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arXiv:2404.03560

A quantum Pascal pyramid and an extended de Moivre-Laplace theorem

Mohamed Sabba

Published Apr 4, 2024• 2 citations
arXivHTMLPDF

Abstract

Pascal's triangle is widely used as a pedagogical tool to explain the "first-order" multiplet patterns that arise in the spectra of $I_N S$ coupled spin-1/2 systems in magnetic resonance. Various other combinatorial structures, which may be well-known in the broader field of quantum dynamics, appear to have largely escaped the attention of the magnetic resonance community with a few exceptions, despite potential usefulness. In this brief set of lecture notes, we describe a "quantum Pascal pyramid" (OEIS https://oeis.org/A268533) as a generalization of Pascal's triangle, which is shown to directly map the relationship between multispin operators of arbitrary spin product rank $q$ ($\hat{Z}_N^q$) and population operators for states with magnetic quantum number $m$ ($\hat{S}_N^m$), and - as a consequence - obtain the general form of the intensity ratios of multiplets associated with antiphase single-quantum coherences, with an expression given in terms of the Jacobi polynomials. An extension of the de Moivre-Laplace theorem, beyond the trivial case $q=0$, is applied to the $q$-th columns of the quantum Pascal pyramid, and is given in terms of a product of the $q$-th order Hermite polynomials and a Gaussian distribution, reproducing the well-known functional forms of the solutions of the quantum harmonic oscillator and the classical limit of Hermite-Gaussian modes in laser physics (Allen et al., $\textit{Phys. Rev. A.}$, $\textbf{45}$, 1992). This is used to approximate the Fourier-transformed spectra of $\hat{Z}_N^q$-associated multiplets of arbitrary complexity. Finally, an exercise is shown in which the first two columns of the quantum Pascal pyramid are used to calculate the previously known symmetry-constrained upper bound on $I_z \rightarrow S_z$ polarization transfer in $I_N S$ spin systems.

