Feynman path integrals for quantum open systems Title (Introductory Workshop on Path Integrals and Pseudo- Differential Operators) Author(s) Mazzucchi, Sonia Citation 数理解析研究所講究録 (2015), 1958: 96-116 Issue Date 2015-07 URL http://hdl.handle.net/2433/224088 Right Type Departmental Bulletin Paper Textversion publisher Kyoto University 数理解析研究所講究録 第 1958 巻 2015 年 96-116 96 Feynman path integrals for quantum open systems By Sonia MAZZUCCHI* Abstract The application of infinite dimensional integration techniques of oscillatory type to the mathematical definition of Feynman path integrals is described. Recent applications to the dynamics of quantum open systems, in particular to the quantum theory of continuous mea- surement, are also presented. \S 1. Introduction Feynman integration techniques are widely used (often at an heuristic level) in many areas of theoretical physics, such as quantum field theory, nonrelativistic quantum me- chanics, statistical mechanics, see e.g. [22, 25, 28, 29, 34, 36, 46, 50]. The origins of these ideas can be found the $40s[23]$ , when R. Feynman, inspired by a suggestion by Dirac [19], proposed an alternative formulation of time evolution in quantum mechanics. Feynman's original aim was a variational Lagrangian formulation of quantum theory, reintroducing the concept of trajectory, which had been banned by the \traditional'` formulation of quantum mechanics [49]. According to Feynman, the wave function $\psi$ of a non relativistic quantum particle, namely the solution of the Schr\"odinger equation (1.1) $\{\begin{array}{l}i\hslash\frac{\partial}{\partial t}\psi=-\frac{\hslash^{2}}{2m}\Delta\psi+V\psi\psi(0, x)=\psi_{0}(x)\end{array}$ (where $\hslash$ is the reduced Planck constant, $m>0$ is the mass of the particle and $F=-\nabla V$ is an external force) should be given by a \sum over all possible histories i.e. by a heuristic integral over the space of all possible paths $\gamma$ (in the configuration space of the system) with finite energy and fixed end point of the form: \ ,, (1.2) $\psi(t, x)=$ $C \int_{\{\gamma|\gamma(t)=x\}}e^{E^{i}}S_{t}(\gamma)\psi_{0}(\gamma(0))D\gamma$ 2010 Mathematics Subject Classification(s): $28C05,35C15,81P15,81S22$ Key Words: Oscillatory integrals, Feynman path integrals, quantum open systems $*$ Dipartimento di Matematica, Universit\`a di Trento, via Sommarive 14, 38123 Povo (Trento), Italy. email: [email protected] SONIA MAZZUCCHI 97 $S_{t}(\gamma)$ In formula (1.2) is the classical action of the system evaluated along the path $\gamma$ : (1.3) $S_{t}( \gamma)\equiv\int_{0}^{t}(\frac{m}{2}\Vert\dot{\gamma}(s)\Vert^{2}-V(\gamma(s)))ds,$ while the symbol $D\gamma$ stands for a heuristic Lebesgue “flat” measure on the space of paths and $C$ for a normalization constant, formally $C= \int e\pi^{i}\int_{0}^{t}\frac{m}{2}\Vert\dot{\gamma}(s)\Vert^{2}dsD\gamma.$ Feynman's approach to the description of quantum dynamics is particularly fasci- nating as it creates a connection between the classical Lagrangian description and the quantum one. Indeed it provides a quantization method, allowing, at least heuristically, to associate a quantum evolution to each classical Lagrangian. Furthermore the study of the semiclassical limit of quantum mechanics, i.e. the analysis of the asymptotic be- havior of the solution of the Schr\"odinger equation (or the transition amplitudes) in the limit where the Planck constant $\hslash$ can be regarded as a small parameter converging to $0,$ is particularly intuitive. Indeed, according to an heuristic application of the stationary phase method to formula (1.2), when $\hbar\downarrow 0$ the paths which give the main contribution to the integral should be those that make stationary the action functional $S_{t}(1.3)$ . These, by Hamilton's least action principle, are exactly the classical orbits of the system. On the other hand formula (1.2), as it stands, is mathematically ill-defined: indeed nei- ther the “infinite dimensional Lebesgue measure" $D\gamma$ , nor the normalization constant in front of the integral are meaningful. In fact, at the beginning of the $60$ 's Cameron $ek^{S_{t}(\gamma)_{D\gamma}}$ [16] proved that the heuristic Feynman measure, formally written as ' cannot $\overline{fe^{l}\hslash^{\mathcal{S}_{t}(\gamma)}D\gamma}$ be realized as a complex $\sigma$ -additive measure with finite total variation on the space of $\gamma\in[0, t]^{R^{d}}$ \paths" , endowed with the $\sigma$ -algebra generated by the cylindrical sets of the form $\{\gamma\in[0, t]^{\mathbb{R}^{d}} : \gamma(t_{1})\in B_{1}, \gamma(t_{k})\in B_{k}\},$ $\mathbb{R}^{d}$ where $0<t_{1}<$ $<t_{k}\leq t$ and $B_{1},$ $B_{k}$ are Borel sets in (see [41] for a dis- cussion of this problem). This technical problem can be solved by changing point of view and trying to realize