The Case Against Quantum Computing

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nsaspook

Joined Aug 27, 2009
12,830
https://spectrum.ieee.org/computing/hardware/the-case-against-quantum-computing
To my mind, quantum computing researchers should still heed an admonition that IBM physicist Rolf Landauer made decades ago when the field heated up for the first time. He urged proponents of quantum computing to include in their publications a disclaimer along these lines: “This scheme, like all other schemes for quantum computation, relies on speculative technology, does not in its current form take into account all possible sources of noise, unreliability and manufacturing error, and probably will not work.”
https://phys.org/news/2013-01-quantum-strategies-capacity-optical-channels.html
Phys.org)—Quantum techniques have been demonstrated to offer improvements in areas such as computing, cryptography, and information processing, among others. But in a new study, researchers from IBM have proven that no quantum trick – no matter how complex or exotic – can improve the capacity of a type of quantum channel that serves as a building block of quantum optical communication systems. Although the result is somewhat surprising and a bit disappointing, it will help guide scientists to explore other ways to enhance channel capacity.
...
"Researchers thought that quantum effects could improve the capacity of Gaussian channels because there are examples of more complicated channels (though somewhat contrived) where entangled signal states can be used to boost the capacity of the channel. Also, in terms of actually proving limits on the possible size of such an effect, there were huge gaps between the best known achievable rates and the best upper limits on the capacity. That gap looked like an opportunity."


However, efforts to improve channel capacity with quantum effects have fallen short. In the new study, König and Smith have finally shown why by providing the first mathematical proof showing that quantum strategies are essentially useless for increasing channel capacity; although the proof doesn't rule out some very small capacity increases, they would be too small to care about.

To reach this conclusion, the researchers considered the concept of entropy, which is a measure of a channel's noisiness and closely related to capacity. They mathematically showed that, when two quantum signals combine at a beamsplitter, then no matter what state each signal contains, the beamsplitter always increases entropy. Through their calculations, the researchers could determine an upper bound on the channel capacity that no quantum effect can improve upon. The results suggest that current technologies for increasing capacity in bosonic Gaussian channels are working at near optimal efficiency.
 
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Thread Starter

nsaspook

Joined Aug 27, 2009
12,830
Some were saying much the same about putting a man on the moon - my dad for a start.
Your dad and the others were just wrong. We've known the 'physics' of putting a 'man' on the moon since Newton. The 'physics' limits of of quantum technology is currently unknown. Theories that give a unique speed or performance (not unique properties) to the quantum attributes of systems seem to be found wanting when actually tested. Maybe nature has a 'no free lunch' rule for quantum interactions.
 

Glenn Holland

Joined Dec 26, 2014
703
In it's simplest form, quantum computing refers to the use of atomic characteristics (such as a change in electron energy levels or a change in magnetic orientation) as digital storage devices. A very crude quantum storage device can be made from multi-color LEDS (such as red representing a 0 and green being a 1).

However, the cost per byte is orders of magnitude more expensive than current microelectronic devices. The volatility of quantum memory devices is another serious problem.
 

Thread Starter

nsaspook

Joined Aug 27, 2009
12,830
https://www.quantamagazine.org/frau...where-our-views-of-reality-go-wrong-20181203/
Despite this lack of empirical evidence, physicists think that quantum mechanics can be used to describe systems at all scales — meaning it’s universal. To test this assertion, Frauchiger and Renner came up with their thought experiment, which is an extension of something the physicist Eugene Wigner first dreamed up in the 1960s. The new experiment shows that, in a quantum world, two people can end up disagreeing about a seemingly irrefutable result, such as the outcome of a coin toss, suggesting something is amiss with the assumptions we make about quantum reality.
HowExperimentWorks_560.jpg
In spontaneous collapse theories, quantum mechanics can no longer to be applied to systems larger than some threshold mass. And while these models have yet to be empirically verified, they haven’t been ruled out either.

Nicolas Gisin of the University of Geneva favors spontaneous collapse theories as a way to resolve the contradiction in the Frauchiger-Renner experiment. “My way out of their conundrum is clearly by saying, ‘No, at some point the superposition principle no longer holds,’” he said.
 

BR-549

Joined Sep 22, 2013
4,928
What is a qubit suppose to do? A bit can only convey presence or non presence. I am no expert, but from what I can gather.....a qubit can show presence, and the direction and rate of the presence. It's like adding a horizontal cross on a one.......left and right hands to a bit....and then adding a rate of change. A bit with a complex value.

And I would surmise.......that with mathematics......one could also assign direction and rate to a non presence value too. Calculating from both ends sorta speak. Many calculations moving both ways. Looking for positives AND negatives.

And all that with selected parallelism, directed by the accumulating result.......itself.

Try to imagine that.

That is a lot to ask. And very scary.
 

bogosort

Joined Sep 24, 2011
696
In it's simplest form, quantum computing refers to the use of atomic characteristics (such as a change in electron energy levels or a change in magnetic orientation) as digital storage devices. A very crude quantum storage device can be made from multi-color LEDS (such as red representing a 0 and green being a 1).

