IEEE International Conference on Quantum Computing and Engineering (QCE)
Entangling Superconducting Qubits over Optical Fiber – Towards Optimization and Implementation
Curtis L. Rau, Akira Kyle, Alexander Kwiatkowski, William D. Warfield, Jacob H. Davidson, Maxime Nurwubusa, John D. Teufel, Ezad Shojaee, Konrad W. Lehnert, Tasshi Dennis
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.
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.
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.
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.
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.
Optical photons play unique role in transmitting information over long distances. Photonic links by the optical fiber networks compose the backbone of today's global internet. Such fiber optics can also provide the most cost-effective quantum channels to distribute quantum information across distant stationary nodes in future large-scale quantum networks. This prospect motivates the recent emerging efforts in developing microwave-optical quantum transduction technology to interconnect microwave quantum processors. Various frequency conversion approaches are investigated to efficiently bridge the enormous electromagnetic frequency gap while preserving quantum coherence. Nonetheless, high-fidelity entanglement generation between remote quantum processing units has remained out of reach to date. Here, we summarize the state-of-the-art progresses in quantum transducer engineering and provide the perspectives on the key challenges and opportunities toward optically heralded quantum entanglement distributions.
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.
The development of scalable quantum networks requires coherent interfaces capable of converting microwave photons used in superconducting quantum processors into optical photons suitable for long-distance fiber transmission. This review surveys recent progress in microwave-to-optical quantum transduction across optomechanical, electro-optic, and magneto-optic platforms, with emphasis on conversion efficiency, bandwidth, added noise, and operating temperature. In addition to standard metrics, we propose the internal efficiency eta_in and the magnon decay rate kappa_m/2pi as normalized parameters that enable fairer comparison across heterogeneous implementations. Optomechanical systems achieve internal phonon-to-photon efficiencies of 93% with sub-quantum added noise of 0.25 quanta at millikelvin temperatures. Electro-optic devices based on LiNbO3 and AlN have advanced from room-temperature efficiencies below 1% to millikelvin systems with internal efficiencies approaching 99.5%, added noise as low as 0.16 quanta at 60 mK, and bandwidths extending to several tens of megahertz. Magneto-optic (optomagnonic) platforms exhibit the lowest efficiencies (typically 10−10 to 10−8), but offer intrinsic non-reciprocity and broadband magnonic operation, with emerging approaches based on topological heterostructures and magnon squeezing predicting enhancements up to 10−4. Optomechanical systems appear promising for high-fidelity quantum state transfer, electro-optic transducers for high-bandwidth coherent links, and magneto-optic devices for non-reciprocal network components. We discuss the fundamental trade-off between efficiency and added noise across all three platforms, and argue that heterogeneous microwave-optical transduction is emerging as a key enabling technology for distributed quantum computing and large-scale quantum networks.