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Articles 61 - 84 of 84
Full-Text Articles in Quantum Physics
Advantages Of Randomization In Coherent Quantum Dynamical Control, Lea F. Santos, Lorenza Viola
Advantages Of Randomization In Coherent Quantum Dynamical Control, Lea F. Santos, Lorenza Viola
Dartmouth Scholarship
Control scenarios have been identified where the use of randomized design may substantially improve the performance of dynamical decoupling methods (Santos and Viola 2006 Phys. Rev. Lett. 97 150501). Here, by focusing on the suppression of internal unwanted interactions in closed quantum systems, we review and further elaborate on the advantages of randomization at long evolution times. By way of illustration, special emphasis is devoted to isolated Heisenberg chains of coupled spin-1/2 particles. In particular, for nearest-neighbor interactions, two types of decoupling cycles are contrasted: inefficient averaging, whereby the number of control actions increases exponentially with the system size, and …
Quantum Pseudorandomness From Cluster-State Quantum Computation, Winton G. Brown, Yaakov S. Weinstein, Lorenza Viola
Quantum Pseudorandomness From Cluster-State Quantum Computation, Winton G. Brown, Yaakov S. Weinstein, Lorenza Viola
Dartmouth Scholarship
We show how to efficiently generate pseudorandom states suitable for quantum information processing via cluster-state quantum computation. By reformulating pseudorandom algorithms in the cluster-state picture, we identify a strategy for optimizing pseudorandom circuits by properly choosing single-qubit rotations. A Markov chain analysis provides the tool for analyzing convergence rates to the Haar measure and finding the optimal single-qubit gate distribution. Our results may be viewed as an alternative construction of approximate unitary 2-designs.
Long-Time Electron Spin Storage Via Dynamical Suppression Of Hyperfine-Induced Decoherence In A Quantum Dot, Wenxian Zhang, N. P. Konstantinidis, V. V. Dobrovitski, B. N. Harmon, Lea F. Santos, Lorenza Viola
Long-Time Electron Spin Storage Via Dynamical Suppression Of Hyperfine-Induced Decoherence In A Quantum Dot, Wenxian Zhang, N. P. Konstantinidis, V. V. Dobrovitski, B. N. Harmon, Lea F. Santos, Lorenza Viola
Dartmouth Scholarship
The coherence time of an electron spin decohered by the nuclear spin environment in a quantum dot can be substantially increased by subjecting the electron to suitable dynamical decoupling sequences. We analyze the performance of high-level decoupling protocols by using a combination of analytical and exact numerical methods, and by paying special attention to the regimes of large interpulse delays and long-time dynamics, which are outside the reach of standard average Hamiltonian theory descriptions. We demonstrate that dynamical decoupling can remain efficient far beyond its formal domain of applicability, and find that a protocol exploiting concatenated design provides best performance …
Quantum Chaos, Delocalization, And Entanglement In Disordered Heisenberg Models, Winton G. Brown, Lea F. Santos, David J. Starling, Lorenza Viola
Quantum Chaos, Delocalization, And Entanglement In Disordered Heisenberg Models, Winton G. Brown, Lea F. Santos, David J. Starling, Lorenza Viola
Dartmouth Scholarship
We investigate disordered one- and two-dimensional Heisenberg spin lattices across a transition from integrability to quantum chaos from both a statistical many-body and a quantum-information perspective. Special emphasis is devoted to quantitatively exploring the interplay between eigen- vector statistics, delocalization, and entanglement in the presence of nontrivial symmetries. The implications of basis dependence of state delocalization indicators (such as the number of principal components) is addressed, and a measure of relative delocalization is proposed in order to robustly characterize the onset of chaos in the presence of disorder. Both standard multipartite and gen- eralized entanglement are investigated in a wide …
Lower Limit To The Scale Of An Effective Quantum Theory Of Gravitation, R. R. Caldwell, Daniel Grin
Lower Limit To The Scale Of An Effective Quantum Theory Of Gravitation, R. R. Caldwell, Daniel Grin
Dartmouth Scholarship
An effective quantum theory of gravitation in which gravity weakens at energies higher than ∼10−3 eV is one way to accommodate the apparent smallness of the cosmological constant. Such a theory predicts departures from the Newtonian inverse-square force law on distances below ∼0.05 mm. However, it is shown that this modification also leads to changes in the long-range behavior of gravity and is inconsistent with observed gravitational lenses.
