A perfect crystal neutron loop cavity

Abstract

Coherent control of neutrons via Bragg diffraction forms the foundation of perfect crystal neutron interferometry, facilitating both fundamental tests of quantum mechanics and applications in quantum information science. In cavity geometries, perfect crystals enable neutron confinement and have been employed in precision measurements of spin-orbit interactions and for neutron electric dipole moment (nEDM) searches. However, in these conventional configurations, neutrons undergo a single pass through the crystal geometry, placing a physical constraint on both crystal and in-flight interaction times and measurement sensitivity. In this work, we introduce a neutron loop cavity that coherently recirculates neutrons through repeated Bragg reflections between perfect silicon crystal blades. This structure is predicted to achieve a neutron survival probability of $\sim64~\%$ for 10,000 Bragg reflections, corresponding to confinement times on the order of seconds. We propose a Schwinger interaction measurement that achieves a $π$ spin rotation in 800 Bragg reflections, representing more than a tenfold improvement in sensitivity over recent measurements. Further applications include high-sensitivity nEDM searches targeting the $10^{-27}~$e$\cdot$cm scale, as well as competitive experimental tests of neutron parity violation, the neutron lifetime, and the quantum Zeno effect with neutrons.

Figures (4)

• Figure 1: Schematic of neutron propagation through three perfect crystal cavity geometries: (a) compact double Bragg blade gentileneutron_cav, (b) extended double Bragg blade vesta, and (c) the proposed loop cavity. In (a-b), the crystals are attached to a common base and $\theta_B$ is near $90\degree$ to maximize the number of bounces. In (c), the four identical Bragg blades are independently mounted and aligned to operate at $\theta_B = 45\degree$, confining neutrons to repeatedly traverse the same path. Note that only a few pathways of diffracted intensity are shown for simplicity.
• Figure 2: (a) Simulated neutron intensity inside the loop cavity after the first four reflections. Neutrons are incident to the rightmost Bragg blade at $\theta_B = 45\degree$, undergoing subsequent reflections in a counter clockwise direction. As the number of reflections within the loop increases (b-c), the neutrons within the Darwin width remain confined, with minimal intensity escaping through the crystals. In these simulations, the crystal thickness and crystal length are compact for visualization purposes.
• Figure 3: Simulation of the designed cavity performance ($t = 100\Delta_H$, $L = 2000\Delta_H$). (a) The evolution of the reflectivity, plotted as ($1-R(N)$), versus the number of reflections $N$. The reflectivity reaches $R = 1-10^{-8}$ at $N = 10^4$ reflections. The escape probability ($1-R(N)$) follows two distinct power-law regimes: an initial rapid decay ($\propto N^{-3}$) transitioning to a shallower dependence ($\propto N^{-1}$), indicated by the dashed lines. The inset demonstrates the confined intensity $I(N)$ versus the number of reflections $N$. (b) Transverse intensity profile after $10^4$ reflections, which is fit to a $\mathrm{sinc}^2$ function. (c) Corresponding transverse momentum distribution showing the rectangular pulse characteristic of the Darwin plateau, with visible Gibbs ringing. The inset demonstrates the neutron transverse coherence $\sigma_\perp$ rapidly saturating to $1~\Delta_H$ as a function of $N$, in agreement with the confined neutron momentum becoming restricted to the Darwin plateau.
• Figure 4: (a) Confined intensity as a function of the number of reflections $N$ for various Bragg blade thicknesses $t$ and $L = 2000\Delta_H$. For $t < 4\Delta_H$, intensity loss increases with $N$, while for $t \geq 4\Delta_H$, confinement is maintained and does not significantly improve with further increases in thickness. (b) Confined intensity as a function of crystal blade surface roughness/defect density. Sensitivity to surface imperfections increases with the number of reflections. For up to $10^2$ reflections, roughness has minimal impact, but for $10^4$ reflections, crystal surfaces must be defect-free to preserve confinement.