Cold ions in traps are well-established, highly controllable systems with a wide variety of applications in quantum science, precision spectroscopy, clocks, and chemistry. Nanomechanical oscillators are used in advanced sensing applications and for exploring the border between classical and quantum physics. Here, we report on the implementation of a hybrid system combining a metallic nanowire with laser-cooled ions in a miniaturized ion trap. Operating the experiment in the classical regime, we demonstrate resonant and off-resonant coupling of the two systems and the coherent motional excitation of the ion by the mechanical drive of the nanowire. The present results prove the feasibility of mechanically coupling ions to nanooscillators and open up avenues for mechanically manipulating the motion of trapped ions as well as for the development of ion-mechanical hybrid quantum systems.
Cold ions in traps are well-established, highly controllable systems with a wide variety of applications in quantum science, precision spectroscopy, clocks, and chemistry. Nanomechanical oscillators are used in advanced sensing applications and for exploring the border between classical and quantum physics. Here, we report on the implementation of a hybrid system combining a metallic nanowire with laser-cooled ions in a miniaturized ion trap. Operating the experiment in the classical regime, we demonstrate resonant and off-resonant coupling of the two systems and the coherent motional excitation of the ion by the mechanical drive of the nanowire. The present results prove the feasibility of mechanically coupling ions to nanooscillators and open up avenues for mechanically manipulating the motion of trapped ions as well as for the development of ion-mechanical hybrid quantum systems.
We study theoretically the mechanical drive of a trapped ultracold ion by a charged nanowire through their mutual Coulomb interaction. We characterize the perturbation of the trapping potential for the ion by the nanowire and discuss the parameters determining the dynamics of the ion under the action of the nanooscillator. We explore the classical dynamics as well as motional quantum states of the ion which can be generated and manipulated with the resonant drive of the nanowire and the effects of anharmonicities of the ion-trap potential on the system. Our modelling indicates that unusual quantum states of the ion motion can be generated with this approach and that sympathetic cooling and quantum entanglement can be realised when both subsystems operate in the quantum regime. The present ion-mechanical hybrid system might prove interesting as a new quantum device, for quantum sensing experiments, for spectroscopy and for mass spectrometry.
Hybrid systems aim toward the combination of different types of physical platforms exhibiting complementary properties and advantages to develop new applications and technologies. In a recent publication [Weegen et al., Phys. Rev. Lett. 133, 223201 (2024)], we demonstrated a hybrid system consisting of two coupled oscillators: laser-cooled atomic Ca+ ions confined in a miniaturized linear radio frequency trap and a charged Ag2Ga nanowire. Here, we detail the design considerations and experimental implementation of the system and characterize the effect of the nanowire assembly on the trapping potential for ions using numerical simulations and experiments. Moreover, we further detail the resonant excitation of trapped ions by the mechanical drive of the nanowire.
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.
Numerous efforts are currently aiming at the implementation of hybrid quantum systems in the quest for developing new devices for quantum technologies. The motivation for a solid state-atomic interface emerges from the possibility to couple otherwise incompatible platforms in order to combine the advantages of both in a complementary fashion. Under this scope, we report in this thesis the implementation of a new quantum device for the interface of an ultracold trapped ion and a conductive nanowire. The experimental measurements obtained here seem promising for the realization of the hybrid dynamics of the ion-nanowire hybrid system. Additionally, we performed numerical calculations in order to characterize the perturbation of the trapping potential for the ion by the nanowire. From our classical and quantum dynamics calculations we explored possibilities of generating Gaussian and non-Gaussian quantum states of the ion from the nanowire drive. Our modelling indicated, also, that sympathetic cooling and quantum entanglement can be realized when both subsystems operate in the quantum regime. The present ion-nanowire hybrid system might prove promising as a new quantum device for quantum information and quantum sensing experiments, for spectroscopy and for mass spectrometry.
Cavity optomechanics represents a frontier research direction in quantum science and technology, centered on the control and exploration of interactions between electromagnetic fields and mechanical motion. Rooted in quantum mechanics, this field realizes efficient coupling and precise manipulation of optical fields and mechanical vibrations through the structural design and precise regulation of optical microcavities and mechanical oscillators, thereby revealing and harnessing novel quantum phenomena. In recent years, cavity optomechanics has evolved into an interdisciplinary frontier integrating quantum optics, condensed matter physics, and quantum precision measurement, exhibiting profound application in both fundamental physics research and quantum information science. With advancements in technologies such as nanomanufacturing and laser cooling, optomechanical interactions have been successfully demonstrated in various experimental systems. Among these platforms, cavity cold-atom systems stand out as one of the ideal platforms for implementing quantum optomechanics. Featuring exceptional environmental isolation, long quantum coherence times, and strong light-matter interaction, these systems provide a crucial testbed for exploring strong and even ultrastrong optomechanical coupling effects as well as rich nonlinear quantum phenomena. This paper reviews the recent progress in optomechanical control and manipulation based on cavity cold-atom systems. We first outline the fundamental principles of standard cavity optomechanical systems. Then, we describe the experimental realization of linear and nonlinear optomechanical couplings in cold-atom systems. After that, we focus on the representative applications of this platform in high-precision quantum sensing, quantum memory, and the preparation of macroscopic nonclassical states. Finally, we give an outlook and challenges in this field.
This dissertation details of the construction of and experiments performed in a hybrid system consisting of a cold neutral atomic magneto-optical trap (MOT) contained within a linear radio-frequency quadrupole ion trap (LQT). The combination of these two workhorses of atomic physics facilitates a variety of developing science such as the controlled investigation of ion-neutral quantum chemistry and the production of ground-state molecular ions. While the primary focus of this work is the production of ultracold molecular ions via sympathetic cooling, there has been investigation of cold (T ~ mK) ion-neutral charge exchange processes in two different ion-neutral pair systems. In addition, a possible method of producing ultracold homonuclear molecular ions via a photo-associative ionization (PAI) pathway is studied in the MOT. The LQT traps ions in spatial overlap with a 40Ca MOT constituting an ultracold buffer gas purposed to sympathetically cool molecular ions.An effective, general method of producing ground-state molecular ions would open a field of physics allowing, for example, precise measurements of molecular transitions which are uniquely sensitive to parity violation, or the possible variation of fundamental constants. Another promising application of cold molecular ions is an implementation of a quantum computing architecture by coupling micro-fabricated strip-line resonators to the microwave rotational transition found in many diatomic molecular ions. The molecular ion BaCl+ is chosen to be used in proof-of-principle sympathetic cooling experiments. Ultimately, a vibrational internal state thermometry experiment shows that the Ca MOT performs very efficiently at quenching the vibrational motion of the BaCl+ molecular ion.As a product of overcoming experimental challenges, this thesis discusses several experiments to characterize trapping and reaction dynamics in the hybrid system. For example, charge transfer measurements are performed with two different laser-cooled atomic ion species, Yb+ and Ba+, which are allowed and disallowed ground-state reaction, respectively. Controlled measurements of these inelastic processes test the cutting edge of quantum theory of ion-neutral interaction in both the ground-state and under optical excitation.