We consider the exchange of spin and orbital angular momenta between a circularly polarized Laguerre–Gaussian beam of light and a single atom trapped in a two-dimensional harmonic potential. The radiation field is treated classically but the atomic centre-of-mass motion is quantized. The internal levels in the atom are taken to be Rydberg circular states. The spin and orbital angular momenta of the field are individually conserved upon absorption, with the result that the electronic and motional degrees of freedom of the atom are entangled. We suggest applications in quantum information processing.
We consider the exchange of spin and orbital angular momenta between a circularly polarized Laguerre–Gaussian beam of light and a single atom trapped in a two-dimensional harmonic potential. The radiation field is treated classically but the atomic centre-of-mass motion is quantized. The internal levels in the atom are taken to be Rydberg circular states. The spin and orbital angular momenta of the field are individually conserved upon absorption, with the result that the electronic and motional degrees of freedom of the atom are entangled. We suggest applications in quantum information processing.
The exchange of orbital angular momentum between a Laguerre-Gaussian beam of light and the center-of-mass motion of an atom or molecule is well known. We show that the orbital angular momentum of light can also be transferred to the internal electronic or rotational motion of an atom or a molecule provided the internal and center-of-mass motions are coupled. However, this transfer does not happen directly to the internal motion, but via center-of-mass motion. If atoms or molecules are cooled down to the recoil limit, then an exchange of angular momentum between the quantized center-of-mass motion and the internal motion is possible during the interaction of cold atoms or molecules with the Laguerre-Gaussian beam. The orientation of the exchanged angular momentum is determined by the sign of the winding number of the Laguerre-Gaussian beam. We present selective results of numerical calculations for the quadrupole transition rates in the interaction of the Laguerre-Gaussian beam with an atomic Bose-Einstein condensate to illustrate the underlying mechanism of light orbital angular momentum transfer. We discuss how the alignment of diatomic molecules will facilitate exploration of the effects of light orbital angular momentum on the electronic motion of molecules.
The angular momentum of light is a useful resource for many applications. In specific physical architectures it can be considered as the sum of two independent terms, the spin and the orbital components, in analogy to particle systems. The spin angular momentum is related to the polarization of the optical beam, that is the direction of the oscillating electric field, whereas the orbital angular momentum is associated with the spatial distribution of the field. Being independent, spin and orbital angular momenta have been discovered and explored in separate contexts for many years, while only recently it has been considered the possibility to address both quantities on the same beam (or individual photons). The interaction between these two quantities gives rise to complex structures of the electromagnetic field, or to the so called classical entanglement in the domain of single photons. The research presented in this work aimed to show that combining spin and orbital angular momenta in light beams or single photons may be a useful tool for a variety of applications, with particular interest to the case of architectures characterized by spin-orbit interaction. This concept was made concrete through the design and the realization of several experiments, in the framework of singular optics, foundations of quantum mechanics, quantum information theory and quantum simulation.
Orbital angular momentum (OAM) of single photons represents a new optical degree of freedom for the entanglement of photons [1,2]. One physical realization of OAM carrying light beams are the so called Laguerre-Gaussian modes of light with the required helical phase structure. One big advantage over the well known polarization degree of freedom is the possibility of realizing entanglement between two photons with very high quantum numbers. However, the creation of photonic OAM entanglement using the widely used spontaneous parametric downconversion process is limited by the strongly reduced efficiency for higher momenta [3]. We have realized a novel method to create entanglement between two photons with a very high difference in their OAM quantum number and momentum respectively. Furthermore we have explored hybrid entanglement of photons in the OAM and polarization degree of freedom.
Using a method of characterizing entanglement in the framework of quantum field theory, we investigate the optical generation and quantitative characterizations of quantum entanglement in an electron-hole system, in presence of spin-orbit coupling, and especially make a theoretical analysis of a recent experimental result. Basically, such entanglement should be considered as between occupation numbers of single-particle basis states, and is essentially generated by coupling between different single-particle basis states in the second-quantized Hamiltonian. Interaction with two resonant light modes of different circular polarizations generically leads to a superposition of ground state and two heavy-hole excitonic states. When and only when the state is a superposition of only the two excitonic eigenstates, the entanglement reduces to that between two distinguishable particles, each with two degrees of freedom, namely, band index, as characterized by angular momentum, and orbit, as characterized by position or momentum. The band-index state, obtained by tracing over the orbital degree of freedom, is found to be a pure state, hence the band index and orbital degrees of freedom are separated in this state. We propose some basic ideas on spatially separating the electron and the hole, so that the entanglement of band indices, or angular momenta, is between spatially separated electron and hole.
It is well established now that light carries both spin and orbital angular momentum which are associated with circular polarisation and helical phase fronts. Orbital angular momentum degrees of freedom recently have been used frequently in quantum information processing as their states are described by vectors in a higher-dimensional Hilbert space which enhances the possibility of realising superior quantum information protocols. On the other hand, quantum coherence, which arises from the superposition principle, is a distinct feature of quantum mechanics that cannot be satisfactorily described by classical physics. Coherence is also identified as essential ingredient for applications of quantum information, computation, and quantum thermodynamics. Three research projects, with their related background information, are presented in this thesis. In the first one, we design a linear optical system to transform the maximally entangled state of a down-converted photon pair into a genuine entangled χ-type state, as this class of genuine entangled states has been showed to have many interesting entanglement properties and can be employed in several quantum information protocols. In the second project, we study the mechanism of angular momentum transfer from light to a dielectric medium when it undergoes total internal reflection. The result shows that the torque associated with angular momentum transfer appears shortly, when the light pulse hits the interface. Finally, we study quantum coherence transfer from a coherence resource initialised in a coherence state to an atomic state by the Jaynes-Cummings model, and we compare it to the coherent operation that uses a resource prepared in a ladder state described by Åberg’s model. We found that a resource in a coherent state is more robust against failures.
We generated orbital angular momentum in atoms with a spin degree of freedom and coherently transferred it to a light field, which resulted in the generation of a Laguerre-Gaussian (LG) beam. In this method, atoms obtain orbital angular momentum through Larmor precession around the quadrupole field. Successive operations of quadrupole and homogeneous magnetic fields lead to a change of sign in the LG beam or even generation of superposition states of the LG beam with different indexes in real time. Possible applications to control orbital angular momentum of arbitrary photons are also discussed.