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).

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27
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2answers
22k views

Quantum entanglement faster than speed of light?

recently i was watching a video on quantum computing where the narrators describes that quantum entanglement information travels faster than light! Is it really possible for anything to move faster ...
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What can the D-Wave quantum computer do?

The media are reporting the commercially sold 128-bit quantum computer from D-Wave http://news.google.com/news?ned=us&hl=us&q=d-wave+quantum&cf=all&scoring=n which of course sounds ...
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How are qubits better than classical bit if they collapse to a classical state after measurement?

Classical computers store information in bits, which can either be $0$ or $1$, but, in a quantum computer, the qubit can store $0$, $1$ or a state that is the superposition of these two states. Now, ...
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Examples of number theory showing up in physics [duplicate]

Are there any interesting examples of number theory showing up unexpectedly in physics? This probably sounds like rather strange question, or rather like one of the trivial to ask but often unhelpful ...
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2answers
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Why doesn't the no-cloning theorem make lasers impossible?

As I understand lasers, you start off with a few photons that are in an identical state, and other photons that are created later tend to have the same quantum numbers due to Einstein-Bose statistics. ...
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1answer
234 views

Meaning of “intensity” in the optical Kerr effect in optical quantum computation

Kerr media, or mediums displaying the optical Kerr effect, are used in some optical quantum computers - on Chuang and Nielsen's Quantum Computing and Quantum Information, pgs. 289 - 290, it says, ...
28
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2answers
9k views

Basic questions in Majorana fermions

Why any fermion can be written as a combination of two Majorana fermions? Is there any physical meaning in it? Why Majorana fermion can be used for topological quantum computation?
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5answers
1k views

direct sum of anyons?

In the topological phase of a fractional quantum Hall fluid, the excitations of the ground state (quasiparticles) are anyons, at least conjecturally. There is then supposed to be a braided fusion ...
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Is the universe a quantum computer - is light speed barrier a computational constraint

There is currently a debate ongoing on leading maths blog Gödel’s Lost Letter, between Gil Kalai and Aram Harrow, with the former arguing that building a quantum computer may not be possible due to ...
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1answer
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Recent breakthroughs in quantum computing?

Can anyone explain to me why we have had no major breakthroughs in the theory of quantum computation in the past 15 years? Shor's algorithm set the standard, since then we've had Grover's algorithm ...
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2answers
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What exactly does No cloning mean, in the context of Quantum Computing?

I am trying to get an intuitive idea of how the No-Cloning theorem affects Quantum computation. My understanding is that given a qubit $Q$ in superposition $Q_0 \left| 0 \right> + Q_1 \left| 1 \...
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Quantum Computing Power Advantages

Currently, the world's fastest supercomputer runs at 17.59 Petaflops, which consumes 9 megawatts of electricity. A qubit-based quantum computer has the potential to operate much more quickly for some ...
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1answer
153 views

Why is the partial trace of this subsystem equal to this? [closed]

I am doing my bachelors dissertation based on an article by David Deutsch. He defines the action of a quantum gate as: $$ U = \sum_{x, y \in \mathcal{Z}_{2}} |x \dot{+}y\rangle|y\rangle\langle x|\...
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Can a parallel computer simulate a quantum computer? Is BQP inside NP?

If you have an infinite memory infinite processor number classical computer, and you can fork arbitrarily many threads to solve a problem, you have what is called a "nondeterministic" machine. This ...
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1answer
850 views

Canonical examples of quantum channels

I have a conjecture about quantum channels. On which examples should I test it before I try to prove it, ask it on StackExchange, or write a paper about it? (Note: This is meant to be a reference ...
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3answers
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How much is quantum computation changing the interpretation of quantum theory, and, if at all, how?

At the beginning of quantum computation, David Deutsch made a strong claim that the Many Worlds interpretation of quantum theory was at the foundation of his ability to do what he did. There was a lot ...
7
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3answers
271 views

Adiabatic Quantum Computing: why not just set the system in its problem Hamiltonian $H_{P}$ immediately?

