Advances in Mathematical Physics

Volume 2018, Article ID 9256320, 10 pages

https://doi.org/10.1155/2018/9256320

## From the Kinematics of Precession Motion to Generalized Rabi Cycles

^{1}University of Architecture, Civil Engineering and Geodesy, 1 Hristo Smirnenski Blvd., 1046 Sofia, Bulgaria^{2}Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Block 4, 1113 Sofia, Bulgaria^{3}Institute of Biophysics and Biomedical Engineering, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Block 21, 1113 Sofia, Bulgaria

Correspondence should be addressed to Ivaïlo M. Mladenov; gb.sab.12oib@vonedalm

Received 29 September 2017; Accepted 16 January 2018; Published 28 February 2018

Academic Editor: Manuel Calixto

Copyright © 2018 Danail S. Brezov et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

We use both vector-parameter and quaternion techniques to provide a thorough description of several classes of rotations, starting with coaxial angular velocity of varying magnitude. Then, we fix the magnitude and let precess at constant rate about the -axis, which yields a particular solution to the free Euler dynamical equations in the case of axially symmetric inertial ellipsoid. The latter appears also in the description of spin precessions in NMR and quantum computing. As we show below, this problem has analytic solutions for a much larger class of motions determined by a simple condition relating the polar angle and -projection of (expressed in cylindrical coordinates), which are both time-dependent in the generic case. Relevant physical examples are also provided.

#### 1. Introduction: Quaternions and Vector-Parameters

The Clifford algebra of quaternions is known to be intimately related to the rotation group in and one way to see it is via the standard spin covering map that is topologically a projection from the unit sphere . The basis of bivectors in is spanned by three units satisfyingwhere denotes the identity element, so one can express each quaternion in the formHence, as a linear space, may obviously be identified with by introducing coordinates as shown above. At the same time, quaternions have a specific Clifford multiplication rule that can be expressed in the above notation aswhere and stand for the dot and cross products in , respectively. Just like in the case of complex numbers, Clifford conjugation in is given by sign inversion of the imaginary (vector) part : that is, , and since there are no zero divisors, the quaternion norm defined as yields the inverse of every nonzero element . In particular, restricting to , one obtains the unit sphere in , which the composition law (3) endows with the group structure of . At this point, rotations in appear quite naturally if we identify three-vectors with pure (imaginary) quaternions and use the adjoint action of the spin group . This effectively transforms the components of via the linear orthogonal mapwhere denotes the identity in , stands for the usual dyadic (tensor) product, and is the skew-symmetric linear map associated with via the Hodge duality: that is, . The above map clearly preserves orientation and thus represents rotation in . Denoting its angle and the unit vector along its axis , one may express the associated quaternion asCentral projection onto the plane then yields the* vector-parameter * that is actually not a vector but rather a point in projective three-space, equipped with an additional group structure. This simple construction appears quite convenient: on the one hand, unlike other known alternatives (such as Euler angles which can be considered as coordinates of points in projective space on the three-torus and lead to singularities, e.g., the so-called* gimbal lock*) it gives a topologically adequate description of the orthogonal group , while, on the other hand, it provides an efficient associative composition lawinherited from the quaternion multiplication (3) upon central projection. This constitutes a (nonlinear) representation of related to the usual matrix realization via , in which the inverse element is given by and the neutral one by the zero vector. Moreover, it yields rational expressions for the matrix entries of that may be obtained directly from (4) or equivalently by means of the famous Cayley transformClearly, the above follows also from Rodrigues’ rotation formula with the aid of Euler’s trigonometric substitution, but the projective approach reveals the major advantages of vector parameterization in a much more direct manner. Recall also that the angular velocity of a rotating rigid body is defined in the body and inertial frames, respectively, asStraightforward differentiation of the Cayley representation (7) yields the relations (see also [1, 2])while in the inverse direction, one easily derives the Riccati equations for the vector-parameterWe refer to [3] for a detailed introduction to quaternions and some of their applications in a wide range or fields, from navigation and attitude control to computer graphics. In quantum mechanics quaternions and Pauli matrices appear naturally in the description of spin systems as we recall below. Moreover, modern physics embraces the idea of quaternionic Hilbert space and quaternionic quantum mechanics (cf. [4, 5]). Many features of complex numbers are preserved except, of course, commutativity, which makes the definition of quaternionic analyticity far nontrivial and yet fruitful (see [6]), for example, in the description of electrodynamics [7] and its relation to the so-called* Fueter analyticity*. Vector parameterization [8, 9], on the other hand, is much less popular although it has proven quite efficient in the description of (pseudo-)orthogonal groups in dimensions three and four [10], as well as in a wide range of problems related to classical and quantum mechanics [2, 11], special relativity and electrodynamics [12, 13], robotics and navigation [3, 14], and various other fields of science and technology. Here we make use of both descriptions.

