Questions tagged [quantum-computer]
The quantum computing tag is relevant for computing that uses quantum states such as superposition and/or entanglement to locate low energy states as solutions to complex problems (rather than laboriously enumerating and checking solutions as would be done with non-quantum traditional computing).
641
questions
1
vote
1answer
37 views
What is the link between regular quantum mechanics and quantum computing?
In the quantum mechanics we are taught first at uni, we learn about schrödinger equation, $i \hbar \partial_t |\Psi\rangle = \hat H |\Psi\rangle,$
eigenvectors $|\Psi \rangle$, and operators on the ...
5
votes
2answers
113 views
Qubits of higher dimension
In Quantum Computing, one usually looks at two-state-systems named qubits to perform a computation. One problem arises when the number of qubits is increased, because the whole system becomes in such ...
0
votes
1answer
36 views
Neutrinos, quasiparticles and Majorana fermions
In connection with topological quantum computing I encountered term Majorana fermions. According to Wikipedia these are:
A Majorana fermion, also referred to as a Majorana particle, is a fermion ...
1
vote
1answer
35 views
Measuring overlap of two quantum states
I have a qubit state $|\psi\rangle = a|0\rangle + b|1\rangle$, and another qubit state $|\psi'\rangle = a'|0\rangle + b'| 1 \rangle$. I want, through quantum gates, to measure the overlap $|\langle \...
0
votes
1answer
55 views
Compute the Schmidt Decomposition of a two-qubit state
I'm trying to compute the schmidt composition of $$ |A\rangle = \frac{1}{2 \sqrt{2}}(|00\rangle + \sqrt{3}|01\rangle + \sqrt3 |10\rangle + |11\rangle) $$
I've calculated the eigenvalues to be $$ 1+\...
2
votes
1answer
45 views
Can we obtain the same result as Deutsch algorithm using a XOR gate?
If I understand correctly, Deutsch's algorithm is useful to know if a function $f(x)$ is constant ($f(0)=f(1)$) or balanced ($f(0) \neq f(1)$). And the advantage is that it only requires the ...
1
vote
1answer
40 views
Action of quantum fourier transform on two-fermion states
In section 2.2 of the paper https://arxiv.org/abs/1807.07112, there appears a Fourier transformation named $F_k^n$ that comes out of a matrix called $F_2$,
$$
F_2 = \begin{pmatrix}
1 & 0 & 0 &...
1
vote
1answer
37 views
Quantum gate fSWAP acting on two-fermion states
When we have identical particles which are fermions, any exchange (swap) among 2 states introduces a minus sign. In the paper https://arxiv.org/abs/1807.07112, the Eq. (7) aims to represent that ...
-1
votes
0answers
37 views
Is there an analog of the Solovay-Kitaev Theorem for approximating quantum states?
The Solovay-Kitaev theorem shows that we can approximate arbitrary unitary transformations with polynomially many quantum gates. Can we approximate the resulting state vectors in the same way by ...
1
vote
1answer
73 views
What is the precise mechanism for the speed of a quantum computer? [closed]
This site may not be the right place to ask this question because it is both a physics and a computing question. In quantum computing, I get that qubits have many more states than zeros and ones. But ...
3
votes
0answers
96 views
Can quantum computing solve the curse of dimensionality?
Edited version
The curse of dimensionality is ubiquitous in machine learning (ML) modeling, stochastic control and reinforcement learning, arising in a probabilistic sense, with strong connections to ...
0
votes
1answer
136 views
What happens to the Universe if a quantum coin in a black box is secretly flipped for zillions times?
First, let us using an IBM quantum processor (QP) to run the following simple quantum circuit:
The IBM $U_3 (\pi/3,0,0) $ gate transforms the initial state $q[0] = \vert0\rangle$ to a biased quantum ...
0
votes
0answers
23 views
Finding the lowest eigenvalue using VQE-like circuits
I want to find the lowest eigenvalue for the following hamiltonian using VQE_like circuits (in python), but I don't want to use quantum libraries.
\begin{eqnarray}
H &=& \frac{-3}{2} I \...
1
vote
0answers
63 views
Prove that there exists a $d \times d$ unitary matrix $U$ which cannot be decomposed as a product of fewer than $d-1$ two-level unitary matrices
This is exercise 4.38 from Nielsen and Chuang:
Prove that there exists a $d \times d$ unitary matrix $U$ which cannot be decomposed as a product of fewer than $d-1$ two-level unitary matrices.
If ...
2
votes
0answers
22 views
What is a Polariton and why are they useful in unconventional computing?
