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Questions tagged [classical-computing]

For questions about the relation between quantum computing and classical computing, such as their relative performance.

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37 votes
6 answers
2k views

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 ...
Steven Sagona's user avatar
179 votes
13 answers
42k 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 ...
John Duffield's user avatar
37 votes
3 answers
3k views

Can a quantum computer simulate a normal computer?

Similar to the question Could a Turing Machine simulate a quantum computer?: given a 'classical' algorithm, is it always possible to formulate an equivalent algorithm which can be performed on a ...
Glorfindel's user avatar
8 votes
3 answers
2k 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?
Tobias Fritzn's user avatar
50 votes
2 answers
7k 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'....
Discrete lizard's user avatar
29 votes
5 answers
8k 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?
Alex Jone's user avatar
  • 633
18 votes
4 answers
12k 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&...
Siu Ching Pong -Asuka Kenji-'s user avatar
10 votes
3 answers
877 views

What is the simplest algorithm to demonstrate intuitively 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 ...
Steven Sagona's user avatar
9 votes
2 answers
436 views

Are we certain that quantum computers are 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?
Adou's user avatar
  • 91
29 votes
4 answers
7k 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' ...
Philip Oakley's user avatar
8 votes
2 answers
944 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 ...
J. Doe's user avatar
  • 241
5 votes
4 answers
4k views

Why do computer scientists care about the phase of qubits?

When I design some classical register, flip-flop, binary counter, small byte of RAM, etc from scratch with classical logic gate, I never deal with such binary direction because classical bit doesn't ...
Muhammad Ikhwan Perwira's user avatar
25 votes
3 answers
6k 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 ...
Paul Turner's user avatar
20 votes
3 answers
1k 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 ...
user1271772 No more free time's user avatar
13 votes
2 answers
996 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;...
kelalaka's user avatar
  • 709
9 votes
1 answer
3k views

Is there anything that can be programmed on a classical computer but not on a quantum computer?

Would we need to create new algorithms that only work on quantum computers or would be simply edit codes in languages such as C++ to involve the new primitives from quantum computing? Are there ...
user avatar
9 votes
1 answer
993 views

Classical algorithm with complexity similar to Shor's discovered: Are there more efficient quantum algorithms than Shor's?

In the article Fast Factoring Integers by SVP Algorithms the author claims that he discovered classical algorithm for factoring integers in polynomial time. The Quantum Report mentioned here that it ...
Martin Vesely's user avatar
8 votes
3 answers
1k views

Compiling a classical function to a quantum circuit in practice

It can be shown that any classical function $f$ can be implemented by a quantum circuit $Q_f$, so that $$ \sum_{x}|x,0^k\rangle \xrightarrow{\mathit{Q_f}} \sum_{x}|x,f(x)\rangle $$ where $f$ has $k$ ...
kgi's user avatar
  • 81
6 votes
1 answer
212 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.
user1271772 No more free time's user avatar
4 votes
1 answer
2k 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 ...
Bidon's user avatar
  • 818
4 votes
2 answers
3k 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 ...
kenorb's user avatar
  • 662
17 votes
2 answers
703 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 ...
Prem's user avatar
  • 223
17 votes
4 answers
4k views

Quantum circuits explain algorithms, why didn't classical circuits?

When explaining a quantum algorithm, many revert to 'circuit-speak' by drawing a diagram of how qubits split off into transformations and measurements, however, rarely if not never would someone ...
develarist's user avatar
14 votes
4 answers
945 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 ...
Discrete lizard's user avatar
13 votes
2 answers
484 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 ...
P. C. Spaniel's user avatar
12 votes
1 answer
2k views

Are circuits with more than 1000 gates common?

I have seen circuits with 30 qubits and around 500 gates. Also circuits with 32 qubits and 6000 gates. Are circuits with more than 1000 gates common in quantum computing? Are there many quantum ...
Alejandro Arcila's user avatar
9 votes
5 answers
3k views

Why can't quantum computation replace classical computation?

I am not a total novice of quantum computation (have read the first 6 chapters of Nielsen and Chuang, though not familiar with every part), but there are some fundamental questions that I don't know ...
Liren Lin's user avatar
  • 191
9 votes
5 answers
2k views

If quantum computing always return random measurement (or uncertain measurement), why do we still need it?

I am very new to quantum computing and currently studying quantum computing on my own through various resources (Youtube Qiskit, Qiskit website, book). As my mindset is still "locked" with ...
KamWoh Ng's user avatar
9 votes
4 answers
5k views

Does a quantum computer have a clock signal and if yes how big is it?

I think there can't be a computer running software without having a clock signal. A fast classical computer has a clock rate between 4 to 5 GHz. If quantum computers are so much faster they must have ...
zomega's user avatar
  • 203
9 votes
2 answers
478 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 ...
fr_andres's user avatar
  • 754
7 votes
1 answer
324 views

What is the relationship between the size of the Hilbert space for boson sampling and the complexity of classical simulating it?

My intuition is that the fastest classical algorithm for simulating some kind of noiseless quantum sampling process should scale roughly with the dimension of the Hilbert space: you would need to ...
tparker's user avatar
  • 2,801
7 votes
3 answers
222 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 ...
Discrete lizard's user avatar
5 votes
2 answers
279 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 ...
M. Al Jumaily's user avatar
4 votes
1 answer
406 views

Will standard programming languages be applicable for quantum computing?

I'm new to quantum computers and computing, so it's possible my question is pointless or unnecessary ... but what about current programming languages such as Java, C++, Python in terms of quantum ...
Mikolaj's user avatar
  • 173
3 votes
1 answer
175 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 ...
Sanchayan Dutta's user avatar
3 votes
2 answers
253 views

Generally speaking, are quantum speedups always due to parallelization of a given problem?

We know that quantum computers use the wave-like nature of quantum mechanics to perform interference. Sometimes we can use this interference to perform specific algorithms that will cause enough ...
Steven Sagona's user avatar
3 votes
1 answer
179 views

How can classical computations be non-unitary?

Given that classical physics emerges from quantum physics on a macroscopic scale, and all quantum operators are unitary, how are we able to perform non-unitary operations (such as setting a register ...
Sideshow Bob's user avatar
3 votes
1 answer
1k 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 ...
Steve Mucci's user avatar
2 votes
1 answer
366 views

No. of bits in 160 qubits computer [duplicate]

I read in a book that (https://hub.packtpub.com/quantum-expert-robert-sutor-explains-the-basics-of-quantum-computing/) 160 qubits (quantum bits) could hold $2^{160} \approx1.46\times 10^{48}$ bits ...
Manu Chadha's user avatar
2 votes
1 answer
734 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 ...
Prasanth Kumar Vemula's user avatar
1 vote
2 answers
632 views

Is there any real world problem where I can see quantum computing being better than classical computing?

Is there any paper or piece of code showing, on a REAL quantum computer, that a specific real world problem (possibly related to computer science, machine learning or finance and possibly NOT related ...
mpro's user avatar
  • 517