Three-level spin system under decoherence-minimizing driving fields: Application to nitrogen-vacancy spin dynamics

Within the framework of a general three-level problem, the dynamics of the nitrogen-vacancy (NV) spin is studied for the case of a special type of external driving consisting of a set of continuous fields with decreasing intensities. Such a set has been proposed for minimizing coherence losses. Each new driving field with smaller intensity is designed to protect against the fluctuations induced by the driving field at the preceding step with larger intensity. We show that indeed this particular type of external driving minimizes the loss of coherence, using purity and entropy as quantifiers for this purpose. As an illustration, we study the coherence loss of an NV spin due to a surrounding spin bath of $^{13}$C nuclei.Comment: 10 pages, 8 figures, to be published in Phys. Rev.

Algebraic characterization of X-states in quantum information

A class of two-qubit states called X-states are increasingly being used to discuss entanglement and other quantum correlations in the field of quantum information. Maximally entangled Bell states and "Werner" states are subsets of them. Apart from being so named because their density matrix looks like the letter X, there is not as yet any characterization of them. The su(2) X su(2) X u(1) subalgebra of the full su(4) algebra of two qubits is pointed out as the underlying invariance of this class of states. X-states are a seven-parameter family associated with this subalgebra of seven operators. This recognition provides a route to preparing such states and also a convenient algebraic procedure for analytically calculating their properties. At the same time, it points to other groups of seven-parameter states that, while not at first sight appearing similar, are also invariant under the same subalgebra. And it opens the way to analyzing invariant states of other subalgebras in bipartite systems.Comment: 4 pages, 1 figur

The temperature dependent bandstructure of a ferromagnetic semiconductor film

The electronic quasiparticle spectrum of a ferromagnetic film is investigated within the framework of the s-f model. Starting from the exact solvable case of a single electron in an otherwise empty conduction band being exchange coupled to a ferromagnetically saturated localized spin system we extend the theory to finite temperatures. Our approach is a moment-conserving decoupling procedure for suitable defined Green functions. The theory for finite temperatures evolves continuously from the exact limiting case. The restriction to zero conduction band occupation may be regarded as a proper model description for ferromagnetic semiconductors like EuO and EuS. Evaluating the theory for a simple cubic film cut parallel to the (100) crystal plane, we find some marked correlation effects which depend on the spin of the test electron, on the exchange coupling, and on the temperature of the local-moment system.Comment: 11 pages, 9 figure

Are multiphase competition & order-by-disorder the keys to understanding Yb2Ti2O7?

If magnetic frustration is most commonly known for undermining long-range order, as famously illustrated by spin liquids, the ability of matter to develop new collective mechanisms in order to fight frustration is no less fascinating, providing an avenue for the exploration and discovery of unconventional properties of matter. Here we study an ideal minimal model of such mechanisms which, incidentally, pertains to the perplexing quantum spin ice candidate Yb2Ti2O7. Specifically, we explain how thermal and quantum fluctuations, optimized by order-by-disorder selection, conspire to expand the stability region of an accidentally degenerate continuous symmetry U(1) manifold against the classical splayed ferromagnetic ground state that is displayed by the sister compound Yb2Sn2O7. The resulting competition gives rise to multiple phase transitions, in striking similitude with recent experiments on Yb2Ti2O7 [Lhotel et al., Phys. Rev. B 89 224419 (2014)]. Considering the effective Hamiltonian determined for Yb2Ti2O7, we provide, by combining a gamut of numerical techniques, compelling evidence that such multiphase competition is the long-sought missing key to understanding the intrinsic properties of this material. As a corollary, our work offers a pertinent illustration of the influence of chemical pressure in rare-earth pyrochlores.Comment: 9 page

Unusual magnetic properties of the low-dimensional quantum magnet Na2V3O7

We report the results of low-temperature measurements of the specific heat Cp(T), ac susceptibility chi(T) and 23Na nuclear magnetic resonance NMR of Na2V3O7. At liquid He temperatures Cp(T)/T exhibits broad field-dependent maxima, which shift to higher temperatures upon increasing the applied magnetic field H. Below 1.5 K the ac magnetic susceptibility chi(T) follows a Curie-Weiss law and exhibits a cusp at 0.086 mK which indicates a phase transition at very low temperatures. These results support the previous conjecture that Na2V3O7 is close to a quantum critical point (QCP) at mu_{0}H = 0 T. The entire data set, including results of measurements of the NMR spin-lattice relaxation 1/T1(T), reveals a complex magnetic behavior at low temperatures. We argue that it is due to a distribution of singlet-triplet energy gaps of dimerized V moments. The dimerization process evolves over a rather broad temperature range around and below 100 K. At the lowest temperatures the magnetic properties are dominated by the response of only a minor fraction of the V moments.Comment: 10.5 pages, 15 figures. Submitted to Phys. Rev.