DOI
10.48550/arxiv.2404.03560

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Pascal's triangle is widely used as a pedagogical tool to explain the "first-order" multiplet patterns that arise in the spectra of INSI_N SIN​S coupled spin-1/2 systems in magnetic resonance. Various other combinatorial structures, which may be well-known in the broader field of quantum dynamics, appear to have largely escaped the attention of the magnetic resonance community with a few exceptions, despite potential usefulness. In this brief set of lecture notes, we describe a "quantum Pascal pyramid" (OEIS https://oeis.org/A268533) as a generalization of Pascal's triangle, which is shown to directly map the relationship between multispin operators of arbitrary spin product rank qqq (Z^Nq\hat{Z}_N^qZ^Nq​) and population operators for states with magnetic quantum number mmm (S^Nm\hat{S}_N^mS^Nm​), and - as a consequence - obtain the general form of the intensity ratios of multiplets associated with antiphase single-quantum coherences, with an expression given in terms of the Jacobi polynomials. An extension of the de Moivre-Laplace theorem, beyond the trivial case q=0q=0q=0, is applied to the qqq-th columns of the quantum Pascal pyramid, and is given in terms of a product of the qqq-th order Hermite polynomials and a Gaussian distribution, reproducing the well-known functional forms of the solutions of the quantum harmonic oscillator and the classical limit of Hermite-Gaussian modes in laser physics (Allen et al., Phys. Rev. A.\textit{Phys. Rev. A.}Phys. Rev. A., 45\textbf{45}45, 1992). This is used to approximate the Fourier-transformed spectra of Z^Nq\hat{Z}_N^qZ^Nq​-associated multiplets of arbitrary complexity. Finally, an exercise is shown in which the first two columns of the quantum Pascal pyramid are used to calculate the previously known symmetry-constrained upper bound on Iz→SzI_z \rightarrow S_zIz​→Sz​ polarization transfer in INSI_N SIN​S spin systems.
The solution of the time-dependent wave equation describing the interaction of two coupled spins with a rotating magnetic field in the presence of a steady field, as in the usual magnetic resonance situation, has been reexamined. By extending the method previously developed by Salwen, an exact expression is derived for all components of the wave function describing a three-state system under magnetic-resonance conditions. This solution describes in analytical form all the single and multiple resonances of which the system is capable for fields of any intensity. It is shown that when zzz is the axis of quantization, the projection of the wave function on an eigenstate of Fx{F}_{x}Fx​ oscillates at twice the resonance frequency at exact double quantum resonance. An approximate form is derived for the transition probability near double quantum resonance, showing that our solution is consistent with Salwen's in the limit of well-separated single and double quantum resonances. The theory is applied to the uppermost three substates of the upper hyperfine multiplet of an alkali metal of arbitrary nuclear spin, a case of interest to the analysis of magnetic resonance in optically pumped vapors.
We introduce a model of interacting bosons exhibiting an infinite collection of fractal symmetries-termed "Pascal's triangle symmetries"-which provides a natural U(1) generalization of a spin-(1/2) system with Sierpinski triangle fractal symmetries introduced in Newman et al., [Phys. Rev. E 60, 5068 (1999).PLEEE81063-651X10.1103/PhysRevE.60.5068]. The Pascal's triangle symmetry gives rise to exact degeneracies, as well as a manifold of low-energy states which are absent in the Sierpinski triangle model. Breaking the U(1) symmetry of this model to Z_{p}, with prime integer p, yields a lattice model with a unique fractal symmetry which is generated by an operator supported on a fractal subsystem with Hausdorff dimension d_{H}=ln(p(p+1)/2)/lnp. The Hausdorff dimension of the fractal can be probed through correlation functions at finite temperature. The phase diagram of these models at zero temperature in the presence of quantum fluctuations, as well as the potential physical construction of the U(1) model, is discussed.
In this paper we consider a quantum version of Pascal's triangle. Pascal's triangle is a well-known triangular array of numbers and when these numbers are plotted modulo 2, a fractal known as the Sierpinski triangle appears. We first prove the appearance of more general fractals when Pascal's triangle is considered modulo prime powers. The numbers in Pascal's triangle can be obtained by scaling the probabilities of the simple symmetric random walk on the line. In this paper we consider a quantum version of Pascal's triangle by replacing the random walk by the quantum walk known as the Hadamard walk. We show that when the amplitudes of the Hadamard walk are scaled to become integers and plotted modulo three, a fractal known as the Sierpinski carpet emerges and we provide a proof of this using Lucas's theorem. We furthermore give a general class of quantum walks for which this phenomenon occurs.
Ever since the introduction of resonating valence bond states by P.\\ W.\\ Anderson in 1970s, the search for exotic many--body quantum behavior in peculiar states of matter known as quantum spin liquids has been a central challenge in the field of condensed matter physics. An important searching ground for such \\emph{quantum--ness} is the low energy physics of frustrated magnets. In a quantum system, frustration---the inability of a physical system to achieve a global state that consistently minimizes its energy locally---enables the unavoidable quantum fluctuations to hybridize an extensive number of classical states into highly entangled quantum states at low temperatures. It can be argued that this is the most practical approach to achieve genuine many--body quantum behavior at large scales. The fundamental source of frustration in many physical systems, is the existence of local non--commuting terms. In the case of antiferromagnet spin theories the simplest examples are triangles of nearest--neighboring sites. Consequently, lattices