Space-time Lattice Research Articles

Relativistic Wigner Function for Quantum Walks

A relativistic Wigner function for free Discrete Time Quantum Walks (DTQWs) on the square 2D space-time lattice is defined. The transport equation obeyed by the relativistic Wigner function is derived in terms of discrete derivatives and degenerates in the continuum limit into the transport equation obeyed by the Wigner function of 2D Dirac fermions. The first corrections to the continuous equation induced by the discreteness of the lattice are also determined and the similarities with non-quantum relativistic transport in phase space are discussed further.

Lattice construction of duality with non-Abelian gauge fields in 2+1D

The lattice construction of Euclidean path integrals has been a successful approach of deriving 2+1D field theory dualities with a U$(1)$ gauge field. In this work, we generalize this lattice construction to dualities with non-Abelian gauge fields. We construct the Euclidean spacetime lattice path integral for a theory with strongly-interacting $SO(3)$ vector bosons and Majorana fermions coupled to an $SO(3)$ gauge field and derive an exact duality between this theory and the theory of a free Majorana fermion on the spacetime lattice. We argue that this lattice duality implies the desired infrared duality between the field theory with an $SO(3)$ vector critical boson coupled to an $SO(3)_1$ Chern-Simons gauge theory, and a free massless Majorana fermion in 2+1D. We also generalize the lattice construction of dualities to models with $O(3)$ gauge fields.

Topologically Protected Quantization of Work.

The transport of a particle in the presence of a potential that changes periodically in space and in time can be characterized by the amount of work needed to shift a particle by a single spatial period of the potential. In general, this amount of work, when averaged over a single temporal period of the potential, can take any value in a continuous fashion. Here, we present a topological effect inducing the quantization of the average work. We find that this work is equal to the first Chern number calculated in a unit cell of a space-time lattice. Hence, this quantization of the average work is topologically protected. We illustrate this phenomenon with the example of an atom whose center of mass motion is coupled to its internal degrees of freedom by electromagnetic waves.

Induced QCD II: numerical results

We numerically explore an alternative discretization of continuum SU(Nc) Yang-Mills theory on a Euclidean spacetime lattice, originally introduced by Budzcies and Zirnbauer for gauge group U(Nc). This discretization can be reformulated such that the self-interactions of the gauge field are induced by a path integral over Nb auxiliary bosonic fields, which couple linearly to the gauge field. In the first paper of the series we have shown that the theory reproduces continuum SU(Nc) Yang-Mills theory in d = 2 dimensions if Nb is larger than Nc − frac{3}{4} and conjectured, following the argument of Budzcies and Zirnbauer, that this remains true for d > 2. In the present paper, we test this conjecture by performing lattice simulations of the simplest nontrivial case, i.e., gauge group SU(2) in three dimensions. We show that observables computed in the induced theory, such as the static q overline{q} potential and the deconfinement transition temperature, agree with the same observables computed from the ordinary plaquette action up to lattice artifacts. We also find evidence that the bound for Nb can be relaxed to Nc − frac{5}{4} as conjectured in our earlier paper. Studies of how the new discretization can be used to change the order of integration in the path integral to arrive at dual formulations of QCD are left for future work.

Few-body physics on a spacetime lattice in the worldline approach

We formulate the physics of two species of non-relativistic hard-core bosons with attractive or repulsive delta function interactions on a space-time lattice in the worldline approach. We show that worm algorithms can efficiently sample the worldline configurations in any fixed particle-number sector if the chemical potential is tuned carefully. Since fermions can be treated as hard-core bosons up to a permutation sign, we apply this approach to study non-relativistic fermions. The fermion permutation sign is an observable in this approach and can be used to extract energies in each particle-number sector. In one dimension, non-relativistic fermions can only permute across boundaries, and so our approach does not suffer from sign problems in many cases, unlike the auxiliary field method. Using our approach, we discover limitations of the recently proposed complex Langevin calculations in one spatial dimension for some parameter regimes. In higher dimensions, our method suffers from the usual fermion sign problem. Here we provide evidence that it may be possible to alleviate this problem for few-body physics

Discrete Geometry from Quantum Walks

A particular family of Discrete Time Quantum Walks (DTQWs) simulating fermion propagation in 2D curved space-time is revisited. Usual continuous covariant derivatives and spin-connections are generalized into discrete covariant derivatives along the lattice coordinates and discrete connections. The concepts of metrics and 2-beins are also extended to the discrete realm. Two slightly different Riemann curvatures are then defined on the space-time lattice as the curvatures of the discrete spin connection. These two curvatures are closely related and one of them tends at the continuous limit towards the usual, continuous Riemann curvature. A simple example is also worked out in full.

