Varying Cu-Ti hybridization near the Fermi energy in Cux_{x}TiSe2_{2}: Results from supercell calculations

The properties of Cu$_{x}$TiSe$_{2}$ are studied by band structure calculation based on the density functional theory for supercells. The density-of-states (DOS) for $x$=0 has a sharply raising shoulder in the neighborhood of the Fermi energy, $E_F$, which can be favorable for spacial charge modulations. The Cu impurity adds electrons and brings the DOS shoulder below $E_F$. Hybridization makes the Ti-d DOS at $E_F$, the electron-phonon coupling and the Stoner factor very large. Strong pressure dependent properties are predicted from the calculations, since the DOS shoulder is pushed to higher energy at a reduced lattice constant. Effects of disorder are also expected to be important because of the rapidly varying DOS near $E_F$.Comment: 5 pages, 4 figures 2 table

Production of superpositions of coherent states in traveling optical fields with inefficient photon detection

We develop an all-optical scheme to generate superpositions of macroscopically distinguishable coherent states in traveling optical fields. It non-deterministically distills coherent state superpositions (CSSs) with large amplitudes out of CSSs with small amplitudes using inefficient photon detection. The small CSSs required to produce CSSs with larger amplitudes are extremely well approximated by squeezed single photons. We discuss some remarkable features of this scheme: it effectively purifies mixed initial states emitted from inefficient single photon sources and boosts negativity of Wigner functions of quantum states.Comment: 13 pages, 9 figures, to be published in Phys. Rev.

Generation of macroscopic superposition states with small nonlinearity

We suggest a scheme to generate a macroscopic superposition state (Schrodinger cat state) of a free-propagating optical field using a beam splitter, homodyne measurement and a very small Kerr nonlinear effect. Our scheme makes it possible to considerably reduce the required nonlinear effect to generate an optical cat state using simple and efficient optical elements.Comment: Significantly improved version, to be published in PRA as a Rapid Communicatio

Conditional Production of Superpositions of Coherent States with Inefficient Photon Detection

It is shown that a linear superposition of two macroscopically distinguishable optical coherent states can be generated using a single photon source and simple all-optical operations. Weak squeezing on a single photon, beam mixing with an auxiliary coherent state, and photon detecting with imperfect threshold detectors are enough to generate a coherent state superposition in a free propagating optical field with a large coherent amplitude ($\alpha>2$) and high fidelity ($F>0.99$). In contrast to all previous schemes to generate such a state, our scheme does not need photon number resolving measurements nor Kerr-type nonlinear interactions. Furthermore, it is robust to detection inefficiency and exhibits some resilience to photon production inefficiency.Comment: Some important new results added, to appear in Phys.Rev.A (Rapid Communication

Generation of distributed entangled coherent states over a lossy environment with inefficient detectors

Entangled coherent states are useful for various applications in quantum information processing, but they are sensitive to loss.We propose a scheme to generate distributed entangled coherent states over a lossy environment in such away that the fidelity is independent of the losses at detectors heralding the generation of the entanglement. We compare our scheme with a previous one for the same purpose [Ourjoumtsev et al., Nat. Phys. 5, 189 (2009)] and find parameters for which our scheme results in superior performance

Coherent manipulation of electronic states in a double quantum dot

We investigate coherent time-evolution of charge states (pseudo-spin qubit) in a semiconductor double quantum dot. This fully-tunable qubit is manipulated with a high-speed voltage pulse that controls the energy and decoherence of the system. Coherent oscillations of the qubit are observed for several combinations of many-body ground and excited states of the quantum dots. Possible decoherence mechanisms in the present device are also discussed.Comment: RevTe

Fault-tolerant linear optical quantum computing with small-amplitude coherent states

Quantum computing using two optical coherent states as qubit basis states has been suggested as an interesting alternative to single photon optical quantum computing with lower physical resource overheads. These proposals have been questioned as a practical way of performing quantum computing in the short term due to the requirement of generating fragile diagonal states with large coherent amplitudes. Here we show that by using a fault-tolerant error correction scheme, one need only use relatively small coherent state amplitudes ($\alpha > 1.2$) to achieve universal quantum computing. We study the effects of small coherent state amplitude and photon loss on fault tolerance within the error correction scheme using a Monte Carlo simulation and show the quantity of resources used for the first level of encoding is orders of magnitude lower than the best known single photon scheme. %We study this reigem using a Monte Carlo simulation and incorporate %the effects of photon loss in this simulation