[Paper Review] Realization of a complete Stern-Gerlach interferometer

Yair Margalit, Zhifan Zhou+5 more|arXiv (Cornell University)|Jan 8, 2018

Optical Polarization and Ellipsometry1 references18 citations

TL;DR

This paper demonstrates the first complete, high-visibility Stern-Gerlach interferometer using macroscopic magnetic fields from an atom chip, achieving 99% spatial interference visibility and 95% spin interference visibility. By precisely controlling magnetic gradients at room temperature, the authors realize a full-loop interferometer and observe the gradual emergence of time-irreversibility, validating the long-theorized Humpty-Dumpty effect in a controlled quantum system.

ABSTRACT

The Stern-Gerlach (SG) effect, discovered almost a century ago, has become a paradigm of quantum mechanics. Surprisingly there is little evidence that the original scheme with freely propagating atoms exposed to gradients from macroscopic magnets is a fully coherent quantum process. Specifically, no high-visibility spatial interference pattern has been observed with such a scheme, and furthermore no full-loop SG interferometer has been realized with the scheme as envisioned decades ago. On the contrary, numerous theoretical studies explained why it is a near impossible endeavor. Here we demonstrate for the first time both a high-visibility spatial SG interference pattern and a full-loop SG interferometer, based on an accurate magnetic field, originating from an atom chip, that ensures coherent operation within strict constraints described by previous theoretical analyses. This also allows us to observe the gradual emergence of time-irreversibility as the splitting is increased. Finally, achieving this high level of control over magnetic gradients may facilitate technological applications such as large-momentum-transfer beam splitting for metrology with atom interferometry, ultra-sensitive probing of electron transport down to shot-noise and squeezed currents, as well as nuclear magnetic resonance and compact accelerators.

Motivation & Objective

• To realize a fully coherent, complete Stern-Gerlach interferometer using macroscopic magnetic fields, overcoming long-standing theoretical skepticism about feasibility.
• To test the hypothesis that time-irreversibility in quantum processes arises from imprecision and instability in quantum operations, as predicted by the Humpty-Dumpty effect.
• To achieve high-visibility spatial and spin interference patterns in a freely propagating atomic beam without relying on Bose-Einstein condensates or laser fields.
• To demonstrate that macroscopic magnets can provide the required precision for coherent quantum operations, enabling new applications in metrology and quantum sensing.

Proposed method

• The experiment uses an atom chip to generate highly accurate, spatially tailored magnetic field gradients that act as a spin-dependent force on a thermal atomic beam.
• A half-loop interferometer configuration uses a π/2 pulse to prepare superposition states and a long stopping pulse to focus wavepackets, enabling spatial interference fringes.
• A full-loop interferometer employs active recombination via a short stopping pulse, requiring high precision to maintain coherence and produce spin population fringes.
• Interference visibility is measured via spatial and spin population imaging using a multi-channel plate and CCD camera, with phase information extracted from polar plots.
• Theoretical modeling accounts for beam velocity spread and wavepacket separation, distinguishing true spatial interference from Ramsey-type phase modulation.
• The authors compare their results with prior experiments, showing that previous observations of transverse modulations were due to Ramsey interferometry, not spatial splitting.

Experimental results

Research questions

• RQ1 Can a complete Stern-Gerlach interferometer be realized using macroscopic magnets at room temperature with high coherence?
• RQ2 To what extent does imprecision in magnetic field gradients lead to time-irreversibility in quantum processes?
• RQ3 Is it possible to observe high-visibility spatial interference with freely propagating atoms in a magnetic gradient field?
• RQ4 How does the coherence of the interferometer depend on wavepacket separation and magnetic field precision?
• RQ5 Do observed transverse modulations in prior experiments represent true spatial interference or are they artifacts of Ramsey interferometry?

Key findings

• The experiment achieves a spatial interference visibility of 99% in a full-loop interferometer, demonstrating high coherence and precision in macroscopic magnetic field control.
• A spin interference visibility of 95% is observed, confirming the time-reversible nature of the process under optimal conditions.
• The wavepacket separation reaches up to 18 times the wavepacket width, significantly exceeding previous experiments where separations were comparable to coherence lengths.
• The authors observe the gradual emergence of time-irreversibility as splitting increases, providing the first experimental demonstration of the Humpty-Dumpty effect.
• Transverse modulations in prior experiments are shown to result from Ramsey interferometry, not spatial interference, resolving a long-standing ambiguity in the literature.
• The study confirms that high-visibility interference is achievable with macroscopic magnets, challenging the long-held belief that such systems are inherently incoherent.