We realize the quantum regime of a surface acoustic wave (SAW) resonator by demonstrating vacuum Rabi mode splitting due to interaction with a superconducting artificial atom. Reaching the quantum regime is physically difficult and technologically challenging since SAW devices consist of large arrays of narrow metal strips. This work paves the way for realizing analogues of quantum optical phenomena with phonons and can be useful in on-chip quantum electronics.
We realize the quantum regime of a surface acoustic wave (SAW) resonator by demonstrating vacuum Rabi mode splitting due to interaction with a superconducting artificial atom. Reaching the quantum regime is physically difficult and technologically challenging since SAW devices consist of large arrays of narrow metal strips. This work paves the way for realizing analogues of quantum optical phenomena with phonons and can be useful in on-chip quantum electronics.
Photon blockade via a giant self-Kerr nonlinearity in an optical resonator induced by an N-type quantum system is one of the main routines towards photon-based quantum information processing. However, such an N-type system for phonons is lacking. We propose an effective N-type quantum system for phonons, forming from a superconducting transmon qubit and a microwave resonator. In the absence of any external driving, the quantum vacuum-qubit interaction causes the surface acoustic wave (SAW) phonon blockade. Such a vacuum-induced phonon blockade paves the way to prepare single SAW phonons from a weak coherent phononic state for SAW accessible superconducting quantum computation.
The past 20 years has seen rapid developments in circuit quantum electrodynamics, where superconducting qubits and resonators are used to control and study quantum light-matter interaction at a fundamental level. The development of this field is strongly influenced by quantum information science and the prospect of realizing quantum computation, but also opens up opportunities for combinations of different physical systems and research areas. Superconducting circuits in the microwave domain offer a versatile platform for interfacing with other quantum systems thanks to strong nonlinearities and zero-point fluctuations, as well as flexibility in design and fabrication. Hybrid quantum systems based on circuit quantum electrodynamics could enable novel functionalities by exploiting the strengths of the individual components.This thesis covers experiments coupling superconducting circuits to surface acoustic waves (SAWs), mechanical waves propagating along the surface of a solid. Strong coupling can be engineered using the piezoelectric properties of GaAs substrates, and our experiments exploit this to investigate phenomena in quantum field-matter interaction. A key property of surface acoustic waves is the slow propagation speed, typically five orders of magnitude slower than light in vacuum, and the associated short wavelength. This enables the giant atom regime where the artificial atom in the form of a superconducting circuit is large compared to the wavelength of interacting SAW radiation, a condition which is difficult to realize in other systems. Experiments described in this thesis use these properties to demonstrate electromagnetically induced transparency for a mechanical mode, as well as non-Markovian interactions between an artificial giant atom and the SAW field. When the SAW field is confined to a resonant cavity, the short wavelength allows multimode spectra suitable for interacting with a frequency comb. We use a multimode SAW resonator to characterize the ensemble of microscopic two-level system defects with a two-tone spectroscopy approach. Finally, we introduce a hybrid superconducting-SAW resonator with applications in quantum information processing in mind. Experiments with this device demonstrate entanglement of SAW modes, and show promising results on the way to engineer cluster states for quantum computation in continuous variables.
Artificial atoms coupled to surface acoustic waves (SAWs) have played a crucial role in the recent development of circuit quantum acoustodynamics. In this paper, we have investigated the interaction of an artificial atom and SAWs beyond the weak-coupling regime, focusing on the role of the interdigital transducer (IDT) that enables the coupling. We find a parameter regime in which the IDT acts as a cavity for the atom, rather than an antenna. In other words, the atom forms its own cavity. Similar to an atom coupled to an explicit cavity, this regime is characterized by vacuum-Rabi splitting, as the atom hybridizes with the phononic vacuum inside the IDT. This hybridization is possible because of the interdigitated coupling, which has a large spatial extension, and the slow propagation speed of SAWs. We work out a criterion for entering this regime from a model based on standard circuit-quantization techniques, taking only material parameters as inputs. Most notably, we find this regime hard to avoid for an atom on top of a strong piezoelectric material, such as lithium niobate (LiNbO3). The SAW-coupled atom on top of LiNbO3 can thus be regarded as an atom-cavity-bath system. On weaker piezoelectric materials, the number of IDT electrodes needs to be large in order to reach this regime.