Finite Difference Solution to the Space-Time Fractional Partial Differential-Difference Toda Lattice Equation

This paper deals with the numerical solution of space-time fractional partial differential-difference Toda lattice equation $\frac{\partial^{2\alpha} u_n}{\partial x^{\alpha}\partial t^{\alpha}}=(1+\frac{\partial^\alpha u_n}{\partial t^{\alpha}})(u_{n-1}-2u_n+u_{n+1})$, $\alpha \in (0,1)$. The finite differences method (FD-method) is used for numerical solution of this problem. According to the method, we approximate the unknown values $u_n$ of the desired function by finite differences approximation. As an application we demonstrate the capabilities of this method for identification of various values of order of fractional derivative $\alpha$. Numerical results show that the proposed version of FD-method allows to obtain all data from the initial and boundary conditions with enough high accuracy.

A time-varying mass metamaterial for non-reciprocal wave propagation

A new type of metamaterial element is proposed to possess time-dependent effective inertial mass, and proved to be valid for the design of the space-time lattice metamaterial that enables non-reciprocal wave propagation. The cell structure is a three-body dynamic system, consisting of a primary body plus two additional bodies that move along the circular orbit. The translational momentum contributed by the orbiting bodies varies periodically depending on their temporal phases, accounting for the time-driven inertial mass observed macroscopically, as verified by the rigorous theoretical derivation. Based on the time-varying mass element, we present the design of the lattice metamaterials with inertial mass that varies periodically in both space and time. Non-reciprocal wave phenomena due to the wave-like modulation of mass are demonstrated by use of the Bloch-based method and the effective-mass representation. The influence of the modulating frequency and amplitude on the asymmetric bandgap is analyzed. The proposed time-varying metamaterial with the non-reciprocal wave behavior is expected to open a new avenue towards unprecedented control over waves and vibrations.

Lattice formulation of N=2* Yang-Mills

We formulate ${\cal N} = 2^*$ supersymmetric Yang-Mills theory on a Euclidean spacetime lattice using the method of topological twisting. The lattice formulation preserves one scalar supersymmetry charge at finite lattice spacing. The lattice theory is also local, gauge invariant and free from doublers. We can use the lattice formulation of ${\cal N} = 2^*$ supersymmetric Yang-Mills to study finite temperature nonperturbative sectors of the theory and thus validate the gauge-gravity duality conjecture in a nonconformal theory.

Nonperturbative study of dynamical SUSY breaking in N=(2,2) Yang-Mills theory

We examine the possibility of dynamical supersymmetry breaking in two-dimensional $\mathcal{N} = (2, 2)$ supersymmetric Yang-Mills theory. The theory is discretized on a Euclidean spacetime lattice using a supersymmetric lattice action. We compute the vacuum energy of the theory at finite temperature and take the zero temperature limit. Supersymmetry will be spontaneously broken in this theory if the measured ground state energy is non-zero. By performing simulations on a range of lattices up to $96 \times 96$ we are able to perform a careful extrapolation to the continuum limit for a wide range of temperatures. Subsequent extrapolations to the zero temperature limit yield an upper bound on the ground state energy density. We find the energy density to be statistically consistent with zero in agreement with the absence of dynamical supersymmetry breaking in this theory.

Ward identities in N = 1 supersymmetric SU(3) Yang-Mills theory on the lattice

The introduction of a space-time lattice as a regulator of field theories breaks symmetries associated with continuous space-time, i.e. Poincaré invariance and supersymmetry. A non-zero gluino mass in the supersymmetric Yang-Mills theory causes an additional soft breaking of supersymmetry. We employ the lattice form of SUSY Ward identities, imposing that their continuum form would be recovered when removing the lattice regulator, to obtain the critical hopping parameter where broken symmetries can be recovered.