Quantum Analysis Of A Linear Dc Squid Mechanical Displacement Detector, M. P. Blencowe, E. Buks
Quantum Analysis Of A Linear Dc Squid Mechanical Displacement Detector, M. P. Blencowe, E. Buks
Dartmouth Scholarship
We provide a quantum analysis of a dc SQUID mechanical displacement detector within the subcritical Josephson current regime. A segment of the SQUID loop forms the mechanical resonator and motion of the latter is transduced inductively through changes in the flux threading the loop. Expressions are derived for the detector signal response and noise, which are used to evaluate the position and force detection sensitivity. We also investigate cooling of the mechanical resonator due to detector back reaction.
Dynamical Control Of Electron Spin Coherence In A Quantum Dot: A Theoretical Study, Wenxian Zhang, V. V. Dobrovitski, Lea F. Santos, Lorenza Viola, B. N. Harmon
Dynamical Control Of Electron Spin Coherence In A Quantum Dot: A Theoretical Study, Wenxian Zhang, V. V. Dobrovitski, Lea F. Santos, Lorenza Viola, B. N. Harmon
Dartmouth Scholarship
We investigate the performance of dynamical decoupling methods at suppressing electron spin decoherence from a low-temperature nuclear spin reservoir in a quantum dot. The controlled dynamics is studied through exact numerical simulation, with emphasis on realistic pulse delays and the long-time limit. Our results show that optimal performance for this system is attained by a periodic protocol exploiting concatenated design, with control rates substantially slower than expected from the upper spectral cutoff of the bath. For a known initial electron spin state, coherence can saturate at long times, signaling the creation of a stable “spin-locked” decoherence-free subspace. Analytical insight into …
Generalized Coherent States As Preferred States Of Open Quantum Systems, S Boixo, L Viola, G Ortiz
Generalized Coherent States As Preferred States Of Open Quantum Systems, S Boixo, L Viola, G Ortiz
Dartmouth Scholarship
We investigate the connection between quasi-classical (pointer) states and generalized coherent states (GCSs) within an algebraic approach to Markovian quantum systems (including bosons, spins, and fermions). We establish conditions for the GCS set to become most robust by relating the rate of purity loss to an invariant measure of uncertainty derived from quantum Fisher information. We find that, for damped bosonic modes, the stability of canonical coherent states is confirmed in a variety of scenarios, while for systems described by (compact) Lie algebras, stringent symmetry constraints must be obeyed for the GCS set to be preferred. The relationship …
Decoherence And Recoherence In A Vibrating Rf Squid, Eyal Buks, M. P. Blencowe
Decoherence And Recoherence In A Vibrating Rf Squid, Eyal Buks, M. P. Blencowe
Dartmouth Scholarship
We study an rf SQUID, in which a section of the loop is a freely suspended beam that is allowed to oscillate mechanically. The coupling between the rf SQUID and the mechanical resonator originates from the dependence of the total magnetic flux threading the loop on the displacement of the resonator. Motion of the latter affects the visibility of Rabi oscillations between the two lowest energy states of the rf SQUID. We address the feasibility of experimental observation of decoherence and recoherence, namely decay and rise of the visibility, in such a system.
