

'Unprecedented' discovery could propel quantum computers to reality - pairing
http://www.thebunsenburner.com/news/unprecedented-discovery-could-propel-quantum-computers-to-reality/

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tux1968
"For qubits, they can hold a value of “1” or “0” as well as both values at the
same time. Described as superposition,"...

Am a bit frustrated reading that same description for years and still having
no clue what it really means. Both values at the same time... huh? What is the
physical property that is existing in two states at once? And how does that
feature translate into speedier computation? Is it so hard to understand that
it can't be explained to a layman in a bit more depth?

Would appreciate any references since everything i've managed to find is
either way too technical, or too vacuous.

~~~
zitterbewegung
Think of it like this.

What if you represented your computer bits as a arrow on a compass. You have
also the plane that the compass is on.

The arrow points in one direction for 1 and in another direction for zero.

Regular computation would have you doing operations on bits. You could only do
one operation at a time on one bit and you are able to flip them given a
recipe.

A probabilistic computer allows you to do some special things. For example you
have this magical random source where you can add a magic variable. This magic
variable is a random variable. This is used in the following algorithm. We
call it fermats little theorem.

Given a prime how do we test its primality? We would have to at minimum divide
each number lower than itself? (there are optimizations I am glossing over for
this exercise).

What if we use this magical random variable. It can be a number from 0 to a
natural number?

If we have a few other algorithmic tools we can make a probabilistic search of
a array. With a random variable we can do the following operation

findingA_LV(array A, n) begin repeat Randomly select one element out of n
elements. until 'a' is found end

This will search a array A randomly and works with probability 1 (this is very
important... if your probability is 0 then you probably don't have a working
algorithm ;) )

Whats the advantage? Well, instead of having a sequential search of an array
you can randomize the search. The probability of success is 1! So, if we are
really lucky we could win very fast. If the array is SMALL then that might be
even better right? If the array has one element then we will always win but
thats not fun :(

But, a more useful example is the fact that quicksort with a randomized sort
can get FAST {O(nlogn)}

Now, lets make a quantum leap. Lets go faster than FAST. Lets go ridiculous.

Lets say that we have a new magical bit. People call it the qubit. The more
you have the better you are off.

Superposition can be described as follows. You can correlate two qubits
together magically. Algorithmically no one cares how . On the Engineering side
the ability to do this is worth trillions of dollars though. The advantage has
the following feature.

If we want to factor a number we can do it faster than any known classical
algorithm. By classical I mean following the rules of your standard computer.

Quantum computers allow you to BREAK the rules of classical ones. They do this
by doing the following things.

Lets review the two computers we have discussed before.

The first kind of computer is called a Turing Machine or a Recursively
Enumerable Language / Problem. This basically follows rules due to a finite
control (a CPU basically) and has a tape which you can write to, go left or
right.

The second kind of computer has the magical randomness. This is a randomized
algorithm. If you are feeling lucky you might get lucky and this reduces your
runtime. In the average case you probably win more often than you lose
hopefully.

Our magical quantum compuer allows for correlation of two or more "variables"
(called qubits) . How is this better?

In quantum physics when you observe a particle you "collapse its wave state".
This is making an observation. This is analogous to a return variable in any C
like language. The difference is how it returns and while in your function it
does some magical things.

The magic is as follows. A qubit can be observed in something called a
computational basis. What does that mean? It means you make two vectors in the
"plane" or the "complex plane" or basically you have a 3d arrow that points
wherever you want. They call this the bloch sphere.

With this magic you can do magic tricks. This magic trick is called sorting a
UNSORTED database in O(sqrt(n)). This sounds ridiculous right?

You are performing operations that are LESS than the size of the array... You
aren't LOOKING at every entry.

This is ridiculous. How do they do this?

Instead of thinking of the database as an array think of it as a function.

We are trying to invert the following function. Encode all of the elements of
the database as a vector in an "n-dimentional state space" this will require a
logarithmic amount of qubits.

Given a function which returns true or false if the element is there you can
write an algorithm that searches it using superposition EXTREMELY fast
O(sqrt(n))

It works by creating a correlation between the state space or basically all of
the qubits are correlated together. Then you start rotating all of the qubits.
Since they are tied to a function which tells you if they are equivalent to
your target variable then this requires sqrt n operations to do so. They
correlate this by doing a "uniform superposition" or entangling all of the
elements and then you start rotating the state space to get what you want.

Thats why we want these things. They are just faster. They break rules but
they are hard to build.

References [http://www.amazon.com/Quantum-Computing-Computer-
Scientists-...](http://www.amazon.com/Quantum-Computing-Computer-Scientists-
Yanofsky/dp/0521879965)

~~~
sp332
_Superposition can be described as follows. You can correlate two qubits
together magically._

That's entanglement not superposition.

~~~
zitterbewegung
Yea, sorry. I haven't studied quantum computing so I am trying to do a plain
English explanation to further my understanding.

I should have described entanglement as the correlation of two qubits at after
their interaction when they are separated.

Superposition should have been described as both particles being able to take
any state in a set of two spherical vectors.

I will correct this when I explain it further. Thanks sp332!

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imrehg
I'm nitpicking just a little, but isn't discovery always about something
unprecedented? Otherwise it's not that different from the rest of the
scientific work people do.

As a disclosure, I'm a physicist, having worked on quantum computing during my
PhD. I think it's really cool, though long qubit lifetime doesn't take you
far, the devil is in the details, for example good (reliable, low noise and
tunable) qubit-qubit interaction (how in this case?), and reproducible devices
(not at all in this case, every diamond will be different).

