We propose a scheme for the generation and reconstruction of entangled states between the internal and external (motional) degrees of freedom of a trapped electron. Such states also exhibit quantum coherence at a mesoscopic level.PACS numbers: 03.65. Bz, 42.50.Vk, 42.50.Dv A single electron trapped in a Penning trap [1] is one of the most fundamental quantum systems. Among its peculiar features, it allows the measurement of fundamental physical constants with striking accuracy. Recently, for instance, the electron cyclotron degree of freedom has been cooled to its ground state, where the electron may stay for hours, and quantum jumps between adjacent Fock states have been observed [2]. It is therefore evident that the manipulation and the characterization of the state of a trapped electron is an important issue, with implications in the very foundations of quantum mechanics. Earlier works [3,4] have dealt with this problem for one (motional) degree of freedom. On the other hand, entanglement [5] has been recognized as one of the most puzzling features of quantum mechanics, being also the basis of quantum information processing [6]. A striking achievement in this rapidly expanding field has been the recent entanglement of four trapped ions [7]. However, it is also possible (and conceptually equivalent) to entangle different degrees of freedom of the same particle [8].In the present work we propose to generate entangled states (combined cyclotron and spin states) of an electron in a Penning trap by using suitable applied fields. The complete structure of the cyclotron-spin quantum state is then obtained with the help of a tomographic reconstruction from the measured data.In a Penning trap an electron is confined by the combination of a homogeneous magnetic field along the positive z axis and an electrostatic quadrupole potential in the xy plane [1]. The spatial part of the electronic wave function consists of three degrees of freedom, but neglecting the slow magnetron motion (whose characteristic frequency lies in the kHz region), here we only consider the axial and cyclotron motions, which are two harmonic oscillators radiating in the MHz and GHz regions, respectively. On the other hand, the spin dynamics results from the interaction between the magnetic moment of the electron and the static magnetic field, so that the free Hamiltonian reads as [1]where the indices z, c, and s refer to the axial, cyclotron and spin motions, respectively.Here, in addition to the usual trapping fields, we consider an external radiation field as a standing wave along the z direction and rotating, i.e. circularly polarized, in the xy plane with frequency Ω [9]. To be more specific, we consider a standing wave within the cylindrical cavity configuration [10] with the (dimensionless) wave vector κ. Then, the interaction Hamiltonian reads [9] H int =hǫ â c e iΩt +â † c e −iΩt cos(κẑ + φ) +hζ σ − e iΩt +σ + e −iΩt sin(κẑ + φ) ,whereσ ± = (σ x ± iσ y )/2, andẑ =â z +â † z . The phase φ defines the position of the center of the axial mo...