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).

Filter by
Sorted by
Tagged with
1
vote
0answers
28 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
63 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
38 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
148 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 ...
2
votes
1answer
55 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
88 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
216 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 ...
1
vote
1answer
266 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
1answer
108 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 ...
185
votes
5answers
24k 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
78 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
73 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
75 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 ...
1
vote
2answers
101 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
57 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 ...
1
vote
2answers
176 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
1answer
39 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
106 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
184 views
2
votes
3answers
136 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
76 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
217 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 ...
0
votes
0answers
36 views

Can someone help in applying covariance condition to this unitary transformation?

I am trying to apply covariance property on this Quantum adder transformation in which a two particle input state is mapped onto a two particle output state. I am trying to represent these states into ...
1
vote
1answer
90 views

Wave function collapse of photon state transformed by beamsplitter

$\newcommand{\ket}[1]{\left|#1\right>}$I am trying to make sense of the math behind transforming the state of a photon with a beam splitter. Suppose I have a beamsplitter crystal which split by ...
0
votes
1answer
42 views

General structure of parity preserving two qubit gate

I am trying to decompose a set of two qubit gates repecting parity, though I am not sure whether parity is the right word for this. The gate has the following structure: $$\begin{bmatrix} u_{11}&...
2
votes
1answer
46 views

Exchange Non-abelian Anyons results in rotation in Vacuum degenerate space?

A basic notion when studying non-abelian anyons is that the system's groundstate is degenerate. Not only that, but exchanging two anyons' position rotates the state in this degenerate subspace. I'm ...
6
votes
4answers
264 views

Does $U^T U$ have real eigenvectors if $U$ is unitary?

I was reading a paper by Kraus and Cirac on general two qubit gates. A crucial step is to decompose an arbitrary unitary operator acting on two qubit space into a special form: $$ U_{AB} = U_A\otimes ...
1
vote
0answers
36 views

Question about implementation of a "reflection" w.r.t a state in quantum computing

I'm going through a paper that uses quantum amplitude estimation, and one of the ingredients to the algorithms is an operator that flips the sign of a state based on the state of a control qubit. ...
-1
votes
1answer
54 views

Can quantum disentanglement be triggered by time dilation?

The question is really one question that leads to the final one: Is it possible to realize a qubit that naturally flips between two quantum states on a definite and fixed period without any ongoing ...
1
vote
0answers
229 views

Why do we need Jordan-Wigner transformation when simulating fermionic systems on quantum circuits?

Why do we need to do a Jordan-Wigner transformation when we want to simulate the fermionic systems with a quantum circuit model? I understand that the spin operator $\sigma_x, \sigma_y, \sigma_z$ on ...
0
votes
1answer
35 views

What distinguish a non-interacting two fermions gate from an interacting one?

We start with a Hilbert space consists of two fermions(a and b), which is 4-dimensional. The basis would be $|vec>$,$b^\dagger|vac>$,$a^\dagger|vac>$,$a^\dagger b^\dagger |vac>$. Denote ...
0
votes
1answer
46 views

Can quDit gates be non-unitary

I have derived a quantum quDit gate that is implemented by photon Fock state inteference. It turns out to be non-unitary. I thought I must have made a mistake, so I have checked several times. I know ...
-1
votes
1answer
98 views

Inverse covariance matrix for a Gaussian state

I was reading an article about Gaussian Boson Sampling (https://arxiv.org/pdf/1801.07488.pdf) and following some calculation appear an inverse covariance matrix when he defines the following matrix A. ...
0
votes
0answers
164 views

Quantum Gates: What is the mathematical difference between the the phase shift gate and the Pauli Z rotated by pi/2?

I come from computer science, and I am trying to get into quantum computing research. I am going through Microsoft's quantum katas and I'm struggling to grasp some of the mathematical nuances ...
1
vote
0answers
34 views

Understanding Skein relations

I'm just studying some concepts in the topology, but I cannot figure out totally how Skein relations work. in the following I've one example: which tries to relate Reidmeister II to the Skein ...
0
votes
0answers
49 views

Is it possible to know whether an arbitrary system is performing a quantum computation?

