
Microsoft’s Quantum Mechanics - finisterre
http://www.technologyreview.com/photoessay/531606/microsofts-quantum-mechanics/
======
DubiousPusher
Some good news, the Majorana fermion has recently been independently observed.

[http://www.sciencedaily.com/releases/2014/10/141002141757.ht...](http://www.sciencedaily.com/releases/2014/10/141002141757.htm)

~~~
duaneb
How certain are the findings? It's so hard to tell with particle physics.

~~~
drostie
The Majorana discoveries are condensed matter physics, not particle physics.

In particular, when they say that they've discovered "a new particle" it's
important to understand that this is not going to appear on the Standard Model
of Particle Physics at any time soon (unless, say, neutrinos, which are
already on that model, turn out to be Majoranas).

We've discovered excitations at the end of nanowires which appear to behave
like Majoranas were supposed to, and we've worked out some theory which
predicts that they were supposed to be there, and if they _are_ there then
they're "topologically" protected from a lot of the usual sources of noise, so
we might be able to make long-lived qubits for a change. That's about what we
know about Majorana fermions, minus some details.

------
Zaheer
They forgot the NSA under the Quantum Projects section.

[http://www.wired.com/2014/03/quantum-crypto-
google/](http://www.wired.com/2014/03/quantum-crypto-google/)

~~~
thinkling
Only the lab in Silicon Valley was closed down. The main lab is in Redmond,
and there are still labs in a variety of other locations:

[http://research.microsoft.com/en-
us/labs/default.aspx](http://research.microsoft.com/en-us/labs/default.aspx)

------
mathgenius
FTA: "Freedman was 30 when he solved a version of one of the longest-standing
problems in mathematics, the Poincaré conjecture."

Not true, he apparently made contributions to the poincare conjecture in
dimension 4. Also, the article has a link from "poincare conjecture" to a clay
institute webpage which is broken.

Meh

~~~
tzs
This is what it says about him on the official list of Fields medalists:
"Developed new methods for topological analysis of four-manifolds. One of his
results is a proof of the four-dimensional Poincaré Conjecture".

The article's statement you quoted is accurate enough to not be "not true".

~~~
mathgenius
Ok, thanks for that. I think I'm done with wikipedia.

------
Arjuna
Can anyone expound further on the practical applications of quantum computing?
In my limited understanding, I think the following are definitely candidates
(presented in no particular order), but I'm sure there are others:

1\. Shor's algorithm could expose all encryption algorithms that are based on
integer factorization.

2\. Quantum simulation could open new avenues of research into how our
universe operates at the quantum level. This could lead to advancements in
materials science, for example.

3\. Quantum computing could open new avenues of research into the P versus NP
problem.

4\. Quantum computing could open the door to the possibility of instantaneous
communication via an understanding of action at a distance / quantum
entanglement.

 _Edit: Thanks for all of the great responses, clarifications and links to
further reading._

~~~
evanb
3\. P vs. NP is a problem phrased in the language of classical Turing
machines. In that sense, quantum computing has no bearing on it. It is unknown
whether NP problems can be solved efficiently on quantum computers.

4\. Is a common misunderstanding about entanglement. Unfortunately, while
entanglement does allow non-classical long-distance correlations, it does not
allow for communication. Think of it this way: suppose we took two random
number generators that start with the same seed, and thus produce the same
stream of bits. Then you take yours over _there_ while I keep mine over
_here_. This doesn't help us communicate faster, because we don't get to pick
the bits that come out of the generator. So too with quantum entanglement.

~~~
tomp
Isn't one of the main idea of entanglement/superposition the fact that the
qubits' values are not pre-determined? I.e. the two random generators don't
have the same actual seed, but as soon as the first one is used, it generates
a seed, and the second one will also use the same seed from then on?

I still have some difficulty understanding how that doesn't rule out some
undetectable determinism, and how/if it's possible to measure if the
superposition has already collapsed (i.e. the seed has already been
determined), something apparently useful in quantum encryption.

~~~
drostie
Right, s/he's giving you a sense of "classical entanglement", but quantum
entanglement can also do some interesting things which classical entanglement
cannot do. However, the scope of that is not as simple as "set those bits over
there to an arbitrary state as seen from over here." In fact, if my bits over
here are entangled with your bits over there, the entanglement manifests in
_spooky coincidences_ between our actions which you won't even notice if you
don't bring our actions together and compare them. From our perspectives
individually it looks like we're both doing random actions; but then you find
out that when you bring the actions lists back together they were both the
same; neither of us had the chance to predict or affect what the other one did
but we both agreed on what we did.

Here's a game for a three-person team: they are all cooperating, but the game
is called "betrayal" because we will secretly force one of them to betray the
other two and measure how gracefully they recover from it. We will put the
team through many "tests", if they win all of them, they get lots of money;
you don't get any money for being a traitor in any given test.

Here's how this goes. We put everybody in relativistically separated rooms;
each room has a computer screen and two buttons labeled 0 and 1. Once they're
all isolated we give the group a test. Then they can come back together and
collaborate before the next test, as they see fit.

Some tests are "control" tests. We flash on the screen, simultaneously to all
3 of them, the command "make the sum of your numbers even." We collect the 1's
and 0's together, add them together, and they pass the test if it's even.

