Coherent quantum state storage and transfer between two phase qubits via a resonant cavity

Abstract: A network of quantum-mechanical systems showing long lived phase coherence of its quantum states could be used for processing quantum information. As with classical information processing, a quantum processor requires information bits (qubits) that can be independently addressed and read out, long-term memory elements to store arbitrary quantum states, and the ability to transfer quantum information through a coherent communication bus accessible to a large number of qubits. Superconducting qubits made with scalable microfabrication techniques are a promising candidate for the realization of a large scale quantum information processor. Although these systems have successfully passed tests of coherent coupling for up to four qubits, communication of individual quantum states between qubits via a quantum bus has not yet been demonstrated. Here, we perform an experiment demonstrating the ability to coherently transfer quantum states between two superconducting Josephson phase qubits through a rudimentary quantum bus formed by a single, on chip, superconducting transmission line resonant cavity of length 7 mm. After preparing an initial quantum state with the first qubit, this quantum information is transferred and stored as a nonclassical photon state of the resonant cavity, then retrieved at a later time by the second qubit connected to the opposite end of the cavity. Beyond simple communication, these results suggest that a high quality factor superconducting cavity could also function as a long term memory element. The basic architecture presented here is scalable, offering the possibility for the coherent communication between a large number of superconducting qubits.

• The paper demonstrates coherent quantum state exchange between superconducting phase qubits using a resonant cavity as a quantum bus.
• It employs rapid flux bias shifts and vacuum Rabi oscillations to achieve high-fidelity state manipulation under strong coupling conditions.
• The work offers a scalable framework for integrating quantum communication and storage in superconducting quantum computing architectures.

Coherent Quantum State Storage and Transfer Between Superconducting Phase Qubits

The paper examines the feasibility and implementation of a quantum communication infrastructure through the coherent transfer and storage of quantum states between two superconducting Josephson phase qubits using a resonant cavity as a quantum bus. The authors, Sillanpää, Park, and Simmonds, present experimental evidence of quantum state exchange facilitated by a superconducting transmission line resonant cavity, contributing significant advancements to the field of superconducting quantum information processing.

Superconducting qubits, particularly those utilizing Josephson junctions, present a favorable pathway towards scalable quantum computing architectures owing to the microfabrication capabilities inherent in their design. The challenge addressed within this research is establishing coherent information transfer between qubits, enhancing the potential for a large-scale quantum processor. This mechanism involves leveraging the foundational dynamics of cavity Quantum Electrodynamics (QED), where superconducting qubits interact with a quantized electromagnetic field within a superconducting cavity.

Experimental Setup and Methodology

The superconducting system described utilizes two phase qubits coupled to a 7 mm long coplanar waveguide resonant cavity. The authors use a technique where the quantum states of an initial qubit are stored temporarily in the form of a photon within the cavity, which acts as a bus for communication, and then retrieved by a secondary qubit. This sequence of state storage and transfer is enabled through the resonant interaction facilitated by precise frequency matching of the qubits with the cavity mode, governed by the Jaynes-Cummings hamiltonian.

The innovation of this work is underlined by successfully demonstrating vacuum Rabi oscillations between qubits and cavity photons—a testament to maintaining strong coupling conditions required for efficient information transfer. They achieve this by employing rapid flux bias shifts that maintain coherence throughout the state transfer process.

Results and Implications

The empirical validation is most prominent in the vacuum Rabi oscillations observed in time-domain measurements, marking coherent population transfer between the states of the involved qubits and the cavity. Through this proficiency, each qubit can serve both as a source and recipient of quantum information, with read and write operations into the cavity resonator. Importantly, the Ramsey-type interference experiments conducted further verified that quantum coherence is preserved during the state transfer, demonstrating high fidelity in the manipulation of superposition states across qubit links.

These results open tangible pathways for developing scalable quantum computers where superconducting systems may utilize quantum buses not only for information transfer but also as repositories for quantum data retention. The exploitation of higher quality factor superconducting cavities, presumably surpassing qubits in coherence times, indicates potential for longstanding quantum memory applications, enhancing the robustness of this technology.

Future Developments

While the research presents a comprehensive framework, future explorations could target refining the fidelity of transfer protocols. This could involve further investigation into mitigating decoherence factors, particularly those introduced by two-level system defects and optimizing the pulse sequences used in shifting qubit energies. Enhancing the purity and manipulation of quantum states within these systems is vital, and should be complemented with advanced state tomography techniques to fully unravel the complex quantum dynamics at play.

In conclusion, this study significantly contributes to the ongoing development of circuit QED and superconducting quantum computing, providing a methodical approach to coherently transferring quantum states in multi-qubit systems interconnected by resonant cavities. Its implications for quantum information science emphasize both immediate practical applications in quantum communication and storage, as well as long-term theoretical advancements in scalable quantum processor designs.