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Superconducting Quantum Networks

Periodic Report Summary 2 - SUPERQUNET (Superconducting Quantum Networks)

Currently, superconducting circuits are one of the prime contenders for building a quantum computer. In the SuperQuNet project, we design, test and realize elements for quantum networks capable of linking the nodes of a network of quantum computers. We realize such networks on a laboratory scale of several tens of meters and explore fundamental and applied networking experiments with superconducting microwave frequency circuits.

One of the motivating experiments includes a fundamental test of quantum mechanics: a loop-hole free Bell-test at microwave frequencies realized with a large data rate. In this experiment, we measure the states of two distant entangled quantum 2-level systems (qubits) faster than a light signal can travel between the two, and show that the results cannot be explained by classical physics.

In order to achieve this goal, we design and construct the world’s longest cryogenic dilution refrigerator with an envisaged length of over 20 meters. We have completed a 4-meter prototype, which was successfully cooled to 0.010 °C above the absolute zero of temperature and includes all of the necessary building blocks for the longer system.

As part of the project, we improved the speed and the fidelity of measurements we perform on our superconducting microwave circuits to be faster and more accurate. Specifically, with our new design concepts, we have measured state of the art fidelities three times faster than in previous work and reached the necessary threshold for the Bell-test (Walter2017).

To create remote entanglement across the network, we have shown that we can control the creation and shape of microwave photons (Pechal2014, Zeytinoglu2015) that we intend to transfer from one node of the network to another one to entangle the two.

In addition, we have demonstrated that our proposed link will be suitable for transmitting microwave quantum signal over the necessary distance with low enough loss (Kurpiers2017).

When the building blocks of the system are successfully assembled to form the backbone for a small quantum network, we expect to successfully complete a non-local Bell-test with data rates surpassing all existing experiments. Using this prototype network, we plan to demonstrate fundamental aspects of quantum key generation and randomness amplification, in which pseudo-random numbers can be made more random. We also plan to develop quantum repeaters and scale our system to include more nodes. Recent theoretical proposals suggest that such networks may also be suitable to help scale quantum computers based on microwave circuits to larger system size. Remote entanglement combined with quantum channels (links) may be capable of extending error correcting algorithms across distant physical or logical qubits.

Microwave-Controlled Generation of Shaped Single Photons in Circuit Quantum Electrodynamics, M. Pechal, L. Huthmacher, C. Eichler, S. Zeytinoğlu, A. A. Abdumalikov Jr., S. Berger, A. Wallraff, and S. Filipp, Phys. Rev. X 4, 041010 (2014)
Microwave-induced amplitude- and phase-tunable qubit-resonator coupling in circuit quantum electrodynamics, S. Zeytinoglu, M. Pechal, S. Berger, A. A. Abdumalikov Jr., A. Wallraff, and S. Filipp, Phys. Rev. A 91, 043846 (2015)
Characterizing the attenuation of coaxial and rectangular microwave-frequency waveguides at cryogenic temperatures, P. Kurpiers, T. Walter, P. Magnard, Y. Salathe, and A. Wallraff, EPJ Quantum Technology 4, 8 (2017)
Realizing Rapid, High-Fidelity, Single-Shot Dispersive Readout of Superconducting Qubits, T. Walter, P. Kurpiers, S. Gasparinetti, P. Magnard, A. Potocnik, Y. Salathe, M. Pechal, M. Mondal, M. Oppliger, C. Eichler, and A. Wallraff, arXiv:1701.06933 (2017) (in print PR Applied)