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arXiv:2008.01642 · Physical Review Letters

Microwave Quantum Link between Superconducting Circuits Housed in Spatially Separated Cryogenic Systems

Paul Magnard, Simon Storz, Philipp Kurpiers, Josua Schär, Fabian Marxer, Janis Lütolf, T. Walter, Jean-Claude Besse, Mihai S. Gabureac, Kevin Reuer, Abdulkadir Akın, Baptiste Royer, Alexandre Blais, Andreas J. Wallraff

Published Dec 21, 2020• 238 citations
arXivHTMLPDF

Abstract

Superconducting circuits are a strong contender for realizing quantum computing systems and are also successfully used to study quantum optics and hybrid quantum systems. However, their cryogenic operation temperatures and the current lack of coherence-preserving microwave-to-optical conversion solutions have hindered the realization of superconducting quantum networks spanning different cryogenic systems or larger distances. Here, we report the successful operation of a cryogenic waveguide coherently linking transmon qubits located in two dilution refrigerators separated by a physical distance of five meters. We transfer qubit states and generate entanglement on demand with average transfer and target state fidelities of 85.8% and 79.5%, respectively, between the two nodes of this elementary network. Cryogenic microwave links provide an opportunity to scale up systems for quantum computing and create local area superconducting quantum communication networks over length scales of at least tens of meters.