Background: In any adiabatic quantum computer (AQC) algorithm, we solve problems in the following manner: We have an initial Hamiltonian, $H_{0}$, whose ground state is easy to find, and a problem ...
6
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1answer
230 views

fixed input qubit state to an arbirary pure state using two variable rotations and one fixed rotation

It is a theorem that any arbitrary unitary transformation in SU(2) can be factored into the following form: $ O = U_X(\theta) U_Y(\phi) U_X(\delta) $ Where $U_X$ is a Bloch sphere rotation. I ...
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1answer
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Quantum and classical scaling of memory

In the literature of Quantum Computation (QC), when discussing the simulation of quantum systems, one usually comes across comparisons with the classical digital analog such as: "Classically the size ...
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1answer
2k views

I am interested in learning Quantum Computing what should I do? [closed]

I wish to learn about quantum computing which seems to be a topic of hot research and overall just intrigues me. I have a strong background in discrete mathematics and number theory. And am a pretty ...
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1answer
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Why do quantum gates have to be reversible?

One possible reason I have come up with is that we are modeling quantum gates by unitary matrices. And since unitary operations are reversible we have to be able reverse the operation in the physical ...
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6answers
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Why do some physicists believe that scalable quantum computing is possible? [closed]

If you drop a glass cup on the ground, it will break and shatter into pieces. This happens all the time and is consistent with quantum mechanics. But it never happens that a shattered glass cup ...
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0answers
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About the Hayden-Preskill circuit

Can someone summarize as to what are the problems and/or the open questions with the Hayden-Preskill circuit? (in the context of understanding black-holes or as a computer science question)It gives a ...
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1answer
690 views

Are quantum computers electronic?

I understand how normal logic gates operate and are implemented using digital circuits and electronics but I wad wondering how qubits are physically implemented. In other words is a physical qubit a ...
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What is “code” in “toric code”?

When I first heard people talking about using Kitaev's toric code to do topological quantum computation, I was thinking how many lines does the toric code have. Then I was told that the "code" really ...
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1answer
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What is quantum discord?

What is quantum discord? I stumbled upon this term on Quantum Computing: The power of discord, but have never heard of it before. Can you give a bit more mathematical explanation of the term here?
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1answer
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Ground state degeneracy of a variation of Toric Code model

We know that the ground state degeneracy of Toric Code model is 4. An easy way of seeing this is the following: Consider a 2D spin model where all the spins live on the links. The Hamiltonian is $H=-...
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3answers
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How are physics and computer science getting united? [closed]

How is theoretical computer science getting united with physics? Phenomena like Quantum Computing uses Quantum Mechanics to be able to compute things, how are computers helping not just to model our ...
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2answers
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How does Landauer's Principle apply in quantum (and generally reversible) computing

I understand that a reversible computer does not dissipate heat through the Landauer's principle whilst running - the memory state at all times is a bijective function of the state at any other time. ...
20
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1answer
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How Does a Quantum Computer Work?

I've read Wikipedia, I've read How Stuff Works, I've read The Singularity is Near, but I still just don't get it. How does a Quantum Computer work? It sounds very intriguing, but I just can't wrap my ...
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2answers
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How to apply a Hadamard gate?

How to apply a Hadamard gate to 3 qubits? by example how to apply $H$ to $(1/\sqrt{2})(\left|000\right> + \left|111\right>)$?
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1answer
415 views

Positivity in the Pauli/Bloch/coherence vector representation

Suppose $\rho$ is an $n$-qubit state and $\vec{x}$ is a vector of coefficients in the Pauli representation (also called the Bloch or coherence vector). That is $$ x_k = {\rm Tr}(\rho \sigma_k), $$ ...
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2answers
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Quantum Computing, Qubit Creation/Entanglement

I am currently a high school student researching quantum computing. I was referred to this site by Google and a friend. Currently I am researching the qubit part of quantum computing. My question is ...
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2answers
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Shor's algorithm and Bohmian Mechanics

