Quantum Systems
A number of platforms are being pursued for developing technologies that exploit the enhanced sensing and measurement capabilities of quantum mechanics. Hybrid systems offer the flexibility of combining and optimizing different platforms. Hong et al. combined optomechanical control of motion and single-phonon counting techniques to probabilistically generate a single-phonon Fock state within a nanomechanical resonator. Chu et al. used electromechanical coupling to address a bulk piezoelectric resonator with a superconducting quantum circuit. Both approaches hold promise for developing hybrid quantum technologies.
Science , this issue p. [203][1], p. [199][2]
[1]: /lookup/doi/10.1126/science.aan7939
[2]: /lookup/doi/10.1126/science.aao1511
Quantum Systems
A number of platforms are being pursued for developing technologies that exploit the enhanced sensing and measurement capabilities of quantum mechanics. Hybrid systems offer the flexibility of combining and optimizing different platforms. Hong et al. combined optomechanical control of motion and single-phonon counting techniques to probabilistically generate a single-phonon Fock state within a nanomechanical resonator. Chu et al. used electromechanical coupling to address a bulk piezoelectric resonator with a superconducting quantum circuit. Both approaches hold promise for developing hybrid quantum technologies.
Optomechanics
The ideal platform for quantum-computing and quantum-sensing applications is likely to be a hybrid system that combines the best features of different components. Superconducting circuits are relatively advanced, and finding components that can control and manipulate the microwaves will be essential. MacCabe et al. explored the use of high-quality microresonators in which the acoustic environment could be engineered such that the phonon lifetime could be extended to more than 1 second. Operating at microwave frequencies of 5 gigahertz, these quantum acoustic-dynamic devices could be coupled with superconducting circuits.
Science , this issue p. [840][1]
[1]: /lookup/doi/10.1126/science.abc7312
Quantum Physics
The development of hybrid quantum systems provides a flexibility that allows for various components to be coupled together, thereby expanding the opportunity to build quantum sensors and devices that can be designed for specific purposes. Key to doing so is being able to strongly couple the different components. Most developments to date have relied on the components being in close proximity, which can hamper design flexibility. Karg et al. used a laser to induce strong coupling between a cloud of atoms and an optomechanical membrane. With the components separated by 1 meter, this approach demonstrates a methodology of coupling quantum systems and easing up restrictions on spatial proximity.
Science , this issue p. [174][1]
[1]: /lookup/doi/10.1126/science.abb0328
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.
Superconducting qubits, realized by incorporating Josephson junctions into superconducting circuits, behave as artificial atoms with anharmonic energy spectra and can be precisely controlled and measured using microwave cavities within the framework of circuit quantum electrodynamics (cQED). Since its emergence in the early 2000s, cQED has established superconducting qubits as leading candidates for scalable quantum devices and has enabled the exploration of hybrid quantum systems that integrate disparate physical platformsThis review surveys superconducting hybrid quantum electromechanical systems in which mechanical resonators are coupled to superconducting qubits, with a focus on two widely used qubit platforms: the transmon and the fluxonium. We provide an overview of the underlying coupling mechanisms arising from interactions through the phase and charge degrees of freedom of the qubit, and discuss how these mechanisms give rise to both longitudinal and transverse qubit-mechanical interactions. We further review extensions of electromechanical platforms to electro-optomechanical architectures, in which optical cavities are integrated to enable coherent interfacing between superconducting circuits and optical photons. This review aims to present a unified framework and perspective on qubit-mechanical and qubit-mechanical-optical hybrid systems in superconducting quantum technologies and applications related to sensors.
There has recently been a surge of interest in the study of mechanical systems near the quantum limit. Such experiments are motivated by both fundamental interest in studying quantum mechanics with macroscopic engineered systems and potential applications as ultra‐sensitive transducers, or even in quantum information processing. A particularly promising system is a microwave cavity optomechanical system, in which a nanomechanical resonator is embedded within (and capacitively coupled to) a superconducting microwave cavity. Here we discuss two schemes for the generation and measurement of quantum states of the nanomechanical resonator. A quantum squeezed state may be generated via mechanical parametric amplification, while a number state may be conditionally generated via continuous measurement and feedback control mediated by a superconducting qubit.
The recently emerged field of quantum technology is aiming to employ quantum coherence and entanglement for realization of next generation sensors, standards, imaging systems, secure communication and computers.Although applications of quantum technologies cover a broad spectrum and their development stage varies from early demonstrations to commercially available devices, many challenges have been identified.The first includes tailoring materials for quantum technologies, which is playing an essential role in technological applications of quantum science.Solid state systems allowing long coherence time are essential for quantum computation and quantum sensing.Ultrapure materials with tailored isotopic content are important for reaching long coherence time of spin qubits (Awschalom et al., 2013).The ability to place single dopants with high precision and form spin qubits at desired location is essential for both quantum computing and quantum sensing applications (McCallum et al., 2012;Smith et al., 2019).The performance of different types of qubits is usually benchmarked in terms of controllability and coherence time.Usually solid-state systems allow fast control, but exhibit fast decoherence owing to complex environment.It is therefore important to search for new types of qubits combining isolation from environment and access via fast coherent control and readout.Novel approaches combining different quantum systems into build hybrid quantum devices for optimal performance is a promising avenue.Examples of such hybrid approaches are spin systems coupled to superconducting qubits (Kubo et al., 2011) and hybrid nuclear-electronic qubits (Morley et al., 2013).Optimal protocols for quantum technologies requires extension of coherence time of qubits beyond the coherence time of the isolated quantum system.The efficient protection toolkit includes dynamical decoupling (Yang et al., 2011) and quantum error correction (Terhal, 2015) based techniques.Although general principles of qubits protection were developed and tested experimentally in different model environments, it is essential to adapt them to realistic environmental noise.In the field of quantum sensing, it is also essential to combine protections against noise with non-reduced sensing performance.Experimental imperfections can be addressed using optimal control tools (Glaser et al., 2015).The development of efficient quantum algorithms is another growing field belonging to quantum software area.On one hand, it is essential to find problems where a quantum computer can outperform classical computers.On the other hand, it is essential to develop an application scenario for a limited number of qubits (Montanaro, 2016).In addition, in order to develop new algorithms, future work must include discoveries of application scenarios of already known algorithms.Application of quantum Fourier transform for sensing is a promising example of such new applications (Vorobyov et al., 2021).Signal processing is another field of quantum software that is becoming essential in applications.Advanced signal analysis protocols, like quantum compressed sensing, can be employed to quantify entanglement in large quantum devices via efficient quantum tomography (Riofrío et al., 2017).