Single ion qubit with estimated coherence time exceeding one hour

Quantum coherence is essential for quantum computing, metrology, and communication. However, decoherence, caused by environmental coupling and control parameter fluctuations, limits the performance of quantum systems. Long coherence times are particularly critical for quantum memories. While progress has been made in extending coherence times in various systems (e.g., ensembles of trapped ions and nuclear spins), achieving long coherence times in single-qubit memories remains challenging. Previous work demonstrated a coherence time of 660 seconds in a single ¹⁷¹Yb⁺ ion qubit using sympathetic cooling and dynamical decoupling. However, the fundamental limitations preventing further enhancements were unclear. This research addresses that gap by focusing on improving the quality of a trapped-ion quantum memory.

Numerous experimental efforts have focused on improving coherence times in quantum memory. In ensembles, coherence times of up to 10 minutes (trapped ions) and 40 minutes (nuclear spins at room temperature) have been reported. Single-qubit memories, essential for quantum computers and communication, have previously shown coherence times on the order of minutes in trapped ions. Limitations in these systems often stemmed from qubit detection infidelity due to heating during photonic laser cooling, a problem addressed by sympathetic cooling, leading to coherence times exceeding one minute. However, this study seeks to further extend these limitations.

The experiment utilizes a four-rod Paul trap containing one ¹⁷¹Yb⁺ ion (qubit) and one ¹³⁸Ba⁺ ion (sympathetic cooling). The ¹⁷¹Yb⁺ qubit is encoded in the S₁/₂ manifold. Ambient magnetic field noise was suppressed using a magnetic-field shielding with a permanent magnet, achieving over 40 dB attenuation at 50 Hz. Microwave frequency stability was improved using a crystal oscillator, and microwave leakage was reduced by over 70 dB with a microwave switch. Laser leakage was also suppressed using acousto-optic modulators (AOMs), electro-optic modulators (EOMs), and a mechanical shutter. Coherence time was measured using Ramsey interferometry with dynamical decoupling (KDDn) pulses, optimizing the pulse interval to suppress noise. Quantum process tomography was performed to systematically characterize the coherence process. The coherence time was extracted by fitting the experimental data to an exponential decay function. Resource theories of quantum memory (RQM) and relative entropy of coherence (REC) were used to further analyze the results.

The researchers achieved a coherence time of approximately 5500 seconds (extrapolated from measurements up to 960 seconds), significantly exceeding previous records. The observed exponential decay in Ramsey contrast indicates the dominant decoherence mechanism is well-characterized and effectively suppressed. Quantum process tomography showed that the main decoherence effects are depolarization and dephasing, quantified by T1 (≈2200 seconds) and T2 (≈4200 seconds), respectively. Analysis using REC and RQM further confirmed the exceptional performance of the quantum memory. Figures 3 and 4 provide the experimental evidence, showing the Ramsey contrast decay and process fidelity over time. Figure 7 illustrates the significant noise suppression achieved through magnetic field shielding and other improvements.

The extremely long coherence time achieved demonstrates significant progress in overcoming limitations in single-qubit quantum memories. The systematic identification and suppression of dominant error sources, coupled with detailed characterization via quantum process tomography and resource-theoretic analysis, provides a comprehensive understanding of the system's performance. This advancement has significant implications for building scalable quantum computers, enabling fault-tolerant operations even in the noisy intermediate-scale quantum (NISQ) era where quantum error correction is not yet fully implemented.

This work presents a trapped-ion-based single-qubit quantum memory with a coherence time exceeding one hour, a major advancement in quantum technology. This breakthrough was achieved through careful suppression of dominant noise sources. Further enhancements, potentially reaching coherence times exceeding 10⁵ seconds, might be possible through improvements in classical oscillator stability and magnetic field fluctuation control. The results pave the way for more sophisticated quantum information processing and applications.

While the achieved coherence time is remarkable, certain limitations exist. The analysis does not account for imperfections in the KDD pulses. The discrepancy between the coherence time obtained from Ramsey measurements and the total dephasing time from process tomography might be attributed to quantum fluctuation noise in the process tomography measurements, which is a statistical limitation inherent to the technique. Furthermore, future research should explore ways to further reduce noise sources and mitigate potential systematic errors.