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arXiv:2610.05005

Arbitrary-phase CZ gate and remote readout across spatially separated superconducting nodes

Anatoly Kulikov, Josua D. Schär, Alonso Hernández-Antón, Aleksandr Grigorev

quant-phSubmitted Oct 4, 2026• Announced Oct 6, 2026• 0 citations
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Abstract

Establishing a complete set of gates across spatially separated quantum nodes is fundamental for distributed quantum computation and communication networks. One of the approaches to do so consists of employing a superconducting device as a universal quantum node that is capable of storing and processing quantum information, and traveling microwave photons as means of communication. In this work, we realize an entangling gate across a primitive two-node superconducting circuit QED network consisting of two dilution cryostats, A and B, connected via a 30-m-long cryogenic quantum link. We perform a bidirectional photon transfer protocol, where a microwave photon emitted by the communication qubit at A acquires a conditional phase based on the state of qubit B. We vary the communication photon frequency, hence tuning the conditional phase acquired during the reflection off the remote node, effectively implementing an arbitrary-phase CZ gate. We use this entangling operation to perform single-shot readout of the qubit in node B from node A. An interesting feature of our scheme is that it does not require precise synchronization of the active node with the remote node; in particular, remote readout can be performed without any active participation by the remote node. These protocols add to the quantum communication toolbox with superconducting circuits and provide a way of realizing basic distributed quantum computing operations.

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Establishing a complete set of gates across spatially separated quantum nodes is fundamental for distributed quantum computation and communication networks. One of the approaches to do so consists of employing a superconducting device as a universal quantum node that is capable of storing and processing quantum information, and traveling microwave photons as means of communication. In this work, we realize an entangling gate across a primitive two-node superconducting circuit QED network consisting of two dilution cryostats, A and B, connected via a 30-m-long cryogenic quantum link. We perform a bidirectional photon transfer protocol, where a microwave photon emitted by the communication qubit at A acquires a conditional phase based on the state of qubit B. We vary the communication photon frequency, hence tuning the conditional phase acquired during the reflection off the remote node, effectively implementing an arbitrary-phase CZ gate. We use this entangling operation to perform single-shot readout of the qubit in node B from node A. An interesting feature of our scheme is that it does not require precise synchronization of the active node with the remote node; in particular, remote readout can be performed without any active participation by the remote node. These protocols add to the quantum communication toolbox with superconducting circuits and provide a way of realizing basic distributed quantum computing operations.
Quantum networks connecting quantum processing nodes via photonic links enable distributed and modular quantum computation. In this framework, quantum gates between remote qubits can be realized using quantum teleportation protocols. The essential requirements for such non-local gates are remote entanglement, local quantum logic within each processor, and classical communication between nodes to perform operations based on measurement outcomes. Here, we demonstrate an unconditional Controlled-NOT quantum gate between remote diamond-based qubit devices. The control and target qubits are Carbon-13 nuclear spins, while NV electron spins enable local logic, readout, and remote entanglement generation. We benchmark the system by creating a Greenberger-Horne-Zeilinger state, showing genuine 4-partite entanglement shared between nodes. Using deterministic logic, single-shot readout, and real-time feed-forward, we implement non-local gates without post-selection. These results demonstrate a key capability for solid-state quantum networks, enabling exploration of distributed quantum computing and testing of complex network protocols on fully integrated systems.
Quantum networks connecting quantum processing nodes via photonic links enable distributed and modular quantum computation. In this framework, quantum gates between remote qubits can be realized using quantum teleportation protocols. The essential requirements for such non-local gates are remote entanglement, local quantum logic within each processor, and classical communication between nodes to perform operations based on measurement outcomes. Here, we demonstrate an unconditional Controlled-NOT quantum gate between remote diamond-based qubit devices. The control and target qubits are Carbon-13 nuclear spins, while NV electron spins enable local logic, readout, and remote entanglement generation. We benchmark the system by creating a Greenberger-Horne-Zeilinger state, showing genuine 4-partite entanglement shared between nodes. Using deterministic logic, single-shot readout, and real-time feed-forward, we implement non-local gates without post-selection. These results demonstrate a key capability for solid-state quantum networks, enabling exploration of distributed quantum computing and testing of complex network protocols on full-stack systems.
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.
We propose a simple interaction protocol to be implemented on a scalable quantum network, in which the quantum nodes consist of qubit systems confined in cavities. The nodes are deterministically coupled by transmission and reflection of a single photon, which is disentangled from the qubits at the end of the coupling operation. This single photon can generate an entangling controlled-phase gate between any selected number of qubits in the network. Our multiqubit gate reaches a much higher fidelity compared to schemes concatenating one-qubit and two-qubit gates; thus it forms an efficient basis for universal quantum computing distributed over multiple processor units. In our analysis we consider atomic qubits coupled to optical photons, while the scheme can be readily generalized to other architectures, such as superconducting qubit nodes coupled by microwave photons.
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.
Direct entangling gates between spatially separated qubits are a key capability for modular quantum computing. They enable nonlocal quantum circuits to be executed across different processor modules, thereby alleviating single-chip scaling constraints and expanding the effective connectivity of the system. Here, we propose and numerically investigate a remote adiabatic controlled-Z gate between two frequency-tunable superconducting qubits connected by a multi-mode coaxial cable. The cable-mediated interaction enables the gate to be implemented without additional tunable couplers, preserving a simple circuit architecture. For a 30-cm cable, simulations incorporating ten standing-wave modes yield an optimized gate infidelity of 4.71×10−74.71\times10^{-7}4.71×10−7 with a duration of 376.7 ns in the absence of decoherence. The gate maintains high fidelity under qubit-frequency fluctuations and millimeter-scale cable-length variations. Calculations for 50- and 100-cm cable situations demonstrate its potential for even larger cryogenic systems. These results provide a promising approach to extending high-fidelity quantum operations beyond individual chips and toward scalable, interconnected superconducting quantum processors.