Quantum networks linking quantum computers are predicted to be extremely useful for a wide range of applications. At the hardware level, superconducting quantum computers have emerged as one of the most promising scalable platforms for general quantum computation, and optical networks are clearly the best choice for long distance communication due to their minimal loss. To leverage the strengths of these two dissimilar platforms, quantum transducers, which are reachingquantum-enabled operation, are needed. This thesis presents progress towards designing a long distance optical network which will entangle two superconducting qubits and the implementation of the optical portion of this network. We have found simple, application independent, necessary and sufficient conditions for transducers to work in quantum networks, and we conclude that some previously demonstrated devices are ready for integration into a quantum network. Optimizing the integration of two transducers into a network is explored and it is found that the most robust topology to transmission loss consists of generating optical Gaussian entanglement and then using the two transducers to down-convert this entanglement to the microwave domain. To implement this topology we have constructed a tunable source of optical two-mode squeezing and two ultra-low noise photodetectors. Finally a new method for transferring and distilling Gaussian entanglement to qubits using the dispersive Jaynes-Cummings interaction is presented. This proof-of-principle two-node network should be able to entangle superconducting qubits at a rate of 5 ebits/second over 4 km of optical fiber using currently available technology.
Quantum networks linking quantum computers are predicted to be extremely useful for a wide range of applications. At the hardware level, superconducting quantum computers have emerged as one of the most promising scalable platforms for general quantum computation, and optical networks are clearly the best choice for long distance communication due to their minimal loss. To leverage the strengths of these two dissimilar platforms, quantum transducers, which are reachingquantum-enabled operation, are needed. This thesis presents progress towards designing a long distance optical network which will entangle two superconducting qubits and the implementation of the optical portion of this network. We have found simple, application independent, necessary and sufficient conditions for transducers to work in quantum networks, and we conclude that some previously demonstrated devices are ready for integration into a quantum network. Optimizing the integration of two transducers into a network is explored and it is found that the most robust topology to transmission loss consists of generating optical Gaussian entanglement and then using the two transducers to down-convert this entanglement to the microwave domain. To implement this topology we have constructed a tunable source of optical two-mode squeezing and two ultra-low noise photodetectors. Finally a new method for transferring and distilling Gaussian entanglement to qubits using the dispersive Jaynes-Cummings interaction is presented. This proof-of-principle two-node network should be able to entangle superconducting qubits at a rate of 5 ebits/second over 4 km of optical fiber using currently available technology.
A quantum network linking superconducting quantum computers would enable highly sought-after applications such as distributed quantum computing. Optical photons are the natural way of encoding information for long distance communication; however, exchanging quantum information between optical and microwave domains requires a quantum transducer. There are many ways transducers could be integrated into a network which leads to the question: Which network is most tolerant of current transducers which are far from perfect? A logical metric for optimizing the first quantum networks is the entanglement threshold which is necessary and sufficient condition on network parameters for microwave-microwave entanglement. Here we first address the problem of finding the network with the least restrictive entanglement threshold on physical parameters. For tractability we restrict our study to networks containing only gaussian states, operations, and measurements which are among the most readily experimentally accessible options. Within this vast space of possible network configurations, we identify a small set of topologies which we believe to be optimal. We find that with previously reported technology some of these topologies are capable of entangling remote microwave modes, despite imperfections in current transduction technology. Finally, progress towards a physical implementation of one of these topologies is discussed.
Doubly parametric quantum transducers (DPTs), such as electro-optomechanical devices, show promise as quantum interconnects between the optical and microwave domains, thereby enabling long-distance quantum networks between superconducting qubit systems. However, any transducer will inevitably introduce loss and noise that will degrade the performance of a quantum network. We explore how DPTs can be used to construct a network capable of distributing remote two-mode microwave entanglement over an optical link by comparing 14 different network topologies. The 14 topologies we analyze consist of combinations of different transducer operations, entangled resources, and entanglement-swapping measurements. For each topology, we derive a necessary and sufficient analytic threshold on DPT parameters that must be exceeded in order to distribute microwave-microwave entanglement. We find that the thresholds are dependent on the given network topology, along with the available entanglement resources and measurement capabilities. In the high-optical-loss limit, which is relevant to realistic networks, we find that down-conversion of each half of an optical two-mode squeezed vacuum state is the most robust topology. Finally, using currently achievable experimental capabilities, we find the encouraging result that several of these topologies could produce microwave-microwave entanglement. However, most of these topologies cannot work given current transducer performance, which demonstrates the importance of thoroughly analyzing all possible networks.