My lecturer has frequently mentioned the term polariton and a quick search on the Internet lead me to its Wikipedia page (which is not surprising but the page is not particularly helpful) as well as a ...
0
votes
2answers
38 views
What operator do we use to measure at the end of a quantum search algorithm?
Perhaps an obvious question, but I do not have the answer to it, so I though I would still ask.
In the below quantum search algorithm, also known as Grover's algorithm, what operator are we ...
0
votes
1answer
36 views
Which technologies are currently known for building quantum gates? [closed]
Which approaches/technologies are currently known for building quantum gates? Apparently, there are different methods proposed in different methods in different papers for this purpose.
2
votes
0answers
38 views
Good resourse for Ising Problem Model
I am student of mathematics, and I will wrote theoretical article about Ising problem in Adiabatic Computation. Do you have a good resource where it is good explaining? In my context I have ...
0
votes
0answers
27 views
How is the Hamiltonian changed in Adiabatic Quantum computing?
In adiabatic quantum computing, we define a time-dependent Hamiltonian, which is then changed gradually to obtain a solution to an optimization problem. Is the Hamiltonian changed automatically, or we ...
2
votes
1answer
312 views
Wick-rotated quantum computers e.g. to be realized with Ising-like systems?
Quantum mechanics is equivalent with Feynman path ensemble, which after Wick rotation becomes Boltzmann path ensemble - and e.g. Ising model is a basic condensed matter model, which is assumed to use ...
1
vote
0answers
55 views
Can a quantum computing advantage exist in a deterministic universe?
Is a deterministic universe with nonlocal hidden variables incompatible with the quantum superpositioning of states that is required for quantum computing?
0
votes
0answers
20 views
How does QC work? [duplicate]
I watched this video https://m.youtube.com/watch?v=g_IaVepNDT4
That says QC has 2 to the N bits. And that is somehow useful. But they don’t say why lots of bits can be useful. Can anyone please ...
1
vote
1answer
45 views
How are topological qubits encoded in the Majorana-fermion-based platform for quantum computing?
Where is “the two level system” of a topological qubit encoded in the Majorana-fermion-based platform of quantum computing?
If the Hamiltonian in a topological quantum field theory is absent (H=0), ...
1
vote
0answers
25 views
Understanding surface code diagrams
I'm trying to understand the diagrams used in the following paper. I'm new to the area so I don't have experience with the stabilizer formalism, but I'm trying to figure out what's going on in the ...
1
vote
0answers
54 views
Nuclear Fusion and Quantum Annealing
This is a pie in the sky idea, and a problem finding sort of question that can probably be dismantled easily, but in light of alternative energy needs with climate change, I think it would be ...
1
vote
0answers
30 views
Degree of Quantum Entanglement
I am a bit confused about the definition of "degree of quantum entanglement".
1- Could anyone give me a generic definition for that?
Moreover, it is not intuitive for me the strategy by which we ...
5
votes
2answers
134 views
Can a Hadamard gate be realized with perfect precision?
This is my first question on the Physics Stack Exchange; I hope it is not be a duplicate… :-|
I am not really a physicist (I am actually a professional mathematician), but I have already read quite a ...
0
votes
0answers
6 views
Does any practical system actually use the Kirchhoff-Law-Johnson-Noise (KLJN) secure key distribution mechanism?
http://www.scholarpedia.org/article/Secure_communications_using_the_KLJN_scheme describes the Kirchhoff-Law-Johnson-Noise (KLJN) secure key distribution scheme, which is based on two switches and an ...
2
votes
1answer
49 views
Is there a common mathematical foundation for quantum and classical computation?
Is there a (natural) formalism that neatly and insightfully includes classical and quantum computation?
I have a mostly sound basis in quantum mechanics (as a physics grad), and a much less sound but ...
-5
votes
1answer
82 views
Classical brain - quantum world [closed]
A noted author (physicist) recently stated that he perceived the human brain as a classical mechanism powered by quantum mechanics. He went on to say that we are reaching the point where we are ...
0
votes
0answers
62 views
Hamiltonian Simulation in Qiskit
I would like to simulate the time evolution of a quantum system Qiskit. Qiskit, however, only supports Hamiltonians that are a sum of hermitian matrices and that can be expanded into tensor products ...
0
votes
0answers
18 views
Non-Abelian Statistic of Majorana Fermions and Quantum Computing
I read in many sources that braiding operations of Majorana excitations in (topological) superconductors do not commute, and that this fact is useful in quantum computing. However, about this last ...