Do quantum computer's tell us anything about the foundations of quantum theory? In particular Shor argued in the famous thread by 't Hooft Why do people categorically dismiss some simple quantum ...
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1answer
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Areas of computer science required for quantum computing

What knowledge of computer science should I have, to be able to pursue research in quantum computing. I am a Physics undergrad and would take three core courses in QM, before the completion of my ...
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1answer
604 views

Extract single qubit state from combined state in QuTiP

I would like to extract the state of single qubits from combined states, using QuTip (the Quantum Toolbox in Python. Is there some QuTiP function that will extract a single qubit state from a ...
11
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2answers
1k views

Can quantum annealing be used for factorization?

It is known that there is a famous quantum factorization algorithm by Peter Shor. The algorithm is thought to be suitable only for quantum gate computer. But can a an adiabatic quantum computer ...
8
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1answer
222 views

are locally unique pure quantum states also ground states of some local hamiltonian?

Let $H=\sum_i H_i$ be some k-local hamiltonian with a unique ground state $|\psi>$. Then it is easily shown that $|\psi>$ is k-locally distinguishable from any other state $|\psi'>$. Is the ...
3
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3answers
474 views

Does processing for a quantum computer take place in other universes?

Apologies in advance if my question seems misinformed. I am a software developer, and neither quantum mechanics nor physics are my specialties. From http://physics.about.com/od/quantumphysics/f/...
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1answer
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Do quantum computers manufactured by D-Wave Systems, Inc. work? [duplicate]

D-wave claims to have built 128 qbit quantum computers which are commercially available? What I don't understand is that have they really been able to do this given that the scientific community is ...
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5answers
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What is the use of a Universal-NOT gate?

The universal-NOT gate in quantum computing is an operation which maps every point on the Bloch sphere to its antipodal point (see Buzek et al, Phys. Rev. A 60, R2626–R2629). In general, a single ...
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3answers
488 views

Quantum computing and quantum control

In 2009, Bernard Chazelle published a famous algorithms paper, "Natural Algorithms," in which he applied computational complexity techniques to a control theory model of bird flocking. Control theory ...
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1answer
518 views

Computability of physics

As far as I understand a Universal Quantum Simulator can simulate any quantum system and thus anything that exists in the universe. Also, a quantum computer can implement such a quantum simulator. ...
8
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1answer
267 views

Characterisation of the generalised Bloch space in spherical coordinates

I'm so confused by the following definition in the "Quantum Error Correction" by Lidar and Brun that not even sure how to formulate the question properly. Let $\mathbf n$ denote a unit vector, i.e.,...
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1answer
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Why do we want to entangle qubits?

The title is pretty much all I want to ask. Why are qubits entangled? To my knowledge (which isn't that deep) a quantum register can be realized without entangling the qubits.
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2answers
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Quantum XOR: How do you generalize it?

Consider the classical XOR Gate: Given a 2 bit system: $G = [u_1, u_2]$ $$XOR(G) = (u_1 + u_2) \ mod \ 2$$ Is the following a good generalizaiton of a Quantum XOR Gate: Given a 2-qubit system: $...
5
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2answers
529 views

Size of a quantum computer to effectively calculate macroscopic reality from quantum mechanics

Assuming the correctness of QM: Would the size of such a computer be smaller than the observable universe? If it were to represent all available information in the universe it seems that it's minimal ...
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2answers
406 views

what breakthrough Physics needs to make quantum computers work?

I read some posts on this forum and some articles which repeatedly state that it is not impossible to build q-comps but to make it successful, physics needs a great breakthrough. I tried finding but ...
8
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2answers
471 views

What nonstandard theory forbids quantum computers?

What would a nonstandard model which reproduces all experimental quantum data so far but still cause quantum computers to fail when implementing Shor's algorithm look like? Would it have to be very ...
8
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1answer
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How many states can a n qubit quantum computer store?

A classical computer composed of '0' or '1' transistors stores $2^n$ states. Is it true that a quantum computer composed of '0' or '1' or '0 & 1' qubits stores $3^n$ states?