Sharing information coherently between nodes of a quantum network is fundamental to distributed quantum information processing. In this scheme, the computation is divided into subroutines and performed on several smaller quantum registers that are connected by classical and quantum channels . A direct quantum channel, which connects nodes deterministically rather than probabilistically, achieves larger entanglement rates between nodes and is advantageous for distributed fault-tolerant quantum computation . Here we implement deterministic state-transfer and entanglement protocols between two superconducting qubits fabricated on separate chips. Superconducting circuits constitute a universal quantum node that is capable of sending, receiving, storing and processing quantum information. Our implementation is based on an all-microwave cavity-assisted Raman process , which entangles or transfers the qubit state of a transmon-type artificial atom with a time-symmetric itinerant single photon. We transfer qubit states by absorbing these itinerant photons at the receiving node, with a probability of 98.1 ± 0.1 per cent, achieving a transfer-process fidelity of 80.02 ± 0.07 per cent for a protocol duration of only 180 nanoseconds. We also prepare remote entanglement on demand with a fidelity as high as 78.9 ± 0.1 per cent at a rate of 50 kilohertz. Our results are in excellent agreement with numerical simulations based on a master-equation description of the system. This deterministic protocol has the potential to be used for quantum computing distributed across different nodes of a cryogenic network.

A robust cryogenic infrastructure in form of a wired, thermally optimized dilution refrigerator (DR) is essential for present and future solid-state based quantum processors. Here, we engineer an extensible cryogenic setup, which minimizes passive and active heat loads, while guaranteeing rapid qubit control and readout. We review design criteria for qubit drive lines, flux lines, and output lines used in typical experiments with superconducting circuits and describe each type of line in detail. The passive heat load of stainless steel and NbTi coaxial cables and the active load due to signal dissipation are measured, validating our robust and extensible concept for thermal anchoring of attenuators, cables, and other microwave components. Our results are important for managing the heat budget of future large-scale quantum computers based on superconducting circuits.

Active qubit reset is a key operation in many quantum algorithms, and particularly in quantum error correction. Here, we experimentally demonstrate a reset scheme for a three-level transmon artificial atom coupled to a large bandwidth resonator. The reset protocol uses a microwave-induced interaction between the |f,0⟩ and |g,1⟩ states of the coupled transmon-resonator system, with |g⟩ and |f⟩ denoting the ground and second excited states of the transmon, and |0⟩ and |1⟩ the photon Fock states of the resonator. We characterize the reset process and demonstrate reinitialization of the transmon-resonator system to its ground state in less than 500 ns and with 0.2% residual excitation. Our protocol is of practical interest as it has no additional architectural requirements beyond those needed for fast and efficient single-shot readout of transmons, and does not require feedback.

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Heralding techniques are useful in quantum communication to circumvent losses without resorting to error correction schemes or quantum repeaters. Such techniques are realized, for example, by monitoring for photon loss at the receiving end of the quantum link while not disturbing the transmitted quantum state. We describe and experimentally benchmark a scheme that incorporates error detection in a quantum channel connecting two transmon qubits using traveling microwave photons. This is achieved by encoding the quantum information as a time-bin superposition of a single photon, which simultaneously realizes high communication rates and high fidelities. The presented scheme is straightforward to implement in circuit QED and is fully microwave-controlled, making it an interesting candidate for future modular quantum computing architectures.Engineering of large-scale quantum systems will likely require coherent exchange of quantum states between distant units. The concept of quantum networks has been studied theoretically [1][2][3][4] and substantial experimental efforts have been devoted to distribute entanglement over increasingly larger distances [5][6][7][8][9][10][11][12][13][14][15]. In practice, quantum links inevitably experience losses, which vary significantly between different architectures and may range from 2×10 −4 dB/m in optical fibers [16] to 5×10 −3 dB/m in superconducting coaxial cables and waveguides at cryogenic temperatures [17]. However, no matter which architecture is used, the losses over a sufficiently long link will eventually destroy the coherence of the transmitted quantum state, unless some measures are taken to mitigate these losses. Possible ways to protect the transmitted quantum information rely, for example, on using quantum repeaters [18,19], error correcting schemes [20][21][22] or heralding protocols [23][24][25][26], which allow one to retransmit the information in case photon loss is detected.Heralding protocols are particularly appealing for near-term scaling of quantum systems since they are implementable without a significant resource overhead and can provide deterministic remote entanglement at predetermined times [27]. In essence, these protocols rely on encoding the transmitted quantum information in a suitably chosen subspace S such that any error, which may be encountered during transmission, causes the system to leave this subspace. On the receiving end, a measurement which determines whether the system is in S but does not distinguish between individual states within S, can be used to detect if an error occurred. Crucially, when the transfer is successful, this protocol does not disturb the transmitted quantum information. As a counter * P.K. and M.P. contributed equally to this work.philipp.kurpiers@phys.ethz.ch, mpechal@stanford.edu, andreas.wallraff@phys.ethz.ch example, a simple encoding as a superposition of the vacuum state |0 and the single photon Fock state |1 is not suitable to detect errors due to photon loss because the error does not cause a transition out ...

Superposition, entanglement and non-locality constitute fundamental features of quantum physics. The fact that quantum physics does not follow the principle of local causality1–3 can be experimentally demonstrated in Bell tests4 performed on pairs of spatially separated, entangled quantum systems. Although Bell tests, which are widely regarded as a litmus test of quantum physics, have been explored using a broad range of quantum systems over the past 50 years, only relatively recently have experiments free of so-called loopholes5 succeeded. Such experiments have been performed with spins in nitrogen–vacancy centres6, optical photons7–9 and neutral atoms10. Here we demonstrate a loophole-free violation of Bell’s inequality with superconducting circuits, which are a prime contender for realizing quantum computing technology11. To evaluate a Clauser–Horne–Shimony–Holt-type Bell inequality4, we deterministically entangle a pair of qubits12 and perform fast and high-fidelity measurements13 along randomly chosen bases on the qubits connected through a cryogenic link14 spanning a distance of 30 metres. Evaluating more than 1 million experimental trials, we find an average S value of 2.0747 ± 0.0033, violating Bell’s inequality with a P value smaller than 10−108. Our work demonstrates that non-locality is a viable new resource in quantum information technology realized with superconducting circuits with potential applications in quantum communication, quantum computing and fundamental physics15.