Casimir Forces And Non-Newtonian Gravitation, Roberto Onofrio
Casimir Forces And Non-Newtonian Gravitation, Roberto Onofrio
Dartmouth Scholarship
The search for non-relativistic deviations from Newtonian gravitation can lead to new phenomena signalling the unification of gravity with the other fundamental interactions. Various recent theoretical frameworks indicate a possible window for non-Newtonian forces with gravitational coupling strength in the micrometre range. The major expected background in the same range is attributable to the Casimir force or variants of it if dielectric materials, rather than conducting ones, are considered. Here we review the measurements of the Casimir force performed so far in the micrometre range and how they determine constraints on non-Newtonian gravitation, also discussing the dominant sources of false …
Generalized Entanglement As A Natural Framework For Exploring Quantum Chaos, Y. S. Weinstein, L Viola
Generalized Entanglement As A Natural Framework For Exploring Quantum Chaos, Y. S. Weinstein, L Viola
Dartmouth Scholarship
We demonstrate that generalized entanglement (Barnum et al., Phys. Rev. A, 68 (2003) 032308) provides a natural and reliable indicator of quantum chaotic behavior. Since generalized entanglement depends directly on a choice of preferred observables, exploring how generalized entanglement increases under dynamical evolution is possible without invoking an auxiliary coupled system or decomposing the system into arbitrary subsystems. We find that, in the chaotic regime, the long-time saturation value of generalized entanglement agrees with random matrix theory predictions. For our system, we provide physical intuition into generalized entanglement within a single system by invoking the notion of extent …
Enhanced Convergence And Robust Performance Of Randomized Dynamical Decoupling, Lea F. Santos, Lorenza Viola
Enhanced Convergence And Robust Performance Of Randomized Dynamical Decoupling, Lea F. Santos, Lorenza Viola
Dartmouth Scholarship
We demonstrate the advantages of randomization in coherent quantum dynamical control. For systems which are either time-varying or require decoupling cycles involving a large number of operations, we find that simple randomized protocols offer superior convergence and stability as compared to deterministic counterparts. In addition, we show how randomization may allow us to outperform purely deterministic schemes at long times, including combinatorial and concatenated methods. General criteria for optimally interpolating between deterministic and stochastic design are proposed and illustrated in explicit decoupling scenarios relevant to quantum information storage.
Exact Casimir Interaction Between Eccentric Cylinders, D. A. R. Dalvit, F. C. Lombardo, F. D. Mazzitelli, R. Onofrio
Exact Casimir Interaction Between Eccentric Cylinders, D. A. R. Dalvit, F. C. Lombardo, F. D. Mazzitelli, R. Onofrio
Dartmouth Scholarship
The Casimir force is the ultimate background in ongoing searches for extragravitational forces in the micrometer range. Eccentric cylinders offer favorable experimental conditions for such measurements as spurious gravitational and electrostatic effects can be minimized. Here we report on the evaluation of the exact Casimir interaction between perfectly conducting eccentric cylinders using a mode summation technique, and study different limiting cases of relevance for Casimir force measurements, with potential implications for the understanding of mechanical properties of nanotubes.
Energy Landscape Of D -Dimensional Q -Balls, Marcelo Gleiser, Joel Thorarinson
Energy Landscape Of D -Dimensional Q -Balls, Marcelo Gleiser, Joel Thorarinson
Dartmouth Scholarship
We investigate the properties of Q-balls in d spatial dimensions. First, a generalized virial relation for these objects is obtained. We then focus on potentials V(ϕϕ†)=∑3n=1an(ϕϕ†)n, where an is a constant and n is an integer, obtaining variational estimates for their energies for arbitrary charge Q. These analytical estimates are contrasted with numerical results and their accuracy evaluated. Based on the results, we offer a simple criterion to classify large and small d-dimensional Q-balls for this class of potentials. A minimum charge is then computed and its dependence on spatial dimensionality is shown to scale as Qmin∼exp(d). We also briefly …
