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Deformation quantization: Quantum mechanics lives and works in phase space

COSMAS K. ZACHOS

Published Jan 1, 2014• 9 citations
Publisher

Abstract

Wigner’s 1932 quasi-probability Distribution Function in phase-space, his first paper in English, is a special (Weyl) representation of the density matrix. It has been useful in describing quantum flows in semiclassical limits; quantum optics; nuclear and physics; decoherence (eg, quantum computing); quantum chaos; “Welcher Weg” puzzles; molecular Talbot–Lau interferometry; atomic measurements. It is further of great importance in signal processing (time-frequency analysis).

DOI
10.1051/epjconf/20147802004

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Wigner's quasi-probability distribution function in phase-space is a special (Weyl) representation of the density matrix. It has been useful in describing quantum transport in quantum optics; nuclear physics; decoherence (e.g. quantum computing); quantum chaos; "Welcher Weg" discussions; semiclassical limits. It is also of importance in signal processing. Nevertheless, a remarkable aspect of its internal logic, pioneered by Moyal, has only emerged in the last quarter-century: It furnishes a third, alternative, formulation of Quantum Mechanics, independent of the conventional Hilbert Space, or Path Integral formulations. In this logically complete and self-standing formulation, one need not choose sides — coordinate or momentum space. It works in full phase-space, accommodating the uncertainty principle. This is an introductory overview of the formulation with simple illustrations.

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Among the equivalent formulations of quantum mechanics, Heisenberg's matrix mechanics and quantum theory, based on the Schrödinger wave equation, are the most technically advanced.Since the advent of quantum mechanics, these two schemes have provided the mathematical tools and the primary basis for the description of quantum phenomena.The path integral method developed by Dirac and Feynman has important advantages in the quantisation of gauge theories.We discuss here the formulation of quantum mechanics in phase space, known as deformation quantisation.Deformation quantisation uses the Wigner-Weyl association rule (Weyl, 1927(Weyl, , 1929(Weyl, , 1931;;Wigner, 1932) to establish a one-to-one correspondence between the functions in the phase space and the operators in the Hilbert space.Wigner's function appears as the Weyl symbol of the density matrix.A consistent dynamical description of the systems with the help of the Wigner function leads to deformation quantization.A useful formulation of the Wigner-Weyl association rule was proposed by Groenewold (1946) andStratonovich (1957).Groenewold introduced a non-commutative associative -product (star-product) of the functions in the phase space (Groenewold, 1946).The evolution of the quantum systems is determined by the antisymmetric part of the -product (Groenewold, 1946;Moyal, 1949), known as the Moyal bracket.The Moyal bracket represents the quantum deformation of the Poisson bracket.Deformation quantisation preserves many features of classical Hamiltonian dynamics.The formulation of deformation quantisation is based on the Wigner function and the Moyal bracket (i.e., the -product).The Wigner-Weyl association rule is necessary to prove the equivalence of deformation quantisation and the standard formalisms of quantum mechanics.Extensive literature has reported on the formulation of quantum mechanics in the phase space and the -product.We refer the reader to excellent reviews by Bayen et al. (1978a, 1978b), Carruthers & Zachariasen (1983), Balazs & Jennings (1984), Hillery et al. (1983), Karasev & Maslov (1991), and Osborn & Molzahn (1995), where one may find additional references.Wigner's function, as a fundamental object of deformation quantisation, has numerous applications in many-body physics, kinetic theory, collision theory, and quantum chemistry.Transport models, originally created to simulate chemical reactions, have