One of the main thrusts of quantum science over the past few decades has been the developmentof quantum networks for the purposes of secure communication, enhanced detector sensitivity, and advanced computing. Realizing such a network of superconducting quantum processors that communicate via optical fibers would leverage the high-fidelity quantum signal processing of superconducting circuits and the thermal robustness of infrared light but requires a transducer capable of connecting these two sections of the electromagnetic spectrum separated by five orders of magnitude in energy. This thesis explores the optimization of a transducer architecture where the mechanical mode of a Si3N4 membrane mediates the coupling of a superconducting lumped element circuit and a Fabry-Perot optical cavity. We aim to maximize the coupling of these three harmonic oscillator to each other while shielding them from noisy processes that would decohere quantum signals. This architecture has led to transducers with unparalleled efficiency and continuous operation. Enhanced cooperativity between the optical cavity and mechanical oscillator enabled optically-detected readout of a superconducting qubit and optomechanical ground state cooling with negligible laser-induced heating of the superconducting qubit and microwave circuit. To surpass the threshold for quantum-enabled operation, we subsequently improved the cooperativity between the microwave circuit and mechanical oscillator by reducing the microwave loss and noise from two-level-system-like defects in the Si3N4 dielectric. When combined with enhanced coupling between the circuit and membrane or improved mechanical isolation, we project that this architecture will be capable of transducing quantum signals between the microwave and optical regimes with a signal-to-noise greater than one.
One of the main thrusts of quantum science over the past few decades has been the developmentof quantum networks for the purposes of secure communication, enhanced detector sensitivity, and advanced computing. Realizing such a network of superconducting quantum processors that communicate via optical fibers would leverage the high-fidelity quantum signal processing of superconducting circuits and the thermal robustness of infrared light but requires a transducer capable of connecting these two sections of the electromagnetic spectrum separated by five orders of magnitude in energy. This thesis explores the optimization of a transducer architecture where the mechanical mode of a Si3N4 membrane mediates the coupling of a superconducting lumped element circuit and a Fabry-Perot optical cavity. We aim to maximize the coupling of these three harmonic oscillator to each other while shielding them from noisy processes that would decohere quantum signals. This architecture has led to transducers with unparalleled efficiency and continuous operation. Enhanced cooperativity between the optical cavity and mechanical oscillator enabled optically-detected readout of a superconducting qubit and optomechanical ground state cooling with negligible laser-induced heating of the superconducting qubit and microwave circuit. To surpass the threshold for quantum-enabled operation, we subsequently improved the cooperativity between the microwave circuit and mechanical oscillator by reducing the microwave loss and noise from two-level-system-like defects in the Si3N4 dielectric. When combined with enhanced coupling between the circuit and membrane or improved mechanical isolation, we project that this architecture will be capable of transducing quantum signals between the microwave and optical regimes with a signal-to-noise greater than one.
Continued development of complementary capabilities in two quantum technologies—information processing with microwave-frequency superconducting circuits and networks based on optical-frequency photonic links—have motivated an effort to bridge the frequency gap between them with a transducer capable of preserving quantum signals. At the same time, mechanical resonators have been engineered to interact with electromagnetic waves across the spectrum in a variety of tailorable physical implementations. This thesis describes advances in a transducer using the mechanical mode of a silicon nitride membrane as an intermediary permitting coupling between a superconducting resonant LC circuit and a Fabry-Pérot optical cavity. With this choice of architecture, we have developed transducers of unparalleled efficiency and the unique capability to run continuously without appreciably degrading the performance of the superconductor from scattered optical photons. We have been able to laser-cool the mechanical mode to an occupation of less than one photon, and achieved transducer performance capable of converting a single microwave photon to optical frequencies with a signal-to-noise ratio of 1/3. Furthermore, operating the transducer does not cause appreciable heating of a superconducting transmon qubit linked to its microwave input port. With progress towards a quantum-enabled interface, we have started to develop capabilities towards implementing protocols that optically verify the quantum performance of the transducer.
A transducer capable of faithfully converting single quanta between microwave and optical frequencies would enable an optical network of superconducting quantum computers. A primary challenge in the ongoing effort to bridge these frequency scales is the detrimental effect that optical photons have on superconducting circuits. This manuscript details the membrane-optomechanical Fabry-P´erot architecture we employ in a high-efficiency electrooptomechanical transducer. We use a chemical bonding process to create an integrated membrane-mirror etalon assembly that is robust against thermal misalignment upon cryogenic cooling. Our choice of input coupling mirror and cavity geometry allow flexible operation of the transducer in the presence of any additional nonidealities in the optical cavity, such as additional scattering loss associated with one mirror. Our transducer is unique in its ability to operate with continuous laser illumination without substantially impacting the superconducting circuitry of the transducer or a superconducting transmon qubit linked to its microwave input. We quantify the effect of laser light on the superconductor, measuring the effective occupancy of the transducer’s superconducting circuit with varying optical power to be less than 0.15 photons even at powers greatly exceeding that needed for transducer operation. We also measure the coherence time T2 of a qubit attached to the transducer for optical readout, and determine that continuous laser pumping of the transducer has no measurable effect.