Dynamical Control Of Qubit Coherence: Random Versus Deterministic Schemes, Lea F. Santos, Lorenza Viola
Dynamical Control Of Qubit Coherence: Random Versus Deterministic Schemes, Lea F. Santos, Lorenza Viola
Dartmouth Scholarship
We reexamine the problem of switching off unwanted phase evolution and decoherence in a single two-state quantum system in the light of recent results on random dynamical decoupling methods [L. Viola and E. Knill, Phys. Rev. Lett. 94, 060502 (2005)]. A systematic comparison with standard cyclic decoupling is effected for a variety of dynamical regimes, including the case of both semiclassical and fully quantum decoherence models. In particular, exact analytical expressions are derived for randomized control of decoherence from a bosonic environment. We investigate quantitatively control protocols based on purely deterministic, purely random, as well as hybrid design, and …
Dynamics Of A Nanomechanical Resonator Coupled To A Superconducting Single-Electron Transistor, M. P. Blencowe, J. Imbers, A. D. Armour
Dynamics Of A Nanomechanical Resonator Coupled To A Superconducting Single-Electron Transistor, M. P. Blencowe, J. Imbers, A. D. Armour
Dartmouth Scholarship
We present an analysis of the dynamics of a nanomechanical resonator coupled to a superconducting single-electron transistor (SSET) in the vicinity of Josephson quasi-particle (JQP) and double Josephson quasi-particle (DJQP) resonances. For weak coupling and wide separation of dynamical timescales, we find that for either superconducting resonances the dynamics of the resonator are given by a Fokker–Planck equation, i.e. the SSET behaves effectively as an equilibrium heat bath, characterized by an effective temperature, which also damps the resonator and renormalizes its frequency. Depending on the gate and drain–source voltage bias points with respect to the superconducting resonance, the SSET can …
Optimal Cooling Strategies For Magnetically Trapped Atomic Fermi-Bose Mixtures, Michael Brown-Hayes, Roberto Onofrio
Optimal Cooling Strategies For Magnetically Trapped Atomic Fermi-Bose Mixtures, Michael Brown-Hayes, Roberto Onofrio
Dartmouth Scholarship
We discuss cooling efficiency for different-species Fermi-Bose mixtures in magnetic traps. A better heat capacity matching between the two atomic species is achieved by a proper choice of the Bose cooler and the magnetically trappable hyperfine states of the mixture. When a partial spatial overlap between the two species is also taken into account, the deepest Fermi degeneracy is obtained for an optimal value of the trapping frequency ratio between the two species. This can be achieved by assisting the magnetic trap with a deconfining light beam, as shown in the case of fermionic 6Li mixed with 23Na, 87Rb, and …
Casimir Force Between Eccentric Cylinders, D. A. R. Dalvit, F. C. Lombardo, F. D D. Mazzitelli, R Onofrio
Casimir Force Between Eccentric Cylinders, D. A. R. Dalvit, F. C. Lombardo, F. D D. Mazzitelli, R Onofrio
Dartmouth Scholarship
We consider the Casimir interaction between a cylinder and a hollow cylinder, both conducting, with parallel axis and slightly different radii. The Casimir force, which vanishes in the coaxial situation, is evaluated for both small and large eccentricities using the proximity approximation. The cylindrical configuration offers various experimental advantages with respect to the parallel planes or the plane-sphere geometries, leading to favourable conditions for the search of extra-gravitational forces in the micrometer range and for the observation of finite-temperature corrections.
Hydrogenic Spin Quantum Computing In Silicon: A Digital Approach, A. J. Skinner, M. E. Davenport, B. E. Kane
Hydrogenic Spin Quantum Computing In Silicon: A Digital Approach, A. J. Skinner, M. E. Davenport, B. E. Kane
Dartmouth Scholarship
We suggest an architecture for quantum computing with spin-pair encoded qubits in silicon. Electron-nuclear spin-pairs are controlled by a dc magnetic field and electrode-switched on and off hyperfine interaction. This digital processing is insensitive to tuning errors and easy to model. Electron shuttling between donors enables multiqubit logic. These hydrogenic spin qubits are transferable to nuclear spin-pairs, which have long coherence times, and electron spin-pairs, which are ideally suited for measurement and initialization. The architecture is scalable to a highly parallel operation.
