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Physical Review Applied

Theory of Deterministic Entanglement Generation between Remote Superconducting Atoms

Kazuki Koshino, K. Inomata, Zhirong Lin, Yuuki Tokunaga, Tsuyoshi Yamamoto, Yasunobu Nakamura

Published Jun 5, 2017• 18 citations
Publisher

Abstract

Hybrid quantum networks of stationary and ``flying'' qubits are essential for distributed quantum information processing. In superconducting quantum computation, two-qubit gates are currently realized by the interaction between neighboring qubits. The authors propose a gate comprising a superconducting ``atom'' and a microwave photon, in which gate operation is completed $d\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}y$ (not probabilistically) upon reflection of the photon. This gate's type can be continuously varied $i\phantom{\rule{0}{0ex}}n$ $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$, enabling $r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e$ entanglement of many ``atoms'' via a single photon, or creation of a quantum domino effect.

DOI
10.1103/physrevapplied.7.064006

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Hybrid quantum networks of stationary and flying'' qubits are essential for distributed quantum information processing. In superconducting quantum computation, two-qubit gates are currently realized by the interaction between neighboring qubits. The authors propose a gate comprising a superconducting atom'' and a microwave photon, in which gate operation is completed d\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}y (not probabilistically) upon reflection of the photon. This gate's type can be continuously varied i\phantom{\rule{0}{0ex}}n s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u, enabling r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e entanglement of many ``atoms'' via a single photon, or creation of a quantum domino effect.
The number of superconducting qubits contained in a single quantum processor is increasing steadily. However, to realize a truly useful quantum computer, it is inevitable to increase the number of qubits much further by distributing quantum information among distant processors using flying qubits. A key element towards this goal is a deterministic quantum interaction between superconducting-atom and propagating microwave-photon qubits. Here, we confirm the bidirectional state transfer between them, which completes by simply reflecting the photon at the atom. The averaged fidelity of the photon-to-atom (atom-to-photon) state transfer reaches 0.826 (0.801), limited mainly by the lifetime of the atom qubit. The present technology would be useful for future distributed quantum computation with superconducting qubits. Published by the American Physical Society 2024
We propose a two-qubit quantum logic gate between a superconducting atom and a propagating microwave photon. The atomic qubit is encoded on its lowest two levels and the photonic qubit is encoded on its carrier frequencies. The gate operation completes deterministically upon reflection of a photon, and various two-qubit gates (SWAP, SWAP\sqrt{\rm SWAP}SWAP​, and Identity) are realized through {\it in situ} control of the drive field. The proposed gate is applicable to construction of a network of superconducting atoms, which enables gate operations between non-neighboring atoms.
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 information processing with hybrid protocols making use of discrete- and continuous-variable currently attracts of great interest because of its promising applications in scalable quantum computer and distant quantum network. By inducing a giant cross-Kerr nonlinearity between two cavities, we propose a general protocol for hybrid quantum gate and quantum entanglement with high fidelity between a stationary, discrete photonic qubit and a flying photonic state. Interestingly, our protocol can be used to conduct a controlled-Z quantum gate between a stationary microwave photon stored in a slowly-decaying microwave cavity and a flying optical photon, and therefore enable to build quantum network for distant superconducting quantum circuits.
Large-scale quantum information processing networks will most probably require the entanglement of distant systems that do not interact directly. This can be done by performing entangling gates between standing information carriers, used as memories or local computational resources, and flying ones, acting as quantum buses. We report the deterministic entanglement of two remote transmon qubits by Raman stimulated emission and absorption of a traveling photon wave packet. We achieve a Bell state fidelity of 73%, well explained by losses in the transmission line and decoherence of each qubit.
The generation of hybrid entangled states between matter qubits and microwave-field qubits is a fascinating area of quantum science and technology. In this work, we propose an efficient method to deterministically prepare hybrid entangled states between multiple superconducting qubits (matter qubits) and microwave-field qubits (flying qubits), by using a system composed of multiple superconducting qutrits and microwave cavities. Here, each superconducting (SC) qubit is encoded with the two lowest levels of a SC qutrit, while each microwave-field (MF) qubit is encoded using discrete-variable (DV) states (e.g., the vacuum and single-photon states) or continuous-variable (CV) states (e.g., coherent states) of a microwave cavity. Due to the nonpopulation of the qutrits' third energy level during the operation, decoherence from this level is significantly reduced. Moreover, the hybrid entangled states can be generated on a large scale. The operation time is independent of the number of SC qubits and MF qubits. Only two steps of the operation are needed for the preparation of the hybrid Greenberger---Horne---Zeilinger (GHZ) states as long as the initial states of the system are ready. As an example, our numerical results demonstrate that high-fidelity creation of hybrid DV-DV entangled states or CV-DV entangled states of three SC qubits and three MF qubits is feasible with current circuit QED technology. This method is universal and can be extended to generate hybrid entangled states between other matter qubits (such as atoms, ions, quantum dots, NV centers, and magnons) and microwave- or optical-field qubits.