Small interstellar payloads

Yes but as I said you have a large mass and a laser. I am envisaging
total mass of arround 1000-2000Kg to put perhaps 1mg on the Alpha
Centauri system. This would comprise a number of seeds. In acceleration
the craft is close to the solar system so there is no real problem
about a large mass. I would envisage perhaps 100mg of antihydrogen and
a few kilos of deuterium and Helium 3.

To get a large mass on an interstellar journey you need laser systems
at both ends. A nano probe could build a laser system after it had
started replicating. Thus to get a large mass there you launch 2 probes
10 years or so apart.

There is a good SciFi plot here. One way for ET to get here would be
for us to build our end. So ET launches a kamikaze probe which will
transmit radio signals for the required time. ET puts AI on the Web by
this method. Having done this the Web gives us the design for a VN
machine which then proceeds to build such a thing.

The President, who as we all know is chosen by the media, which ET now
effectively controls. The president announces a new star on the flag -
A terraformed Venus. The first Venusians arrive and find that a large
probe is coming towards then at c/2.

This is a good story but I think nanotechnology is the way it will be
done.