Second order connections and stochastic horizontal lifts

In this paper we develop a theory of second order connections with a view towards the associated stochastic calculus. Connections in principal fiber bundles are defined as sections of the tangent space of second order differential operators. We prove existence and uniqueness of stochastic horizontal lifts for semimartingales with respect to these connections. Finally, the parallel transport along semimartingales on the base space is studied.     

Semi-classical quantum mechanics and stochastic calculus of variations

Within the framework of the general theory of stochastic calculus of variations, we examine mainly the notion of second variation in the stochastic mechanics of E. Nelson, a representative of quantum mechanics in which the concept of path for particles keep a sense. We show that the two approaches used in classical calculus of variation to know if a path is not only an extremum but also the minimum of the action, namely, the local one (weak minimum) and the global one (strong minimum), can be generalized to include the quantum-mechanical paths. Thus, we can prove that locally, a solution of the classical equation of motion is really the minimum, even in a large class of quantum paths containing the semi-classical trajectories. By introducing a stochastic version of the excess function of Weierstrass, we show the analogous global property. There, of course, one can speak of the principle of least action in a strict sense. Several explicit examples are discussed.     

Quantum stochastic evolutions

Quantum stochastic differential inclusions of hypermaximal monotone type are studied, under very general conditions, by means of certain discrete schemes which approximate them. The existence of an evolution operator corresponding to each such inclusion is proved.     

Forward and backward adapted quantum stochastic calculus and double product integrals

We show that iterated stochastic integrals can be described equivalently either by the conventional forward adapted, or by backward adapted quantum stochastic calculus. By using this equivalence, we establish two properties of triangular (causal) and rectangular double quantum stochastic product integrals, namely a necessary and sufficient condition for their unitarity, and the coboundary relation between the former and the latter.     

Fermion Ito's formula and stochastic evolutions

An Ito product formula is proved for stochastic integrals against Fermion Brownian motion, and used to construct unitary processes satisfying stochastic differential equations. As in the corresponding Boson theory [10, 11] these give rise to stochastic dilations of completely positive semigroups.Work completed in part while the first author was supported by an SERC research studentship, and in part while the second author was visiting the Physics Department of the University of Texas at Austin supported by NSF grant PHY 81-07381     

Quantum Ito's formula and stochastic evolutions

Using only the Boson canonical commutation relations and the Riemann-Lebesgue integral we construct a simple theory of stochastic integrals and differentials with respect to the basic field operator processes. This leads to a noncommutative Ito product formula, a realisation of the classical Poisson process in Fock space which gives a noncommutative central limit theorem, the construction of solutions of certain noncommutative stochastic differential equations, and finally to the integration of certain irreversible equations of motion governed by semigroups of completely positive maps. The classical Ito product formula for stochastic differentials with respect to Brownian motion and the Poisson process is a special case.Parts of this work were completed while the first author was a Royal Society-Indian National Science Academy Exchange Visitor to the Indian Statistical Institute, New Delhi, and visiting the University of Texas supported in part by NSF grant PHY81-07381, and part while the second author was visiting the Mathematics Research Centre of the University of Warwick     

Constructing the davies process of resonance fluorescence with quantum stochastic calculus

The starting point is a given semigroup of completely positive maps on the 2×2 matrices. This semigroup describes the irreversible evolution of a decaying two-level atom. By using the integral-sum kernel approach to quantum stochastic calculus, the two-level atom is coupled to an environment, which in this case will be interpreted as the electromagnetic field. The irreversible time evolution of the two-level atom then stems from the reversible time evolution of the atom and the field together. Mathematically speaking, a Markov dilation of the semigroup has been constructed. The next step is to drive the atom by a laser and to count the photons emitted into the field by the decaying two-level atom. For every possible sequence of photon counts, a map is constructed that gives the time evolution of the two-level atom implied by that sequence. The family of maps obtained in this way forms a so-called Davies process. In his book, Davies describes the structure of these processes, which brings us into the field of quantum trajectories. Within the model presented in this paper, the jump operators are calculated and the resulting counting process is briefly described.     

Unitary quantum groups,quantum projective spaces andq-oscillators

We discuss the parametrization of quantum groups in terms of independent operators. We find that this consideration leads to the parametrization ofSU _q(2) in terms of aq-oscillator plus a commuting phase. The commuting phase is naturally identified with the subgroupU(1) and the remaining cosetSU _q(2)/U(1)=CP _q(1) consists of aq-oscillator. For unitary quantum groupsSU _q (n), the analogous construction results in the quantum projective spaceSU _q(n+1)/U _q (n)=CP _q (n) being identified with then-dimensionalq-oscillator. This yields a nonlinear action of the quantum groupSU _q(n+1) on then-dimensionalq-oscillator.     

Unification of fermion and Boson stochastic calculus

Fermion annihilation and creation processes are explicitly realised in Boson Fock space as functions of the corresponding Boson processes and second quantisations of reflections. Conversely, Boson annihilation and creation processes can be constructed from the Fermion processes. The existence of unitary stochastic evolutions driven by Fermion and gauge noise is thereby reduced to an equivalent Boson problem, which is then solved.This work was carried out while both authors were participating in the Symposium on Stochastic Differential Equations at the University of Warwick. The first author acknowledges conversations with R.F. Streater during the same Symposium     

Unitary quantum three-body problem in a harmonic trap

We consider either 3 spinless bosons or 3 equal mass spin-1/2 fermions, interacting via a short-range potential of infinite scattering length and trapped in an isotropic harmonic potential. For a zero-range model, we obtain analytically the exact spectrum and eigenfunctions: for fermions all the states are universal; for bosons there is a coexistence of decoupled universal and efimovian states. All the universal states, even the bosonic ones, have a tiny 3-body loss rate. For a finite range model, we numerically find for bosons a coupling between zero angular momentum universal and efimovian states; the coupling is so weak that, for realistic values of the interaction range, these bosonic universal states remain long-lived and observable.     

Differential calculus on quantum spheres

Three-dimensional differential calculus on quantum spheres S _infc ^sup2 ,]–1, 1[{0}, c[0, ], is introduced and investigated. Spectra of generalized Laplacians are found. These operators are expressed by generalized directional derivatives. Classical limits of these objects are obtained and a simple approach to quantum mechanics on a quantum sphere is presented.     

Spinor calculus for quantum groups

An outline of a quantum group covariant spinor calculus is presented. The use of the calculus is demonstrated, for example, by setting up the commutation relations of coordinates of differentq-planes.     

On the correspondence of semiclassical and quantum phases in cyclic evolutions

Based on the exactly solvable case of a harmonic oscillator, we show that the direct correspondence between the Bohr-Sommerfeld phase of semiclassical quantum mechanics and the topological phase of Aharonov and Anandan is restricted to the case of a coherent state. For other Gaussian wave packets the geometric quantum phase strongly depends on the amount of squeezing.We dedicate this paper to our friend Professor Asim O. Barut—a great scientist, an extraordinary human being, and a man who enjoys the whole world as his home.     

Unitary bounds and controllability of quantum evolution in NMR spectroscopy

The effect of restricted control of unitary quantum evolution is investigated with specific attention to NMR spectroscopy. It is demonstrated that in cases where the Hamiltonian through commutation fails to span the entire Lie algebra su(n) for an n-level quantum system, the maximum transfer efficiency may be reduced significantly relative to previously known unitary bounds on spin dynamics. The paper describes methods to determine the degree of controllability and the conditional unitary bounds induced by restricted control. These features are exemplified in relation to heteronuclear coherence transfer by planar and isotropic mixing in liquid state NMR.