14-qubit entanglement: creation and coherence (1009.6126v2)

Abstract: We report the creation of Greenberger-Horne-Zeilinger states with up to 14 qubits. By investigating the coherence of up to 8 ions over time, we observe a decay proportional to the square of the number of qubits. The observed decay agrees with a theoretical model which assumes a system affected by correlated, Gaussian phase noise. This model holds for the majority of current experimental systems developed towards quantum computation and quantum metrology.

• The paper demonstrates the successful creation of 14-qubit GHZ states using Mølmer-Sørensen interactions in an ion-trap processor.
• The paper finds that coherence decay scales quadratically with qubit count, evidencing superdecoherence from correlated Gaussian phase noise.
• The paper provides detailed fidelity metrics and discusses implications for scalable quantum computing and noise mitigation strategies.

Insights on 14-Qubit Entanglement: Creation and Coherence

This paper addresses the creation and decay of Greenberger-Horne-Zeilinger (GHZ) states involving up to 14 qubits, achieved within an ion-trap quantum processor. The researchers focus on understanding the entanglement properties and coherence times, exploring the implications of phase noise on quantum states.

Creation of GHZ States

The experiment utilizes a string of $^{40}$Ca$^{+}$ ions, confined within a linear Paul trap. Each ion encodes quantum information as a qubit in its ground and metastable states. M{\o}lmer-S{\o}rensen (MS) interactions facilitate the generation of GHZ states from the initial state $|1 \ldots 1\rangle$. These GHZ states exhibit populations, coherence, and fidelity metrics that are systematically studied in the paper.

Coherence and Decay of GHZ States

A key observation is the quadratic scaling of decoherence with respect to the number of qubits, a phenomenon termed ``superdecoherence.'' The paper attributes this to correlated Gaussian noise, exemplified by phase fluctuations impacting multi-qubit systems. This insight presents coherence decay as proportional to $N^2$, a hypothesis validated through comparison with initial single-qubit decay measurements.

Numerical Observations and Implications

Table I of the paper provides detailed populations, coherence, and fidelity values for GHZ states comprising 2 to 14 ions. Metrics reveal decreasing entanglement fidelity with increasing qubit numbers, attributed to the superdecoherence effect. The empirical validation of a fidelity decline scaling with $N^2$ aligns with the theoretical model proposed, emphasizing the significance of correlated phase noise in quantum systems.

Broader Implications

The implications of this paper are twofold—practical and theoretical. Practically, the meaningful hindrance of superdecoherence may challenge the scalability of quantum metrology and computation. Noise mitigation or encoding in decoherence-resistant states could be vital for progressing towards higher qubit registers. Theoretically, understanding quantum-to-classical transition phenomena requires exploring the effects of superdecoherence.

Future Developments and Applications

Efficiency in producing large-scale entangled states marks significant progress in quantum computing, opening avenues for quantum simulation experiments. Potential applications range from understanding quantum sensing mechanisms, such as bird magnetoreception, to simulating cosmological models. The experimental realization of substantial qubit entanglement establishes foundational capabilities for more advanced quantum computational implementations.

In summary, the experimental creation of a 14-qubit GHZ state and the subsequent examination of its coherence and entanglement metrics leads to valuable insights into the quantum information arena. The identification of superdecoherence as a significant limiting factor underscores the ongoing challenge in scaling quantum systems while maintaining coherent entangled states. This work provides a promising platform for future inquiries into noise-resistant quantum computation and metrology applications.