Lenta: 17 Metres per Second

Quantum Foundations arc, page 19 of 193. Back to The Pixelated Physics Universe

Who they are

Lenta walks a beam of light on a leash, slowed to bicycle speed.

Lenta is a character of the Quantum Foundations arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.

The physics

Electromagnetically induced transparency uses a control laser to make an opaque atomic cloud transparent over a narrow range of colours. Across that window the refractive index changes extremely steeply, so a light pulse's group velocity drops enormously. In 1999 Hau, Harris, Dutton and Behroozi slowed light to 17 m/s in ultracold sodium, about 18 million times slower than in vacuum; in 2001 light was stopped.

$$ \begin{gathered} v_g = \frac{c}{n + \omega\,\dfrac{dn}{d\omega}} \end{gathered} $$

Sources

• Hau, Harris, Dutton & Behroozi (1999) Light speed reduction to 17 metres per second in an ultracold atomic gas, Nature 397, 594
• Liu, Dutton, Behroozi & Hau (2001) Observation of coherent optical information storage in an atomic medium using halted light pulses, Nature 409, 490
• Fleischhauer, Imamoglu & Marangos (2005) Electromagnetically induced transparency: Optics in coherent media, Rev. Mod. Phys. 77, 633

Crossovers

Lenta meets Superlumen (Phase Velocity Faster Than Light). Lenta slows a light pulse's group velocity to bicycle speed in an atomic cloud. Superlumen points to the phase velocity, which in a dispersive medium can exceed c. Neither of them carries information faster than light.

Lenta meets Superlumen (Phase Velocity Faster Than Light). In a dispersive medium the phase and group velocities part ways. Superlumen's phase velocity can exceed c, while Lenta's group velocity can crawl. The signal front never beats c.

Wear the lore

• Lenta – 17 Metres per Second Physics T-Shirt

Concept keywords: 17 Metres per Second, Lenta, Quantum Foundations, physics lore, Pixelated Physics