The text is organized as follows: after this brief introductory part, we consider the kinematical problem of a rotating rigid body with fixed direction of the angular velocity vector using the projective vector-parameter construction discussed above, thus extending an old result due to O’Reilly and Payen [15]. Then, in Section 3, we switch back to quaternions, which lead to linear, rather than Riccati equations, in order to deal with the more complicated settings. Namely, we consider the case of a precessing (about a fixed axis) angular velocity and the most general case, given conveniently in cylindrical coordinates, using techniques from the arsenal of quantum mechanics, which allow us to derive a simple integrability condition. Finally, after a brief note on dynamics, we relate these results to the Rabi oscillator that appears in quantum computation, showing how our solutions yield more freedom in the entire process.

#### 2. The Coaxial Angular Velocity (CAV) Motion

Now, let us consider the first equation in (10) and look for solutions with a fixed direction of . Without loss of generality, we may choose for convenience a reference frame, in which is aligned with the -axis: that is,This allows for expressing the matrix ODE (10) explicitly in components asFrom the third equation, one obviously has (we set the initial moment at )while for the first two we may use the substitution , which yieldsThe latter clearly possesses a trivial solution that is a well-known result: in this case, the rotation takes place about the axis determined by the angular velocity vector with magnitude . However, for , one hasCombining the above two cases, one may express the general solution in the form (here the amplitude and the two phases , are constants of integration)from which it is straightforward to write Changing the parameter , it is not hard to show that the integral curves (17) obtained above are actually rays in described bywhere the constants and are most naturally given in polar coordinates asIn this way, we end up with a two-parameter family of rays in , whose intersection points with the planes and constitute isosceles right triangles with the origin. In other words, if we denote in (18), the constant vectors and are perpendicular and , where is the projection of in the -plane. Those are all orbits still periodic in . In particular, the vertical -axis and the horizontal projective line at infinity are limiting cases corresponding to and as discussed below in detail. Note that one may redefine via an overall phase shift introduced by a proper choice of initial time, which yields , so in physical terms one has only two relevant constants (amplitude and phase) and the general solution of (12) is generated by the time flow of as orbits of all possible initial points. On the other hand, it is not difficult to see that the whole manifold is covered by the above map. Moreover, each point in projective space is reached by the initial data setting since is obviously covered by the family of rays (18), in which we may use as an intrinsic parameter. Now, in order to lift the whole construction up to the spin cover, we may express the two images corresponding to the vector-parameter (17) in the formwith a suitable choice of a quaternion basis and using the notationNote that the -orbits on the unit sphere project to circles in the and planes with equal frequencies but generally different phases and amplitudes. In particular, for and the former (resp., the latter) circle shrinks to a point. From the perspective of the Hopf map, one has quaternion -orbits on whose holonomy shifts the phase by a whole period for each revolution about the -axis of the base . This becomes more apparent if we write with its coordinates on asStereographic projection (from the south pole onto the equatorial plane) yields also the so-called* Wiener-Milenkovic conformal rotation vector* (see [9] for more details)which does not involve infinities at least for , so one may plot the corresponding curves in Euclidean space. Moreover, the rotation angle is not difficult to derive from formula (17): namely,As for the corresponding invariant axis, ignoring the common prefactor in (17), we see that it precesses about the vector . Besides, the rate of precession and nutation are synchronized and in the case , equal to half the magnitude of the angular velocity . It is convenient also to introduce spherical coordinates for the unit invariant vector in the formUsing formula (7) and taking into account (21), we express the corresponding rotation matrix explicitly asNote that the angles and differ only by a phasewhile is a constant of integration, which makes the time dependence of the above matrix entries relatively simple. Moreover, we can manipulate both and via specific choice of initial data to obtain some particular cases. Firstly, we discuss the three distinct types of solutions pointed out in [15] for the case . The most obvious one, for which with (Type I), corresponds to () and is usually referred to as* steady rotation*. It suggests a constant invariant axis oriented along the angular velocity vector and linearly evolving angle : namely,On the other hand, setting , that is, , we obtain the half-turn example considered in [15] (Type II rotations) as , where , which may be written explicitly in matrix terms directly from (26) in the form (note that the -dependence in (26) is completely factored out by demanding to be a right angle)This time the rotation takes place about a varying axis that remains perpendicular to and precesses about it, while the angle remains at each particular instant of time.

