Two-level States Research Articles

Rabi oscillation damping of two-level states in quantum dots

Within a general approach suitable to describe a coherently driven two-level system interacting with a dephasing reservoir, we have proposed various mechanisms to explain the nature of the damping of Rabi oscillations with increasing driving-pulse area in localized two-level semiconductor systems. We have shown that the non-Markovian character of the reservoir leads to the dependence of the dephasing rate on the driving-field intensity, as observed experimentally. Furthermore, we have also shown that stationary as well as non-stationary effects resulting from the coupling to the environment may give rise to intensity-dependent damping of oscillations.

Thermal and electronic transport in La 0.7Sr 0.3−xAg xMnO 3 compounds between 50 and 450 K

Temperature dependence of electrical and thermal conductivity of nanocrystalline La 0.7Sr 0.3− x Ag x MnO 3 ( x = 0.0 5 , 0.10, 0.20, 0.25) pellets prepared by pyrophoric method is reported between 20 and 300 K. Silver doping in LSMO influences strongly both the electrical and thermal conductivity. The metal–insulator transition temperature shows significant enhancement from 303 K ( x=0.05) to 363 K ( x=0.25). Electrical conduction in La 0.7Sr 0.3− x Ag x MnO 3 above T C is consistent with adiabatic small polaron hopping, while, in the ferromagnetic region ( T < T C ) the resistivity data shows a nearly perfect fit for all the samples to an expression containing, the residual resistivity, two-magnon scattering term and the term associated with small-polaron metallic conduction, which involves a relaxation time due to a soft optical phonon mode. Heat conduction in this compound is dominated by phonons and electronic part is only ∼1% of the total λ. Lattice thermal conductivity ( λ ph) between 20 and 250 K, estimated using relaxation time approach gives an excellent agreement (within 2%) with the measured values, only when Umklapp scattering and scattering by two-level states are included along with phonon scattering by grain boundaries, sheet-like faults and point defects in the phonon relaxation time expression. Strengths of these phonon scattering rates are also estimated individually. Two-dimensional sheet-like faults appear to be the most important phonon scatterer in La 0.7Sr 0.3− x Ag x MnO 3 compounds.

Decoherence in Josephson Qubits from Dielectric Loss

Dielectric loss from two-level states is shown to be a dominant decoherence source in superconducting quantum bits. Depending on the qubit design, dielectric loss from insulating materials or the tunnel junction can lead to short coherence times. We show that a variety of microwave and qubit measurements are well modeled by loss from resonant absorption of two-level defects. Our results demonstrate that this loss can be significantly reduced by using better dielectrics and fabricating junctions of small area . With a redesigned phase qubit employing low-loss dielectrics, the energy relaxation rate has been improved by a factor of 20, opening up the possibility of multiqubit gates and algorithms.

Template matching of mixed quantum states

We consider the problem of optimal classification of an unknown input mixed quantum state f ^ with respect to a set of predefined patterns C i , each represented by a known mixed quantum template g ^ i . The performance of the matching strategy is addressed within a Bayesian formulation where the cost function, as suggested by the theory of monotone distances between quantum states, is chosen to be the fidelity or the relative entropy between the input and the templates. We investigate various examples of quantum template matching for the case of a finite number of copies of a two-level input state f ^ and for a generic, group covariant, set of two-level template states.

Can We Build a Large-Scale Quantum Computer Using Semiconductor Materials?

AbstractThe following article is based on the Symposium X presentation given by Bruce E. Kane (University of Maryland) at the 2004 Materials Research Society Spring Meeting in San Francisco. Quantum computing has the potential to revolutionize our ability to solve certain classes of difficult problems. A quantum computer is able to manipulate individual two-level quantum states (“qubits”) in the same way that a conventional computer processes binary ones and zeroes. Here, Kane discusses some of the most promising proposals for quantum computing, in which the qubit is associated with single-electron spins in semiconductors. While current research is focused on devices at the one- and two-qubit level, there is hope that cross-fertilization with advancing conventional computer technology will enable the eventual development of a large-scale (thousands of qubits) semiconductor quantum computer.The author focuses on materials issues that will need to be surmounted if large-scale quantum computing is to be realizable. He argues in particular that inherent fluctuations in doped semiconductors will severely limit scaling and that scalable quantum computing in semiconductors may only be possible at the end of the road of Moore's law scaling, when devices are engineered and fabricated at the atomic level.

