 |
SUSANNE YELIN Associate Professor of Physics |
||
University of Connecticut U-3046 |
Room No: P-111 |
||
overview |
Research
Quantum information processing, cooling, and nonlinear optics with polar molecules Polar molecules present a promising new platform for quantum computation and simulation, because they incorporate the prime advantage of both neutral atoms and trapped ions, i.e., long coherence times and strong interactions, respectively. In particular, recent advances of preparing ultracold molecules in their ground states make this topic part of a burgeoning research direction. We have made numerous proposals, in collaboration with my colleague R. Côté, to efficiently cool molecules. This involves complex STIRAP (“stimulated adiabatic Raman p” – the dissipation-less population transfer between metastable states via excited states based on quantum coherence effects), FOPA (“Feshbach-assisted photo-association” – using the large amplitude of states near Feshbach resonances to considerably enhance dipole transition rates), and DC-field assisted resonances. In addition, we have been exploring phase gates between molecular qubits by switching “on” and “off” the strong interactions between polar molecules by taking advantage of the fact that the magnitude (and sign) of a molecular dipole moment can change depending on the state of the molecule. By exciting the molecule between a state that exhibits a strong dipole moment and a state with a dipole moment of zero, the interactions can be effectively turned on and off, thus helping to simplify phase gates and minimize decoherence. In a second project, the dipole-dipole interactions present between polar molecules are utilized in order to create single-photon nonlinearities. In order to accomplish this, single “slow-light polaritons,” i.e., quanta of light propagating through a medium prepared by electromagnetically induced transparency, are created. The nonlinear dipole-dipole interaction imposes a nonlinear phase on those and thus on the outcoming single photons. This phase can serve for single-photon switching or transitors and is the signature of “optical quantum computing.” Quantum coherence effects, imaging, and nonlinear optics in atomic gases and ions An intriguing challenge for modern science and technology is the coherent manipulation of quantum systems. My interest in these problems is stimulated by fundamental aspects of quantum control and decoherence, and by recent developments in quantum information science. In particular, our work on “light storage,” i.e., the mapping of quantum information between light and matter, serves as a starting point for new projects. One avenue of my research involved the investigation of novel technologies for the coherent manipulation of trapped quantum states of light. Specifically we were studying the concept of a single-photon multiple-beam splitter. In addition, we use these techniques for the investigation of imaging and image storage through hot vapor. Coherence effects can even be used to mitigate the effects of diffusion. In addition, it turns out that by using masks for the strong coupling fields image manipulation can be done considerably faster than with more traditional means. Another project is aiming at improving the quality of very noisy lasers by quantum optically employing noise correlations. This project is carried out in collaboration with R. Walsworth's group at Harvard. A new project, in collaboration with I. Cirac's group in Munich, involves using collective excitation in Coulomb crystals in an optical cavity in order to produce single-photon nonlinearities (related to the analogous project with polar molecules) or efficiently and effectively couple single photons to single ions and thus, for example, create optimal control for quantum computing or simulation. This setup has all the advantages of single ions in a trap in terms of controllability and coherence times combined with the strong collective coupling parameters present between photon and an ensemble of atoms. Correlation and cooperative effects in light matter interaction – superradiance Traditionally, quantum optics has dealt with the coupling of light to dilute atomic samples in which atomic interactions are negligible. Today, however, the forefront of quantum optics includes the treatment of dense atomic ensembles, for which atomic correlations may lead to a wide variety of effects ranging from completely incoherent (such as radiation trapping) to fully coherent manifestations (such as superradiance and atom entanglement). A careful theoretical description of such effects is, in general, a complex problem on the interface of quantum optics and many-body physics. It should account for coherent interactions with external electromagnetic fields and for correlations and entanglement among interacting atoms. We are developing such a novel theoretical framework to describe such phenomena and applying this theory to several experimentally relevant situations. Specifically we aim to describe effects involving multi-atom quantum correlations due to dipole-dipole coupling mediated by radiation fields. The basis for this work is a formalism that can successfully describe atom-field interactions in dense gases by reducing a many-body multi-mode Hamiltonian to an effective single- or two-atom master equation. Our previous work demonstrated that this formalism can be used to describe a diverse range of phenomena including radiation trapping, enhanced spontaneous emission, and optical bistability. These investigations differ from earlier work on the subject in that this approach allows us to treat collective