Observation of multi-component atomic Schrödinger cat states of up to 20 qubits (1905.00320v1)

Abstract: We report on deterministic generation of 18-qubit genuinely entangled Greenberger-Horne-Zeilinger (GHZ) state and multi-component atomic Schr\"{o}dinger cat states of up to 20 qubits on a quantum processor, which features 20 superconducting qubits interconnected by a bus resonator. By engineering a one-axis twisting Hamiltonian enabled by the resonator-mediated interactions, the system of qubits initialized coherently evolves to an over-squeezed, non-Gaussian regime, where atomic Schr\"{o}dinger cat states, i.e., superpositions of atomic coherent states including GHZ state, appear at specific time intervals in excellent agreement with theory. With high controllability, we are able to take snapshots of the dynamics by plotting quasidistribution $Q$-functions of the 20-qubit atomic cat states, and globally characterize the 18-qubit GHZ state which yields a fidelity of $0.525\pm0.005$ confirming genuine eighteen-partite entanglement. Our results demonstrate the largest entanglement controllably created so far in solid state architectures, and the process of generating and detecting multipartite entanglement may promise applications in practical quantum metrology, quantum information processing and quantum computation.

• The paper presents the deterministic creation of an 18-qubit GHZ state with 52.5% fidelity, confirming genuine multipartite entanglement.
• It details the synthesis of multi-component atomic Schrödinger cat states, including a 5-component cat state achieved via a one-axis twisting Hamiltonian.
• The work employs a superconducting 20-qubit processor with uniform coupling, advancing scalable quantum information processing architectures.

Observations of Multi-component Atomic Schrödinger Cat States Using a 20-Qubit Quantum Processor

The paper presents significant advancements in the synthesis and analysis of large-scale quantum entanglement within superconducting circuit architectures. The central result is the deterministic generation of an 18-qubit genuinely entangled Greenberger-Horne-Zeilinger (GHZ) state and Schrödinger cat states involving up to 20 qubits. This achievement was realized using a quantum processor composed of superconducting qubits interconnected via a bus resonator, which the researchers manipulated to engineer a one-axis twisting Hamiltonian.

Key Findings

• GHZ State Generation: An 18-qubit GHZ state was successfully prepared with a fidelity of $0.525 \pm 0.005$. This demonstrates genuine multipartite entanglement and marks a notable milestone in qubit interconnectivity and state control within solid-state quantum systems. The fidelity measure confirms that the entangled state is achieved despite potential perturbative noise.
• Atomic Schrödinger Cat States: By exploiting a nonlinear one-axis twisting Hamiltonian, the researchers observed multi-component atomic Schrödinger cat states — superpositions of atomic coherent states — such as a 5-component cat state. These observations indicate that superpositions with a higher number of components can be detected by increasing the number of qubits, thereby reducing the overlap between the states.
• Implementation: This work uses a superconducting quantum processor with 20 Xmon qubits, which are frequency-tunable and coupled via a central bus resonator. The key improvement in this system design was to ensure uniform coupling across the qubits, achieving a more stable and symmetric configuration crucial for coherent quantum state evolutions.

Implications and Future Research

• Quantum Information Processing: The high controllability and efficiency showcased by this experiment emphasize the potential of interconnected superconducting circuits to perform scale quantum information processing tasks. This architecture could facilitate advanced quantum algorithms that require extensive entanglement.
• Potential Applications: The ability to generate large-scale GHZ states and cat states is not only important for testing the foundations of quantum mechanics but also has practical implications in quantum metrology, cryptography, and computation. Better control of multipartite entanglement could enable more robust quantum error correction and fault-tolerant quantum computation.
• Scaling Opportunities: Future work could explore increasing qubit counts and further reducing coupling noise, thus enhancing scalability. Incorporating error correction techniques directly into these architectures may allow for even more complex quantum computing tasks. Further development in this line could significantly accelerate the implementation of noisy intermediate-scale quantum (NISQ) computing devices.

In conclusion, this paper delineates a methodological framework for achieving complex quantum states using highly controllable superconducting quantum systems. The outcomes present a robust platform not only for advancing quantum computing hardware but also for opening new avenues in quantum algorithm implementation.