Questions tagged [classical-computing]
For questions about the relation between quantum computing and classical computing, such as their relative performance.
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questions
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Are there many practical problems for which Grover's algorithm beats the best heuristic classical algorithm?
It's well known that, given an oracle for a function $f$ from a very large set $S$ (of order $N \gg 1$) to $\{0, 1\}$, Grover's algorithm can find an element of $S$ that maps to 1 with $\sim \sqrt{N}$ ...
4
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2answers
100 views
A quantum computer that doesn't use entangled qubits has the same computational power as a classical one?
Is the statement in the question correct?
I would love to receive an explanation as to why it is or isn't.
I have a computer science degree background and I am a beginner learning the fundamental of ...
2
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1answer
41 views
Classical and quantum limits to classical copying?
The no-cloning theorem can be sharpened to give quantitative bounds on the fidelity with which an arbitrary quantum state can be copied. Is there a similar picture available for classical copying? ...
2
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1answer
450 views
Can a quantum computer run classical algorithms?
I realize that fundamentally speaking quantum and classical computers might as well be apples and oranges, and that for very specific problems such as integer factorization with Shor's algorithm ...
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0answers
73 views
Comparing CPU to QPU In terms of processing power
The current processors are limited by the speed of the electrons but quantum processors take advantage of the properties of subatomic particles. But the question is how to compare the processing power ...
3
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1answer
147 views
What exactly makes quantum computers faster than classical computers?
What feature of a quantum algorithm makes it better than its classical counterpart? Are quantum computers faster than classical ones in all respects?
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0answers
40 views
References on quantum arithmetic circuit complexity
In classical computing, arithmetic circuit complexity is apparently a big topic. But I couldn't find much about the complexity of quantum arithmetic circuits. Almost all references like arXiv:1805....
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1answer
49 views
Implementing piecewise functions on a quantum computer
I am curious about how to do implement functions like $$f(x)=\begin{cases}
2x &\text{if} &0\leq x <0.5 \\
x/2 &\text{if} &0.5\leq x<1
\end{cases}$$
Do we implement this like ...
-1
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1answer
119 views
Large size of matrices for little outcome [closed]
I am stuck in a dilemma about how to proceed with a quantum computing algorithm that changes the original state of a system to another.
Say I have a superposition of all $8$ bit integer values that ...
2
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1answer
61 views
Is it possible to construct an equivalent quantum circuit from a CORDIC-based digital circuit?
DaftWullie mentions an interesting point here:
let's assume that we know an efficient classical computation of $f(x)$. That means we can build a reversible quantum computation that runs in the same ...
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0answers
30 views
What is the difference in computational power between qubit and bit (transistor)? [duplicate]
If I use for example 5-qubit IBM-Q quantum computer and we accept the law of quantum parallelism ($n$ qubit = $2^n$ ordinary bits), so does it mean that computational power of this computer is equal ...
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4answers
452 views
Implementing “Classical AND Gate” and “Classical OR Gate” with a quantum circuit
Quantum cNOT Gate (Classical XOR Gate)
A "Controlled NOT (cNOT) Gate" flips the 2nd qubit if the 1st qubit is $\left|1\right>$, and returns the 2nd qubit as-is if the 1st qubit is $\left|0\right&...
3
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1answer
62 views
Aren't reversible logic gates a necessity for efficiently executing quantum algorithms?
The Wikipedia article on logical reversibility says:
...reversible logic gates offered practical improvements of
bit-manipulation transforms in cryptography and computer graphics.
But I guess ...
1
vote
1answer
133 views
Reversibility and irreversibility of logic gates (quantum vs classical)
I have been told that one of the great keys that unlock quantum computing's potential is the reversibility of quantum logic gates as for classical gates there's some loss of information, but I cannot ...
4
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2answers
128 views
Does quantum computing relate to stochastic computing in any way?
I'm a bit familiar with the concept of stochastic computing, where numbers are stored in large bit streams called "Stochastic Numbers", which represent numbers in the domain $[0,1]$ typically.
The ...
