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97

Your question is sort of "opinion-based", but yet, there are a few important points to be made. Theoretically, there's no convincing argument (yet) as to why quantum computers aren't practically realizable. But, do check out: How Quantum Computers Fail: Quantum Codes, Correlations in Physical Systems, and Noise Accumulation - Gil Kalai, and the ...

38

Is quantum computing just pie in the sky? So far it is looking this way. We have been reaching for this pie aggressively over the last three decades but with not much success. we do have quantum computers now, but they are not the pie we wanted, which is a quantum computer that can actually solve a problem faster or with better energetic efficiency than a ...

35

Classical computing has been around longer than quantum computing. The early days of classical computing is similar to what we are experiencing now with quantum computing. The Z3 (First Turing complete electronic device) built in the 1940s was the size of a room and less powerful than your phone. This speaks to the phenomenal progress we have experienced in ...

23

If you take as definition "the number of transistors in a dense integrated circuit doubles about every two years", it definitely does not apply: as answered here in Do the 'fundamental circuit elements' have a correspondence in quantum technologies? there exist no transistors-as-fundamental-components (nor do exist fundamental-parallel-to-transistors) in a ...

22

According to Wikipedia of Timeline of quantum computing, here are the main events: 1960 Stephen Wiesner invents conjugate coding. 1968 A quantum computer with spins as quantum bits was also formulated for use as a quantum spacetime in 1968. Finkelstein, David (1968). "Space-Time Structure in High Energy Interactions". In Gudehus, T.; Kaiser, G. ...

21

Early classical computers were built with existing technology. For example, vacuum tubes were invented around four decades before they were used to make Colossus. For quantum computers, we need to invent the technology before we make the computer. And the technology is so beyond what had previous existed, that just this step has taken a few decades. Now we ...

18

TL,DR: Engineering and physics arguments have already been made. I add a historical perspective: I argue that the field of quantum computation is really only a bit more than two decades old and that it took us more than three decades to build something like the MU5. Since you mention the timeline, let's have a closer look: The beginnings First of all, the ...

15

When you ask whether it is pie in the sky, that rather depends on what promises you think quantum technologies are trying to fulfil. And that depends on who the people are making those promises. Consider why you are even aware of quantum computation, given that it hasn't yet managed to produce any devices (or to be more fair, not very many devices) which ...

12

Anyone who has written a paper, and asked themselves whether they could improve the notation, or present the analysis a bit differently to make it more elegant, is familiar with the fact that choices of notation, description, and analysis can be an accident — chosen without deep motivations. There's nothing wrong with it, it just doesn't have a strong ...

12

The phase kickback trick appears in this paper: Richard Cleve, Artur Ekert, Chiara Macchiavello, Michele Mosca. Quantum Algorithms Revisited. Proceedings of the Royal Society of London A, 454(1969): 339-354, 1998. The authors credit Alain Tapp for independently discovering the same improvement to Deutsch's algorithm that results from using this trick. (...

12

To answer part of the question, "will I ever buy a quantum computer", etc. I think there is a fundamental misunderstanding. Quantum computing isn't just classical computing but faster. A quantum computer solves certain kinds of problems in a short time that would take a classical super computer a thousand years. This isn't an exaggeration. But regular kinds ...

11

TL;DR: I've been working on the theory of quantum computers for about 15 years. I've seen nothing convincing to say that they won't work. Of course, the only real proof that they can work is to make one. It's happening now. However, what a quantum computer will do and why we want it does not match up with the public perception. Is quantum computing just ...

10

Like all good questions, the point is what you mean. As the CTO of a startup developing a quantum computer, I have to emphatically disagree with the proposition that quantum computing is just pie in the sky. But then you assert "You won't be buying one in PC World any time soon." This I not only agree with but would suggest that in the foreseeable future, ...

10

Preliminary - The DiVincenzo criteria for a 'normal' quantum computer The DiVincenzo criteria, as originally proposed by DiVincenzo, are $5$ criteria that he originally proposed in his seminal 2000 paper. In this paper, he proposed five criteria, which are widely considered to be the five (sufficient and necessary) criteria that any physical quantum computer ...

9

See the timeline on Wikipedia, and ask yourself where's the parallel adder? It seems to me that your answer lies in your question. Looking at the timeline on Wikipedia shows very slow progress from 1959 until about 2009. It was mainly theoretical work until we went from zero to one. In the only 9 years since then, the pace of progress has been ...