been modified and are widely used to describe heavy-ion collisions.www.intechopen.comSome Applications of Quantum Mechanics 68 Deformation quantisation does not have many recognised successful applications in quantum theory.Recently, attempts have been made to use specific properties of the formalism to investigate semiclassical expansion (Osborn & Molzahn, 1995;McQuarrie et al., 1998) and to calculate the determinants of one-loop operators in field theory (Pletnev & Banin, 1999;Banin et al., 2001) and the high-order corrections to the Bohr-Sommerfeld quantisation rule (Gracia-Saz, 2004;Cargo et al., 2005).Potential applications of deformation quantisation in transport models are presented in this review.Transport models for heavy-ion physics are designed for phenomenological descriptions of the complex dynamics of nuclear collisions.Several types of advanced transport models are based on the Boltzmann-Uhlenbeck-Ueling equations (BUU) (Blaettel et al., 1993), (relativistic) quantum molecular dynamics (QMD/RQMD) (Sorge et al., 1989;Aichelin, 1991;Faessler, 1992), or antisymmetrised molecular dynamics (AQMD) (Feldmeier & Schnack, 1997).These approaches have the correct classical limit and they contain special plug-ins and quantum-mechanical attributes, such as Pauli blocking for binary collisions of fermions.Numerical solutions are implemented through the distribution of test particles (BUU) or centroids of wave packets (QMD, AQMD) for classical trajectories in the phase space.For AQMD, the wave packets are antisymmetrised in their parameters.The transport models provide a solid basis for a phenomenological description of a variety of complex nuclear collisions phenomena.However, quantum coherence effects and non-localities are beyond the scope of these models.The internal consistencies of the approximations in the models remain a subject of debate, promoting further developments (see, for example, papers by Kohler (1995) andFeldmeier &Schnack (1997) and references therein).The most striking feature of the transport models is the depiction of the trajectories in the phase space, which are test particles or centroids of wave packets.The evolution of the system of classical particles can be calculated using standard programs to solve first-order ordinary differential equations (ODEs).At the same time, the evolution of wave functions of many-body systems is a field-theoretic problem with an infinite number of degrees of freedom that cannot be solved either analytically or numerically.Any simulation of many-body quantum dynamics must be based on a concept of trajectories, which is the only attribute allowing access to an approximate description of complex quantum systems.The concept of phase-space trajectories arises naturally in the formalism of deformation quantisation through the Wigner transformation of the operators of the canonical coordinates and momenta in the Heisenberg representation.These trajectories satisfy the quantum version of Hamilton's equations (Osborn & Molzahn, 1995;Krivoruchenko & Faessler, 2006b) and are the characteristics by which the time-dependent Weyl's symbols for the other operators can be determined (Krivoruchenko & Faessler, 2006b;Krivoruchenko et al., 2006c Krivoruchenko et al., , 2007)).In the classical limit, quantum characteristics reduce to classical trajectories.Knowledge of the quantum phase flow, i.e., the quantum trajectories, is equivalent to a complete knowledge of the quantum dynamics.In this chapter, we provide an introduction to deformation quantisation and demonstrate the usefulness of the formalism in solving the evolution problem for many-body systems in terms of semiclassical expansion.We show that, in any fixed order of expansion over the Planck's constant, the evolution problem can be reduced to a statistical-mechanics problem of calculating an ensemble of quantum characteristics in the phase space and their Jacobi fields.In comparison with the corresponding rules of classical statistical mechanics, the rules for computing the probabilities and time-dependent averages of observables are www.intechopen.com