However, the cost per byte is orders of magnitude more expensive than current microelectronic devices. The volatility of quantum memory devices is another serious problem.
Processing, not storage, is the point of quantum computing. Quantum states are far too fragile to be useful as a storage medium. Rather, the hope is that quantum entanglement can be leveraged to solve certain types of problems faster than a classical computer. The thrilling idea is this: if you can prepare the solution space of a problem as a set of entangled quantum states, then quantum interference will reinforce the true solution and make the false solutions cancel out. This is in contrast to the classical method of having to traverse the solution space solution by solution. The hope is that certain 2^n exponential-time problems can be solved by an n-qubit quantum computer in polynomial time (most of which is spent preparing the states).

So far, this speedup has only been shown theoretically for a handful of problems (most famously prime factorization). But we're not even sure if similar speedups can't be accomplished with classical algorithms -- no one has proved quantum supremacy on paper, let alone in an actual machine.
 

bogosort

Joined Sep 24, 2011
696
What is a qubit suppose to do? A bit can only convey presence or non presence. I am no expert, but from what I can gather.....a qubit can show presence, and the direction and rate of the presence. It's like adding a horizontal cross on a one.......left and right hands to a bit....and then adding a rate of change. A bit with a complex value.
The short of it is that a qubit, as you'd expect, represents a state. Once a qubit has been measured, it can only represent a classical 0 or 1 state. Boring. But before measurement, a qubit can be in a superposition of states, which means that -- in a sense -- it can be in multiple states at the same time. Each of these states has a probability associated with it, called an amplitude, which is a complex number. Which specific state it ends up in (once measured) depends on the amplitudes of the states.

Now, a single qubit is useless for computing. But if you entangle a bunch of them together, you suddenly have a superposition of superpositions. All those probability amplitudes will interact -- and since they are complex numbers, they can be negative. So amplitude interaction actually means amplitude interference, with some states destructively interfering and others constructively interfering. Upon measurement, interference will do its thing and the qubits will end up in a classical state. If you had prepared the quantum states properly, you now have an answer to your problem.

Long story short, the two key things that distinguish qubits from bits are the mixing of states in superposition and the complex-valued probability amplitudes. The great hope for quantum computing is that the universe gives us these two key things for free; it's up to the engineers to figure out how to make one work.
 

Thread Starter

nsaspook

Joined Aug 27, 2009
12,830
An interesting article, relevant to the discussion in this thread:

https://www.sciencedaily.com/releases/2018/12/181203111603.htm
Very interesting but it's just another physical realization of quantum mechanical properties that somehow must interface with our mainly classical physics world.
I think they will hit much the same problems as the linked quantum optical communication systems in #1. It will work but won't provide a usable speed or performance improvement over classical methods when scaled beyond a proof of theory experiment.
 

Thread Starter

nsaspook

Joined Aug 27, 2009
12,830
That's a jelly doughnut with chocolate, whipped cream, nuts and bacon on top.

As usual the engineering problem is making those special quantum properties do something special at the computing practical level that can't be done with classical computers. It's potential might be like the potential energy of a frozen glacier, huge but locked in a way that's impossible to efficiently tap beyond experiments to prove it's possible.

There is no fundamental reason why quantum computers should be good at factoring, or why classical computers should be bad at it beyond a narrow set of 'needle in a haystack' problems where there is a complex but deterministic set of rules for the needle location. If the needle location is totally random or inherently chaotic there is no advantage to quantum computing. The research is important and valuable for classical computing because as possible QM systems are emulated on classical machines we are finding ways to push the processing requirements for quantum computational supremacy farther down the road.

https://arxiv.org/pdf/1807.10749.pdf
Quantum Supremacy entails an inherently adversarial protocol that asymmetrically favors quantum computers — a computational problem is being selected that can be solved by quantum yet not classical computers [1, 2, 3, 4, 5, 6, 7, 8, 9]. Reliably defeating Quantum Supremacy requires more than a handful of opportunistic simulations as one has to anticipate modifications of problem instances that complicate simulation more than they complicate quantum evolution. It is sometimes easy to confuse Quantum Supremacy with almost the opposite, i.e., showing that classical computers can solve tasks that present-day quantum computers cannot. The later is trivial and can be demonstrated on many common tasks for which classical software and hardware excel, while quantum computers have no algorithmic advantage and are currently much smaller and error-prone. For example, sorting 2n numbers requires Ω(n2 n ) time on both classical and quantum computers [31] and can be accomplished just as quickly.
 

cmartinez

Joined Jan 17, 2007
8,181
That's a jelly doughnut with chocolate, whipped cream, nuts and bacon on top.

As usual the engineering problem is making those special quantum properties do something special at the computing practical level that can't be done with classical computers. It's potential might be like the potential energy of a frozen glacier, huge but locked in a way that's impossible to efficiently tap beyond experiments to prove it's possible.