Electrical resistivity in Zr48Nb8Cu12Fe8Be24 glassy and crystallized alloys

The electrical resistivity of Zr48Nb8Cu12Fe8Be24 bulk metallic glassy and crystallized alloys in the temperature range of 4.2–293 K is investigated. It is found that the resistivity in glassy and crystallized states shows opposite temperature coefficients. For the metallic glass, the resistivity shows a negative logarithmic dependence at temperatures below 16 K, whereas it has more normal behavior for the crystallized alloy. At higher temperatures, the resistivity in both glassy and crystallized alloys shows dependence upon both T and T2, but the signs of the T and T2 terms are opposite. The results are interpreted in terms of scattering from two-level tunneling states in glasses and the generalized Ziman diffraction model.

Electron Spin-Lattice Relaxation in Polymers and Crystals Related to Disorder and Structure Defects

Temperature dependences (4-300 K) of the electron spin-lattice relaxation have been determined by electron spin echo technique for free radicals in two polymeric systems: phenol-formaldehyde resin and polyhydrazodisulphide. The dependences are described in terms of dynamics specific for amorphous systems involving two-level tunnelling states at low temperatures, exchange-coupled clusters of paramagnetic centres and local oscillators. Some universal temperature behaviour of the spin-lattice relaxation of amorphous systems is suggested, with a strong increase in relaxation rate with temperature at low temperatures and much weaker increase above 50 K with characteristic linear temperature dependence in a broad temperature range and cosech(Δ/kT)-type behaviour. It is also shown that the amorphous-type behaviour appears in low temperatures relaxation studies of single crystals but it is due to a non-uniform distribution of doped paramagnetic ions. Such behaviour we have found in Tutton salt crystals doped with Cu 2 + , as well as for free radical centres produced by ionising irradiation in (NH 4 ) 3 H(SeO 4 ) 2 and Li(N 2 H 5 )SO 4 single crystals, where extended phonons are suppressed around radiation damage centres suggesting a local amorphisation of the crystal structure.

Magnetic field influence on the low temperature heat capacity and heat release of (TMTSF)2PF6

We have measured under applied magnetic field the heat capacity c p and heat release $${\dot Q}$$ of the quasi-1D conductor (TMTSF)2PF6 in its spin density wave (SDW) ground state. The low-temperature heat capacity (T < 0.5 K) is dominated by a Schottky anomaly contribution. A corresponding term was also found for the heat release. In this T-range both properties are strongly sensitive to moderate magnetic fields (i.e., below 0.5 T): their amplitudes have a sharp maximum for H c=0.2 T. We show that the corresponding density of two-level states N S undergoes a sharp maximum at H c, which rules out an extrinsic origin for the two-level states. Instead we suppose an intrinsic origin, such as defects of commensurability of the SDW, as the (slow) excitations related to the domain walls. This effect is found to be independent on the field orientations H ∥ a and H ⊥ a.

Grüneisen parameter of D-doped Nb 37Ti 63 at temperatures below 10 K

Glasslike anomalies were observed in the low-temperature thermal properties of the polycrystalline Nb 37Ti 63 alloy and Nb 37Ti 63 doped with deuterium. In (Nb-Ti) 92D 8, a very high spectral density of two-level states of 7.5×10 45 J −1 m −3 was found. After rapid cooling of this system, both a giant long-time heat release and a length relaxation with a logarithmic time dependence were observed, corresponding to a time- and temperature-independent Grüneisen parameter Γ=−2. From the measurements of specific heat and thermal expansion coefficient, a contribution of the tunneling systems to the Grüneisen parameter of Γ=−57 is calculated. The origin of the discrepancy between the two values is discussed.

Elastic properties of amorphous and nanocrystalline silicon

We review the present status of the elastic properties of amorphous silicon (a-Si) and germanium (a-Ge) thin films produced by either plasma-assisted chemical-vapor-deposition (PECVD) or hot-wire (cat) CVD (HWCVD). Using an extremely sensitive c-Si mechanical oscillator, we measure the low temperature oscillator damping for a variety of amorphous and nanocrystalline Si thin films deposited on the oscillator and thus determine their internal friction. This internal friction is a direct measure of the phonon loss mechanisms. We find that HWCVD a-Si with H content below approximately 3 at.% is the first example that the usual two-level tunneling states (TLS) found in all amorphous solids can be reduced by nearly three orders of magnitude. However, the TLS are present in a-Si prepared by other methods. This low H material also shows an enhanced resistance to the Staebler–Wronski effect. We will present results of light soaking on the elastic properties of both HWCVD and PECVD a-Si. Although the low temperature elastic properties of PECVD and HWCVD are significantly different, their room temperature thermal conductivity is virtually the same.

