

Scientists create first electronic quantum processor - dlnovell
http://www.physorg.com/news165418586.html

======
gjm11
Already on HN at <http://news.ycombinator.com/item?id=678236> with a link to
_Nature_ at
[http://www.nature.com/news/2009/090628/full/news.2009.603.ht...](http://www.nature.com/news/2009/090628/full/news.2009.603.html)
.

The physorg article does have one advantage over the Nature one: it has a
picture of the solid-state device.

------
Dilpil
Can anyone explain in technical detail how the phone book problem works?

~~~
gjm11
<http://en.wikipedia.org/wiki/Grover%27s_algorithm>

In _handwavy_ technical detail: arrange that the things you're searching form
a basis a1, a2, ..., aN for your state space, with your have-I-found-it test
as an operator that negates the state you're looking for and maps the others
to themselves. In other words, it's reflection in the hyperplane perpendicular
to the state you're looking for. Now let a = a1+...+aN, and define a new
operator that reflects in the hyperplane perpendicular to a. And now put your
system in state a, and repeatedly apply those two operators, one after the
other.

The composition of two reflections is a rotation. The angle of that rotation
is known (and quite small); the angle between the two vectors (your desired
state and a) is close to a right angle; by doing the rotation an appropriate
number of times, you bring the system's state vector close to the desired
state. Now you measure the system's state, and with high probability it's the
state you're looking for.

A little calculation shows that the number of rotations needed is roughly
proportional to sqrt(N), and the probability of getting the wrong state is
roughly proportional to 1/N. Now do the process as many times as it takes for
1/N^k to be small enough for your satisfaction. Congratulations: you've now
found what you were looking for, taking about sqrt(N) time, with error
probability as small as you like.