1
vote
1answer
56 views
Hamiltonian for 2-qubit system
What is the general form of a Hamiltonian for a 2 qubit system?
The Hamiltonian for D-Wave system is:
$H = K_1\sigma_x^1 + K_2\sigma_x^2 + H_1\sigma_z^1 + H_2\sigma_z^2 + J_{12}\sigma_z^1\sigma_z^2 ...
0
votes
0answers
28 views
Many Worlds Interpretation and Quantum Key Distribution / Quantum Computing
I'm trying to understand how to interpret Quantum Key Distribution and Quantum Computing in terms of the Many Worlds Interpretation of Quantum Mechanics.
So according to the many worlds ...
0
votes
1answer
76 views
Is quantum entanglement contradictory with Schrödinger's cat?
As I understand, an electron spin is in a superposition state until the spin is observed. Due to the observation, it will either be spin up or spin down.
Due to quantum entanglement, if one electron ...
165
votes
5answers
22k views
Why is Google's quantum supremacy experiment impressive?
In the Nature paper published by Google, they say,
To demonstrate quantum supremacy, we compare our quantum processor against state-of-the-art classical computers in the task of sampling the output ...
0
votes
1answer
60 views
Is there a geometrical representation of classical bits, like that of the Bloch Sphere for quantum bits?
Consider only a single qubit. We know that it can be accurately described using the so called Bloch Sphere. Is there a similar geometric construction used to describe classical bits?
1
vote
1answer
56 views
What problems (other than crypto) can quantum computing solve?
So my (limited) understanding of quantum computing lends its self very well to breaking public-private crypto; by being able to factor products of large primes.
However in some press releases (google ...
2
votes
1answer
60 views
Prove that the Quantum Fourier Transform satisfies $F_N(\sum_x |xr \rangle) \simeq \sum_y |y N/r \rangle $ when $r$ divides $N$
Before I start, this is a homework question, but I have spent the last 7 hours on it and gotten nowhere. I will mention what I've tried and noticed, but it's not much. At this point, I'm wondering if ...
0
votes
2answers
91 views
What does it mean to simulate a quantum system?
What does it mean by simulating quantum system? Can I simulate Young's double slit experiment with a quantum computer? If yes, won't it violate no-cloning theorem?
1
vote
1answer
50 views
Hamiltonian capable of quantum computation
Suppose we have a 1D spin chain evolving in time according to some Hamiltonian $H_t(p_0, p_1, p_2 \ldots)$, where $p_i$ are classical parameters ``set by the lab equipment". Divide time into discrete ...
2
votes
1answer
102 views
Why a quantum computer simulation is not a quantum computer?
Is it because the inability to generate true random numbers (I dont think so) in the host machine or something else?
My understanding is that the ability to emulate a quantum computer in a ...
0
votes
0answers
53 views
Why do quantum computation models based on open quantum systems receive so little attention?
In almost all research on (universal) quantum computation the common models assumed from the outset are either the quantum circuit model with unitary gates, the measurement-based one-way model or the ...
0
votes
1answer
38 views
Is there a theoretical limit on the smallest angle that can be used in a quantum rotation gate?
Assuming ideal conditions and no error, what is the smallest $\theta>0$ that can be used in $R_x(\theta)$ or $R_y(\theta)$.
1
vote
2answers
75 views
Why doesn't using quantum error-correction increase the production of entropy and decohrence?
Error-correction in quantum computing is designed to get around the decoherence "washing out" the answer to a computation. But wouldn't the introduction of error-correction procedures or apparatus ...
-2
votes
1answer
109 views
0
votes
0answers
12 views
Spintronics: How can we inject a spin current into/onto a sample without physically touching it, ie: modulation with a field?
So background, I'm imagining a black box scenario where we have some spintronic system.
It might be a topological quantum computer that uses spin currents running across it's memory to connect ...
2
votes
3answers
118 views
Do rotating phases limit the possibility to perform quantum computing?
Let me try to make several statements which I believe to be true - I'm basically hoping that somebody will point out where I make an error (or errors).
1) per wikipedia, any two-level quantum ...
1
vote
1answer
40 views
Can a Controlled-Swap gate control itself in a Quantum Computer?
Let there be a system of three qubits, $a,b,c,d$ where $d$ is an ancilla qubit. We can add more ancilla if required. The operation I want to implement is as follows:
"If $a$ and $b$ are both in the '$...
3
votes
5answers
197 views
How does quantum computing work if the wavefunction has no reality?
This is probably a dumb layman question but as I understand it quantum computing makes use of the superposition principle in which a qbit can be in more states at once until it’s observed. A ...
