The 3D-printed fitting could help to miniaturize cold-atom sensors by reducing the need for continuous pumping.
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Making Hidden States Visible
Experiments with programmable electroacoustic cavities reveal that a multistable system can be steered into states that are unreachable with conventional control methods.
Many physical systems can be in more than one stable state: A laser can be on or off, and a magnetic bit up or down. This multistability can appear in nonlinear resonators—such as ferromagnets and genetic toggle switches in cells—that are driven into different steady states, or “branches,” by ramping up and down the driving parameter [1]. This behavior is often pictured using a familiar hysteresis loop that traces the system’s trajectory between a lower branch and higher branch (Fig. 1). It is easy to imagine that additional steady states might coexist with those sampled, but experiments have largely ignored that possibility, assuming instead that slow, quasistatic parameter sweeps reveal all “physically relevant” behavior.
In a new acoustic experiment, Kun Zhang from the Wuhan University in China and colleagues challenge that assumption [2]. They show that a pair of coupled acoustic cavities can host a fully “folded” steady state that is perfectly stable yet invisible to conventional sweeps. This hidden branch can, however, be reached with carefully designed sound pulses, the team shows. These results—combined with those from another recent study [3]—turn the abstract idea of hidden multistability into a concrete and controllable feature of nonlinear resonator networks, which might one day be used to securely store sensitive information.
A Very Stable Mirror
To make an ultrastable laser beam for applications such as gravitational-wave detectors, the frequency of a beam confined within an optical cavity is locked to the cavity’s resonant frequency. This frequency is determined by the cavity’s length. The stability of the laser beam’s frequency and the quality of the cavity’s resonance depend on the thermal noise of the mirrors that define that length. Dahyeon Lee at JILA and the University of Colorado Boulder and his colleagues have now demonstrated a crystalline mirror coating with superior thermomechanical properties compared to conventional coatings [1]. The new coating could lead to ultrastable cavities for optical clocks and next-generation interferometers.
Recently, mirrors coated with crystalline alloys of gallium arsenide (GaAs) have emerged as promising candidates to replace those with conventional amorphous dielectric coatings. GaAs-coated mirrors have excellent optical qualities and exhibit low thermal noise at room temperature. But previous studies found that these crystalline coatings suffer from additional noise contributions, which undermine their potential usefulness.
The origins of some of those noise contributions remain unclear. Nevertheless, Lee and colleagues have demonstrated that crystalline GaAs-based coatings can still be superior at cryogenic temperatures. The researchers constructed a 6-cm-long cavity bounded by two mirrors made of alternating layers of GaAs and aluminum gallium arsenide on silicon substrates. They used more layers compared to previous experiments, which reduced photon loss. Operating the cavity at 17 K, where the thermal expansion coefficient of the silicon substrate is zero, they achieved a frequency stability of 2.5 × 10−17. This stability is 4 times better than the expected limit for conventional coatings and sets a new record for cavity-stabilized lasers.
2.8 Days to Disaster: Low Earth Orbit Could Collapse Without Warning
A new analysis suggests modern satellite networks could suffer catastrophic collisions within days of losing control during a major solar storm. The phrase “House of Cards” is often associated today with a Netflix political drama, but its original meaning refers to a structure that is inherently