DOI
10.1103/physrevlett.125.260502

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Superconducting circuits are a strong contender for realizing quantum computing systems and are also successfully used to study quantum optics and hybrid quantum systems. However, their cryogenic operation temperatures and the current lack of coherence-preserving microwave-to-optical conversion solutions have hindered the realization of superconducting quantum networks spanning different cryogenic systems or larger distances. Here, we report the successful operation of a cryogenic waveguide coherently linking transmon qubits located in two dilution refrigerators separated by a physical distance of five meters. We transfer qubit states and generate entanglement on demand with average transfer and target state fidelities of 85.8% and 79.5%, respectively, between the two nodes of this elementary network. Cryogenic microwave links provide an opportunity to scale up systems for quantum computing and create local area superconducting quantum communication networks over length scales of at least tens of meters.
Quantum technologies promise a radically new way to solve classically intractable computing problems. Superconducting circuits as a platform are at the forefront of this field. The cryogenic operation temperatures of superconducting circuits however impose challenges for the further scaling to many connected quantum information processing units into a local area or global network. In this work, we present a hardware solution for connecting quantum devices operating at microwave frequencies into local area networks, which enable the exchange of quantum information between spatially separated parties. Specifically, we demonstrate a modular system spanning distances of 5, 10 and 30 meters operated at cryogenic temperatures and connecting two superconducting circuit systems, located in individual dilution refrigerators, through a quantum communication channel. We develop a thermal model to evaluate the heat transfer processes in the setup, optimize the design and select appropriate materials for its construction. The assembled 30-meter-long system achieves operating temperatures of below 50 mK after a cooldown time of about six and a half days. This link enables the execution of distributed quantum computing and communication algorithms. It also adds the resource of non-locality, certified by a loophole-free Bell test, to the field of quantum science and technology with superconducting circuits.
Scalable quantum information processing with superconducting circuits is expected to advance from individual processors located in single dilution refrigerators to more powerful distributed quantum computing systems. The realization of hardware platforms for quantum local area networks (QLANs) compatible with superconducting technology is of high importance in order to achieve a practical quantum advantage. Here, we present a fundamental prototype platform for a microwave QLAN based on a cryogenic link connecting two separate dilution cryostats over a distance of 6.6 m with a base temperature of 52 mK in the center. Superconducting microwave coaxial cables are employed to form a quantum communication channel between the distributed network nodes. We demonstrate the continuous-variable entanglement distribution between the remote dilution refrigerators in the form of two-mode squeezed microwave states, reaching squeezing of 2.10 ± 0.02 dB and negativity of 0.501 ± 0.011. Furthermore, we show that quantum entanglement is preserved at channel center temperatures up to 1 K, paving the way towards microwave quantum communication at elevated temperatures. Consequently, such a QLAN system can form the backbone for future distributed quantum computing with superconducting circuits.
Superconducting quantum processors are a leading platform for implementing practical quantum computation algorithms. Although superconducting quantum processors with hundreds of qubits have been demonstrated, their further scaling up is constrained by the physical size and cooling power of dilution refrigerators. This constraint can be overcome by constructing a quantum network to interconnect qubits hosted in different refrigerators, which requires microwave-to-optical transducers to enable low-loss signal transmission over long distances. Despite that various designs and demonstrations have achieved high-efficiency and low-added-noise transducers, a coherent photonic link between separate refrigerators has not yet been realized. In this work, we experimentally demonstrate coherent signal transfer between two superconducting circuits housed in separate dilution refrigerators, enabled by a pair of frequency-matched aluminum nitride electro-optic transducers connected via a 1-km telecom optical fiber. With transducers at each node achieving >0.1% efficiency, an overall 80 dB improvement in transduction efficiency over commercial electro-optic modulators is attainable, paving the way towards a fully quantum-enabled link. This work provides critical design guidelines towards scalable superconducting quantum networks interconnected by photonic links.
In this study, we investigate the unconditional microwave quantum teleportation of Gaussian states and the feasibility of its realization with real microwave elements between two dilution refrigerators. Our approach involves employing the Braunstein–Kimble protocol for continuous variables, represented in the symplectic domain, with microwave components integrated into a teleportation circuit. This circuit is established between two dilution refrigerators, which are interconnected by a cryogenic link and incorporate a homodyne detector. The proposed framework offers controllability, suitable for implementation within a single refrigerator or between two separate refrigerators. It operates effectively across a temperature range spanning from milliKelvin to 4 K. This platform is useful for advancing superconducting quantum communication within and between refrigeration systems. Its potential extends to facilitating quantum local area networks and enabling distributed quantum computing protocols. Furthermore, we briefly discuss the complex realm of long-range open-air quantum microwave communication under realistic conditions. We also introduce a protocol designed to enhance entanglement distillation of two-mode squeezed states between two refrigerators. This enhancement targets the squeezing factor of the resource and improves the protocol efficiency.
Quantum communication in the microwave regime is set to play an important role in distributed quantum computing and hybrid quantum networks. However, typical superconducting quantum circuits require millikelvin temperatures for operation, which poses a significant challenge for large-scale microwave quantum networks. Here, we present a solution to this challenge by demonstrating the successful quantum teleportation of microwave coherent states between two spatially separated dilution refrigerators over a thermal microwave channel in the temperature range up to 4 K. We distribute two-mode squeezed states over this noisy channel and employ the resulting quantum entanglement for quantum teleportation of coherent states with fidelities of 72.3±0.5% at 1 K and 59.9±2.5% at 4 K, exceeding the no-cloning and classical communication thresholds, respectively. We successfully model the teleportation protocol using a Gaussian operator formalism that includes losses and noise. Our analysis shows that the teleportation infidelity mainly stems from a parasitic heating of the cold quantum nodes due to the hot network connection. These results demonstrate the experimental feasibility of distributed superconducting architectures and motivate further investigations of noisy quantum networks in various frequency regimes.
ABSTRACT Based on the hybrid system, we propose a network device to achieve entanglement transfer between multiple superconducting qubits and spatially separated optical cavities. The network model consists of nodes, each equipped with an optical fiber linked to the remote optical cavity. In the central microwave cavity of the network, several superconducting qubits are placed. The protocol begins by initializing superconducting qubits in an entangled state (Bell or GHZ state). By leveraging microwave‐optical photon converters and optical fibers, the entangled states are transferred to spatially separated optical cavities. As a specific example, we numerically demonstrate that the high‐fidelity transfer of a Bell state of superconducting qubits onto spatially separated optical cavities is achievable within the present‐day circuit QED technology. Besides, it is worthy to note that when the dissipation is negligible, the entangled Bell state of spatially separated optical cavities can be transferred back onto superconducting qubits by performing reverse operations. Finally, this proposal is quite general and can be extended to other hybrid systems with resembling level structure. This approach provides a promising candidate for long‐distance entanglement distribution in hybrid quantum networks with potential applications in scalable quantum communication and information processing.