Re: Randomness from a Quantum source?
- From: David Eather <eather@xxxxxxxxxx>
- Date: Fri, 28 Aug 2009 02:15:02 +1000
Unruh wrote:
Wolfgang Draxinger <wdraxinger@xxxxxxxxxxxxxxxx> writes:
David Eather wrote:
Please have a look at this thing:
http://www.idquantique.com/products/quantis.htm
It makes a claim of 4 or 16 Mbits per second of random data based a
quantum process. I don't dispute the method but is that kind of data
rate feasible / compatible with the concept of measuring one photon at a
time?
Of course. All you need is a very weak laser that produces that amount of
photons per second with a total random polarisation. At a wavelength of
640nm this amounts to a power of 0.7pW.
Getting a laser to produce "totally random polarizations is hard.
This laser is directed to a 50:50 beamsplitter, and at each branch a
avalance photodiode (those can detect single photons). One diode yields a
0-bit the other a 1-bit.
The question is whether or not the photodiode can recover on the 10s of
nanoseconds timescale. Also are you sure that the beamsplitter is really
50-50( biasing the 1s and 0s)? How are you sure? Are you sure that the photodiodes do not become
more/less sensitive after detecting a photon for a while (biasing the
consecutive 1 or 0), or that the dark noise changes after a detection?
Or that the diode produces 1/f noise? All physical systems have biases of some sort
or another. Now they are often predictable or correctable. But if you
are not careful, they bite you.
We have such a thingy at my university, and there's even a service for
students, that they can bring a mobile storage (HDD, USB flashdrive, etc.)
which is filled up with a perfect random bitstream.
Wolfgang
Bill,
If I may ask, how big a doubt would you have that this thing -
http://www.idquantique.com/products/quantis.htm
might not be producing true random data at the data rate suggested?
.
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