Principal component analysis for fermionic critical points

We use determinant Quantum Monte Carlo (DQMC), in combination with the principal component analysis (PCA) approach to unsupervised learning, to extract information about phase transitions in several of the most fundamental Hamiltonians describing strongly correlated materials. We first explore the zero temperature antiferromagnet to singlet transition in the Periodic Anderson Model, the Mott insulating transition in the Hubbard model on a honeycomb lattice, and the magnetic transition in the 1/6-filled Lieb lattice. We then discuss the prospects for learning finite temperature superconducting transitions in the attractive Hubbard model, for which there is no sign problem. Finally, we investigate finite temperature charge density wave (CDW) transitions in the Holstein model, where the electrons are coupled to phonon degrees of freedom, and carry out a finite size scaling analysis to determine $T_c$. We examine the different behaviors associated with Hubbard-Stratonovich auxiliary field configurations on both the entire space-time lattice and on a single imaginary time slice, or other quantities, such as equal-time Green's and pair-pair correlation functions.

Dynamic programming-based multi-vehicle longitudinal trajectory optimization with simplified car following models

Jointly optimizing multi-vehicle trajectories is a critical task in the next-generation transportation system with autonomous and connected vehicles. Based on a space-time lattice, we present a set of integer programming and dynamic programming models for scheduling longitudinal trajectories, where the goal is to consider both system-wide safety and throughput requirements under supports of various communication technologies. Newell's simplified linear car following model is used to characterize interactions and collision avoidance between vehicles, and a control variable of time-dependent platoon-level reaction time is introduced in this study to reflect various degrees of vehicle-to-vehicle or vehicle-to-infrastructure communication connectivity. By adjusting the lead vehicle's speed and platoon-level reaction time at each time step, the proposed optimization models could effectively control the complete set of trajectories in a platoon, along traffic backward propagation waves. This parsimonious multi-vehicle state representation sheds new lights on forming tight and adaptive vehicle platoons at a capacity bottleneck. We examine the principle of optimality conditions and resulting computational complexity under different coupling conditions.

Quantum walks and gravitational waves

A new family of discrete-time quantum walks (DTQWs) propagating on a regular (1+2)D spacetime lattice is introduced. The continuum limit of these DTQWs is shown to coincide with the dynamics of a Dirac fermion coupled to an arbitrary relativistic gravitational field. This family is used to model the influence of arbitrary linear gravitational waves (GWs) on DTQWs. Pure shear GWs are studied in detail. We show that on large spatial scales, the spatial deformation generated by the wave induces a rescaling of the eigen-energies by a certain anisotropic factor which can be computed exactly. The effect of pure shear GWs on fermion interference patterns is also investigated, both on large scales and on scales comparable to the lattice spacing.

Spin on a 4D Feynman Checkerboard

We discretize the Weyl equation for a massless, spin-1/2 particle on a time-diagonal, hypercubic spacetime lattice with null faces. The amplitude for a step of right-handed chirality is proportional to the spin projection operator in the step direction, while for left-handed it is the orthogonal projector. Iteration yields a path integral for the retarded propagator, with matrix path amplitude proportional to the product of projection operators. This assigns the amplitude $i^{\pm T}\, {3}^{-B/2}\,2^{-N}$ to a path with $N$ steps, $B$ bends, and $T$ right-handed minus left-handed bends, where the sign corresponds to the chirality. Fermion doubling does not occur in this discrete scheme. A Dirac mass $m$ introduces the amplitude $i\epsilon m$ to flip chirality in any given time step $\epsilon$, and a Majorana mass similarly introduces a charge conjugation amplitude.

Induced QCD I: theory

We explore an alternative discretization of continuum SU(N_c) Yang-Mills theory on a Euclidean spacetime lattice, originally introduced by Budzcies and Zirnbauer. In this discretization the self-interactions of the gauge field are induced by a path integral over N_b auxiliary boson fields, which are coupled linearly to the gauge field. The main progress compared to earlier approaches is that N_b can be as small as N_c. In the present paper we (i) extend the proof that the continuum limit of the new discretization reproduces Yang-Mills theory in two dimensions from gauge group U(N_c) to SU(N_c), (ii) derive refined bounds on N_b for non-integer values, and (iii) perform a perturbative calculation to match the bare parameter of the induced gauge theory to the standard lattice coupling. In follow-up papers we will present numerical evidence in support of the conjecture that the induced gauge theory reproduces Yang-Mills theory also in three and four dimensions, and explore the possibility to integrate out the gauge fields to arrive at a dual formulation of lattice QCD.

