A thread about the very weird world that we all are part of.
A world that make no sense to anybody but is very real.

@ William Rothamel

What specifically could a quantum computer do that a conventional computer cannot do?

Factorization is one thing that a quantum computer is very good at.
A method to make encrypted transactions that is totally secure from evesdroppers.
Banks and the military are very interested in that technology.
Applications for quantum computers.
Optimization
Machine learning
Sampling / Monte Carlo
Pattern recognition and anomaly detection
Cyber security
Image analysis
Financial analysis
Software / hardware verification and validation
Bioinformatics / cancer research

But apart from that ordinary computer are better to do what we are used to.

Where is there a quantum computer?

There is one at MIT.
And D-Wave has a couple.
Link with many answers to your questions.
https://www.dwavesys.com/quantum-computing
Technical details.
https://www.dwavesys.com/sites/default/files/D-Wave%202000Q%20Tech%20Collateral_0117F.pdf
IBM has a couple as well
https://www.research.ibm.com/ibm-q/learn/what-is-ibm-q/

Here you can test a quantum computer.
https://quantumexperience.ng.bluemix.net/qx/editor

You do all realize that transistors use tunneling between electrons and "Holes"--call it "quantum" if you like.

Yes. It's called Quantum tunnelling and works very well.
https://en.wikipedia.org/wiki/Quantum_tunnelling

Your responses to this question were all things that are commonly done on today's digital computers.

What specifically could a quantum computer do that a conventional computer cannot do?

D-Wave has claimed to be the world's first company to sell computers which exploit quantum effects in their operation, although some researchers disagree.

"Quantum computing is the use of quantum-mechanical phenomena such as superposition and entanglement to perform computation. A quantum computer is used to perform such computation, which can be implemented theoretically or physically." Huh more gobbledy gook--explain why and exactly how these work.

I visited the Quantum computer simulation but would need some instruction on how to use it. This proves that a "quantum" computer can be simulated on a digital machine which proves my point (as in Threshold logic and Neural modeling) it all decomposes into boolean logic which is implemented now at mind blowing speeds in todays computers and super computers.

Some of the links hint that a Quantum computer would be good for only a specific task such as face recognition. Maybe like the past efforts of machines such as the "Perceptron" which once again is well done conventionally much more cheaply and faster--see Apples new phones.

Your responses to this question were all things that are commonly done on today's digital computers.

Correct... very, very slowly. We don't use single-core CPUs and onboard unaccelerated VGA anymore for this reason. I think that trying to slow the inevitable progress of quantum computing is going to be about as successful as bringing them back as the standard.

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To do a Level set and to help dispel my ignorance on the subject--let me just ask how a a four bit (QUBIT ?) full adder would work. Show the truth table, logic diagram and then the hardware please with the registers and the adder..

To do a Level set and to help dispel my ignorance on the subject--let me just ask how a a four bit (QUBIT ?) full adder would work. Show the truth table, logic diagram and then the hardware please with the registers and the adder..

Here you go.

A qubit too much to post here, really. :^) Also not the specified number of bits but it's going to have to do.

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What specifically could a quantum computer do that a conventional computer cannot do?

Actually nothing.
But a quantum computer does parallell computing much faster then a conventional computer. Exponentially much faster.
Explained here:
The Mathematics of Quantum Computers
https://www.youtube.com/watch?v=IrbJYsep45E

D-Wave's computers seems to be still in it's infancy.

D-Wave machines have attracted scepticism as well as excitement since they went on sale six years ago. So far, researchers have proved that, for a problem crafted to suit the machine’s abilities, the quantum computer can offer a huge increase in processing speed over a classical version of an algorithm (V. S. Denchev et al. Phys. Rev. X 6,031015; 2016). But the computers do not beat every classical algorithm, and no one has found a problem for which they outperform all classical rivals.
D-wave’s qubits are much easier to build than the equivalent in more traditional quantum computers, but their quantum states are also more fragile, and their manipulation less precise. So although scientists now agree that D-wave devices do use quantum phenomena in their calculations, some doubt that they can ever be used to solve real-world problems exponentially faster than classical computers — however many qubits are clubbed together, and whatever their configuration.

BTW. How To Make a Quantum Bit
https://www.youtube.com/watch?v=zNzzGgr2mhk

To do a Level set and to help dispel my ignorance on the subject--let me just ask how a a four bit (QUBIT ?) full adder would work. Show the truth table, logic diagram and then the hardware please with the registers and the adder..

There are very few technical differences between the computer types. "Only" for the quantum registers that handle qubits instead of classical bits. "Merely" a design problem with different interfaces.
The problem is though with a quantum computer (not to mention the difficulties to get a result at all) you never get an exact result. So you run the computation several times.
Otherwise truth table, logic diagram and adders (adders, which I think you mean how arithmetics work) works the same.

