Nano-spacecraft: Alpha Centauri in 20 years?

Fascinating idea:

Breakthrough Starshot’s spacecrafts, which they call “nanocrafts,” will be gram-scale computer chips that will include “cameras, photon thrusters, power supply, navigation and communication equipment,” Avi Loeb, a Harvard scientist involved in the operation, told reporters.
A rocket would deliver a “mother ship” carrying a thousand or so of the nanocrafts into space. Once in orbit, the crafts would be propelled with thin sails and hyper-powerful laser beams shot from Earth into the universe to explore and discover. There the crafts would take pictures of their surroundings, which would take around four years to be sent back to Earth.
The nanocrafts would travel at around 20% of the speed of light, Loeb said. At that rate it would be possible to reach Alpha Centauri in around 20 years, and the potentially habitable planets within 70. Using the best currently existing technology, it would take some 78,000 years.

It is estimated the project could cost $5 billion to $10 billion, but Milner is initially investing $100 million for research and development.
Each nanocraft will cost roughly the same to make as a high-end smartphone, allowing a massive number to be sent on the journey.

There are still a number of challenges facing the creation of the nanocrafts, including the creation of the hyper-powerful laser beam — which, in order to launch the crafts, must generate the same amount of power needed for launching a large space shuttle — as well as making the crafts resistant to space accidents, such as impact with dust particles and space debris.

“As far as we can tell,” Loeb assured the crowd of reporters, “there are no deal-breakers in the physics behind this project.”