the \Feynman integral"' in terms of a linear continuous func- tional on a Banach algebra of \integrable" functions, in the spirit of the Rietz-Markov theorem, that states a one to one correspondence (on suitable topological spaces $X$ ) between complex bounded measures and hnear continuous functionals on $C_{\infty}(X)$ (the continuous functions on $X$ vanishing at $\infty$ ). Nowadays several implementation of this program can be found in the physical and in the mathematical literature, for instance by means of analytic continuation of Wiener integrals [16, 44, 34, 47, 35, 20, 37, 43, 34], or as an infinite dimensional distribution in the framework of Hida calculus [30], via \complex Poisson measures"' [38, 2], or via non standard analysis [4] or as a infinite dimensional oscillatory integral. The latter method is particularly interesting as it is the only one allowing a systematic implementation of an infinite dimensional version of 98 FEYNMAN PATH INTEGRALS FOR QUANTUM OPEN SYSTEMS the stationary phase method, as well as its application to the study of the semiclassical limit of quantum mechanics[7]. Such an approach has its roots in a couple of papers by Ito [32, 33] and was developed by S. Albeverio and R. $H\emptyset egh$-Krohn [7, 8], D.Elworthy and A.Ruman [21], S. Albeverio and Z. Brze$\acute{z}niak[1]$ . It is based on a generalization of the definition (and the main properties) of classical oscillatory integrals [31] to the case where the integration is performed on an infinite dimensional Hilbert space. It provides a mathematical definition of Feynman's heuristic formula (1.2) in the case where the initial datum $\psi_{0}$ and the potential $V$ are bounded continuous function which can be written as the Fourier transform of $a$ (complex) finite measure on $\mathbb{R}^{d}[21]$ . Recently more general quantum dynamical systems have been investigated by means of these functional integration techniques, see e.g. [9, 39]. Particularly interesting applications can be found in the quantum theory of open systems, a challenging area of theoretical physics which intertwines foundational issues of quantum theory (such as phenomena as decoherence and the process of measurement of a physical observable) and modern applications (such as, for instance, the theory of quantum computing) [22]. In the traditional formulation of quantum mechanics the continuous time evolution described by the Schr\"odinger equation (1.1) is valid if the quantum system is \undis- turbed i.e. if it is isolated. On the other hand the concept of isolated system is not realistic. We should not forget that all the informations we can have on the state of a quantum system are the result of some measurement process. According to the tradi- tional Copenhagen formulation of quantum mechanics, when the particle interacts with the measuring apparatus, its time evolution is no longer continuous: the state of the system after the measurement is the result of a random and discontinuous change, the so-called \collapse of the wave function which cannot be described by the Schr\"odinger equation. Indeed, let us consider an observables $\mathcal{A}$ represented by a self-adjoint operator $A$ on a complex separable Hilbert space $\mathcal{H}$ , whose unitary vectors represent the states of the system, and let us assume for simplicity that $A$ is bounded and its spectrum is discrete. Let $\{a_{i}\}_{i\in N}\subset \mathbb{R}$ and $\{\psi_{i}\}_{i\in N}\subset \mathcal{H}$ be the corresponding eigenvalues and eigenvectors. According to the traditional mathematical formulation by von Neumann the consequences of the measurement are: 1. the decoherence of the state of the quantum system: because of the interaction with the measuring apparatus an initial pure state $\psi$ becomes a mixed state, described by the density operator $\rho^{\rho rior}(t)=\sum_{i}w_{i}P_{\psi_{i}}$ , where $P_{\psi_{i}}$ denotes the projector operator onto the eigenspace which is spanned by the vector $\psi_{i}$ and $w_{i}=|\langle\psi_{i},$ $\psi\rangle|^{2}.$ Considering another observable $\mathcal{B}$ (represented by a bounded self-adjoint operator $B)$ , its expectation value at time $t$ , after the measurement of the observable $\mathcal{A}$ (but without the information of the result of the measurement of $\mathcal{A}$), is given by $\mathbb{E}(B)_{t}^{prior}=Tr[\rho^{prior}(t)B]$ 99 SONIA MAZZUCCHI The transformation mapping $\psi$ to the so-called \prior state"' $\rho^{prior}(t)$ is named \prior dynamics"' or non selective dynamics. 2. The so-called (collapse of the wave function" : after the reading of the result of $\mathcal{A}$ the measurement of (i.e. the real number $a_{i}$ ) the state of the system is the corresponding eigenstate of the measured observable: $\rho(t)_{a_{i}}^{post}=P_{\psi_{i}}.$