2
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1answer
138 views
FANOUT with Toffoli Gate
Figure 1.16: FANOUT with the Toffoli gate, with the second bit being the input to the FANOUT (and the other two bits standard ancilla states), and the output from the FANOUT appearing on the second ...
6
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2answers
157 views
Quantum Supremacy: How do we know that a better classical algorithm doesn't exist?
According to the Wikipedia (Which quotes this paper https://arxiv.org/abs/1203.5813 by Preskill) the definition of Quantum Supremacy is
Quantum supremacy or quantum advantage is the potential ...
7
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2answers
211 views
Understanding (theoretical) computing power of quantum computers
I am very new to quantum computing and just try to understand things from a computer scientist's perspective.
In terms of computational power, what I have understood,
100 ideal qubits ... can ...
3
votes
1answer
62 views
Could we use varying voltage with programmable gates?
One of the benefits I'm reading about qubits is that they can be in an infinite number of states. I'm aware of Holevo's bound (even though I don't fully understand it). However, it made me think of ...
9
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2answers
270 views
Can we speed up the Grover's Algorithm by running parallel processes?
In classical computing, we can run the key search (for example AES) by running parallel computing nodes as many as possible.
It is clear that we can run many Grover's algorithms, too.
My question is;...
8
votes
1answer
70 views
Can a stored programming model be applied to a Quantum Computer?
A stored programming computer model is that where a central memory is used to store both instructions and data that they operate on. Basically all the classical computers of today that follow the von ...
7
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3answers
187 views
What is quantum computing vs. what is not quantum computing
That is to say,
what are some common or popular misconceptions about what constitutes quantum computing?
and
how are those things misconceptions?
It could help in explaining to frame this ...
8
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3answers
398 views
Simplest algorithm for intuitively demonstrating quantum speed-up?
What's the simplest algorithm (like Deutsch's algorithm and Grover's Algorithm) for intuitively demonstrating quantum speed-up? And can this algorithm be explained intuitively?
Ideally this would be ...
3
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4answers
206 views
Are classical bits quantum?
A bit is a binary digit, typically 0 or 1.
Until a value is assigned (or a measurement is made) a bit is in a superposition of the entangled binary pair, is it not?
8
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2answers
332 views
The process for transferring qubits between locations
I understand that right now qubits are physical entities in a Quantum Computer and I am playing around on the IBM Quantum Computer as well as the Q# language and dipping my toes into the Quantum world ...
15
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4answers
1k views
Are quantum computers just a variant on Analog computers of the 50's & 60's that many have never seen nor used?
In the recent Question "Is Quantum Computing just Pie in the Sky" there are many responses regarding the improvements in quantum capabilities, however all are focussed on the current 'digital' ...
19
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6answers
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If quantum speed-up is due to the wave-like nature of quantum mechanics, why not just use regular waves?
The intuition I have for why quantum computing can perform better than classical computing is that the wavelike nature of wavefunctions allow you to interfere multiple states of information with a ...
6
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1answer
123 views
What classical public key cryptography protocols exist for which hacking is QMA complete or QMA hard?
Such a public key cryptosystem would be "quantum safe" in the sense that quantum computers cannot efficiently solve QMA hard problems.
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13answers
30k views
Is quantum computing just pie in the sky?
I have a computer science degree. I work in IT, and have done so for many years. In that period "classical" computers have advanced by leaps and bounds. I now have a terabyte disk drive in my bedroom ...
6
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2answers
226 views
Are we certain that quantum computers more efficient than classical computers can be built?
I mean are we certain that they will be able to provide us a huge improvements (in some tasks) compared to clasical computers?
6
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3answers
161 views
Will Moore's Law be no longer effective once quantum computers are created? [duplicate]
Moore's law states that computer power doubles in every 18 months (more formally: "the number of transistors in a dense integrated circuit doubles about every two years."). Statistics suggest that ...
19
votes
2answers
3k views
Is quantum cryptography safer than classical cryptography?
Quantum computing allows us to encrypt information in a different way compared to what we use today, but quantum computers are much more powerful than today's computers. So if we manage to build ...