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Wigner’s 1932 quasi-probability Distribution Function in phase-space, his first paper in English, is a special (Weyl) representation of the density matrix. It has been useful in describing quantum flows in semiclassical limits; quantum optics; nuclear and physics; decoherence (eg, quantum computing); quantum chaos; “Welcher Weg” puzzles; molecular Talbot–Lau interferometry; atomic measurements. It is further of great importance in signal processing (time-frequency analysis).
Wigner's quasi-probability distribution function in phase-space is a special (Weyl) representation of the density matrix. It has been useful in describing quantum transport in quantum optics; nuclear physics; decoherence (e.g. quantum computing); quantum chaos; "Welcher Weg" discussions; semiclassical limits. It is also of importance in signal processing. Nevertheless, a remarkable aspect of its internal logic, pioneered by Moyal, has only emerged in the last quarter-century: It furnishes a third, alternative, formulation of Quantum Mechanics, independent of the conventional Hilbert Space, or Path Integral formulations. In this logically complete and self-standing formulation, one need not choose sides — coordinate or momentum space. It works in full phase-space, accommodating the uncertainty principle. This is an introductory overview of the formulation with simple illustrations.
Among the equivalent formulations of quantum mechanics, Heisenberg's matrix mechanics and quantum theory, based on the Schrödinger wave equation, are the most technically advanced.Since the advent of quantum mechanics, these two schemes have provided the mathematical tools and the primary basis for the description of quantum phenomena.The path integral method developed by Dirac and Feynman has important advantages in the quantisation of gauge theories.We discuss here the formulation of quantum mechanics in phase space, known as deformation quantisation.Deformation quantisation uses the Wigner-Weyl association rule (Weyl, 1927(Weyl, , 1929(Weyl, , 1931;;Wigner, 1932) to establish a one-to-one correspondence between the functions in the phase space and the operators in the Hilbert space.Wigner's function appears as the Weyl symbol of the density matrix.A consistent dynamical description of the systems with the help of the Wigner function leads to deformation quantization.A useful formulation of the Wigner-Weyl association rule was proposed by Groenewold (1946) andStratonovich (1957).Groenewold introduced a non-commutative associative -product (star-product) of the functions in the phase space (Groenewold, 1946).The evolution of the quantum systems is determined by the antisymmetric part of the -product (Groenewold, 1946;Moyal, 1949), known as the Moyal bracket.The Moyal bracket represents the quantum deformation of the Poisson bracket.Deformation quantisation preserves many features of classical Hamiltonian dynamics.The formulation of deformation quantisation is based on the Wigner function and the Moyal bracket (i.e., the -product).The Wigner-Weyl association rule is necessary to prove the equivalence of deformation quantisation and the standard formalisms of quantum mechanics.Extensive literature has reported on the formulation of quantum mechanics in the phase space and the -product.We refer the reader to excellent reviews by Bayen et al. (1978a, 1978b), Carruthers & Zachariasen (1983), Balazs & Jennings (1984), Hillery et al. (1983), Karasev & Maslov (1991), and Osborn & Molzahn (1995), where one may find additional references.Wigner's function, as a fundamental object of deformation quantisation, has numerous applications in many-body physics, kinetic theory, collision theory, and quantum chemistry.Transport models, originally created to simulate chemical reactions, have been modified and are widely used to describe heavy-ion collisions.www.intechopen.comSome Applications of Quantum Mechanics 68 Deformation quantisation does not have many recognised successful applications in quantum theory.Recently, attempts have been made to use specific properties of the formalism to investigate semiclassical expansion (Osborn & Molzahn, 1995;McQuarrie et al., 1998) and to calculate the determinants of one-loop operators in field theory (Pletnev & Banin, 1999;Banin et al., 2001) and the high-order corrections to the Bohr-Sommerfeld quantisation rule (Gracia-Saz, 2004;Cargo et al., 2005).Potential applications of deformation quantisation in transport models are presented in this review.Transport models for heavy-ion physics are designed for phenomenological descriptions of the complex dynamics of nuclear collisions.Several types of advanced transport models are based on the Boltzmann-Uhlenbeck-Ueling equations (BUU) (Blaettel et al., 1993), (relativistic) quantum molecular dynamics (QMD/RQMD) (Sorge et al., 1989;Aichelin, 1991;Faessler, 1992), or antisymmetrised molecular dynamics (AQMD) (Feldmeier & Schnack, 1997).These approaches have the correct classical limit and they contain special plug-ins and quantum-mechanical attributes, such as Pauli blocking for binary collisions of fermions.Numerical solutions are implemented through the distribution of test particles (BUU) or centroids of wave packets (QMD, AQMD) for classical trajectories in the phase space.For AQMD, the wave packets are antisymmetrised in their parameters.The transport models provide a solid basis for a phenomenological description of a variety of complex nuclear collisions phenomena.However, quantum coherence effects and non-localities are beyond the scope of these models.The internal consistencies of the approximations in the models remain a subject of debate, promoting further developments (see, for example, papers by Kohler (1995) andFeldmeier &Schnack (1997) and references therein).The most striking feature of the transport models is the depiction of the trajectories in the phase space, which are test particles or centroids of wave packets.The evolution of the system of classical particles can be calculated using standard programs to solve first-order ordinary differential equations (ODEs).At the same time, the evolution of wave functions of many-body systems is a field-theoretic problem with an infinite number of degrees of freedom that cannot be solved either analytically or numerically.Any