with triangles as building blocks have long been studied in hopes of discovering spin liquids physics. In particular the kagome lattice, which is composed of corner--sharing triangles, has been the center of at least four decades of spin liquid research. This is due to the underconstrained nature of system made out of corner--sharing triangles. After the first three chapters, introduction in chapter~\\ref{chap:intro}, preliminary physics in chapter~\\ref{chap:prelim}, and a brief introduction to spin--liquids in chapter~\\ref{chap:QSLs}, in chapter~\\ref{chap:kagome} I tackle a simpler version of the long-standing problem of spin--1/21/21/2 Heisenberg antiferromagnets by considering a quasi--1D lattice consisting entirely of corner--sharing triangles, \\emph{kagome strip}. I illustrate that the standard Heisenberg antiferromagnet Hamiltonian over kagome strip is an extended gapless quantum phase, that is well characterized by two fermionic/bosonic gapless modes and power--law decaying spin and bond--energy correlations. I also demonstrate that the correlation functions oscillate at tunably incommensurate wave vectors. It turns out that this phase can be identified by a particular marginal instability of a two-band spinon Fermi surface coupled to an emergent U(1) gauge field. This interpretation is supported by analytic Abelian bosonization and with extensive numerical large--scale density matrix renormalization group study as well as variational Monte Carlo calculations on Gutzwiller ans\\"atz wave functions. This intriguing result is the first numerical demonstration of emergent fermionic spinons in a simple SU(2) invariant nearest-neighbor Heisenberg model beyond the strictly 1D (Bethe chain) limit. The unexpected success of the fermionic spinons in describing the low energy physics of kagome antiferromagnet, reintroduces the questions about the validity of spinon physics as an effective theory of quantum spin liquids. However, numerical methods are still far behind the theoretical advances. And still even today, an accurate description of the quantum spin liquid states using tensor network methods is notoriously challenging. It is known that for large quasi-1D systems, the density matrix renormalization group and related methods usually require significant computational resources and sometimes fail to converge to a satisfactory state. On the other hand, variational wavefunctions acquired from the Gutzwiller projection of gaussian fermionic theories have long served as both a theoretical starting point for the construction of such spin liquid states and as an inspiration for numerical variational Monte Carlo (VMC) to calculate observables of interest. Noting this observation I examine a different method by exploring the possibility of constructing a matrix product state (MPS) representation for a Gutzwiller--projected state from two given MPS representations of gaussian fermionic theories in the \\ref{chap:gutz} chapter. I investigate the complexity of different approaches to achieve Gutzwiller projection for MPSs and introduce the novel algorithm which we call the Guzwiller zipper method. The performance of the algorithm is tested against two copies of a single half-filled band of spin-1/2 fermionic spinons. In a successful attempt to describe the nature of spin liquid states on quasi-1D strips of triangular and kagome-like lattices, I apply this method to two MPS of multi-band fermionic spinon theories and compare with the complexity of the traditional VMC approach. In particular, we methodically disprove the conjecture of a spinon fermi surface spin liquid for the triangular lattice. Finally, I conclude this thesis by laying out a bird's eye view of the current and future of spin liquid physics in kagome and triangular lattices and pointing out numerous possible applications of the novel Gutzwiller zipper method. In addition, I would also discuss its possible extensions to more complicated tensor networks as well as versions for higher spatial dimensions.
Matrix diagonalization has long been a setback in the numerical simulation of the magnetic resonance spectra of multispin systems since the dimension of the Hilbert space of such systems grows exponentially with the number of spins -- a problem commonly referred to as the of In this paper, we propose two mathematical instruments which, when harmoniously combined, could greatly help surmount to a fair degree and in a systematic manner the curse of dimensionality. These are: 1) the Holstein-Primakoff bosons and 2) what we have termed the index compression maps. These two allow a bijective mapping of (multi)spin states to integers. Their combination leads to the block diagonalization of the multispin Hamiltonian, thus a computationally exact way of diagonalizing the latter but which also scales polynomially with the number of spins. We also show that the eigenvectors and eigenvalues of the Liouvillian operator can be easily determined once those of the related multispin Hamiltonian are known. Interestingly, the method also enables an analytical characterization of the multispin Hilbert space -- a feat hardly attainable with other approaches. We illustrate the method here by showing how a general static isotropic multispin Hamiltonian could be exactly diagonalized with very less computational cost. Nonetheless, we emphasize that the method could be applied to study numerous quantum systems defined on finite Hilbert spaces and embodied with at most pairwise interactions.
We consider the Heisenberg spin triangle with general coupling coefficients and general spin quantum number s. The corresponding classical system is completely integrable. In the quantum case the eigenvalue problem can be reduced to that of tridiagonal matrices in at most 2s+1 dimensions. The corresponding energy spectrum exhibits what we will call spectral symmetries due to the underlying permutational symmetry of the considered class of Hamiltonians. As an application we explicitly calculate six classes of universal polynomials that occur in the high temperature expansion of spin triangles and more general spin systems.