Without knowing in advance that a physical system has been configured to do so, is it possible to determine whether that system is preforming, or has just performed a quantum computation? That is, if ...
1
vote
0answers
31 views

Can all CFT state be prepared through scale invariant MERA

It is known that in numeric computation, scale invariant MERA is useful for representing a CFT vacuum state. Is the converse true? i.e. all CFT vacuum state (the quantum state with translation and ...
1
vote
0answers
56 views

What is the simplest circuit on a quantum computer that "proves" superposition/entanglement

I am a Physics Teacher at pre university level. Tunneling, wavefunctions and uncertainty are on the syllabus on a very vague way. Quantum superposition and entanglement are not but tie in well and I ...
1
vote
0answers
42 views

5 qubit quantum error correction property [closed]

Having $$\eqalign{ |v_0⟩&=|00000⟩+|11000⟩+|01100⟩+|00110⟩+ {}\cr &\quad |00011⟩+|10001⟩−|10010⟩-|10100⟩− {}\cr &\quad |01001⟩−|01010⟩−|00101⟩−|11110⟩− {}\cr &\quad |...
0
votes
1answer
82 views

What is a quantum random walk?

I am an undergraduate doing research on QC/QI. My current topic to learn is continuous-time quantum walks, but first I must learn the random quantum walk. That being said, I was wondering if someone ...
1
vote
0answers
37 views

For Grover's search: How can a quantum computer store the data, and how can it overall be faster than a classical computer

I think Grover's algorithm can be used to find which entry in a list of names contains a given name, say "Fred" in entry number 12345. Each name can be seen as a binary number, so we might expect to ...
1
vote
0answers
23 views

In quantum search algorithm, how to interpret the effect of $U(t)$ as a rotation on the Bloch sphere?

In Nielsen's QCQI, in page 259, it reads, $$U \left ( \Delta t \right ) = \left ( \cos^2 \left ( \frac {\Delta t} 2 \right ) - \sin ^2 \left ( \frac {\Delta t} 2 \right ) \vec \psi \cdot \hat z \...
1
vote
0answers
107 views

Topological superconductors

I have been reading a lot about quantum computation by using topological materials and I could see that the standard approach is to engineer a p-wave superconductor by using a 1-D semiconductor wire ...
0
votes
1answer
208 views

Universe simulation

Is it theoretically possible to simulate the universe? I know that some philosophical theories say that our universe is a computer simulation, but is this feasible? Would it ever be possible for a ...
1
vote
1answer
41 views

What happens to physical qubits when they decohere during quantum error correction?

Say you have a scaled up system with 98% fidelity so 2% decoheres. These are physical qubits doing error correction on information encoded on logical qubits. 98% of these entangled qubits will be ...
1
vote
1answer
63 views

Understanding Anyonic Exchange

In the book of "Introduction To Topological Quantum Computation" by Jiannis K. Pachos, in chapter 5, it tries to explain anyonic exchange. In the following, the $m$ and $e$ quasi-particles are ...
0
votes
2answers
163 views

What does $e^{i\alpha}$ stand for in the general expression for a qubit gate $e^{i\alpha}R_n(\theta)$?

All qubit gates can be written in the form of: $$U = \exp(i\alpha)R_n(\theta).$$ I know $R_n(\theta)$ is a rotation of $\theta$ about an arbitrary axis n in Bloch sphere, but what does $\exp(i\alpha)...
-1
votes
1answer
113 views

"Quantum information and quantum computation": quantum period finding algorithm

The procedure for a quantum period-finding algorithm is described on page 236 of "Quantum Computation and Quantum Information" by Isaac Chuang and Michael Nielsen. In step 3 of the procedure, authors ...
2
votes
0answers
46 views

Fluxes on a finite group $G$

So I've been studying about topological quantum computation and I have a few questions I haven't been able to solve. The first one is why fluxes take values on a finite group $G$? Does it have to do ...
0
votes
1answer
147 views

Can any 1D critical state be represented by a MERA tensor network?

My understanding of the Multiscale Entanglement Renormalisation Ansatz (MERA) is that it is designed to represent highly entangled, but low complexity states. Is MERA capable of representing high ...

1 2 3
4
5
16