simulation of many-body quantum dynamics must be based on a concept of trajectories, which is the only attribute allowing access to an approximate description of complex quantum systems.The concept of phase-space trajectories arises naturally in the formalism of deformation quantisation through the Wigner transformation of the operators of the canonical coordinates and momenta in the Heisenberg representation.These trajectories satisfy the quantum version of Hamilton's equations (Osborn & Molzahn, 1995;Krivoruchenko & Faessler, 2006b) and are the characteristics by which the time-dependent Weyl's symbols for the other operators can be determined (Krivoruchenko & Faessler, 2006b;Krivoruchenko et al., 2006c Krivoruchenko et al., , 2007)).In the classical limit, quantum characteristics reduce to classical trajectories.Knowledge of the quantum phase flow, i.e., the quantum trajectories, is equivalent to a complete knowledge of the quantum dynamics.In this chapter, we provide an introduction to deformation quantisation and demonstrate the usefulness of the formalism in solving the evolution problem for many-body systems in terms of semiclassical expansion.We show that, in any fixed order of expansion over the Planck's constant, the evolution problem can be reduced to a statistical-mechanics problem of calculating an ensemble of quantum characteristics in the phase space and their Jacobi fields.In comparison with the corresponding rules of classical statistical mechanics, the rules for computing the probabilities and time-dependent averages of observables are www.intechopen.com
This is a text on quantum mechanics formulated simultaneously in terms of position and momentum, i.e. in phase space. It is written at an introductory level, drawing on the remarkable history of the subject for inspiration and motivation. Wigner functions density -- matrices in a special Weyl representation -- and star products are the cornerstones of the formalism. The resulting framework is a rich source of physical intuition. It has been used to describe transport in quantum optics, structure and dynamics in nuclear physics, chaos, and decoherence in quantum computing. It is also of importance in signal processing and the mathematics of algebraic deformation. A remarkable aspect of its internal logic, pioneered by Groenewold and Moyal, has only emerged in the last quarter-century: it furnishes a third, alternative way to formulate and understand quantum mechanics, independent of the conventional Hilbert space or path integral approaches to the subject. In this logically complete and self-standing formulation, one need not choose sides between coordinate or momentum space variables. It works in full phase-space, accommodating the uncertainty principle; and it offers unique insights into the classical limit of quantum theory. The observables in this formulation are c-number functions in phase space instead of operators, with the same interpretation as their classical counterparts, only composed together in novel algebraic ways using star products. This treatise provides an introductory overview and supplementary material suitable for an advanced undergraduate or a beginning graduate course in quantum mechanics. Readership: Advanced undergraduates and graduates studying quantum mechanics.
The Weyl-Wigner transformation enables one to construct a representation of quantum motion equations using functions in phase space. The states of the system are determined bv the quasi-probability distribution in phase space and the motion is described by an orthogonal integral operator. This formalism is employed in the study of the classical meaning of the discrete symmetry groups, in the problem of defining the current probability and in the proving of the expression of cross section from the S-matrix. (GE)
This thesis describes quantum mechanics in the phase space formulation. We introduce quantization and in particular the Weyl quantization. We study a general class of phase space distribution functions on phase space. The Wigner distribution function is one such distribution function. The Wigner distribution function in general attains negative values and thus can not be interpreted as a real probability density, as opposed to for example the Husimi distribution function. The Husimi distribution however does not yield the correct marginal distribution functions known from quantum mechanics. Properties of the Wigner and Husimi distribution function are studied to more extent. We calculate the Wigner and Husimi distribution function for the energy eigenstates of a particle trapped in a box. We then look at the semi classical limit for this example. The time evolution of Wigner functions are studied by making use of the Moyal bracket. The Moyal bracket and the Poisson bracket are compared in the classical limit. The phase space formulation of quantum mechanics has as advantage that classical concepts can be studied and compared to quantum mechanics. For certain quantum mechanical systems the time evolution of Wigner distribution functions becomes equivalent to the classical time evolution stated in the exact Egerov theorem. Another advantage of using Wigner functions is when one is interested in systems involving mixed states. A disadvantage of the phase space formulation is that for most problems it quickly loses its simplicity and becomes hard to calculate.
Quantum phase‐space distributions offer a royal road into the fascinating quantum–classical interface; the Wigner function being the first and best example. However, the subject is frequent with subtleties and textbook‐level misinformation; e.g. “The Wigner distribution can give wrong answers for some operator expectation values” . Since the Wigner distribution is just another representation of the density matrix, it must yield correct answers. To that end, Marlan Scully has asked at several international conferences (the 2015 Prague conference being one of them) the following question: “Starting with the density matrix (not the Moyal characteristic function), could you give me a simple direct derivation of the Wigner distribution?” Section contains his answer. In Appendix D , we give a related treatment and make contact with other approaches. We hope that as a result of our studies, the Wigner distribution will become more deeply appreciated.