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7answers
6k views
Why is it harder to build quantum computers than classical computers?
Is it because we don't know exactly how to create quantum computers (and how they must work), or do we know how to create it in theory, but don't have the tools to actually execute it in practice? Is ...
15
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2answers
322 views
Is it possible to “calculate” the absolute value of a permanent using Boson Sampling?
In boson sampling, if we start with 1 photon in each of the first $M$ modes of an interferometer, the probability of detecting 1 photon in each output mode is: $|\textrm{Perm}(A)|^2$, where the ...
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5answers
5k views
Does Moore's law apply to quantum computing?
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?
6
votes
3answers
134 views
Can the theory of quantum computation assist in the miniaturization of transistors?
In his inaugural lecture, Ronald de Wolf states
People are working with quantum objects, but trying to make them behave as classical as possible. (...) Instead of suppressing them to make systems ...
6
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0answers
106 views
Empirical Algorithmics for Near-Term Quantum Computing
In Empirical Algorithmics, researchers aim to understand the performance of algorithms through analyzing their empirical performance. This is quite common in machine learning and optimization. Right ...
6
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1answer
51 views
Assessing speed-up via Quantum-Stochastic correspondence
You can make a natural correspondence between a quantum state vector and a classical probability vector, and between a quantum unitary operator and a classical stochastic matrix. There is also a ...
14
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2answers
521 views
In a Quantum Turing Machine, how is the decision to move along the memory tape made?
Let, for a Quantum Turing machine (QTM), the state set be $Q$, and the alphabet of symbols be $\sum=\{0,1\}$, which appear at the tape head. Then, as per my understanding, at any given time while the ...
7
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1answer
151 views
How does the Curry-Howard correspondence apply to quantum programs?
In words of Wikipedia,
The Curry–Howard correspondence is the observation that two families of seemingly unrelated formalisms—namely, the proof systems on one hand, and the models of computation ...
3
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1answer
109 views
What impact would have introducing the quantum switch effect in classical computing?
This divulgation article by Prof. Brukner talks about the possibility of creating a situation where
"A causing B" and "B causing A" which we call a quantum switch. Such a setup is similar to some ...
18
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3answers
2k views
What makes quantum computers so good at computing prime factors?
One of the common claims about quantum computers is their ability to "break" conventional cryptography. This is because conventional cryptography is based on prime factors, something which is ...
6
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2answers
351 views
Can classical algorithms be improved by using quantum simulation as an intermediary step?
I'm wondering whether even if we cannot create a fast quantum computer, simulating quantum algorithms can be a reasonable method for classical algorithms.
In particular, I'd like to see any results ...
5
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3answers
623 views
Why do classical bits perform calculations at a scale that expands linearly and qubits at exponential scale in the number of (qu)bits?
What does one mean by saying that classical bits perform operations at the scale of $2n$ and quantum computers perform operations at the scale of $2^n$? In both cases, $n$ = Number of bits/qubits.
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1answer
109 views
Quantum memory assisting classical memory
Consider a classical computer, one making, say, a calculation involving a large amount of data. Would quantum memory allow it to store that information (in the short term) more efficiently, or better ...
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4answers
526 views
Is the common Computer Science usage of 'ignoring constants' useful when comparing classical computing with quantum computing?
Daniel Sank mentioned in a comment, responding to (my) opinion that the constant speed-up of $10^8$ on a problem admitting a polynomial time algorithm is meager, that
Complexity theory is way too ...
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2answers
4k views
Is there proof that the D-wave (one) is a quantum computer and is effective?
I'm admittedly a novice in this field, but I have read that, while the D-wave (one) is an interesting device, there is some skepticism regarding it being 1) useful and 2) actually a 'quantum computer'....
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1answer
541 views
How much memory is required to simulate a 48-qubit circuit?
This CDMTCS Report 514, 2017 entitled "The Road to Quantum Computational Supremacy" states (in Section 6) that the amount of memory needed to simulate random quantum circuits on classical computers ...
