arXiv subjects

Sabre Kais

Publications and source records attributed to Sabre Kais.

At least 181 records · Page 10Linked to original sources

arXiv2014

Sensitivity and Entanglement in the Avian Chemical Compass

The Radical Pair Mechanism can help to explain avian orientation and navigation. Some evidence indicates that the intensity of external magnetic fields plays an important role in avian navigation. In this paper, based on a two-stage strategy, we demonstrate that birds could reasonably detect the directions of geomagnetic fields and gradients of these fields using a yield-based chemical compass that is sensitive enough for navigation. Also, we find that the lifetime of entanglement in this proposed compass is angle-dependent and long enough to allow adequate electron transfer between molecules.

physics.bio-ph↗

arXiv2013

Persistence of entanglement in thermal states of spin systems

We study and compare the persistence of bipartite entanglement (BE) and multipartite entanglement (ME) in one-dimensional and two-dimensional spin XY models in an external transverse magnetic field under the effect of thermal excitations. We compare the threshold temperature at which the entanglement vanishes in both types of entanglement. We use the entanglement of formation as a measure of the BE and the geometric measure to evaluate the ME of the system. We have found that in both dimensions in the anisotropic and partially anisotropic spin systems at zero temperatures, all types of entanglement decay as the magnetic field increases but are sustained with very small magnitudes at high field values. Also we found that for the same systems, the threshold temperatures of the nearest neighbour (nn) BEs are higher than both of the next-to-nearest neighbour BEs and MEs and the three of them increase monotonically with the magnetic field strength. Thus, as the temperature increases, the ME and the far parts BE of the system become more fragile to thermal excitations compared to the nn BE. For the isotropic system, all types of entanglement and threshold temperatures vanish at the same exact small value of the magnetic field. We emphasise the major role played by both the properties of the ground state of the system and the energy gap in controlling the characteristics of the entanglement and threshold temperatures. In addition, we have shown how an inserted magnetic impurity can be used to preserve all types of entanglement and enhance their threshold temperatures. Furthermore, we found that the quantum effects in the spin systems can be maintained at high temperatures, as the different types of entanglements in the spin lattices are sustained at high temperatures by applying sufficiently high magnetic fields.

quant-ph↗

arXiv2013

A Universal Quantum Circuit Scheme For Finding Complex Eigenvalues

We present a general quantum circuit design for finding eigenvalues of non-unitary matrices on quantum computers using the iterative phase estimation algorithm. In particular, we show how the method can be used for the simulation of resonance states for quantum systems.

quant-ph↗

arXiv2013

Influence of the intensity gradient upon HHG from free electrons scattered by an intense laser beam

When an electron is scattered by a tightly-focused laser beam in vacuum, the intensity gradient is a critical factor to influence the electron dynamics, for example, the electron energy exchange with the laser fields as have been explored before [P.X.Wang et al.,J. Appl. Phys. 91, 856 (2002]. In this paper, we have further investigated its influence upon the electron high-harmonic generation (HHG) by treating the spacial gradient of the laser intensity as a ponderomotive potential. Based upon perturbative QED calculations, it has been found that the main effect of the intensity gradient is the broadening of the originally line HHG spectra. A one-to-one relationship can be built between the beam width and the corresponding line width. Hence this finding may provides us a promising way to measure the beam width of intense lasers in experiments. In addition, for a laser pulse, we have also studied the different influences from transverse and longitudinal intensity gradients upon HHG.

physics.optics↗

arXiv2013

Degree Distribution in Quantum Walks on Complex Networks

In this theoretical study, we analyze quantum walks on complex networks, which model network-based processes ranging from quantum computing to biology and even sociology. Specifically, we analytically relate the average long time probability distribution for the location of a unitary quantum walker to that of a corresponding classical walker. The distribution of the classical walker is proportional to the distribution of degrees, which measures the connectivity of the network nodes and underlies many methods for analyzing classical networks including website ranking. The quantum distribution becomes exactly equal to the classical distribution when the walk has zero energy and at higher energies the difference, the so-called quantumness, is bounded by the energy of the initial state. We give an example for which the quantumness equals a Renyi entropy of the normalized weighted degrees, guiding us to regimes for which the classical degree-dependent result is recovered and others for which quantum effects dominate.

quant-ph↗

arXiv2013

Manipulation of Molecules with Electromagnetic Fields

The goal of the present article is to review the major developments that have led to the current understanding of molecule-field interactions and experimental methods for manipulating molecules with electromagnetic fields. Molecule-field interactions are at the core of several, seemingly distinct, areas of molecular physics. This is reflected in the organization of this article, which includes sections on Field control of molecular beams, External field traps for cold molecules, Control of molecular orientation and molecular alignment, Manipulation of molecules by non-conservative forces, Ultracold molecules and ultracold chemistry, Controlled many-body phenomena, Entanglement of molecules and dipole arrays, and Stability of molecular systems in high-frequency super-intense laser fields. The article contains 853 references.

