We propose a technique that allows to laser cool a nanomechanical resonator mode to its motional ground state. The method is based on resonant laser excitation of a phonon sideband of an embedded self-assembled quantum dot. The strength of this sideband coupling is determined directly by the difference between the electron-phonon couplings of the initial and final states of the quantum dot (QD) optical transition. When compared with the analogous sideband-cooling of a trapped-ion (TI), we find novel quantum interference effects in the cooling cycle and that the finite Q-value can lead to regimes where the final occupancy is proportional to the initial one -- with their ratio determined by the product of the "effective Lamb-Dicke" parameter and the Q-value. The interactions underlying this cooling scheme also provide a tool-box for quantum state engineering in these systems.
We propose a technique that allows to laser cool a nanomechanical resonator mode to its motional ground state. The method is based on resonant laser excitation of a phonon sideband of an embedded self-assembled quantum dot. The strength of this sideband coupling is determined directly by the difference between the electron-phonon couplings of the initial and final states of the quantum dot (QD) optical transition. When compared with the analogous sideband-cooling of a trapped-ion (TI), we find novel quantum interference effects in the cooling cycle and that the finite Q-value can lead to regimes where the final occupancy is proportional to the initial one -- with their ratio determined by the product of the "effective Lamb-Dicke" parameter and the Q-value. The interactions underlying this cooling scheme also provide a tool-box for quantum state engineering in these systems.
Abstract We propose a ground state cooling scheme for an optomechanical resonator based on the system of a V-type three-level QD ensemble trapped in a two-mode cavity. The two-mode cavity was pumped by two different deriving fields. We studied the optomechanical cooling under the highly unresolved-sideband and beyond Lamb-Dicke regime (coupling strength between the cavity field and the phonon field not far less than the vibrational frequency). Our results show that the noise spectrum appeared a single peak when the detuning of the QD’s high excitation state and high-frequency driving field equals that of the QD’s low excitation state and low-frequency driving field, otherwise, then appeared double peaks. In the case of double peaks, we can realize the transition between optomechanical cooling and heating by changing the coupling strength of QD with cavity field, which is not available in the case of a single peak. We also studied the evolution of the mean phonon number with time, which is helpful to analyze ground state optomechanical cooling.
Summary form only given. The preparation of quantum systems in pure states is a prerequisite for coherent control. In this context laser cooling of trapped particles to the ground state of the confining potential is a powerful tool for preparing e.g. ions for quantum logic. In particular sideband cooling has been demonstrated to be the most efficient technique, allowing to achieve more than 99% of occupation of the ground state of the trap. This cooling technique relies on two conditions: the Lamb-Dicke regime, and the strong confinement regime. This latter condition, which allows the spectral resolution of individual sidebands of the motion, is however a limitation on the cooling time. We propose a model for cooling single ions inside a trap as an alternative to sideband cooling. The scheme uses electromagnetically induced transparency (EIT) in a /spl Lambda/-level scheme, where one of the transitions is dressed by a strong laser field. For detuning of the dressing laser, the absorption spectrum for the transition shows EIT when the cooling (probe) laser has detuning. Then, by tuning the cooling laser on the dark resonance (or EIT), for a suitable set of parameters and in the Lamb-Dicke regime the atoms are cooled to the ground state by the sideband transitions which are resonantly excited. The scheme achieves efficient ground state cooling and fast cooling rates using a continuous driving, instead of a pulsed one as in Raman sideband cooling schemes. An example is illustrated, where the average vibrational number is plotted as a function of the cooling time, and the final probability of occupation is plotted as a function of the vibrational state. This method may also be applied to reduce phase errors in coherent manipulations of trapped ions for quantum logic.
The coupling of nanomechanical resonators to controllable quantum systems represents a growing and promising area of research. Recent advances in this field have brought to astonishing results, such as the initialization of a mechanical resonator into its quantum ground state and even the preparation of non-classical coherent states of motion. These achievements open up appealing scenarios to quantum information technologies and to the exploration of the quantum-classical boundary. From an application perspective, such quantum-mechanical hybrid systems provide a versatile and attractive tool for a variety of precision measurements, like ultra-sensitive detection of force, mass, and displacement. \n \nQuantum control over a mechanical resonator, or, conversely, the prospect of using resonator's motion for probing quantum states, both involve some tight requirements. First of all, the interaction between the mechanics and its quantum partner has to be large on the scale of the decoherence rates of the coupled systems. In most cases, the coupling mechanism has to be activated by engineering the resonator with electrodes, magnets, or mirrors, or by using tailored laser fields. For this reason, the search for a strong coupling typically competes with the requirement of overcoming decoherence effects. In addition, for quantum effects to be observable, strong coupling has to be accompanied by the preparation of zero entropy initial states, for instance by cooling the resonator into its ground state of motion. \n \nThe motivation pursued by this thesis is to contribute to the inspiring field of hybrid systems by walking through each of the aforementioned directions. We in fact demonstrate a promising system in which optically active quantum dots, embedded in fully self-assembled core-shell nanowires, are coupled to the nanowire motion. Mechanical vibrations of the nanowire modulate the quantum dot emission energy over a broad range exceeding 14 meV, by means of deformation potential coupling. In our system, therefore, both the coupling mechanism and the quantum states themselves are intrinsic to the resonator's structure. Besides revealing unusually strong, such a built-in opto-mechanical interaction produces a hybrid system whose inherent coherence is unspoiled by any functionalization or external field and whose fabrication is simpler than top-down techniques. \n \nWe further demonstrate the use of a quantum point contact as a displacement transducer to measure and control the