Two Qubits in the Dirac Representation

A general two qubit system expressed in terms of the complete set of unit and fifteen traceless, Hermitian Dirac matrices, is shown to exhibit novel features of this system. The well-known physical interpretations associated with the relativistic Dirac equation involving the symmetry operations of time-reversal T, charge conjugation C, parity P, and their products are reinterpreted here by examining their action on the basic Bell states. The transformation properties of the Bell basis states under these symmetry operations also reveal that C is the only operator that does not mix the Bell states whereas all others do. In a similar fashion, expressing the various logic gates introduced in the subject of quantum computers in terms of the Dirac matrices shows for example, that the NOT gate is related to the product of time-reversal and parity operators.Comment: 11 page

Calculation of quantum discord for qubit-qudit or N qubits

Quantum discord, a kind of quantum correlation, is defined as the difference between quantum mutual information and classical correlation in a bipartite system. It has been discussed so far for small systems with only a few independent parameters. We extend here to a much broader class of states when the second party is of arbitrary dimension d, so long as the first, measured, party is a qubit. We present two formulae to calculate quantum discord, the first relating to the original entropic definition and the second to a recently proposed geometric distance measure which leads to an analytical formulation. The tracing over the qubit in the entropic calculation is reduced to a very simple prescription. And, when the d-dimensional system is a so-called X state, the density matrix having non-zero elements only along the diagonal and anti-diagonal so as to appear visually like the letter X, the entropic calculation can be carried out analytically. Such states of the full bipartite qubit-qudit system may be named "extended X states", whose density matrix is built of four block matrices, each visually appearing as an X. The optimization involved in the entropic calculation is generally over two parameters, reducing to one for many cases, and avoided altogether for an overwhelmingly large set of density matrices as our numerical investigations demonstrate. Our results also apply to states of a N-qubit system, where "extended X states" consist of (2^(N+2) - 1) states, larger in number than the (2^(N+1) - 1) of X states of N qubits. While these are still smaller than the total number (2^(2N) - 1) of states of N qubits, the number of parameters involved is nevertheless large. In the case of N = 2, they encompass the entire 15-dimensional parameter space, that is, the extended X states for N = 2 represent the full qubit-qubit system.Comment: 6 pages, 1 figur

PERSEPSI MASYARAKAT TERHADAP RENCANA RELOKASI AKIBAT BENCANA BANJIR (STUDI KASUS KELURAHAN DENDENGAN DALAM, KECAMATAN PAAL DUA)

This research aimed to describe culture, social, physical, and economy condition, to find out public perception of the relocation plan, and to identify relationship between public perception and influence factors at Lingkungan I. This research was done for three months from Agustus to October 2015 at Kelurahan Dendengan Dalam Lingkungan I, Paal Dua Sub-District, Manado City, North Sulawesi Province. This research uses descriptive qualitative with method or case study approach. Data analysed in quantitative and qualitative with statistical analysis in tabulation, percentage, and scoring and then continued with chi-squared analysis. Result showed that people condition at Lingkungan I after flood disaster didn't affect familty economy which still in passably economy but physical condition changed to weak and socio-cultural condition became good while organization condition (government socialization effort) was in passably condition because public didn't understand enough or had awareness about the relocation plan. People of Lingkungan I, Kelurahan Dendengan Dalam, doubt and didn't agree with resettlement plan of government. Most of the public attitude were attitude of passably economy condition, weaker physical and fixed condition, low and passably good organization condition, and lack of (enough) public awareness about the whole relocation plan. Factors which have connection with public perception at Lingkungan I, Kelurahan Dendengan Dalam such as organization condition (government socialication effort) and the level of knowledge about the whole relocation plan, while economy, physic, and socio-cultural condition were not affect by that

PERSEPSI MASYARAKAT TERHADAP RENCANA RELOKASI AKIBAT BENCANA BANJIR (STUDI KASUS KELURAHAN DENDENGAN DALAM, KECAMATAN PAAL DUA)

This research aimed to describe culture, social, physical, and economy condition, to find out public perception of the relocation plan, and to identify relationship between public perception and influence factors at Lingkungan I. This research was done for three months from Agustus to October 2015 at Kelurahan Dendengan Dalam Lingkungan I, Paal Dua Sub-District, Manado City, North Sulawesi Province. This research uses descriptive qualitative with method or case study approach. Data analysed in quantitative and qualitative with statistical analysis in tabulation, percentage, and scoring and then continued with chi-squared analysis. Result showed that people condition at Lingkungan I after flood disaster didn't affect familty economy which still in passably economy but physical condition changed to weak and socio-cultural condition became good while organization condition (government socialization effort) was in passably condition because public didn't understand enough or had awareness about the relocation plan. People of Lingkungan I, Kelurahan Dendengan Dalam, doubt and didn't agree with resettlement plan of government. Most of the public attitude were attitude of passably economy condition, weaker physical and fixed condition, low and passably good organization condition, and lack of (enough) public awareness about the whole relocation plan. Factors which have connection with public perception at Lingkungan I, Kelurahan Dendengan Dalam such as organization condition (government socialication effort) and the level of knowledge about the whole relocation plan, while economy, physic, and socio-cultural condition were not affect by that