coherent interactions and incoherent phenomena on an equal footing. More recent work is aimed at the application of this formalism to quantum optical phenomena in novel experimental systems. We are exploring novel nonlinear optical phenomena in such a strongly interacting system. In another avenue we were investigating superradiant behavior in an ultracold gas of Rydberg atoms. This project is carried out in collaboration with my colleagues P. Gould, E. Eyler, and R. Côté. Future projects will include investigation of quantum interference effects in elongated samples, and studies of superradiant effects in novel systems such as ensembles of excitons or Bose-condensed atoms. In collaboration with G. Giedke and I. Cirac's group at the Max-Planck-Institute for Quantum Optics in Munich we are exploring superradiant effects of nuclear spin polarization. In particular, all the nuclear spins are directly coupled to one electron spin (e.g., of a diamond NV center or a quantum dot). The effective interaction (upon tracing over the electron's degrees of freedom) has a similar effect as the dipole-dipole interaction in traditional superradiance. In addition, it turns out that this effect is influenced by the symmetry of the collective states, and that, in turn, can be controlled by changing parameters such as the energy mismatch between electronic and nuclear transitions. Absorptionless negative refraction and response in different media
Materials with unusual light refraction properties, in particular with a negative index of refraction are a hot research topic these days. They seem to deny the laws of optics when they, for example, refract light in the “wrong” direction, which Veselago predicted already in the late 60's. Because of their unusual properties these materials promise unheard-of applications, such as “superlenses,” or even an “invisibility cloak.” The problem: There are no known natural substances which could achieve this feat.
Our own research suggests that this feat could also be accomplished using atomic gases and quantum interference effects. In particular, two of the main problems to date in creating metamaterials have been the difficulty of manipulating to the necessary degree magnetic response and inherent absorption. Both of these problems promise to be solved by our approach that is based on chirality and electromagnetically induced transparency. This research has been carried out in collaboration with the groups of M. Fleischhauer in Kaiserslautern and R. Walsworth at Harvard. In an attempt to use these ideas for massive particles, it was recently suggested that the graphene dispersion relation, that, for undoped graphene, is linear (i.e., photon-like) close to the Fermi surface, allows to imagine unitary negative refraction for electrons. Since this dispersion relation can be simulated with atoms in optical lattices, we want to investigate the resulting possibilities of creating negative refraction for atomic matter waves. As a starting point, however, we have been studying coherent phenomena such as the so-called “Goos-Hänchen shift” and electron-wave storage (akin to light storage) in (ballistic) graphene. Optical and spin physics for quantum optics in semiconductors Much progress has been made in the controlled manipulation of light and matter using isolated atomic systems., However, the complex environment of a solid-state system makes it significantly more challenging to achieve a similar degree of control. We have been exploring quantum optical control of electronic and spin degrees of freedom associated with impurity bound excitons and quantum dots in semiconductors. Such structures hold the promise to act like “artificial atoms” with excellent optical properties.
We are investigating specific properties such as homogeneity of optical frequencies and We also have been investigating the generation and probing of electronic spin coherence via optical excitation of donor bound excitons. Specific projects include a feasibility study of electromagnetically induced transparency (EIT) via bound excitons and electron spin states in Si, GaAs,and others, and its use for controlling light propagation. Future work will include investigation of collective properties of strongly interacting ensembles of excitons. While the above project has included collaboration with resesarchers from Stanford University and HP research laboratories, another avenue is followed in collaboration with I. Cirac's group in Munich (see “superradiance” above), concerning cooperative effects of nuclear spin polarization in diamond NV centers and quantum dots. Changing the mirror position in a cavity with a high frequency can lead to the creation of photons of half that frequency. This effect, known as “dynamic Casimir effect,” can also be seen by changing the effective index of refraction inside the cavity with the same frequency. So far, none of these effects have been seen experimentally, since the cavity lifetime has to be very long and at the same time the necessary frequencies have to be high enough, which is very challenging. We have been suggesting to use a two-level system inside the cavity with a transition frequency close to the cavity resonance. Changing this transition frequency (for example using a driving field on a second transition) has the effect of changing the index of refraction but can potentially be done very fast. We are suggesting several setups and architectures for this project, for example a “Cooper box” artificial atom inside a stripline cavity. A visiting student, A. Dodonov, from Sao Paolo, Brazil is collaborating with M. Lukin and me on this project. |
||
|
|