There is no fundamental reason why quantum computers should be good at factoring, or why classical computers should be bad at it beyond a narrow set of 'needle in a haystack' problems where there is a complex but deterministic set of rules for the needle location. If the needle location is totally random or inherently chaotic there is no advantage to quantum computing. The research is important and valuable for classical computing because as possible QM systems are emulated on classical machines we are finding ways to push the processing requirements for quantum computational supremacy farther down the road.

https://arxiv.org/pdf/1807.10749.pdf
You were doing so well... until you said "bacon on top" ... :p
 

BBee

Joined Dec 6, 2018
35
It's a little beyond me at the moment, excuse my ignorance, but the way I have read things is that a quantum state cannot be read as the act of reading it changes it. That said, things can generally be said to be in a certain state hence statistically predictable. Working with the vaugness, yet predictability of statistics could well speed things up as actual states need never be read? Maybe I am confusing myself? I have noticed recently that Visual Studio is being prepared for these applications though.

Tracy
 

bogosort

Joined Sep 24, 2011
696
It's a little beyond me at the moment, excuse my ignorance, but the way I have read things is that a quantum state cannot be read as the act of reading it changes it. That said, things can generally be said to be in a certain state hence statistically predictable. Working with the vaugness, yet predictability of statistics could well speed things up as actual states need never be read? Maybe I am confusing myself? I have noticed recently that Visual Studio is being prepared for these applications though.

Tracy
There's a common misconception that Heisenberg's uncertainty principle means "You disturb what you measure", hence we cannot measure a quantum system. Neither of those claims are accurate. In particular, when we measure a quantum system -- like the state of a qubit -- we read a classical state (e.g., on or off). Before the measurement the system did not have a well-defined classical state, but it had a perfectly well-defined mixture of quantum states (e.g., 70% on and 30% off). The act of measurement simply caused a transition from quantum to classical state. If we repeat the same experiment several times, we'll find that the distribution of classical readings approaches 70% on and 30% off.

Some physicists believe that the act of measurement causes the quantum system's wave function (which determines its time evolution through quantum states) to collapse to a single, classical point (the measured value). Others believe that there is no wave function collapse; the act of measurement causes our instruments (and ourselves!), which are also quantum systems, to become entangled with the quantum system under measurement. But, in either case, we can definitely measure quantum systems. The goal of quantum computing is to prepare the quantum states such that the correct answer approaches 100% when read and the wrong answers approach 0%. As nsaspook said, this can only work if the problem itself has the right kind of internal structure. For every other type of problem (most!), quantum computing is beyond worthless.
 

BBee

Joined Dec 6, 2018
35
Thank you. I think from what you say, we are maybe not that far apart here as my main point about it which I did not state very well is that these quantum states cannot be readily individually defined but statistically they are far more definite. Ie, my view of things as being of a statistical rather than concrete has a better feel. It's just that, although I have interest in quantum mechanics, I have no formal training. I later thought that in any form of processing the states of the system can be largely irrelevant at time of processing and only need to become so at the point of results (which is unlike current processing in which a processes individual logical position can be largely well determined at any point in time, although they often don't really need to be).

I do think though that the main idea of quantum theory which I don't get on with is one I have seen before, and you also imply - 'Before the measurement the system did not have a well-defined classical state'. Who is to say what the state was? It may have been rock stable, just not determinable by measurement. Hence the disturbance of measurement. Please don't take this point (unless you have an easy answer) as it is not the point of this thread, just something which confuses me, which I will persue elsewhere.

Tracy
 

BR-549

Joined Sep 22, 2013
4,928
From what I get.....the quantum model, mathematically ties/combines the "duality" of particle characteristics and wave characteristics into a "quantum state".

There is no duality......a field is not a wave. A field can emit or absorb a wave. The particle's field is what interacts with the environment and other particles. The particles momentum and power is in the fields.

Field RPM tells one everything, about the particle. We just need to listen to it. They all hum the same notes.
 

bogosort

Joined Sep 24, 2011
696
. . . these quantum states cannot be readily individually defined but statistically they are far more definite.
On the contrary, the quantum states are perfectly well-defined. We can even prepare a system with an exact quantum state. The only difference is that the mixture of quantum states do not correspond to any classical state, so we have to use the formal methods of quantum mechanics to describe them. But we can definitely describe them. :)

'Before the measurement the system did not have a well-defined classical state'. Who is to say what the state was? It may have been rock stable, just not determinable by measurement. Hence the disturbance of measurement.
Physicists have come up with formal proofs and a whole bunch of really clever experiments to test whether there's actually some "hidden" classical state that we simply can't determine. In every experiment, the results show that at the quantum scale the universe is fundamentally non-classical. In other words, it's not a matter of ignorance (because of measurement disturbance or something else); the universe really is that weird. Check out Bell's theorem for more.
 
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