Observation of quantum jumps in a superconducting artificial atom (1009.2969v3)

Abstract: A continuously monitored quantum system prepared in an excited state will decay to its ground state with an abrupt jump. The jump occurs stochastically on a characteristic time scale T1, the lifetime of the excited state. These quantum jumps, originally envisioned by Bohr, have been observed in trapped atoms and ions, single molecules, photons, and single electrons in cyclotrons. Here we report the first observation of quantum jumps in a macroscopic quantum system, in our case a superconducting "artificial atom" or quantum bit (qubit) coupled to a superconducting microwave cavity. We use a fast, ultralow-noise parametric amplifier to amplify the microwave photons used to probe the qubit state, enabling continuous high-fidelity monitoring of the qubit. This technique represents a major step forward for solid state quantum information processing, potentially enabling quantum error correction and feedback, which are essential for building a quantum computer. Our technology can also be readily integrated into hybrid circuits involving molecular magnets, nitrogen vacancies in diamond, or semiconductor quantum dots.

• The paper presents a successful high-fidelity Quantum Non-Demolition (QND) measurement scheme to directly observe quantum jumps in a superconducting qubit.
• Using a circuit quantum electrodynamics architecture and an ultralow-noise amplifier, the experiment achieved real-time observation of quantum jumps in a macroscopic quantum system.
• This work validates the QND measurement technique for superconducting qubits, providing a critical step towards robust quantum feedback, error correction, and reliable quantum computing.

Observation of Quantum Jumps in a Superconducting Artificial Atom

This paper presents significant advancements in the real-time observation of quantum jumps in superconducting qubits, a key element in the broader agenda of developing quantum computing technologies. The work demonstrates a successful implementation of a high-fidelity, Quantum Non-Demolition (QND) measurement scheme within a superconducting circuit, allowing for direct observation of state transitions in a quantum system originally anticipated by Bohr.

The experimentation involved a circuit quantum electrodynamics (cQED) architecture, where a superconducting qubit is dispersively coupled to a readout cavity. For signal amplification, an ultralow-noise parametric amplifier was used, achieving high noise performance levels essential for discerning quantum jumps. With the experimental constraints met, such as sample temperature anchored at the millikelvin scale and precision in frequency tuning, the successful observation of quantum jumps in real time has been achieved� notably the first of its kind for a macroscopic quantum system.

Superconducting qubits are advantageous due to their scalability, tunability, and relative ease of manipulation compared to their counterparts, despite their comparatively short relaxation times. These issues can be mediated by quantum error correction technologies, making them potential frontrunners in the realization of scalable quantum computing architectures. This paper illustrates how such a challenging predicament regarding qubit short life spans can be navigated with innovative solutions.

Detailed performance metrics from their setup included an achieved system noise temperature of approximately 142 mK, in stark contrast to the 10-30 K range of typical systems. This improved signal-to-noise ratio (SNR) facilitated single-shot measurement capability, capturing quantum state transitions of the qubit accurately. Despite some minor discrepancies in expected and observed SNR levels, mainly attributed to amplifier saturation, the results strongly validate the efficacy of this measurement technique.

Remarkably, the paper's experiment also highlighted the application of coherent drive to the qubit amidst ongoing measurement, producing a random telegraph signal capturing the quantum Zeno effect. Not only does this showcase the coherence of the setup, but it also fervently suggests new potential research avenues.

The statistical analysis revealed that the histogram of jump times conformed to expected exponential decay, with the decay constants aligning convincingly with predicted qubit relaxation times. Such consistency underscores the reliability of the QND measurement technique implemented. This finding implies substantive perspectives for practical applications in quantum feedback control and error correction schemes, integral components for maintaining stable quantum systems.

In the future, further enhancements in the SNR could be achieved by leveraging alternative qubit geometries or additional optimization of amplification techniques, potentially advancing toward infallible quantum operations. Furthermore, the paper's framework could be adapted for other quantum systems, extending its applicability far beyond the confines of the demonstrated setup.

The research exemplifies a significant stride toward bringing reliable quantum information processing systems to fruition. The path set by this endeavor could transform quantum computing by enabling robust real-time error correction, elevating the theoretical underpinnings of quantum mechanics into a domain of tangible, practical application.