Some explainations and thoughts from scientists that are in the forefront about quantum computers.
Quantum Computing - Prof. Andrea Morello
https://www.youtube.com/watch?v=7susESgnDv8
The Future of Quantum Computing - Prof. Seth Lloyd at MIT
https://www.youtube.com/watch?v=5xW49CzjhgI

I've taken three online courses on quantum computers, one by a Japanese University and two by the Nederlands Delft Technical University. These two were more difficult that the Japanese course and on the first one I failed to obtain the 60% average on tests, so I could not claim a diploma. On the second Delft course I obtained a 62% average, but since they wanted an image of my face taken by a webcam and also that of a document like a driver license, I still could not obtain a diploma. They said my images were blurry. But I have the diplomas from Keio University, Japan, Paris Diderot University for a course on Gravitation and gravitational waves, and a Statement of Participation from the University of Edinurgh on the discovery of the Higgs boson. None of them asked for an image.
Tullio

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Ah.The irony.
TU Delft are now very much studying the science of facial recognition. Blurry images or not:)
https://techacute.com/tu-delft-digital-id-video/
https://www.tudelft.nl/en/2012/tu-delft/from-facial-recognition-to-medical-imaging/
And of course also trying to utilize quantum computers to do it.
Have they forgot? It's about probabilities not exact results.

Thank you for the links I will study the diagrams and work the truth tables etc. I want to know if this will go the way of Neural networks 50 years ago or whether this will be useful for commerce. As for digital computers--it is, I am sure, the desire to shrink the basic switch (Transistor) and the memory down to atomic size. This you might call "Quantum" since this term seems to be applied just to anything small.

If we need liquid helium and microwaves to make a single atom work then I doubt anything more than research interest will come out of these efforts.
We shall see. I never thought that in the era of large IBM mainframes desktops would be anything more than a toy ==LOL.

If we need liquid helium and microwaves to make a single atom work then I doubt anything more than research interest will come out of these efforts.
We shall see. I never thought that in the era of large IBM mainframes desktops would be anything more than a toy ==LOL.

Indeed... the (almost) first electronic computer, ENIAC, required an air conditioner to stay within thermal limits that weighed about sixty tons.

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When I was at Trieste Area Science Park and managed a BULL/MIPS minicomputer, a RISC machine, CERN made a test on it after asking me for an account and it resulted it had the same speed on the Linpack test of a DEC VAX9000 which cost ten time as much. The DEC was soon scrapped and so the CRAY of the Trieste University, all substituted by networks of workstations like mine.
Tullio

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At last. I have managed to do a program that change the state of a five qubit register.
|00000> to |00101>, that is changing the value from 0 to 5.
I used IBM Q Experience for this and the "computation" was done on their device ibmqx4.
https://quantumexperience.ng.bluemix.net/qx/editor?codeId=f77e352ad8aa679ef4c1052263d8d960

include "qelib1.inc";
qreg q[5];
creg c[5];

x q[0];
measure q[0] -> c[0];
x q[2];
measure q[2] -> c[2];

The result after 1024 shots
0 - 0.8%
1 - 7.1%
4 - 8.4%
5 - 83.7%

Pretty close:)

I found a Youtube channel that are showing interviews with many scientists, that are big thinkers all about from the quantum world to outer space and even the origin of the universe.
Questions like "Why isn't there nothing at all? Why is it not the case that there is no cosmos, no laws of nature, no consciousness, literally nothing at all? Scientists claim that the universe came from nothing. But what's the nature of that kind of nothing?" are also discussed:)
And Jill Tarter - Why aren't Aliens Already Here?
Closer To Truth
https://www.youtube.com/channel/UCl9StMQ79LtEvlrskzjoYbQ

It's Sunday and I think I'm in a Superpositional State...
Not really. But the concept is quite difficult to grasp.
Here is what I think a better visual experiment than the Schrödinger's cat thought experiment.
A coin can have two states, either head or tails, when it's on a flat surface.
But if you spin the coin you cannot tell if it's head or tails.
https://www.youtube.com/watch?v=sOznUxFZW34
The coin is now in a Superpositional State with an infinite number of states between head or tails.
It's not until after it's stop spinning you can tell the outcome.
Scientist call it that before you measure the state it's uncertain what the state is.
When the measurement is done it's called a wave function collaps.
It was btw Schrödinger who came up with the wave function that describe the propabilities of an outcome for particles in the quantum world.

The Chinese Micius Foundation has given prizes for 2018 and 2019 to scientists working in the fields of quantum computing, quantum cryptography and quantum communications also via satellite. A few I know are Peter Shor, David Deutsch, Arthur Ekert, Anton Zeilinger and Pan Jian-Wei, the only Chinese.
Prizes are 150000 $ each.
Tullio

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Another of the Micius Quantum laureates is David J. Wineland.
"For his groundbreaking experiments that opened the way to quantum computing and quantum metrology with trapped ions"
http://www.miciusprize.org/index/lists/003001
He was also awarded the 2012 Nobel Prize in Physics, jointly with Serge Haroche.
"For their ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems".

When the measurement is done it's called a wave function collaps.

Garbage talk.

It's Wednesday:)
https://en.wikipedia.org/wiki/Wave_function_collapse

In quantum mechanics, wave function collapse is said to occur when a wave function—initially in a superposition of several eigenstates—appears to reduce to a single eigenstate due to interaction with the external world; this is called an "observation". It is the essence of measurement in quantum mechanics and connects the wave function with classical observables like position and momentum.

Read Lee Smolin's book "Albert Einstein's unfinished revolution: the search fo what lies beyond the quantum", Penguin Press, 2019.
Tullio

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