Deterministic generation of a 20-qubit two-dimensional photonic cluster state (2409.06623v1)

Abstract: Multidimensional cluster states are a key resource for robust quantum communication, measurement-based quantum computing and quantum metrology. Here, we present a device capable of emitting large-scale entangled microwave photonic states in a two dimensional ladder structure. The device consists of a pair of coupled superconducting transmon qubits which are each tuneably coupled to a common output waveguide. This architecture permits entanglement between each transmon and a deterministically emitted photonic qubit. By interleaving two-qubit gates with controlled photon emission, we generate 2 x n grids of time- and frequency-multiplexed cluster states of itinerant microwave photons. We measure a signature of localizable entanglement across up to 20 photonic qubits. We expect the device architecture to be capable of generating a wide range of other tensor network states such as tree graph states, repeater states or the ground state of the toric code, and to be readily scalable to generate larger and higher dimensional states.

• The paper demonstrates a deterministic protocol that produces a 20-qubit 2D photonic cluster state, marking a significant improvement over probabilistic methods.
• It employs superconducting transmon qubits with controlled photon emissions and two-qubit gates to achieve reliable entanglement across the state.
• Matrix product state tomography reconstructs density matrices effectively, revealing measurable localizable entanglement despite fidelity challenges as the state scales.

Deterministic Generation of a 20-Qubit Two-Dimensional Photonic Cluster State

The paper under examination delineates the deterministic generation of a photonic cluster state comprising 20 qubits structured in a two-dimensional configuration. The significance of multidimensional cluster states in quantum technologies, particularly for robust quantum communication, measurement-based quantum computing, and quantum metrology, underpins this paper.

Summary of Content

At the core of the research is a device built using superconducting transmon qubits to generate large-scale entangled states efficiently. The transmons are coupled to a common waveguide, allowing entanglement between each transmon and deterministically emitted photonic qubits. By ingeniously interweaving two-qubit gates with controlled photon emission, the authors construct 2x$n$ grids of time- and frequency-multiplexed cluster states of itinerant microwave photons.

The experimental setup involves two transmon qubits coupled to emission and readout lines, enabling precise photon state manipulation. Through phased control gates, the paper successfully demonstrates the emission of 20-qubit cluster states, recording signatures of localizable entanglement across the entirety of the photonic ensemble.

Key Results

• The cluster states were generated via a deterministic protocol, a significant advancement over probabilistic generation methods, which face challenges in scalability due to efficiency constraints in heralded methods.
• The research achieves a fidelity of 0.84, 0.77, and 0.59 for four-, six-, and eight-qubit states, respectively. Although fidelity decreases as the state size increases to 20 qubits, resulting in a fidelity of 0.12, the presence of measurable entanglement across all qubits suggests the robustness of the technique.
• The paper employs matrix product state tomography to reconstruct density matrices efficiently for states of significant size, circumventing challenges posed by high-dimensional Hilbert spaces.

Implications and Future Directions

The implications of this research are multifaceted. Practically, the ability to deterministically generate such entangled photonic states paves the way for advances in quantum communication systems—facilitating secure information transfer over extended networks. Theoretically, these findings contribute to the ongoing discourse on quantum state engineering and the operationalization of measurement-based quantum computing paradigms.

From a future development perspective, the system's capability to produce more complex tensor network states intimates potential scalability, which could accommodate states capable of more sophisticated quantum computations. The integration of quantum memory and enhanced qubit coherence times could permit larger-dimensional state generation, aligning quantum photonics more closely with the demands of next-generation quantum computing architectures.

Ultimately, this work represents a critical development in the field of quantum information science, offering both a practical tool for current applications and a foundation for further research into scalable quantum technologies and state synthesis.