9

Why would you expect two different technologies to advance at the same rate? Simply put, quantum computers can be immensely more powerful but are immensely harder to build than classical computers. The theory of their operation is more complicated and based on recent physics, there are greater theoretical pitfalls and obstacles that inhibit their scaling up ...

8

Around 1960-1973 the idea was beginning to form, but the field really started spreading in the 1980s. One of the biggest pioneers was Richard P. Feynman. He proposed a model of a quantum computer in his talk. From that talk, many other scientists pushed the field further (Toffoli created one of the first quantum gates; Shor, at Bell Labs, created one of the ...

8

tl;dr- Moore's law won't necessarily apply to the quantum computing industry. A deciding factor may be if the manufacturing processes can be iteratively improved to exponentially increase something analogous to transistor count or roughly proportional to performance. Background: Moore's law and why it worked It's important to note that Moore's law was ...

8

What is a qubit? And what is a quantum computer? Any claim about about which is first will depend on our definitions. One suggestion might be the 1981 experiment by Aspect, Grangier and Roger to demonstrate a violation of Bell’s inequality. My arguments for this are: It uses a physical degree of freedom (photon polarization) which has since been ...

8

The sad truth for most of the people here is that John Duffield (the asker) is right. There is no proof that a quantum computer will ever be of any value. However, for the companies that have invested in quantum computing (IBM, Google, Intel, Microsoft, etc.), it is entirely worth it to try to build one, because if they are successful they will be able to ...

7

The difficulty with explaining quantum computing is that quantum objects and processes have no direct classical analogue; they're an entirely new ontological category. For example, you might have learned in high school physics that light "is both a particle and a wave" in an attempt to relate it to two classical objects you can intuitively understand. In ...

7

There are many technical challenges to developing a universal quantum computer consisting of with many qubits, as pointed out in the other answers. See also this review article. However, there may be workaround ways to get certain nontrivial quantum computing results before we get to the first truly universal quantum computer. Note that classical computing ...

7

I managed to get access to the paper mentioned in the question. Schrödinger in 1935 (the same year the original EPR paper was published) wrote in English: "By the interaction the two representatives (or $\psi$-functions) have become entangled." This was in the abstract. He also wrote later in the paper: "What constitutes the entanglement is that $\psi$ is ...

6

The first thing to understand about Moore’s law is that it is not a law in the absolute sense, mathematically provable, or even postulated (like a law of physics). Really, it was just a rule of thumb that said the number of transistors in a processor would double every x years. This can be seen in the way that the value x has changed over time. Originally, ...

6

It's difficult to define the point where an experimental setup is a quantum computer. But the crucial feature of a quantum computer is that it's able to perform a quantum computation. The first experimental realization of an algorithm was indeed Jones' and Mosca's implementation of the Deutsch algorithm in 1998 using an NMR setup. Of course previous ...

6

The Hadamard gate has close ties to the discrete Fourier transform. Consider the DFT for an $N$-level system: $$\vert j \rangle = \frac{1}{\sqrt{N}} \sum\limits_{k=0}^{N-1} e^{\frac{i2 \pi j k}{N}} \vert k \rangle.$$ For $N=2$ this is simply \vert j \rangle = \frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\ 1 & -1 \end{bmatrix} \, \vert k \rangle = H \...

6

Semantics aside, I’m assuming that your question is essentially “why do we use a matrix formulation of quantum mechanics rather than a continuous variable/differential equation/integral formulation” (I may be wrong and would welcome clarification) and nothing to do with the interaction picture and the like, which some other answers seem to be touching upon. ...

5

Apparently $\vert W \rangle$ was first reported (and the naming convention first adopted) by Dür, Vidal and Cirac in this preprint on May 26, 2000 (version 1 of 2). This is supported by the footnote on page 4 of this preprint on June 25, 2000 (version 3 of 3, this footnote did not appear in the earlier versions), which states (in part) Very recently Dürr ...

4

Plain and simple. Does Moore's law apply to quantum computing, or is it similar but with the numbers adjusted (ex. triples every 2 years). Also, if Moore's law doesn't apply, why do qubits change it? A great question, with great answers; still, I will try my hand at it. No, most quantum computers do not have qubits created in silicon; even the few that do ...

4

According to Matthias Christandl (who did some research on this to resolve a bet with Artur Ekert), while the term "entanglement" was first used in 1935, as already relayed in other answers, the concept was discussed by Schrodinger in 1932. This set of slides (slides 3-8 in particular) from a talk reproduce part of a document that details this. The full ...

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