Dynamics of silica glass:two-level tunnelling states and low-energy floppy modes

We present the results of a computational study of the low-energydynamics of silica glass. Molecular dynamics simulation resultsshow that parts of the glass structure can undergo large cooperativereorientations of SiO4 tetrahedra. These motions involvereorientations of about 30 tetrahedra with an energy barrier ofabout 0.06 eV. We relate these motions to the presence ofdouble-well potentials which give rise to two-level tunnellingstates in the model, thereby providing the mechanism for theanomalous low-temperature thermal properties of glasses. Simulationof larger structures of silica glass shows that jump events becomemore frequent and uncorrelated with each other. In addition tostudying the flexibility of silica glass in terms of the largetetrahedral rearrangements, we also address the flexibility ofsilica glass in terms of its ability to sustain low-ωfloppy modes. The latter part of the study is supported byinelastic neutron scattering data, and we compare experimental andcalculated dynamic structure factors in the energy range 0-10 meVand scattering vector range 0-8 Å-1. By applying theanalysis of the rigid-unit-mode model as initially developed forcrystalline silicates to structures of silica glass we find thatsilica glass is surprisingly similar to its correspondingcrystalline phases in its ability to support low-ω floppymodes. The same conclusion follows from the comparison ofcalculated vibrational densities of states of silica glass and itscrystalline phases, and is borne out in the inelastic neutronscattering data.

Phase-covariant quantum cloning

We consider an $\stackrel{\ensuremath{\rightarrow}}{N}M$ quantum cloning transformation acting on pure two-level states lying on the equator of the Bloch sphere. An upper bound for its fidelity is presented, by establishing a connection between optimal phase-covariant cloning and phase estimation. We give the explicit form of a cloning transformation that achieves the bound for the case $N=1,$ $M=2,$ and find a link between this case and optimal eavesdropping in the quantum cryptographic scheme BB84.

Mechanism and kinetics of the reversible transformation lda-hda of amorphous ice under pressure

An ultrasonic investigation of pressure-induced direct and reverse transformations lda-hda between the amorphous phases of low-and high-density ice is performed and the change in the shear modulus is determined. It is found that elastic softening of the amorphous network precedes both transformations, and the behavior of lda phase is similar to that of hexagonal 1h ice before amorphization. At the same time a number of features of the lda-hda transformation are due to the topological disordering of the amorphous phases, and the kinetics of the transformation is different from the standard behavior at a first-order phase transition and also from the compression kinetics of “ classic” glasses of the type a-SiO2 and a-GeO2. It is shown that the difference in the behavior of the lda and hda amorphous phases under pressure can be attributed to characteristic features of their spectra of low-energy two-level states.

Annealing effects in quench-condensed neon and argon films

Films of argon and neon were prepared on a cold substrate by quench-condensation from the gas phase onto a surface acoustic wave device operated at 630 MHz. On annealing above the condensation temperature strong changes of the sound velocity occur indicating structural rearrangement of the film. The annealing procedure has also significant influence on the temperature dependence of the sound velocity and attenuation which is governed by the presence of two-level tunneling states.

Light-Induced Increase in Two-Level Tunneling States in Hydrogenated Amorphous Silicon

AbstractWe observe an increase of the low-temperature internal friction of hydrogenated amorphous silicon prepared by both hot-wire and plasma-enhanced chemical-vapor deposition after extended light-soaking at room temperature. This increase, and the associated change in sound velocity, can be explained by an increase of the density of two-level tunneling states, which serves as a measure of the lattice disorder. The amount of increase in internal friction is remarkably similar in both types of films although the amount and the microstructure of hydrogen are very different. Experiments conducted on a sample prepared by hot-wire chemical-vapor deposition show that this change anneals out gradually at room temperature in about 70 days. Possible relation of the light-induced changes in the low-temperature elastic properties to the Staebler-Wronski effect is discussed.