The QCD critical point: an exciting Odyssey in the Femto-world

Strongly interacting matter, which makes up the nuclei of atoms, is described by a theory called quantum chromodynamics (QCD). A critical point in the phase diagram of QCD, if established either theoretically or experimentally, would be as profound a discovery as the familiar gas–liquid critical point discovered in the nineteenth century. Due to the extremely short-lived nature of the concerned phases, novel experimental techniques are needed to search for it. The Relativistic Heavy Ion Collider (RHIC) in USA has an experimental programme which can fit the bill to do so. Theoretical techniques of Lattice QCD, which is QCD defined on a discrete space-time lattice, have provided glimpses into where the QCD critical point may be, and how to search for it in the experimental data. A brief overview of the theoretical and experimental attempts is provided.

MODELING OF HEAT AND MASS TRANSFER PROCESS OF HUMIDIFIED AIR IN THE REGENERATIVE HEAT EXCHANGER

Abstract. It is proved that the design and the choice of mode require to consider the possibility of condensation from the exhausting air on the cold surfaces of the heat exchangers. The paper considers the numerical method of the regenerative heat exchanger calculation, taking into account the phase transitions (evaporation, condensation). Based on the operating principle of regenerative heat exchangers is proved the need to consider the heat transfer process inside the nozzle. It is proved that the calculation of heat and mass transfer processes in the regenerative heat exchanger of the air-conditioning systems is reduced to the solution of three nonlinear differential equations with appropriate boundary conditions. In the developed model are used the equation of state of an ideal gas and convective heat and mass transfer. The algorithm to solve the nonlinear safe equations on the space-time lattice using Seidel approach is proposed.

S Bosons and Dark Particles of Space Field

The spacetime lattice model involves time lattice (static lattice) model and space lattice (dynamic lattice) model, both of which have the same lattices’ domains and the same fractal structures. The behaviors of the space field obey the uncertainty relations, which gauge invariance shows the space field is a gauge field, making the electromagnetic field, gravitowagnetic field and the fermion field be gauged, and the Lorentz condition and Lorentz gauge are the intrinsic attributes of the spacetime. The quantization of the classical space field produces S bosons of spin-1, which stimulated states by charges and masses are respectively photons and gravitons. The S bosons in thermal excitation are immeasurable and their energies may be dark. The principle of partition of independent freedom degrees regularizes the degrees for all particles including neutrino, which must have mass. By the S bosons, we interpret newly the virtual photons. Using the spacetime lattice model, we investigate the breaking of the symmetry of the gradient fields and the symmetry of the curl fields for the potential functions of the space field, and the creations and the annihilations of the dark photons and the dark gravitons. The complexity requires us to rename the electroweak phase transition as electro-gravito-weak phase transition. Finally, antiparticles are discussed. Our approach for the lattice models is a kind of renormalization group theory, signifying the breaking of symmetries can be renormalized.

Supersymmetry on a Lattice

Attempts to formulate supersymmetric field theories on discrete spacetime lattices have a long history. However, until recently most of these efforts have failed. In this talk I will review some of the new ideas that have finally allowed a solution to this problem for certain supersymmetric theories. I will focus my attention, in particular, on N=4 super Yang-Mills which forms one of the pillars of the AdSCFT correspondence connecting gravitational theories in anti-deSitter space to gauge theories living on the boundary of that space. It is also integral to the scattering amplitudes program and is under intensive study using conformal bootstrap techniques. I will review the theoretical construction of the model and describe what is known analytically about the lattice theory. Following on from this I will present preliminary results stemming from the first large scale lattice study of N=4 Yang-Mills. Our results are consistent with the existence of a single deconfined phase for the theory in which the static fermion potential shows only a Coulomb-like behavior. Unlike QCD there is no confining, chirally broken phase even at strong coupling.