Almost 60 years ago Dicke introduced the term superradiance to describe a signature quantum effect: N atoms can collectively emit light at a rate proportional to N(2). Structures that superradiate must also have enhanced absorption, but the former always dominates in natural systems. Here we show that this restriction can be overcome by combining several well-established quantum control techniques. Our analytical and numerical calculations show that superabsorption can then be achieved and sustained in certain simple nanostructures, by trapping the system in a highly excited state through transition rate engineering. This opens the prospect of a new class of quantum nanotechnology with potential applications including photon detection and light-based power transmission. An array of quantum dots or a molecular ring structure could provide a suitable platform for an experimental demonstration.
Almost 60 years ago Dicke introduced the term superradiance to describe a signature quantum effect: N atoms can collectively emit light at a rate proportional to N(2). Structures that superradiate must also have enhanced absorption, but the former always dominates in natural systems. Here we show that this restriction can be overcome by combining several well-established quantum control techniques. Our analytical and numerical calculations show that superabsorption can then be achieved and sustained in certain simple nanostructures, by trapping the system in a highly excited state through transition rate engineering. This opens the prospect of a new class of quantum nanotechnology with potential applications including photon detection and light-based power transmission. An array of quantum dots or a molecular ring structure could provide a suitable platform for an experimental demonstration.
The counterintuitive properties of quantum mechanics have the potential to produce revolutionary new technology. The applications of these devices are both vital and diverse: the efficient generation of energy from light, sensing and measuring with exquisite precision, and information processing with unparalleled speed. In this thesis, I use the theory of open quantum systems to investigate quantum technologies for enhanced sensing and light absorption. In the first research chapter, we develop a new method for describing qubit dynamics in the Rabi model. We obtain a new expression for the ac Stark shift, which enables practical and precise qubit thermometry of an oscillator. In the second research chapter, we demonstrate that it is possible to invert the phenomenon of Dicke Superradiance using nanostructures and quantum control. This creates the possibility of a new class of quantum light absorption technologies with a super-linear scaling in the absorption rate. In the final research chapter, we investigate another means of enhancing light absorption. We show that phonon assisted transitions to ratchet states in rings allow absorbed excitions to be protected from reemission.
Collective behaviour of the components of a quantum system can significantly alter the rates of dynamical processes within the system. A paradigmatic collective effect is superradiance, the enhancement in the rate that radiation is emitted by a group of emitters relative to that emitted by independent emitters. Less studied are collective effects in energy transport, notably supertransfer, the enhancement of the rate of energy transfer from donors to acceptors due to delocalised excitations. Despite its generality as a collective enhancement mechanism in transport, there has been no direct experimental detection of supertransfer because, in most physical platforms where delocalisation arises naturally, delocalisation cannot be turned on and off to evaluate its effect on energy transfer. Here, we show that supertransfer could be directly observed using a quantum device based on a superconducting circuit. The programmability and control offered by an engineered device would allow controllable delocalisation of quantum states, giving full tunability over supertransfer. Our guidelines for engineering supertransfer could inform the design of future quantum-enhanced devices.
This work focuses on the many-body properties of semiconductor nanostructures for quantum engineering of infrared optoelectronic devices. In particular, we theoretically investigate the optical response accounting for collective effects in systems of tunnel-coupled quantum wells. A clear manifestation of these effects appears in the optical spectrum of highly doped quantum wells, where the absorption peaks are at completely different energies with respect to the single-particle electronic transitions. We calculate light-matter interaction in two steps. First, we consider the microscopic polarization associated with the electronic transitions between confined levels of the wells, which are all coupled by dipole-dipole Coulomb interaction. Then we calculate the interaction of the resulting collective states with the electromagnetic field. The absorption spectrum is finally expressed in terms of microscopic currents, describing the collective charge oscillations. The theoretical model is applied to a series of relevant systems, and its outcomes are compared with experimental results. As the collective states are issued from the coherent superposition of several electronic excitations, they have the properties of superradiant states. They are thus a promising entity for the realization of efficient light emitters in the mid- and far-infrared frequency range.
The rate at which matter emits or absorbs light can be modified by its environment, as markedly exemplified by the widely studied phenomenon of superradiance. The reverse process, superabsorption, is harder to demonstrate because of the challenges of probing ultrafast processes and has only been seen for small numbers of atoms. Its central idea—superextensive scaling of absorption, meaning larger systems absorb faster—is also the key idea underpinning quantum batteries. Here, we implement experimentally a paradigmatic model of a quantum battery, constructed of a microcavity enclosing a molecular dye. Ultrafast optical spectroscopy allows us to observe charging dynamics at femtosecond resolution to demonstrate superextensive charging rates and storage capacity, in agreement with our theoretical modeling. We find that decoherence plays an important role in stabilizing energy storage. Our work opens future opportunities for harnessing collective effects in light-matter coupling for nanoscale energy capture, storage, and transport technologies.
An exciting frontier in quantum information science is the integration of otherwise "simple" quantum elements into complex quantum networks. The laboratory realization of even small quantum networks enables the exploration of physical systems that have not heretofore existed in the natural world. Within this context, there is active research to achieve nanoscale quantum optical circuits, for which atoms are trapped near nano-scopic dielectric structures and "wired" together by photons propagating through the circuit elements. Single atoms and atomic ensembles endow quantum functionality for otherwise linear optical circuits and thereby enable the capability of building quantum networks component by component. Toward these goals, we have experimentally investigated three different systems, from conventional to rather exotic systems : free-space atomic ensembles, optical nano fibers, and photonics crystal waveguides. First, we demonstrate measurement-induced quadripartite entanglement among four quantum memories. Next, following the landmark realization of a nanofiber trap, we demonstrate the implementation of a state-insensitive, compensated nanofiber trap. Finally, we reach more exotic systems based on photonics crystal devices. Beyond conventional topologies of resonators and waveguides, new opportunities emerge from the powerful capabilities of dispersion and modal engineering in photonic crystal waveguides. We have implemented an integrated optical circuit with a photonics crystal waveguide capable of both trapping and interfacing atoms with guided photons, and have observed the collective effect, superradiance, mediated by the guided photons. These advances provide an important capability for engineered light-matter interactions, enabling explorations of novel quantum transport and quantum many-body phenomena.
We report experimental realization of superabsorption as a time reversed process of superradiance. We utilize the single-atom superradiance experimental setup based on the cavity-QED microlaser, where two-level atoms prepared in the same superposition state traverse a high-Q cavity one by one. Without an input field to the cavity, the system generates a superradiant output field with a phase determined by that of the atomic superposition state. By injecting an input field to the cavity whose phase is opposite to that of the superradiant field that atoms would emit without the input field, we can then reverse the superradiance process to realize superabsorption. In our experiment, the number of photons completely absorbed during a fixed time interval was proportional to the square of the number of atoms in the cavity. The latest experimental data and analysis will be presented.