Higher-Order Evaluation Of The Critical Temperature For Interacting Homogeneous Dilute Bose Gases, Frederico F. F. De Souza Cruz, Marcus Pinto, Rudnei O. Ramos, Paulo Sena
Higher-Order Evaluation Of The Critical Temperature For Interacting Homogeneous Dilute Bose Gases, Frederico F. F. De Souza Cruz, Marcus Pinto, Rudnei O. Ramos, Paulo Sena
Dartmouth Scholarship
We use the nonperturbative linear δ expansion method to evaluate analytically the coefficients c1 and c''2 that appear in the expansion for the transition temperature for a dilute, homogeneous, three-dimensional Bose gas given by Tc=T0(1+c1an1/3+[c′2ln(an1/3)+c''2]a2n2/3+O(a3n)), where T0 is the result for an ideal gas, a is the s-wave scattering length, and n is the number density. In a previous work the same method has been used to evaluate c1 to order δ2 with the result c1=3.06. Here, we push the calculation to the next two orders obtaining c1=2.45 at order δ3 and c1=1.48 at order δ4. Analyzing the topology …
A First Principles Warm Inflation Model That Solves The Cosmological Horizon And Flatness Problems, Arjun Berera, Marcelo Gleiser, Rudnei O. Ramos
A First Principles Warm Inflation Model That Solves The Cosmological Horizon And Flatness Problems, Arjun Berera, Marcelo Gleiser, Rudnei O. Ramos
Dartmouth Scholarship
A quantum field theory warm inflation model is presented that solves the horizon and flatness problems. The model obtains, from the elementary dynamics of particle physics, cosmological scale factor trajectories that begin in a radiation dominated regime, enter an inflationary regime, and then smoothly exit back into a radiation dominated regime, with non-negligible radiation throughout the evolution.
Strong Dissipative Behavior In Quantum Field Theory, Arjun Berera, Marcelo Gleiser, Rudnei O. Ramos
Strong Dissipative Behavior In Quantum Field Theory, Arjun Berera, Marcelo Gleiser, Rudnei O. Ramos
Dartmouth Scholarship
We study the conditions under which an overdamped regime can be attained in the dynamic evolution of a quantum field configuration. Using a real-time formulation of finite temperature field theory, we compute the effective evolution equation of a scalar field configuration, quadratically interacting with a given set of other scalar fields. We then show that, in the overdamped regime, the dissipative kernel in the field equation of motion is closely related to the shear viscosity coefficient, as computed in scalar field theory at finite temperature. The effective dynamics is equivalent to a time-dependent Ginzburg-Landau description of the approach to equilibrium …
Mass Spectrum And Correlation Functions Of Non-Abelian Quantum Magnetic Monopoles, E. C. Marino, Rudnei O. Ramos
Mass Spectrum And Correlation Functions Of Non-Abelian Quantum Magnetic Monopoles, E. C. Marino, Rudnei O. Ramos
Dartmouth Scholarship
The method of quantization of magnetic monopoles based on the order-disorder duality existing between the monopole operator and the Lagrangian fields is applied to the description of the quantum magnetic monopoles of't Hooft and Polyakov in the SO(3) Georgi-Glashow model. The commutator of the monopole operator with the magnetic charge is computed explicitly, indicating that indeed the quantum monopole carries 4πg units of magnetic charge. An explicit expression for the asymptotic behavior of the monopole correlation function is derived. From this, the mass of the quantum monopole is obtained. The tree-level result for the quantum monopole mass is shown to …
Microphysical Approach To Nonequilibrium Dynamics Of Quantum Fields, Marcelo Gleiser, Rudnei O. Ramos
Microphysical Approach To Nonequilibrium Dynamics Of Quantum Fields, Marcelo Gleiser, Rudnei O. Ramos
Dartmouth Scholarship
We examine the nonequilibrium dynamics of a self-interacting λφ4 scalar field theory. Using a real time formulation of finite temperature field theory we derive, up to two loops and O(λ2), the effective equation of motion describing the approach to equilibrium. We present a detailed analysis of the approxi- mations used in order to obtain a Langevin-like equation of motion, in which the noise and dissipation terms associated with quantum fluctuations obey a fluctuation-dissipation relation. We show that, in general, the noise is colored (time-dependent) and multiplicative (couples nonlinearly to the field), even though it is still Gaussian distributed. The noise …