlsruhe Institute of Technology, D-76128 Karlsruhe, Germany(Dated: July 16, 2014)Experiments with one-dimensional Josephson junction arrays in the regime of dominating chargingenergy show that the Coulomb blockade is lifted at the threshold voltage, which is proportional tothe array’s length and depends strongly on the Josephson energy. We explain this behavior bydepinning of the charges in the array. We assume strong charge disorder and argue that physicsaround the depinning point is governed by a disordered sine-Gordon-like model. This allows us toemploy the well-known theory of charge density wave depinning. Our model is in good agreementwith the experimental data.

Influence of two-level fluctuators on adiabatic passage techniques

We study the process of stimulated Raman adiabatic passage (STIRAP) under the influence of a nontrivial solid-state environment, particularly the effect of two-level fluctuators (TLFs) as they are frequently present in solid-state devices.

Insulating Josephson Junction Chains as Pinned Luttinger Liquids

Quantum physics in one spatial dimension is remarkably rich, yet even with strong interactions and disorder, surprisingly tractable. This is due to the fact that the low-energy physics of nearly all one-dimensional systems can be cast in terms of the Luttinger liquid, a key concept that parallels that of the Fermi liquid in higher dimensions. Although there have been many theoretical proposals to use linear chains and ladders of Josephson junctions to create novel quantum phases and devices, only modest progress has been made experimentally. One major roadblock has been understanding the role of disorder in such systems. We present experimental results that establish the insulating state of linear chains of submicron Josephson junctions as Luttinger liquids pinned by random offset charges, providing a one-dimensional implementation of the Bose glass, strongly validating the quantum many-body theory of one-dimensional disordered systems. The ubiquity of such an electronic glass in Josephson-junction chains has important implications for their proposed use as a fundamental current standard, which is based on synchronization of coherent tunneling of flux quanta (quantum phase slips).

Excitation and state transfer through spin chains in the presence of spatially correlated noise

We investigate the influence of environmental noise on spin networks and spin chains. In addition to the common model of an independent bath for each spin in the system we also consider noise with a finite spatial correlation length. We present the emergence of decoherence-free subspaces and different dynamics with increasing correlation length for both dephasing and dissipating environments. This leads to relaxation blocking of one spin by uncoupled surrounding spins. We then consider perfect state transfer through a spin chain in the presence of decoherence and discuss the dependence of the transfer quality on spatial noise correlation length. We identify qualitatively different features for dephasing and dissipative environments in spin-transfer problems.

Stochastic Bloch-Redfield theory: Quantum jumps in a solid-state environment

We discuss mapping the Bloch-Redfield master equation to Lindblad form and then unraveling the resulting evolution into a stochastic Schrodinger equation according to the quantum-jump method. We give two ¨ approximations under which this mapping is valid. This approach enables us to study solid-state systems of much larger sizes than is possible with the standard Bloch-Redfield master equation, while still providing a systematic method for obtaining the jump operators and corresponding rates. We also show how the stochastic unraveling of the Bloch-Redfield equations becomes the kinetic Monte Carlo algorithm in the secular approximation when the system-bath-coupling operators are given by tunneling operators between system eigenstates. The stochastic unraveling is compared to the conventional Bloch-Redfield approach with the superconducting single-electron transistor (SSET) as an example.

Thermally activated conductance in arrays of small Josephson junctions

We present measurements of the temperature-dependent conductance for series arrays of small-capacitance SQUIDs. At low bias voltages, the arrays exhibit a strong Coulomb blockade, which we study in detail as a function of temperature and Josephson energy

Approximate solutions to Mathieu's equation

E_J

Coulomb drag and depinning in bilinear Josephson junction arrays

. We find that the zero-bias conductance is well described by thermally activated charge transport with the activation energy on the order of

De-pinning of disordered bosonic chains

\Lambda E_C