Superconducting quantum circuits are a leading hardware candidate for realizing a large-scale fault-tolerant quantum computer. However, scaling superconducting quantum computers will soon approach the physical limits imposed by their cryogenic operating environments. Consequently, interconnects between multiple processor modules will become increasingly necessary for scaling quantum computers. Optical fiber is a particularly promising candidate for these interconnects due to its low loss and negligible thermal noise at room temperature. However, interfacing microwave-frequency superconducting circuits with optical fibers presents several challenges. In this thesis, we present a modular architecture which addresses many of the current challenges in building this interface. We develop an optomechanical quantum transducer which is capable of generating microwave- and optical-frequency photons in pairs, and demonstrate quantum entanglement between these microwave-optical photon pairs. Further, we develop a superconducting qubit module optimized for efficient absorption of itinerant microwave photons, and demonstrate a high-efficiency photon catch protocol. Finally, we connect the superconducting qubit module to the transducer module via a superconducting microwave cable. We drive the microwave-optical photon pair generation process in the transducer module and employ the photon catch protocol in the qubit module to facilitate state transfer between the qubit and the flying microwave photon from the transducer. We observe correlations between the final qubit state and the detection of an optical photon, demonstrating a crucial step toward establishing quantum entanglement between superconducting qubits and light. We discuss ongoing progress and challenges in realizing hybrid entanglement and in extending this approach to entangle remote superconducting qubits over optical networks.
Quantum networks of superconducting circuit quantum processors are an exciting prospect but are unfeasible without an efficient quantum interface between optical light and superconducting qubits. This problem motivates the development of a quantum microwave-to-optical transducer, a device that can faithfully transfer the state of a superconducting circuit to a photonic flying qubit and vice versa. This thesis describes our progress towards building a microwave-to-optical transducer mediated by phonons in a high-overtone bulk acoustic resonator (HBAR). Novel aluminum nitride-on-quartz HBARs are designed, fabricated, and tested. The acoustic loss rates are found to be near the state-of-the-art. Transduction-compatible transmon qubits are designed and tested. These qubits are capable of operating inside an optical cavity as well as achieving electromechanical interactions in the strong-coupling regime. An initial experiment studied how superconducting qubits recover after exposure to infrared light and demonstrated that the transduction-compatible qubits should be able to operate in a transducer with a reasonable duty cycle. The concept of microwave-to-optical transduction using multiple acoustic modes is established, and new simulation methods are developed to rapidly optimize HBARs. These tools are used to create a transduction-compatible qubit-HBAR device operating in the multi-mode strong coupling regime. Separately, a transduction compatible optical cavity was built and used to perform optomechanical measurements of a transduction-compatible HBAR and confirm that this device can reach the high-cooperativity regime. Finally, a measurement for verifying that a transducer is operating as a source of single photons is discussed. This can be accomplished by measuring the optical second-order correlation function of transduced photons. A filter cavity cascade is constructed to perform this measurement without being overwhelmed by optical pump leakage. These results establish bulk acoustic transduction as a promising platform for low-noise quantum microwave-to-optical transduction.
Superconducting circuits are important technological tools that have found various applications, notably hypothetical future superconducting quantum computers. Scaling such systems further is, however, proving challenging because of thermal heat load and space constraints in cryostats. One way of improving the scalability of such systems would be to resort to optical fibers for controlling the circuits in cryostats, as well as mediating long-distance interaction between them while preserving quantum coherence. Such a strategy relies on building a coherent interface between GHz frequencies and optical frequencies. This interface, referred to as "microwave-optical transducer", would need to exhibit high photon conversion efficiencies and very low added noise, ideally less than a quantum, in a cryogenic environment. Since the prospects of weaving networks of optically connected superconducting quantum processors have now attracted the attention of the scientific community for more than a decade, very efficient transducers have already been proposed and realized. However, despite their impressive performances against selected metrics, often equivalent to optimizing the on-chip conversion efficiency alone, none of the approaches pursued so far really solved the qubit interconnection problems in ways that could be applied to large-scale systems in the near future. The aim of this thesis was to develop transduction approaches that would ultimately enable optical driving of superconducting circuits and quantum state transfer between microwave and optical frequencies, while at the same time ensuring good scalability, both in terms of wafer manufacturability and integration in cryogenic environments. Integrated photonic circuits based on silicon nitride appeared as a promising platform, as silicon nitride is a CMOS-certified material, has a transparency window compatible with optical fibers and lacks two photon absorption. Realizing the coupling between microwave and optical frequencies in a way similar to what is done in optomechanics is desirable, because the field of optomechanics has seen the establishement of techniques that enable control of the low frequency degree of freedom at the quantum mechanical level, including ground state cooling, control of the susceptibility by dynamical backaction, quantum state transfer and entanglement. The heterogeneous integration of nonlinear crystalline thin films, such as lithium niobate and lithium tantalate, onto silicon nitride photonic circuitry endows it with an electro-optic activity that emulates the optomechanical interaction, while offering an inherent access to the microwave degree of freedom. However, material nonlinearities, including photorefraction, charge screening, and hysteresis, introduce complications that motivate exploring alternative approaches. Here, alongside an electro-optic implementation, a transducer that leverages the interaction of bulk acoustic resonances excited by micro-electromechanical systems with whispering-gallery-like modes of microresonators is presented. Coherent transduction between microwave and optical fields is demonstrated, and optomechanical dynamical backaction is observed. The transducers fast bandwidth, compatibility with multiplexing and ability to down-convert microwaves suggest interesting prospects, particularly upon further improvements of the optomechanical coupling.