The third type of rotations proposed in [15], in which both the rotation axes and angle are periodic functions of time, is also given by formula (26) if we simply set . We may consider, for example, the case of coherent phases , that is, , in whichand express the respective rotational matrix explicitly: that is,Specific solutions for different values of the phase shift are constructed similarly. Note also that the above matrices have such a simple form only in the reference frame, in which the angular velocity is aligned with one of the axes. Otherwise, the vector-parameter (17) needs to be rotated correspondingly, relative to the coordinate change.

The compact explicit form of the solution (26) allows for expressing the three parameters , , and , which we use as a substitute for the usual Euler angles, in the generic case aswhere stands for the proper quadrant inverse tangent (note that , so and is monotonous, thus invertible in this interval) and the ’s denote the matrix entries of given by formula (26) in the standard basis. Moreover, sinceas it follows from formula (17), it is straightforward to obtain also the azimuthal angle in the spherical representation of the invariant unit vector and the angle of rotation , respectively, asHowever, in the limiting cases of vanishing and infinite amplitude of precession, corresponding to and , respectively, (32) yield indeterminacy, so these two cases need to be treated separately. In particular, for , one haswhile the polar angle becomes obsolete. Similarly, the limit yieldsand is no longer relevant as it has been already discussed.

#### 3. Quaternion Description of Precessions

To derive the kinematic equations in quaternion terms, it suffices to recall that and substitute it in formula (10), which yields upon multiplication by Then, taking a dot product with and using the identitiesyield the derivative of the scalar (real) part of the quaternionCombining both expressions, we obtain the vector (imaginary) part in the formso the kinematic equations have the block-matrix representationIdentifying quaternions and four-vectors allows for writing the above result as and, thus, derive the general solution for the propagator in terms of time-ordered matrix exponents. In the particular case , however, we end up with a system of linear homogeneous ODEs with constant coefficients, hence the solutionNote that , so we have two double roots , which yields, in the canonical basis, synchronized rotations (with equal frequencies) in two mutually perpendicular planes in as we have seen in the case . Moreover,and, thus, the propagator may be expressed aswhere we let denote the identity in any dimension and with . Bearing this in mind, for the time evolution of an arbitrary quaternion , one haswhere stands for the unit vector in the direction of and is simply the initial condition.

Denoting also , we obtain the time dependence of the vector-parameterIn particular, if is parallel to , it remains such along the flow determined by (46) as one hasso we end up with the trivial orbit , which gives the solution also in the case of a steady initial state (). Similarly, if and , one has , so remains perpendicular to . More generally, we may denote and , as well as , and obtainNote that, just as before, we may weaken the condition of constant angular velocity by demanding only , thus letting . This introduces just a slight modification of the above solution: namely, in formula (46), we make the substitutionIt is useful to point out that the matrix (and therefore, its exponent) may be expressed also in a block form. In particular, if , one haswhich yields the solutions obtained in the previous section. More generally, we havethat is, a real representation of a pure unit quaternion and may be mapped onto as . The action of on is naturally realized via matrix multiplication if the latter is represented as a point in : namely,Then, the kinematic equation for may be written as , or in componentsand for fixed the corresponding unitary evolution operator takes the form

#### 4. Precessing Angular Velocity

Let us now consider the case of varying directional vector . One relatively simple setting is that of an angular velocity precessing about a fixed axis: for example,This time the magnitude is constant, so one hasand due to the nonvanishing commutator , obtaining the propagator via exponentiating the above matrix generally involves time-ordering or some other cumbersome procedure. Nevertheless, one may still derive the exact solution transforming system (53) into a single ODE. Namely, after differentiating the first equation and substitutingfrom the second one, the time dependence of the coefficients factors out and we end up with a linear homogeneous second-order ODE with constant coefficientsWith the notationwe may express the general solution in the compact formand, respectively, from the above substitution, we haveHere, and are complex constants determined by the initial conditions and asThis allows for writing the propagator in the simple formwhere for the first row we substitute the expressions for in the equation for , while for the second one, we use the fact that takes values in . Considering, for example, a steady initial state from the above, one easily obtainsThis allows for expressing also the evolution of the vector-parameter in the form (see Figure 1)Taking into account the fact that , one may express the entries of the corresponding rotation matrix using formula (7). In particular, in the resonant limit , one has and ; hence, , which finally yieldsAs we show in the next section, a similar expression holds also for variable and (under certain restrictions) if we substitute and . One may also derive the rotation matrix using directly formula (4), written in components as