News and Views

Comments about the article in Nature: Quantum Computers

Following is a discussion about this "REVIEWS" article in Nature Vol 464 4 March 2010 by T.D. Ladd etc.
In the last paragraph I explain my own opinion.

Quantum computers

The article starts with the following text:
In the past decade there has been tremendous progress in the experimental development of a quantum computer.
IMO (again) this introduction is much to optimistic.

Next we read:

Likewise, a quantum computer (QC) will not be faster, bigger or smaller version of an ordinary computer. Rather, it will be a different kind of computer, engineered to control coherent quantum mechanical waves for different applications.
I have a different point of view. IMO a QC will be a different kind of computer solving certain problems much faster than an ordinary computer using quantum mechanical physical concepts.

The next sentence supports this point of view:

This (Shor's quantum algorithm) is one among several quantum algorithms that would allow modestly sized quantum computers to outperform the largest classical supercomputers in solving some specific problems important for data encryption.
In stead of largest IMO there should have been written fastest. At the other side the idea that QC's are modestly sized is wrong. QC's are large to solve identical sized problems.

Next we read:

in the long term, another application may have a higher technological impact: Feynman's 1980s proposal of using quantum computers for the efficient simulation of quantum systems.
This same idea is also discussed in "Quantum Computing" In Nature Vol 463 28 January 2010 by Emanuel Knill. See for my comments: Quantum Computing . IMO this is impossible.

Next we read:

The engineering of the ultra-small will continue to advance and change our world in coming decades and as this happens we might use QC's to understand and engineer such technology at atomic level.
IMO this are all dreams. Anyway the article does not give any glue what they specific mean.

Next we read:

Quantum information research promises more than computers as well. Similar technology allows quantum communication, which enables the sharing of secrets with security guaranteed by the laws of physics.
The laws of physics are the description of the physical reality. Those laws guarantee nothing. Those laws describe what the experiments tell you. If you use quantum mechanics to perform communication that you have to demonstrate by experiment to what extend the communication is secure. The fact that you use quantum mechanics is no guarantee that this communication is in agreement with our definition of security.

Next we read.

Quantum computing 'software' is discussed elsewhere, such as in ref 1.
The problem with QC's is that it does not support the concept of software. A QC is a machine that special has to be build in order to solve each problem. If you want to modify (extend) your problem than you have to modify your hardware. This is because each problem is solved in real time.
You can only use software in order to design (draw) your QC.

Requirements for quantum computing

In this paragraph we read:
No system is fully free of decoherence, but small amounts of decoherence may be removed through various techniques gathered under the name of 'quantum error correction' (QEC)
In fact this name is misleading. It should have been called 'quantum error detection and correction' or (QEDAC). This name identifies much better the huge problems involved i.e. detection of an error and correction of that same error. In an ordinary computer this is a rather straight forward issue (by adding extra bits and logic in memory words) but in a real time physical system this is extremely difficult because errors can happen everywhere and they cannot be localized.


In this paragraph we read: Looking to the future which type of hardware has the most promise for achieving large scale QC? To answer this very difficult question etc.
Yes this is an extremely difficult question. The problem is that the whole QC should work based on that same type of hardware in cluding the QUbits, the communication between those QUbits and the logic used to control the QC.
It is "interesting" to read that currently the hardware issue is still open for discussion.

Next we read:

Also in practice it may arise that imperfections in the coherent control of qubits are more likely to limit a QC's performance than decoherence.
Qubit control is of course a very difficult task. When I read this line I ask my self the following questions:

Next we read:

one must also consider constraints for connectivity between qubits, the specific noise-processes for that hardware, and the relative speed of qubit-to-qubit communication, control, initialization and measurement.
A whole string of problems. None of them new to me. Again which tremendous progress is made?

Next we read:

In the end, a valuable comparison must examine complete architectures of QC's that enumerate the resources required to finish algorithms of relevant sizes with negligible error.
This wil never happen in practice.

Next we read:

Increased effort in this direction is needed to move towards realizing devices based on quantum principles that are actually more powerful, more efficient or less costly than their classical counterparts.
This is like a match between a rabbit (PC) and a turtle (QC) where the rabbit is already in front.

Next we read at the end:

When we have mastered technology enough to scale up a quantum computer, we will have tamed the quantum world and become inured (used NV) to a new form of technological reality.


If this article is a good representation of the present state of art in Quantum Computers than the future is gloomy.

The last sentence of the article starts with the word: When. I expect this will never happen.

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Created: 10 March 2010

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