SUSANNE YELIN Professor of Physics
University of Connecticut U-3046
Room No: P-111
Control over propagation and scattering of light fields plays a central role in optical science and photonics. In particular, it is well known that strongly modified linear and nonlinear optical responses are exhibited by emitters at their resonance. Recently, it has been shown that thin 2D metasurfaces can drastically alter the transmitted field by enabling spatial control of its amplitude, phase and polarization. As a rule, the separation between the microfabricated array elements is typically much smaller than the operating wavelength of the light.
Opposed to that, we are exploring light scattering from ordered and dilute arrays of atoms, with a lattice constant of the order of a wavelength, as can be realized, e.g., using ultra-cold atoms loaded into optical lattices. In such a case near-resonant operation can still lead to strong scattering. This still allows the possibility of strong scattering in dilute media, based on the important effect of interactions between the atomic dipoles.
Experimental realizations using ultracold arrays of trapped atoms and excitons in 2D semiconductor materials can be imagined as well as potential applications ranging from atomically thin metasurfaces to single photon nonlinear optics and nanomechanics.
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, using mostly coherent methods. 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.
These ideas can also be utilized in a many-body context: one idea is to utilize the dipole-dipole interactions present between polar molecules 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 transistors and is the signature of “optical quantum computing.” Another idea is to use molecular dipoles aligned in-plane in an optical lattice to study phases that arise from non-isotropic interactions. Even with a quasi-1D setup, this could lead to frustration and topologically interesting phases.
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.
Collaborations and comparisons with experimental work have been done in various novel experimental systems, such as an ultracold gas of Rydberg atoms (with Gould, Eyler at UConn), molecules in the electronic ground state with regard to radiative decay of vibrational states, which is very slow due to their terahertz-range energy differences (with Weidemüller, Heidelberg). The questions here are whether (i) superradiance can help cool the vibrational states, (ii) their nearly clean harmonic nature can introduce interesting variations of the usual superradiance patterns, and (ii) the collective nature of superradiant (“cooperative”) effects allows for manipulation like laser cooling. Lately, we have been collaborating with the group of Field (MIT) on a strongly superradiating gas of Ba Rydberg atoms, where, for the first time in a dense strongly excited medium, cooperative shifts have been seen.
In collaboration with the Cirac group in Munich, we have been exploring superradiant effects of nuclear spin polarization. It turns out that the effective, electron-spin mediated and mostly distance-independent interaction between nuclear spins shows the same basic signatures as the more traditional dipole-dipole interaction. This is expected to be the case for any other type of interaction as well.
Finally, it turns out that entanglement and entanglement classes can be studied in such. First of all, we have been showing, that, while superradiance all by itself does neither create nor need entanglement, in conjunction with other elements such as an external driving field, entanglement can be well controlled. For example, it is possible to spin squeeze an ensemble of atoms this way. (While a formal proof for this exists so far only for idealized Dicke samples, it can be shown that this is an effects that is still present in a realistic system with dipole-dipole interactions.) Starting from that, we are now working on entanglement classes and phase transitions in a cooperative system as a prime example of open non-equlibrium systems.
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 such example, (with Cirac, Munich) involves using collective excitation in Coulomb crystals in an optical cavity in order to produce single-photon nonlinearities.
A particular example, however, are materials with unusual light refraction properties, in particular with a negative index of refraction. 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. One solution, however, is to build custom-made arrays of electric and magnetic dipoles, metamaterials, a research direction which has grown immensely in the last two decades. 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 electromagnetically induced transparency. This research has been carried out in collaboration with the groups of Fleischhauer (Kaiserslautern) and Walsworth (Harvard). While this is an interesting and promising approach, it suffers from two major drawbacks regarding possible experimental application: first, it needs very specialized atomic level systems that can only be found in atoms (or molecules) involving very high relativistic effects, and second, the densities needed are forbidding. We have thus been studying an extension to this work, involving (phase-coherent) four-wave mixing schemes to alleviate possibly both of these problems.
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 color centers in and quantum dots in semiconductors and similar materials. Such structures hold the promise to act like “artificial atoms” with excellent optical properties. We are investigating electromagnetic transitions and electronic spin coherence, for example for so-called nitrogen-vacancy centers in diamond crystals.
In such systems, coherence effects, in this case in the guise of so-called “coherent population trapping” (CPT), can be utilized to influence nuclear spins in condensed matter systems such as diamond NV-centers or quantum dots. In particular, CPT can lead to substantial decrease of the line broadening always accompanies otherwise coherent transitions in such systems. CPT (and, in some cases, the correct repumping techniques) can trap all the nuclear spins in the close environment of the NV center or quantum dot in a single state, and thus alleviate effective random magnetic Overhauser fields and thus broadening. This work happens in collaboration with Giedke and Cirac (Munich) as well as Imamoglu (Zurich).
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 “dynamical Casimir effect,” can also be seen by changing the effective index of refraction inside the cavity with the same frequency. So far, these effects have been seen experimentally only in a marginal case, 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.
More importantly, one of the limiting factors in traditional setups for the dynamical Casimir effect has been the fact, that all these processess produce photons of about half the frequency of the modulation frequency, which makes seeing this effect for optical freqencies all but impoosible. We have been exploring a new set of resonances that allow very different frequency regimes and might result in much simpler experimental setups.