low-temperature thermal motion of a nearby micromechanical cantilever. We show that by including the QPC in a suitable feedback loop, we are able to cool the cantilever's fundamental mechanical mode down to the level of the measurement noise, achieving a squashing of the QPC noise at high gain. Due to its off-board design, our system is particularly versatile and suitable to force-sensing applications. Since the QPC transducer is sensitive to local modifications of the nearby electric field, our approach is in principle compatible with any nanoscale resonator, without requiring any functionalization to activate the coupling. By improving the performance of the QPC as a one-dimensional conductor and the cantilever-QPC capacitive coupling, our system has the potential to achieve quantum-limited displacement resolution and ground state cooling. We then report on some ongoing attempts to overcome the current limitations of our system and couple mechanical motion to different mesoscopic transport devices. \n \nWe remark that our demonstration of an as-grown quantum-dot-in-nanowire hybrid system opens up bright prospects on future experiments, as also testified by the very recent ferment of the scientific community around this topic. We find that the opto-mechanical coupling rate is not far from the nanowire mechanical frequency. This fact makes our system particularly promising for the quantum non-demolition readout of a quantum dot state through a measurement of the nanowire position. On the other hand, we show that the coupling can be further optimized, and the nanowire mechanical heating rate reduced, thus disclosing the captivating perspective of using quantum dots to probe and control the mechanical state of a nanowire. In this context, it would be important to investigate a possible spin-oscillator coupling in our system, given the long coherence time offered by a spin state in a quantum dot. The wide range of control over the quantum dots emission energy and the ability to tune two neighboring dots into resonance pave the way to mechanically induced emitter-emitter coupling. In addition, our monolithic opto-mechanical system constitutes a good candidate for recent proposals of a tripartite hybrid system, which would integrate an optical cavity, a quantum two-level system and a mechanical resonator. The intrinsically strong coupling between the quantum dots and the nanowire motion would enhance the interaction between the mechanics and the cavity field, leading for instance to efficient cooling of the resonator to the ground state. Finally, the quantum dot sensitivity to the nanowire resonant vibration could also support the use of our system as an integrated force probe or as a nanomechanical mass sensor. In other words, the results reported in this dissertation constitute not only a relevant proof-of-principle, moreover they open up intriguing challenges for future research.
We propose a rapid ground-state optomechanical cooling scheme in a hybrid system, where a two-level quantum dot (QD) is placed in a single-mode cavity and a nanomechanical resonator (NMR) is also coupled to the cavity via radiation pressure. The cavity is driven by a weak laser field while the QD is driven by another weak laser field. Due to the quantum destructive interference arisen from different transition channels induced by simultaneously driving the QD-cavity system in terms of the two different lasers, two-photon absorption for the cavity field can be effectively eliminated by performing an optimal quantum interference condition. Furthermore, it is demonstrated that the QD-cavity system can be unbalancedly prepared in two single-polariton states with different eigenenergies. If the frequency of the NMR is tuned to be resonant with transition between two single-polariton states, it is found that a fast ground-state cooling for the NMR can also be achieved, even when the QD-cavity system is originally in the moderate-coupling regime. Thus the present ground-state cooling scheme for the NMR may be realized with currently available experimental technology.
Although they have enabled several advances in the field of optomechanics, optomechanical disk resonators have not yet been operated in the quantum regime. We present the first experimental demonstration of an optomechanical disk resonator prepared in the quantum ground state. With a gigahertz frequency, the mechanical breathing mode of the investigated semiconductor disk reaches a level of excitation below a single phonon when cooled in a dilution refrigerator. We quantify the phonon occupancy of the mechanical mode by performing Brillouin sideband spectroscopy: a conical optical fiber is evanescently coupled to the disk optical whispering-gallery mode, and Stokes and anti-Stokes photons scattered by phonon emission and absorption are counted on a single-photon detector. We measure a suppression of the absorption process corresponding to a phonon occupancy of 0.66±0.20. We experimentally investigate the mechanisms ruling laser-induced heating, which limits the lowest measurable phonon occupancy, and notably witness an extra-cavity heating effect.
In this thesis novel methods for the laser cooling of quantum systems are presented. The use of quantum interference allows for the tailored cancelation of heating processes, so that an approximation to a cooling operator is possible that does not rely on the rotating wave approximation. This makes these schemes considerably faster and more efficient than existing ground state cooling methods, and allow for a significant relaxation of current experimental constraints. Several approaches are investigated in different systems. On the one hand, a special laser configuration, applicable to trapped ions, atoms or cantilevers, generates a double dark state that eliminates both the blue sideband and the carrier transition. As a consequence, vanishing phonon occupation up to first order in the perturbative expansion is achieved. Underlying this scheme is a combined action of two cooling schemes which makes the proposal very stable under parameter fluctuations. Its suitability as a cooling scheme for several ions in a trap or for a cloud of atoms in a dipole trap is shown. On the other hand, a pulsed cooling scheme for optomechanical systems is presented. It can be implemented for both strongly and weakly coupled optomechanical systems in both weakly and highly dissipative cavities. Its underlying mechanism is based on interferometric control of optomechanical interactions, and its efficiency is demonstrated with pulse sequences that are obtained by using methods from optimal control. Finally, it is shown how this pulsed method can be combined with continuous measurement to drive mechanical oscillators to highly squeezed steady states. Its mechanism relies on the modification of the dissipation and measurement terms, which drive the system towards a specific quadrature eigenstate. The scheme is robust to measurement inefficiencies and works also with highly dissipative cavities, which makes it accessible to implementation with state of the art technology.