Surface acoustic waves (SAWs) are mechanical vibrations that propagate on the surface of solids while dissipating little power, consequently enabling them to propagate freely over long distances. The speed and wavelength of SAWs are reduced a five order of magnitude compared to when light is used as a carrier at gigahertz frequencies. The unique properties of SAWs combined with the possibility to let them interact with artificial atoms, discovered and shown for the very first time in appended Paper I of this thesis, open up for exploration of new regimes of quantum physics. The appended Paper II is a book chapter providing an overview of many of the new areas of research, as well as going into depth of the method and significance of the results ofthe appended Paper I.The essential interaction between artificial atoms and SAWs was further investigated by using Autler-Townes splitting to achieve fast control of the interactions. The appended Paper IV, shows a transmitted field extinction of 80 %, and provides proof of concept for a SAW router in the quantum regime. In addition, due to the artificial atom's highly frequency dependent coupling to SAWs, electromagnetically induced transparency (EIT) could be observed in the appended Paper V. Furthermore, the EIT region was distinguished from the Autler-Townes splitting region by a threshold in the applied power. The results produce parallel findings to quantum optics, but are perhaps best described as part of a different field, quantum acoustics.Among the many possible areas of research emerging as an outcome of this work, a variety of potential quantum experiments would benefit greatly from a higher conversion efficiency between electric signals and SAWs. Due to this, focus was put on improving this conversion efficiency by studying superconducting unidirectional transducers (UDTs), making use of advances in classical SAW devices. The appended Paper III shows that 99.4~\\% of the acoustic power can be focused in a desired direction and that the conversion between electric signals and SAWs is greatly improved by using UDTs, thereby eliminating the largest source of loss of symmetric inter-digital transducers. There is, however, a trade-off between conversion efficiency and bandwidth. This finding allows tailoring of quantum experiments based on SAWs that may pave the way towards measuring quantum sound.
Abstract The experimental investigation of quantum devices incorporating mechanical resonators has opened up new frontiers in the study of quantum mechanics at a macroscopic level. It has recently been shown that surface acoustic waves (SAWs) can be piezoelectrically coupled to superconducting qubits, and confined in high-quality Fabry–Perot cavities in the quantum regime. Here we present measurements of a device in which a superconducting qubit is coupled to a SAW cavity, realising a surface acoustic version of cavity quantum electrodynamics. We use measurements of the AC Stark shift between the two systems to determine the coupling strength, which is in agreement with a theoretical model. This quantum acoustodynamics architecture may be used to develop new quantum acoustic devices in which quantum information is stored in trapped on-chip acoustic wavepackets, and manipulated in ways that are impossible with purely electromagnetic signals, due to the 10 5 times slower mechanical waves.
Abstract We experimentally study a circuit quantum acoustodynamics system with a superconducting artificial atom coupled to both a two-dimensional surface acoustic wave resonator and a one-dimensional microwave transmission line. The strong coupling between the artificial atom and the acoustic wave resonator is confirmed by the observation of the vacuum Rabi splitting at the base temperature of dilution refrigerator. We show that the propagation of microwave photons in the microwave transmission line can be controlled by a few phonons in the acoustic wave resonator. Furthermore, we demonstrate the temperature effect on the measurements of the Rabi splitting and temperature induced transitions from high excited dressed states. We find that the spectrum structure of two-peak for the Rabi splitting could become into those of several peaks under some special experimental conditions, and gradually disappears with the increase of the environmental temperature T. The continuous quantum-to-classical crossover is observed around the crossover temperature T c, which is determined via the thermal fluctuation energy k B T and the characteristic energy level spacing of the coupled system. Experimental results agree well with the theoretical simulations via the master equation of the coupled system at different effective temperatures.