Low Energy Dynamics and Tunneling States in Silica Glass

We present the results of a simulation study of silica glass which addresses the issue of largeamplitude low energy motions involving reorientations of SiO4 tetrahedra. Using methods developed for crystalline silicates we find that random networks of linked SiO 4 tetrahedra are as flexible as crystalline phases for infinitesimal displacements. Patches of the networks can also undergo large sudden distortions involving reorientations of the SiO 4 tetrahedra, with little change in energy and an energy barrier of around 0.06 eV. These may act as tunneling states to provide the mechanisms for the anomalous low-temperature thermal properties of glasses. [S0031-9007(98)07382-7] PACS numbers: 61.43.Fs, 63.20.Pw, 63.50. + x, 66.35. + a The structure of silica glass is well described as an infinite random network of SiO4 tetrahedra linked together at the corners. Evidence comes from diffraction studies [1], reverse Monte Carlo [2] and molecular dynamics simulations [3], and model building [4]. Over short length scales there are similarities between the structures of amorphous silica and the cristobalite and tridymite crystalline phases of silica [5], and the quantitative extent of these similarities and the associated length scale has recently been deduced [6]. A description of the vibrational dynamics of silica glass is less developed [7 ‐10]. In this Letter we address the issue of the excitations between 0 ‐ 5 meV, which will include those believed to be responsible for two-level tunneling states [11] and associated with the anomalous thermal behavior at low temperatures [12]. We find here, rather surprisingly, that silica glass has the same degree of flexibility as dynamically disordered [13] crystalline phases. We also find that it is able to undergo large sudden rotational

Temperature dependence of the electronic specific heat of superconductors with quantum defects

The electronic specific heat of metals with quantum defects in the superconducting state is examined. The role of the electron-polaron effect, as well as that of the level population factor of two-level states, is analyzed in an adiabatic approach. The cases of intermediate and strong coupling are discussed.

Atomic two-level states and relaxations observed in a computer model of amorphousNi81B19

We study the atomic motion associated with low-frequency localized vibrational modes in a computer model of amorphous ${\mathrm{Ni}}_{81}{\mathrm{B}}_{19}.$ We observe that the excitation of such a mode can lead to a transition of the system into a nearby potential-energy minimum. Such a ``jump'' has a cooperative character involving some tens of atoms. During the jump the potential-energy changes for atoms located at the jump site can be considerably larger than the total introduced excitation energy. The jumps are shown to take place at sites where so-called two-level states are located, whose existence in noncrystalline matter has been experimentally verified. The jumps can be of a reversible or of an irreversible nature, and a distinction can be made between two-level states that allow reversible atomic motion to take place and two-level states that disappear after the jump. The jumps can be identified with the local relaxations that have been predicted to occur in amorphous materials between 2 and 10 K and are correlated to low-frequency localized modes. The existence of double-well locations is argued to play an important role in diffusion at elevated temperatures, because they are sites that have a potentially high atomic mobility.

The possibilities of modeling neural networks in the framework of the thermodynamics of genetically disordered systems (glasses).

Non-spin glasses possess a number of specific features which, in structural and dynamic aspects, are close to conditions necessary for neural networks to function. In a disordered network there exists a plurality of structural parameters and a number of two-level states defined by double-well potentials. Their characteristics are specified by the conditions of glass formation, i.e. by genesis. The thermodynamic description of glass as a self-organizing system (that does not require introducing an interacting potential model) leads to an unambiguous conclusion that its frequency spectrum is predetermined by the structure, which is characterized by zero-point entropy. Glass is a natural system of oscillators which form a disordered network. In this sense, glass conforms to a known model of a disordered neural network formed by interconnected oscillators. If one assumes that in living organisms the structure of a neural network (the brain) is inherited according to a genetic mechanism, the quickness of learning and recognition of patterns, the stability of associative memory and other capabilities have to be inherited genetically. The more ordered a neural network formed by distinguishable neurons, the better its capabilities.

Evidence for two-level states and cooperative atomic jumps in a computer model of amorphous Ni 81B 19

Following a new approach we study the atomic motion associated with double-well potential energy minima, whose existence in non-crystalline matter has been predicted by the two-level states theory. These minima are located at the same sites as low-frequency localized vibrational modes and the ‘launching’ of such a mode can result in a jump into a new potential energy minimum. The jump has a cooperative character involving some tens of atoms. The potential energy changes during a jump indicate that the new local configuration is only stable because the displacement of all atoms involved creates a new potential energy minimum, rather than that the atoms move into an already existent neighboring potential minimum. The significance of this type of jumps for diffusion is discussed.