Other tests, we randomly choose one as a traitor. We flash on the screen to
the traitor the same "make the sum of your numbers even" prompt, but we flash
to the other two, "traitor round! make the sum of your numbers odd" \-- and
they pass the test if, when we add together the three numbers they press, that
sum is odd.

There's no classical probability distribution on the six random variables
A_even, B_even, C_even, A_odd, B_odd, C_odd which satisfies all of those tests
100% of the time, so that A_even + B_even + C_even is even but A_odd + B_odd +
C_even is odd and so forth. Just add all of the equations together; you'll get
2 * (A_e + B_e + C_e + A_o + B_o + C_o) on the left hand side, but (even +
3*odd) on the right hand side and thus even = odd, which is impossible. There
is no classical 100%-solution.

There is a quantum 100%-solution. If |+> is the state |0> \+ |1> and |−> is
the state |0> − |1> then the (entangled, GHZ) state

    
    
        |+++> + |−−−> = |000> + |011> + |101> + |110>
    

guarantees that any measurement will have an even sum, while the state

    
    
        |+++> − |−−−> = |001> + |010> + |100> + |111>
    

guarantees that any measurement will have an odd sum. The two people who know
there is a traitor in their midst can do the unitary transform which takes |+>
to |+> and |−> to i |−> (which is a "controlled phase rotation" combined with
some "Hadamard gates"), and this defining property of complex numbers that i^2
= -1 causes the total state to switch between those two parameters when any
two people make those transformations.

So if they're playing with an entangled quantum state, two of them can make a
local change to their own state which induces the right sort of change in the
global state that, when we bring the data together and correlate, we find out
that they can do something which classical observers cannot ever do: win a
game with 100% probability. Each of them locally appears to be producing 0 or
1 with a 50/50 probability but globally when we compare their bits we find out
that they could choose together whether that sum was odd or even in a crazy
new way.

------
ianstallings
I think I have some facts twisted up. I thought Microsoft closed it's R&D lab?

[http://www.zdnet.com/microsoft-to-close-microsoft-
research-l...](http://www.zdnet.com/microsoft-to-close-microsoft-research-lab-
in-silicon-valley-7000033838/)

Do they have others?

~~~
tkmcc
Yes, there are MSR campuses around the world! Only the Silicon Valley
satellite campus was closed. See [http://research.microsoft.com/en-
us/labs/](http://research.microsoft.com/en-us/labs/) for the full list.

------
tatqx
Does anyone know or can explain simplistically why if we can get supercomputer
like power from 100 qubits then why not a desktop computer like power with 1
or 2 qubits? I assume that the computational capacity increases exponentially
with the number of qubits, but how?

~~~
gjm11
A quantum computer isn't really much like a super-powered classical computer.
There are certain specific things it's amazingly good at (most notably,
factorizing large integers), some other things it's quite good at (e.g., some
kinds of search operation), and lots of things for which no one knows how to
make quantum computers any faster than classical ones.

So, if you have a quantum computer, you're probably going to want to use it
for breaking RSA or similar cryptography by factorizing large numbers.

There are (ordinary, non-quantum) algorithms for factorizing large-ish
numbers. They work pretty well for numbers of modest size. But they don't
scale well as the numbers get very large. Something like exp(log(n)^1/3
log(log(n))^2/3) to factorize a number near to n. Or, in terms of the number
of bits, something like exp(b^1/3 log(b)^2/3).

So this works fine for numbers up to, let's say, 200 bits on a single machine
or 1000 bits for a team with a substantial network. (Note: all numbers here
are kinda made up but of the right order of magnitude.)

With a quantum computer, you can factorize a b-bit number with something like
2b qubits, b^3 quantum gates, and a runtime that scales more manageably with
b.

So there's not much point in using a quantum computer until the number of
qubits it has gets large enough to use for factorizing numbers bigger than you
can do on your PC. Which means somewhere on the order of 100 qubits. As the
number goes up, the advantage of the quantum computer over the classical
computer increases. The transition from "slower than your PC" to "faster than
any classical computer that will ever be built" takes place over a fairly
small range of qubit-count.

Down with much smaller numbers of qubits, though, the quantum computer can
maybe manage to find the factors of, say, 15. Which is, shall we say, quite a
lot less than the computer on your desktop can do.

~~~
ConceptJunkie
I'm confused, you say you could factor a b-bit number with "something like 2b
qubits", but then go on to say you'd only need 100 qubits to factor bigger
numbers than a PC.

Factoring a 50-bit number is not that big a deal. I used my Python command-
line calculator program (all the hard stuff is done by mpmath, pyprimes and
other open-source code I didn't write) to factor a 50-bit number just now:

c:\>rpn -t 2 50 __7 + factor

[ 37, 30429727211963 ]

29.569 seconds

I'm sure Mathematica is much faster.

It seems to me several hundred cubits would be necessary to outperform
classical algorithms on commodity hardware.

~~~
archgoon
Why on earth would it take 30 seconds to factor a random 50 bit number? You
just check all numbers between 2 and 2^25 (about 32 million). This should take
less then a second, maybe two in python.

The parent poster likely was thinking 100 bit number (200 qubits), which,
while technically tractable, is quite a bit of computing power (assuming that
we don't end up with huge coefficients with quantum computers).

------
peter303
Windows will only take 2 minutes to boot on a quantum computer then.

~~~
_broody
When was the last time you used a Windows PC? Windows 8 boots and shows you
the login screen in under 10 seconds.