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.
The complementary features of different qubit platforms for computing and communicating impose an intrinsic hardware heterogeneity in any quantum network, where nodes, while processing and storing quantum information, must also communicate through quantum links. Indeed, one of the most promising hardware platforms at quantum node scale for scalable and fast quantum computing is the superconducting technology, which operates at microwave frequencies. Whereas, for communicating at distances of practical interest beyond few meters, quantum links should operate at optical frequencies. Therefore, to allow the interaction between superconducting and photonic technologies, a quantum interface, known asquantum transducer, able to convert one type of qubit to another is required. This paper aims to provide a tutorial treatise on the fundamental research challenges of quantum transduction. The tutorial is structured around a communications engineering framework, thereby shedding light on its fundamental role in quantum network design and deployment—a perspective often overlooked in existing literature. This framework allows us to categorize different transduction modalities and to reveal an unorthodox one where the transducer itself can act as an entanglement source. From this standpoint, it is possible to conceive different source-destination link archetypes, where transduction plays a crucial role in the communication performances. The analysis also translates the quantum transduction process into a proper functional block within a new communication system model for a quantum network.
A quantum network consisting of computational nodes connected by high-fidelity communication channels could expand information-processing capabilities beyond those of classical networks. Superconducting qubits hold promise for scalable quantum computation at microwave frequencies, but must operate in an isolated cryogenic environment; meanwhile, quantum communication over long distances has been demonstrated with optical photons. A fast, quantum-coherent interface between the two would be a key element of a large-scale quantum network or distributed quantum computer.In this thesis, we describe the theoretical basis as well as the practical design and development of a device incorporating a silicon optomechanical nanobeam and an aluminum-nitride-based electromechanical transducer. We find that this class of device has the potential to approach ideal quantum microwave-optical transduction. Finally, we experimentally demonstrate classical, continuous-wave operation of such a device with internal conversion efficiencies above 1%. This device also has a larger bandwidth than previous efficient microwave-optical transducers, allowing us to operate in the time domain with 20 ns pulses.
Doubly parametric quantum transducers, such as electro-optomechanical devices, show promise for providing the critical link between quantum information encoded in highly disparate frequencies such as in the optical and microwave domains. This technology would enable long-distance networking of superconducting quantum computers. Rapid experimental progress has resulted in impressive reductions in decoherence from mechanisms such as thermal noise, loss, and limited cooperativities. However, the fundamental requirements on transducer parameters necessary to achieve quantum operation have yet to be characterized. In this work we find simple, protocol-independent expressions for the necessary and sufficient conditions under which doubly parametric transducers in the resolved-sideband, steady-state limit are capable of entangling optical and microwave modes. Our analysis treats the transducer as a two-mode bosonic Gaussian channel capable of both beamsplitter-type and two-mode squeezing-type interactions between optical and microwave modes. For the beamsplitter-type interaction, we find parameter thresholds that distinguish regions of the channel's separability, capacity for bound entanglement, and capacity for distillable entanglement. By contrast, the two-mode squeezing-type interaction always produces distillable entanglement with no restrictions on temperature, cooperativities, or losses. Counterintuitively, for both interactions, we find that achieving quantum operation does not require either a quantum cooperativity exceeding one, or ground-state cooling of the mediating mode. Finally, we discuss where two state-of-the-art implementations are relative to these thresholds and show that current devices operating in either mode of operation are in principle capable of entangling optical and microwave modes.