Staying adiabatic with unknown energy gap

We introduce an algorithm to perform an optimal adiabatic evolution that operates without an apriori knowledge of the system spectrum. By probing the system gap locally, the algorithm maximizes the evolution speed, thus minimizing the total evolution time. We test the algorithm on the Landau-Zener transition and then apply it on the quantum adiabatic computation of 3-SAT: The result is compatible with an exponential speed-up for up to twenty qubits with respect to classical algorithms. We finally study a possible algorithm improvement by combining it with the quantum Zeno effect.Comment: 4 pages, 4 figure

Impulsive quantum measurements: restricted path integral versus von Neumann collapse

The relation between the restricted path integral approach to quantum measurement theory and the commonly accepted von Neumann wavefunction collapse postulate is presented. It is argued that in the limit of impulsive measurements the two approaches lead to the same predictions. The example of repeated impulsive quantum measurements of position performed on a harmonic oscillator is discussed in detail and the quantum nondemolition strategies are recovered in both the approaches.Comment: 12 pages, 3 figure

GaN and InN nanowires grown by MBE: a comparison

Morphological, optical and transport properties of GaN and InN nanowires grown by molecular beam epitaxy (MBE) have been studied. The differences between the two materials in respect to growth parameters and optimization procedure was stressed. The nanowires crystalline quality has been investigated by means of their optical properties. A comparison of the transport characteristics was given. For each material a band schema was shown, which takes into account transport and optical features and is based on Fermi level pinning at the surface.Comment: 5 pages, 5 figure

Quantum computations with atoms in optical lattices: marker qubits and molecular interactions

We develop a scheme for quantum computation with neutral atoms, based on the concept of "marker" atoms, i.e., auxiliary atoms that can be efficiently transported in state-independent periodic external traps to operate quantum gates between physically distant qubits. This allows for relaxing a number of experimental constraints for quantum computation with neutral atoms in microscopic potential, including single-atom laser addressability. We discuss the advantages of this approach in a concrete physical scenario involving molecular interactions.Comment: 15 pages, 14 figure

Are violations to temporal Bell inequalities there when somebody looks?

The possibility of observing violations of temporal Bell inequalities, originally proposed by Leggett as a mean of testing the quantum mechanical delocalization of suitably chosen macroscopic bodies, is discussed by taking into account the effect of the measurement process. A general criterion quantifying this possibility is defined and shown not to be fulfilled by the various experimental configurations proposed so far to test inequalities of different forms.Comment: 7 pages, 1 eps figure, needs europhys.sty and euromacr.tex, enclosed in the .tar.gz file; accepted for publication in Europhysics Letter

Tunable thermal quenching of photoluminescence in Mg-doped p-type GaN

We have studied the thermal quenching of the ultraviolet luminescence band with a maximum at about 3.25 eV in p-type Mg-doped GaN. The characteristic temperature of the thermal quenching of photoluminescence (PL) gradually shifted to higher temperatures with increasing excitation intensity. This effect is explained by a population inversion of charge carriers at low temperatures, which suddenly converts into a quasiequilibrium population as the temperature increases above the characteristic value. Tunable quenching of PL has been observed only in some of the GaN:Mg samples. The absence of the tunable quenching of PL in another group of GaN:Mg samples is preliminarily attributed to different types of dominant nonradiative defects in the two groups of samples

Quantum point contact due to Fermi-level pinning and doping profiles in semiconductor nanocolumns

We show that nanoscale doping profiles inside a nanocolumn in combination with Fermi-level pinning at the surface give rise to the formation of a saddle-point in the potential profile. Consequently, the lateral confinement inside the channel varies along the transport direction, yielding an embedded quantum point contact. An analytical estimation of the quantization energies will be given

Theoretical analysis of the implementation of a quantum phase gate with neutral atoms on atom chips

We present a detailed, realistic analysis of the implementation of a proposal for a quantum phase gate based on atomic vibrational states, specializing it to neutral rubidium atoms on atom chips. We show how to create a double--well potential with static currents on the atom chips, using for all relevant parameters values that are achieved with present technology. The potential barrier between the two wells can be modified by varying the currents in order to realize a quantum phase gate for qubit states encoded in the atomic external degree of freedom. The gate performance is analyzed through numerical simulations; the operation time is ~10 ms with a performance fidelity above 99.9%. For storage of the state between the operations the qubit state can be transferred efficiently via Raman transitions to two hyperfine states, where its decoherence is strongly inhibited. In addition we discuss the limits imposed by the proximity of the surface to the gate fidelity.Comment: 9 pages, 5 color figure

Green luminescence in Mg-doped GaN

A majority of the point defects in GaN that are responsible for broad photoluminescence (PL) bands remain unidentified. One of them is the green luminescence band (GL2) having a maximum at 2.35 eV which was observed previously in undoped GaN grown by molecular-beam epitaxy in Ga-rich conditions. The same PL band was observed in Mg-doped GaN, also grown in very Ga-rich conditions. The unique properties of the GL2 band allowed us to reliably identify it in different samples. The best candidate for the defect which causes the GL2 band is a nitrogen vacancy (VN). We propose that transitions of electrons from the conduction band to the +/2+ transition level of the VN defect are responsible for the GL2 band in high-resistivity undoped and Mg-doped GaN