physics.atom-ph↗

arXiv2013

Quantum Circuit Design for Solving Linear Systems of Equations

Recently, it is shown that quantum computers can be used for obtaining certain information about the solution of a linear system Ax=b exponentially faster than what is possible with classical computation. Here we first review some key aspects of the algorithm from the standpoint of finding its efficient quantum circuit implementation using only elementary quantum operations, which is important for determining the potential usefulness of the algorithm in practical settings. Then we present a small-scale quantum circuit that solves a 2x2 linear system. The quantum circuit uses only 4 qubits, implying a tempting possibility for experimental realization. Furthermore, the circuit is numerically simulated and its performance under different circuit parameter settings is demonstrated.

quant-ph↗

arXiv2013

Quantum Confinement and Negative Heat Capacity

Thermodynamics dictates that the specific heat of a system is strictly non-negative. However, in finite classical systems there are well known theoretical and experimental cases where this rule is violated, in particular finite atomic clusters. Here, we show for the first time that negative heat capacity can also occur in finite quantum systems. The physical scenario on which this effect might be experimentally observed is discussed. Observing such an effect might lead to the design of new light harvesting nano devices, in particular a solar nano refrigerator.

cond-mat.stat-mech↗

arXiv2013

Quantum criticality analysis by finite size scaling and exponential basis sets

We combine the finite size scaling method with the meshfree spectral method to calculate quantum critical parameters for a given Hamiltonian. The basic idea is to expand the exact wave function in a finite exponential basis set and extrapolate the information about system criticality from a finite basis to the infinite basis set limit. The used exponential basis set -though chosen intuitively- allows handling a very wide range of exponential decay rates and calculating multiple eigenvalues simultaneously. As a benchmark system to illustrate the combined approach, we choose the Hulthen potential. The results show that the method is very accurate and converges faster when compared with other basis functions. The approach is general and can be extended to examine near threshold phenomena for atomic and molecular systems based on even-tempered exponential and Gaussian basis functions.

quant-ph↗

arXiv2013

Dynamics of Entanglement in one and two-dimensional spin systems

We present a review of dynamics of entanglement in one and two dimensional systems under the effect of external magnetic field and different degrees of anisotropy at zero and finite temperatures. Different techniques for treating the spin systems with large Hilbert space dimensions are discussed such as trace minimization algorithm, time-evolution matrix transformation and step by step projection technique.

quant-ph↗

arXiv2013

Reducing the number of ancilla qubits and the gate count required for creating large controlled operations

In this paper we show that it is possible to adapt a qudit scheme for creating a controlled-Toffoli created by Ralph et al. [Phys. Rev. A 75 011213] to be applicable to qubits. While this scheme requires more gates than standard schemes for creating large controlled gates, we show that with simple adaptations it is directly equivalent to the standard scheme in the literature. This scheme is the most gate-efficient way of creating large controlled unitaries currently known, however it is expensive in terms of the number of ancilla qubits used. We go on to show that using a combination of these standard techniques presented by Barenco et al. [Phys. Rev. A 52 3457 (1995)] we can create an n-qubit version of the Toffoli using less gates and the same number of ancilla qubits as recent work using computer optimization. This would be useful in any architecture of quantum computing where gates are cheap but qubit initialization is expensive.

quant-ph↗

arXiv2013

Entanglement creation in cold molecular gases using strong laser pulses

While many-particle entanglement can be found in natural solids and strongly interacting atomic and molecular gases, generating highly entangled states between weakly interacting particles in a controlled and scalable way presents a significant challenge. We describe here a one-step method to generate entanglement in a dilute gas of cold polar molecules. For molecules in optical traps separated by a few micrometers, we show that maximally entangled states can be created using the strong off-resonant pulses that are routinely used in molecular alignment experiments. We show that the resulting alignment-mediated entanglement can be detected by measuring laser-induced fluorescence with single-site resolution and that signatures of this molecular entanglement also appear in the microwave absorption spectra of the molecular ensemble. We analyze the robustness of these entangled molecular states with respect to intensity fluctuations of the trapping laser and discuss possible applications of the system for quantum information processing.

physics.atom-ph↗

arXiv2013

Decomposition of Unitary Matrices for Finding Quantum Circuits: Application to Molecular Hamiltonians

Constructing appropriate unitary matrix operators for new quantum algorithms and finding the minimum cost gate sequences for the implementation of these unitary operators is of fundamental importance in the field of quantum information and quantum computation. Evolution of quantum circuits faces two major challenges: complex and huge search space and the high costs of simulating quantum circuits on classical computers. Here, we use the group leaders optimization algorithm to decompose a given unitary matrix into a proper-minimum cost quantum gate sequence. We test the method on the known decompositions of Toffoli gate, the amplification step of the Grover search algorithm, the quantum Fourier transform, and the sender part of the quantum teleportation. Using this procedure, we present the circuit designs for the simulation of the unitary propagators of the Hamiltonians for the hydrogen and the water molecules. The approach is general and can be applied to generate the sequence of quantum gates for larger molecular systems.