Due to its high coherence in transmission over a large distance in the ambient environment, the quantum optical system has been a prevailing platform for long-distance quantum communication, which was recently realized over a continental distance with a low earth orbit satellite and ground stations [102, 70]. However, the pure quantum optical system has so far shown weak interactions between photon and matter, which makes it inefficient in carrying out deterministic quantum gates for quantum repeater based scalable quantum network and quantum computing. On the other hand, superconducting quantum systems operating in the microwave domain with Josephson junction transmon qubits have proven to be capable of efficient deterministic quantum operations on quantum states [86, 87, 66]. Nevertheless, such architecture is prone to errors and decoherence due to cross-talk between microwave elements in a large-scale superconducting quantum circuit. Furthermore, superconducting systems, in general, also have large footprint (100s um) elements (resonators and superconducting quantum bits) [92, 60] that limit the ability to scale up a superconducting quantum system. Moreover, microwave quantum circuits require cooling to around 10 mK, making it unsuitable for communicating quantum information outside a dilution refrigerator (DF). Micro- and nano- acoustic elements have been extensively used in conventional integrated information processing systems due to their compactness and high coherence [97]. Acoustic systems in quantum engineering also have the advantage of being a platform for universal couplings between various quantum systems including spins, optical photons, and superconducting circuits. As it will be discussed in this thesis, elements critical to scalable optical quantum network and superconducting quantum circuit can be constructed relying on the cavity optomechanics and piezoelectric interactions. Optomechanical interaction is concerned with the light pressure coupling of cavity mechanical deformation to a strong optical field. This interaction has allowed the close to mechanical ground state cooling of mechanical resonators using laser and the ultra-sensitive displacement measurement that led to the detection of gravitational waves in the LIGO collaboration [125, 25]. Optomechanical crystals (OMCs) are lithographically patterned devices which contain a periodic structure that host bandgaps for both optical band electromagnetic waves and microwave band acoustic waves. A properly engineered defect in the crystal can confine and localize acoustic and electromagnetic modes of similar wavelengths into a small mode volume [17, 20, 21]. A strong optomechanical coupling, which can be achieved between such strongly confined co-localized optical and acoustic modes, can be used in engineering the quantum state of mechanical motion to realize useful quantum devices such as a high-coherence quantum memory [74] and an optomechanical high efficiency optical isolator for unidirectionally connecting distant optical cavities via an acoustic bus [37]. To strongly couple the mechanical degree of freedom with a superconducting quantum circuit, various methods can be used, ranging from electromechanic coupling (electric coupling to a mechanically compliant capacitor), magnetomechanical coupling (magnetic coupling to a vibrating SQUID loop), and piezoelectric coupling. The recent advent of quantum acoustics [23, 8, 9] was realized with the strong piezoelectric coupling between a superconducting transmon qubit and a high-coherence mechanical resonator. The engineered strong piezoacoustic coupling provides the possibility to carry out deterministic ultra-high fidelity two-qubit quantum gates on non-classical mechanical quantum states [52]. This ability together with the recent demonstration of ultra-long phonon lifetime mechanical resonators show the possibility of integrating the ultra-high quality mechanical resonator as a compact quantum memory element and even a new ultra-compact (10s um) quantum bit architecture for scalable superconducting quantum circuits. Furthermore, the strong piezoelectric coupling that can transduce quantum state in a superconducting circuit into mechanical wave also makes it possible to efficiently transduce a quantum state between a superconducting quantum circuit and a telecommunication band optical channel via a mechanical waveguide connected to an optomechanical crystal cavity.