~~~
sp332
I think a discovery that improves the storage time of qubits by 6 orders of
magnitude is unprecedented. Also, even if this technique has terrible qubit-
qubit interaction properties, it would still be useful for storage. You could
just read the qubit out, do computation in a different medium, and store the
result back.

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tete
It's always a bit creepy to see this. I mean, if one is really close to this
it could mean NSA or DARPA could have the know how and especially the money to
already have one.

Well it's actually just the money. Money to buy knowhow, researchers,
facilities and what else they'd need. It's not only about such things, but if
you see how EFF built the DES cracker with donations or a group of security
researchers breaks stuff with playstations and then see how much money and
power government organizations without any transparency have then it's really
creepy.

It's also a bit creepy how much power the bosses of such institutions
potentially have when compared to something like a president. No wonder there
are so many conspiracy theories.

~~~
borlak
have you read Cryptonomicon? :) you might like it

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perlgeek
Original publication at <http://www.sciencemag.org/content/336/6086/1283.full>
(not sure if it's behind a paywall, I can read it but I'm in a University
network right now).

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tete
Can someone maybe describe the details of "could propel" and what exactly
"reality" mean?

I've always been told "when quantum computers become reality you can forget
about everything you learned on computer science".

For me reality would be that one can use it in production, even if it's a
complete niche.

In other words is it "we got a step closer" or "we potentially solved our only
(major) problem"?

~~~
freehunter
Quantum computers are used in production, as you say in a complete niche.
Lockheed Martin bought a quantum computer (and associated research &
development) from D-Wave for use in their "most complex computations". [1]

There's no further notice of what Lockheed has done with it, but at least
there's one quantum computer in private hands designed to do production work.

[1] [http://www.marketwire.com/press-release/d-wave-systems-
sells...](http://www.marketwire.com/press-release/d-wave-systems-sells-its-
first-quantum-computing-system-lockheed-martin-corporation-1518805.htm)

~~~
riffraff
in my understanding the jury is still out on whether D-Wave actually has a
quantum computer or not, e.g. <http://www.scottaaronson.com/blog/?p=954>

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Difwif
We always hear about the ramifications a quantum computer would have if used
to crack enterprise level security. Does there exist encryption that would be
troublesome even for a quantum computer to crack or are we just SOL if one of
these falls into the wrong hands?

~~~
dvdkhlng
Lattice-based cryptography (<https://en.wikipedia.org/wiki/Lattice-
based_cryptography>) is currently believed to be quantum-computer resistant
(i.e. requires super-polynomial time to break even on a sufficiently large
quantum computer).

There are only very few problems were quantum computers achieve an exponential
speedup vs. classical computers, factoring being one example.

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kokey
Apart from the fairly wonderful applications in computing, it will be
interesting to see what it does for the enhancement of synthetic diamond
technology. I am not going to invest my pension in De Beers, unless they're
going large in the synthetic diamond business.

~~~
hessenwolf
I developed the system years ago that De Beers used to track the recipes for
all of their synthetic diamonds. Small world.

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loceng
This I believe is how certain aspects of the mind exists, information being
temporarily stored and interactive with other systems. Theoretically then
systems of the brain/mind should be able to be duplicated. This would allow
for managing of large amounts of information, at very high speed, with no-heat
generation. The only heat generated would be through the support systems /
foundation - I suppose much like how the brain functions as a foundation for
the mind.

A brain made out of a diamond? Indiana Jones anyone?

~~~
Bakkot
Not sure if you meant to imply the brain is an inherently quantum system or
not, but if so:

> Theoretically then systems of the brain/mind should be able to be
> duplicated.

<http://en.wikipedia.org/wiki/No-cloning_theorem>

It's actually theoretically _impossible_.

~~~
loceng
Interesting. I wasn't implying it's a quantum system, not through entanglement
anyhow. I'm not sure it would need to be linked in such a way for it to play
its role in overall functioning.

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swombat
There's something really awesome and science-fiction-like about the idea that
when we finally build super-awesome AIs (like Iain M Banks' "Minds"), who then
either destroy us or become our best pals in the whole universe, their brains
will be built of diamond.

~~~
dvanduzer
The bar is set pretty low so far. All the diamond has to do is factorize 21.

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vasco
If you can store values for a given time frame with such time frame higher
than the delay of the circuit, why don't we just build a latch and extend the
time frame indefinitely? Or are these diamonds only settable once, and then
you have to build another one?

~~~
Sharlin
Because when you read a qubit, you collapse the superposition and end up with
a boring old 0 or 1. Which is what you want when the algorithm is completed,
but during the execution of the algorithm you want to do operations on the
superpositions. So you cannot read the value and feed it back because in the
process, you destroy it.

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rsanchez1
Qubits in the Sky with Diamonds

If they can achieve two seconds with diamonds, you have to wonder what they
will be able to achieve with graphene, another allotrope of carbon. Maybe they
will be able to find metamaterials that can store information in qubits
indefinitely. This is indeed an exciting development.

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hexagonal
Flagged.

~~~
freehunter
If you're going to take the time to make a comment, how about making a comment
worth reading? "Flagged" doesn't mean anything useful.