quant-ph↗

arXiv2013

Experimental realization of quantum algorithm for solving linear systems of equations

Quantum computers have the potential of solving certain problems exponentially faster than classical computers. Recently, Harrow, Hassidim and Lloyd proposed a quantum algorithm for solving linear systems of equations: given an $N\times{N}$ matrix $A$ and a vector $\vec b$, find the vector $\vec x$ that satisfies $A\vec x = \vec b$. It has been shown that using the algorithm one could obtain the solution encoded in a quantum state $|x$ using $O(\log{N})$ quantum operations, while classical algorithms require at least O(N) steps. If one is not interested in the solution $\vec{x}$ itself but certain statistical feature of the solution ${x}|M|x$ ($M$ is some quantum mechanical operator), the quantum algorithm will be able to achieve exponential speedup over the best classical algorithm as $N$ grows. Here we report a proof-of-concept experimental demonstration of the quantum algorithm using a 4-qubit nuclear magnetic resonance (NMR) quantum information processor. For all the three sets of experiments with different choices of $\vec b$, we obtain the solutions with over 96% fidelity. This experiment is a first implementation of the algorithm. Because solving linear systems is a common problem in nearly all fields of science and engineering, we will also discuss the implication of our results on the potential of using quantum computers for solving practical linear systems.

quant-ph↗

arXiv2013

Avalanches in the Raise and Peel model in the presence of a wall

We investigate a non-equilibrium one-dimensional model known as the raise and peel model describing a growing surface which grows locally and has non-local desorption. For specific values of adsorption ($u_a$) and desorption($u_d$) rates the model shows interesting features. At $u_a = u_d$, the model is described by a conformal field theory (with conformal charge $c=0$) and its stationary probability canbe mapped to the ground state of the XXZ quantum chain. Moreover, for $u_a \geq u_d$, the model shows a phase in which the the avalanche distribution is scale invariant. In this work we study the surface dynamics by looking at avalanche distributions using Finite-size Scaling formalism and explore the effect of adding a wall to the model. The model shows the same universality for the cases with and without a wall for an odd number of tiles removed, but we find a new exponent in the presence of a wall for an even number of avalanches released. We provide new conjecture for the probability distribution of avalanches with a wall obtained by using exact diagonalization of small lattices and Monte-Carlo simulations.

cond-mat.stat-mech↗

arXiv2012

Implementation of Quantum Logic Gates Using Polar Molecules in Pendular States

We present a systematic approach to implementation of basic quantum logic gates operating on polar molecules in pendular states as qubits for a quantum computer. A static electric field prevents quenching of the dipole moments by rotation, thereby creating the pendular states; also, the field gradient enables distinguishing among qubit sites. Multi-Target Optimal Control Theory (MTOCT) is used as a means of optimizing the initial-to-target transition probability via a laser field. We give detailed calculations for the SrO molecule, a favorite candidate for proposed quantum computers. Our simulation results indicate that NOT, Hadamard and CNOT gates can be realized with high fidelity for such pendular qubit states.

quant-ph↗

arXiv2012

Ground-state stability and criticality of two-electron atoms with screened Coulomb potentials using the B-splines basis set

We applied the finite-size scaling method using the B-splines basis set to construct the stability diagram for two-electron atoms with a screened Coulomb potential. The results of this method for two electron atoms are very accurate in comparison with previous calculations based on Gaussian, Hylleraas, and finite-element basis sets. The stability diagram for the screened two-electron atoms shows three distinct regions: a two-electron region, a one-electron region, and a zero-electron region, which correspond to stable, ionized and double ionized atoms. In previous studies, it was difficult to extend the finite size scaling calculations to large molecules and extended systems because of the computational cost and the lack of a simple way to increase the number of Gaussian basis elements in a systematic way. Motivated by recent studies showing how one can use B-splines to solve Hartree-Fock and Kohn-Sham equations, this combined finite size scaling using the B-splines basis set, might provide an effective systematic way to treat criticality of large molecules and extended systems. As benchmark calculations, the two-electron systems show the feasibility of this combined approach and provide an accurate reference for comparison.

physics.atom-ph↗

arXiv2012

Quantum Coherence and Entanglement in the Avian Compass

The radical pair mechanism is one of two distinct mechanisms used to explain the navigation of birds in geomagnetic fields. However, little research has been done to explore the role of quantum entanglement in this mechanism. In this paper, we study the lifetime of radical pair entanglement corresponding to the magnitude and direction of magnetic fields to show that the entanglement lasts long enough in birds to be used for navigation. We also demonstrate that, due to a lack of orientational sensitivity of the entanglement in the geomagnetic field, the birds are not able to orient themselves by the mechanism based directly on radical-pair entanglement. To explore the entanglement mechanism further, we propose a model in which the hyperfine interactions are replaced by local magnetic fields of similar strength. The entanglement of the radical pair in this model lasts longer and displays an angular sensitivity in weak magnetic fields, both of these factors are not present in the previous models.

quant-ph↗