104 votes

Is quantum computing just pie in the sky?

I'll be trying to approach this from a neutral point of view. Your question is sort of "opinion-based", but yet, there are a few important points to be made. Theoretically, there's no ...
46 votes
Accepted

Is quantum computing just pie in the sky?

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 ...
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35 votes

Is quantum computing just pie in the sky?

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 ...
29 votes
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Can a quantum computer simulate a normal computer?

Yes, it can do so in a rather trivial way: Use only reversible classical logical gates to simulate computations using boolean logic (for instance, using Toffoli gates to simulate NAND gates), use only ...
28 votes
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Is there proof that the D-wave (one) is a quantum computer and is effective?

There is still a search for problems where the D-Wave shows improvement over classical algorithms. One might recall media splashes where the D-Wave solved some instances $10^8$ times faster than a ...
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24 votes
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Does Moore's law apply to quantum computing?

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 ...
  • 3,667
22 votes

Is quantum computing just pie in the sky?

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 ...
22 votes
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Are circuits with more than 1000 gates common?

I'd say it's far more common for quantum algorithms to use billions of gates than thousands. And that's assuming you're ignoring Clifford gates as well as error correction overhead! If you want to ...
  • 24.8k
19 votes

Is quantum computing just pie in the sky?

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 ...
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19 votes

Is quantum computing just pie in the sky?

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. ...
19 votes

If quantum speed-up is due to the wave-like nature of quantum mechanics, why not just use regular waves?

Your primary assertion that the mathematics of waves mimic that of quantum mechanics is the right one. In fact, many of the pioneers of QM used to refer to it as wave mechanics for this precise reason....
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19 votes
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Quantum circuits explain algorithms, why didn't classical circuits?

You might find this analogy helpful: the development of quantum algorithms is still in the Booth's multiplication algorithm stage; we haven't quite reached dynamic programming or backtracking. You'll ...
18 votes
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Is quantum cryptography safer than classical cryptography?

If you are talking specifically about quantum key distribution (quantum cryptography being an umbrella term that could apply to lots of stuff), then once we have a quantum key distribution scheme, ...
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16 votes
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What makes quantum computers so good at computing prime factors?

The short answer $\newcommand{\modN}[1]{#1\,\operatorname{mod}\,N}\newcommand{\on}[1]{\operatorname{#1}}$Quantum Computers are able to run subroutines of an algorithm for factoring, exponentially ...
16 votes
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Aren't qubits just ternary?

Qubits have more than three distinct states. Here are six examples of such states: \begin{align} |0\rangle\tag{1}\\ |1\rangle\tag{2}\\ |+\rangle = \frac{|0\rangle + |1\rangle}{\sqrt{2}}\tag{3} \\ |-\...
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13 votes

Are quantum computers just a variant on Analog computers of the 50's & 60's that many have never seen nor used?

Here is a quick list of notable differences between analog and quantum computers: Analog computers can't pass Bell tests. The state space of an analog computer with N sliders is N dimensional. The ...
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13 votes
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Implementing "Classical AND Gate" and "Classical OR Gate" with a quantum circuit

Your construction by gueswork in this answer is OK but not really elegant. Moreover, it's a convention to start in the state $|0\rangle$; we usually don't initialize a qubit with the state $|1\rangle$...
12 votes

Is quantum computing just pie in the sky?

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 ...
12 votes

Is quantum computing just pie in the sky?

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 ...
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12 votes
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The process for transferring qubits between locations

You are totally right in your assumption about transporting qubits from Alice to Bob implies something physical. Usually problems/situations that have this setup of a transmission between two parties ...
12 votes
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Does quantum computing already possess the level of abstraction to be explicable even without knowledge of physics?

I don't think you need to know quantum physics to understand quantum computing - similarly to how you don't think about the hardware implementation of the classical computers when you write high-level ...
12 votes
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Is there anything that can be programmed on a classical computer but not on a quantum computer?

Short answer: no. Any classical algorithm can be transformed into quantum algorithm. This result has little practical value, because you don't obtain quantum speedup, but it is important from ...
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11 votes

Why is it harder to build quantum computers than classical computers?

There's many reasons, both in theory and implementation, that make quantum computers much harder to build. The simplest might be this: while it is easy to build machines that exhibit classical ...
11 votes

Is quantum cryptography safer than classical cryptography?

Most attacks now on classical computers don't actually break the encryption, they trick the systems / communication protocols into using it in a weak way, or into exposing information via side ...
10 votes

Quantum memory assisting classical memory

In summary, no. If you think about it, this makes sense. When measuring a quantum system with $n$ qubits, you get $n$ bits of information. the $2^n$ figure exists only when the system is in ...
  • 3,349
10 votes
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Is the common Computer Science usage of 'ignoring constants' useful when comparing classical computing with quantum computing?

The common Computer Science usage of 'ignoring constants' is only useful where the differences in performance of various kinds of hardware architecture or software can be ignored with a little bit of ...
10 votes

Is quantum computing just pie in the sky?

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. ...
9 votes

Is it possible to "calculate" the absolute value of a permanent using Boson Sampling?

It appears to be true, up to a point. As I read Scott Aaronson's paper, it says that if you start with 1 photon in each of the first $M$ modes of an interferometer, and find the probability $P_S$ that ...
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