modulation

A sound signal is conveniently represented as a complex number: its modulus (amplitude) reflects loudness, while its argument (phase) indicates the wave's position in time: its instantaneous shift.

In embryonic development, a cell resembles a musician in a vast orchestra: it must play its part, following a conductor — the gradient of signaling molecules. But the conductor is unreliable: morphogen concentrations constantly fluctuate. How then does a cellist not veer into a violin melody?

An ordinary photometry detector can't tell a left photon from a right one. But let it start moving, and everything changes. Imagine a rower moving between two flutists playing the same note. His ear is perfectly tuned to that note and ignores all others. As he moves, the pitch from the flutist ahead sounds higher, and from the one behind sounds lower.

Show the same photo of a cat to two different neural networks, and they often reach a similar conclusion: "cat." But ask them for more detail, and their answers may diverge. One model focuses on the whiskers, another on the coat color, a third on the pose. So even within the "visual" world of neural networks, there's no complete unity.

An ordinary camera captures light reflected from objects and builds a picture from it. But if the object is hidden behind an obstacle or does not reflect light, radio waves come to the rescue: they pass through walls and fog, and a computer reconstructs the image from scattered signals.

Quantum computers and sensors operate at microwave frequencies, but transmitting their signals over long distances is conveniently done via optical fibers, which use light.

A steady laser beam is like a taut string: if it vibrates even a little, the music turns into cacophony. In a quantum computer, the laser holds atoms in a trap, and any frequency jitter ruins the computation. That's why laser setups used to occupy entire tables, and tuning took weeks.

A typical accelerator is like an assembly line where electrons are the workpieces. To get the desired X-ray pulse, the workpiece is fine-tuned after it's already on the line: squeezed, filtered, corrected. In the new work, engineers decided to take a different approach — shaping the workpiece before it even hits the conveyor. At LCLS-II, electrons are knocked out of a metal by a short laser pulse.

A green flashlight beam dives into a pool and reaches the bottom with barely any loss. Water is transparent to green light, which is why it's used for underwater communication. But to send video or sensor data, the light needs to be switched on and off very quickly — billions of times per second. Ordinary lasers are too slow and unstable for this.

A beam of light passing through a transparent plate usually keeps its color. But if you stack the plate from thin alternating layers, it becomes a grating that reflects and transmits waves selectively. This has long been used in lasers and optical filters. Now such gratings have been made to vibrate, and light started changing color.

If you compress a plate with accordion-like cells, a kink wave runs through it, like a wrinkle in a rug when you push it with your foot. Such waves can switch the material between two stable shapes, and this is the basis of mechanical memory. But if a table leg gets in the way of the wrinkle, it gets stuck. In a new study, researchers showed how to nudge it loose without brute force.

You are counting sand grains falling in an hourglass, but each time the wind adds or removes a few. In the most sensitive cameras, the task is similar: you need to figure out how many "sand grains" (electrons) the sensor itself added, and how many came from light. This inherent sensor noise is called readout noise, and it hinders seeing weak signals — for example, the light of a distant star.

Light passing through a thin crystal can change color and the direction of its oscillations. In a molybdenum disulfide flake just hundreds of atoms thick, an infrared beam turns green, and its polarization, the way the light wave oscillates, obeys the crystal's geometry.

A laser pointer shines a neat spot: all its light waves march in step, like soldiers on parade. Such light is called coherent. But nature and technology also feature another kind of light—'messy' light, where waves are out of sync, like a crowd at a market. It's called partially coherent.