# Tagged Questions

Questions concerning computations performed on a quantum-computer. DO NOT USE THIS TAG just because the question concerns a computation of a quantum-system!

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### What is three-photon interference?

Whilst reading this paper on a quantum processor that performs a type of matrix computation, I came across the concept of 'three-photon interference'. A quick Google search shows that this process is ...
1answer
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### 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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### Creating matrix Hamiltonian for Feynman's CNOT

I'm trying to read Quantum Mechanical Computer and to implement the CNOT logical gate with some analytical software. Since i wish to use the SWITCH implementation of the CNOT [Fig.8] i've realized ...
1answer
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### How are qubits better than classical bit?

WHAT I KNOW: classical computers store information in bits which can either be 0 or 1, but in quantum computer the qubit can store 0 , 1 or a state that is the superposition of these two states. Now ...
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### Deutsch-Jozsa algorithm [closed]

How many calls are required to determine is the function balanced or not on the classical computer with probability of error < 50%. Ref: Deutsch-Jozsa algorithm.
2answers
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### Qubit projections

Given the qubit: $$\frac{|0\rangle+i|1\rangle}{\sqrt{2}}$$ What is the corresponding point on the extended complex plane and Bloch sphere? How to perform calculations and get the point representing ...
1answer
56 views

### Two qubits problem [closed]

Given the 2 qubit state: (a/b) |00> + (c/b) |01> + (c/b) |10> + (d/b) |11> What is the probability that 2 qubits are equal? Thanks much!
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### Constructing a Toffoli gate from CNOT and single-qubit gates [closed]

Toffoli gate in terms of CNOT and single-qubit gates. Thanks much!
0answers
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### The physical implementation of quantum annealing algorithm

From that question about differences between Quantum annealing and simulated annealing, we found (in comments to answer) that physical implementation of quantum annealing exists (D-Wave quantum ...
1answer
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### 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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### Is it ever necessary to extend an analysis of Grover's algorithm beyond $k/N = 1/2$?

Is it ever necessary to extend an analysis of Grover's algorithm beyond $k/N = 1/2$, where $k$ is the number of "hits" in a total of $N$ possible values for $|\,x\rangle$? If we know $k$, and know ...
1answer
153 views

### Studying Feynman articles nowadays

I'm curious to know if it's useful to study Feynman article "Quantum Mechanical Computer" nowadays. I'm a computer scientist, and i don't know any of the literature in quantum computers. Since long ...
3answers
367 views

### Does quantum computing rely on particular interpretations of quantum mechanics?

It is my understanding that quantum computing relies on quantum superposition and entanglement to work--qbits must exist in all states simultaneously before giving a particular result when observed. ...
1answer
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### 2 following gates, inverse circuit

I have a circuit that has 4 wires and 2 following each other Toffoli gates. The first Toffoli gate occupies 3 wires from above, the following Toffoli gate occupies 3 wires from below. What will look ...
1answer
97 views

### 2 following gates, permutation matrix

I have a circuit that has 4 wires and 2 following each other Toffoli gates. I have permutation matrix for each Toffoli gate (A and B). Do I have to multiply that 2 matrices to get the entire ...
0answers
61 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 ...
2answers
307 views

### Entropy of a state subject to the action of a set of random unitaries

Suppose that we have a known set of unitaries $U_1,...,U_n$ randomly selected from the Haar measure and suppose that each unitary is applied with probability $\frac{1}{n}$ to some input state $\rho$ ...
2answers
465 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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1answer
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### Scaling of quantum error correction

I'm having a question regarding quantum error correction. Using a large number of imperfect (but already very good) quantum gates, it is in theory possible to build an equivalent, error-corrected ...
4answers
211 views

### Quantum computers: are they possible or impossible?

I know quantum computers are very complicated and my question is is there any way in "Principle" to create one? Are there already quantum computers being created?
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472 views

### How to apply a Hadamard gate?

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

### Help on controlled quantum gates?

Obviusly a controlled-not gate is possible, is a not-controlled gate possible? I need a gate to flip the first qbit and leave the second unchanged, but in literature I have never seen such a gate. ...
2answers
189 views

### Quantum Teleportation Fidelity

I understand that quantum teleportation fidelity is the overlap of the initial quantum state with the teleported quantum state. If the teleportation is perfect, then the fidelity would equal 1 or 100% ...
0answers
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### Finding out resulting state [closed]

Suppose we have two qubits in the state $x|00\rangle+y|11\rangle$. What is the resulting state of the second qubit in that case? Use and to denote and respectively.
1answer
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### Reversible gates

Is it possible to make any gate reversible merely by retaining the input bits in the output and introducing ancilla bits as necessary? That is, given an irreversible gate with $k$ inputs and $l$ ...
1answer
175 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. ...
2answers
153 views

### Constructing a Toffoli gate with 2-and 1-qubit gates?

I'm looking through Nielson's book on quantum computation and information and in part of it he says that any $C^2(U)$ gate can be constructed from two qubit and one qubit gates. I can't figure out how ...
1answer
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### Quantum computing problem

Suppose that a qubit is in the state $|\varphi\rangle=a|0\rangle+\sqrt{1-a^2}|1\rangle$, where $a\in[-1,1]$. If we first perform a standard basis measurement on this qubit and then perform a ...
1answer
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### Uneven Quantum Measurement

$\newcommand{\ket}[1]{\left| {#1} \right> }$ I have no academic background in physics, but I'm attempting to study quantum computation. I have read that a quantum system of two qubits is ...
0answers
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### Finding all marked element by Grover search(not in superposition)

Quantum search enables square-sped up search for marked element. When there are multiple maked element, grover search provides only superposition of them. If I want to find all the marked elements, ...
2answers
404 views

### 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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### Non-destructive measurement of qbits

Yale news "New qubit control bodes well for future of quantum computing" (Original paper) says: "The Yale physicists successfully devised a new, non-destructive measurement system for observing, ...
3answers
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### Controlled-measurement of a quantum register

Given a state vector $\left[\alpha,\beta,\gamma,\delta\right]$ which is not known a priori, does there exist an operation, which I will call "controlled-measurement", which results in the ensemble ...
4answers
180 views

### If quantum computation is reversible, what is the point of Grover's search algorithm?

Wikipedia et al say the following about Grover's algorithm: Although the purpose of Grover's algorithm is usually described as “searching a database”, it may be more accurate to describe it as ...
1answer
317 views

### If the ground states of interacting QFTs are so complicated, how did Nature find them?

My question was inspired by trying to understand the paper Quantum Algorithms for Quantum Field Theories, by Jordan, Lee, and Preskill. The main result of that paper is that scattering experiments in ...
1answer
106 views