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-4
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2answers
48 views

How to resolve “bottleneck” of encoding input of quantum computer? [on hold]

As far as I understand, all quantum computing purpose is to accelerate exponentially (upon input length) computation time of given task. But here user faces bottleneck to serialize input, and ...
2
votes
3answers
58 views

Two qubits system in polar co-ordinates

I know that I can write a single qubit state in terms of polar co-ordinates $(r,\theta,\phi)$ on a Bloch sphere. \begin{equation} \rho = \begin{pmatrix} \frac{1+r \cos\theta}{2} &\frac{r \exp(-i\...
-1
votes
0answers
38 views

Interpreting conical intersection in quantum adiabatic evolution [on hold]

Recently, I have studied a particular quantum adiabatic algorithm. When I plot the eigenvalues of the ground and first excited state against normalized time $s$, there appears a conical intersection. ...
1
vote
1answer
60 views

How to choose proper measurement operator?

Let's assume I have two states inside the Bloch sphere, at radial vectors $r_1$ and $r_2$ respectively $(r_1<r_2<1)$. Their angular location is same. These are like: \begin{equation} \rho = \...
0
votes
1answer
54 views

Intuition for defining basis for Hamiltonian in momentum representation

I am going through Quantum information approach to the Ising model: Entanglement in chains of qubits by Stelmachovic et al. In Section A.4, the authors determines the eigenvalues and eigenstates of ...
1
vote
1answer
56 views

rotation of a state in Bell basis

Suppose I have a state in Bell basis. For example \begin{equation} \rho = \begin{pmatrix} \rho_{11} &0 & 0 & \rho_{14} \\ 0 &\rho_{22} & \rho_{23} & 0 \\ 0 &\rho_{32} &...
1
vote
0answers
44 views

Toric code vs Shor's code

I study computer science and wanted to get into the field of quantum computing. Right now, I'm trying to tackle the subject of quantum error correcting codes. As is the usual, I've started with Shor's ...
2
votes
1answer
68 views

Are the 'clock' and 'shift' operators used in Qudit codes physically realizable?

I've recently started doing some reading on the subject of qudit codes. In particular, i'm interested in the frequently used clock and shift operators. Can these operators be physically realized? Or,...
1
vote
0answers
37 views

Why does the Quantum Approximate Optimization Algorithm work?

I am trying to understand the structure of the QAOA algorithm (see here) which is seen as one of the candidates to demonstrate early quantum supremacy. Yet Farhi, Goldstone and Gutmann only describe ...
-1
votes
0answers
48 views

Improved gap estimates for quantum adiabatic evolution

In his PhD thesis, Daniel Nagaj mentioned that Deift† et al has tightened the relationship between the adiabatic evolution time $T$ and the energy gap between the ground state and first excited state, ...
0
votes
0answers
30 views

Questions on the Lechner-Hauke-Zoller quantum annealing architecture

The Lechner-Hauke-Zoller quantum annealing architecture was first introduced in A quantum annealing architecture with all-to-all connectivity from local interactions. While going through the paper, I ...
1
vote
0answers
37 views

Probability of measuring a pure qubit state after some unitary rotation [closed]

Suppose I have the prepared state $$|+\rangle = \frac{|0\rangle + |1\rangle}{\sqrt{2}}$$ and the unitary $Z_{\pi/2}$ which rotates a state in the Bloch sphere by $+\pi/2$ about the $z$-axis. As I ...
0
votes
1answer
29 views

Can we perform matrix operations of CNOT on 2 qubit systems? [closed]

I am trying to get started on quantum computing. I find that 2x2 matrices like Pauli X,Y,Z,or gates like H,S can be used to perform operations on single qubits as direct matrix multiplication. For e.g ...
1
vote
2answers
75 views

Variation of schrodinger cat replaced by quantum computer

In the "classical" imaginary Schrodinger's cat experiment, which seems to be no longer serious, or at least irrelevant, by many (some?) people, everything is explained away by decoherence. Now, let ...
0
votes
2answers
65 views

What's Bob's state after this quantum circuit? [closed]

As shown in the picture, we know Alice's state will be intact after this circuit, but what about Bob's state, will it be $|0\rangle$ or $(|0\rangle+|1\rangle)/\sqrt{2}$ and why? I think it will be $(|...
0
votes
1answer
27 views

Heisenberg Representation of Quantum Computers explain observable transformations

The Heisenberg Representation of Quantum Computers (Daniel Gottesman) http://arxiv.org/abs/quant-ph/9807006 Suppose we have a quantum computer in the state $|\psi\rangle$, and we apply the ...
2
votes
0answers
33 views

Can anyone recommend some recent journal articles on the limitations of Quantum Computing

More specifically, I would like some reading material on what the issue is with quantum computers, why can we not yet create them? Are there any journal articles that comprehensively list the reasons ...
3
votes
2answers
99 views

How do we know WHEN to get the result from quantum computers?

So I always hear that you can't disturb quantum computers because this will ruin the particles superstate. Well, how do we know WHEN to retrieve the result from the calculation? How can we determine ...
2
votes
2answers
296 views

Quantum Grover's algorithm: constructing the black box gate

Grover's Algorithm uses a black box gate that can recognize an x such that $f(x) = 1$ for a certain function f. It inverts this x and leaves all other inputs unchanged. My question is about the ...
2
votes
1answer
30 views

How can a quantum dot be used as a qubit?

Many people say that quantum dot is a potential physical representation of qubit. A qubit should have two distinguishable states which may carry quantum information. What are the two states of a ...
0
votes
0answers
21 views

Is there a quantum computing model accounting for uncertainty of a qubit state?

Any physical quantum computer would have a limit on the fidelity with which it can create qubit superposition states. If we're trying to create $|\Psi\rangle = c_0|0\rangle + c_1|1\rangle$, the ...
0
votes
2answers
75 views

What do operations on single Qubits of Unfactorable Superpositions Do?

So suppose I have the following Quantum Circuit: A ---- |Control| -----|Hadamard|---- B ---- |xxxxxxx|------------------------ Which is a 2 input Controlled Gate (applying some gate of two choices ...
0
votes
3answers
93 views

Quantum computing entanglement dimensions question

While trying to understand the basics of how quantum computers work, I recently read this statement. "...consider that single-qubit states can be represented by a point inside a sphere in 3-...
-2
votes
2answers
67 views

Can a non-entangled qubit be teleported by entangling it?

Let's say I have a qubit that is not entangled in state $\psi$. I want to teleport this qubit by entangling it with another qubit but still getting $\psi$ back in the end. Is this possible, or would ...
4
votes
2answers
121 views

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 \...
3
votes
1answer
50 views

Is QC with Superpositioned Quantum Gates any different than normal Quantum Computation?

This might be more appropriate for theoretical CS stackexchange, but it feels sufficiently low level to be relevant here. Consider the following thought experiment: I have a Quantum FPGA, it is a ...
1
vote
0answers
57 views

Approximate cloning of a quantum state, informed by past measurements

Suppose I give you a state $|\psi\rangle$, and tell you a sequence of measurements that have been performed on it. The measurements are not guaranteed to be orthogonal to each other, or to cover the ...
1
vote
0answers
21 views

When will a useable universal quantum computer first be realised? [closed]

Ideally with the reference to the type of technology, when do experts predict a small universal quantum computer of a couple hundred qubits will be developed? From my reading the leading techology is ...
1
vote
5answers
149 views

How Are Quantum Computers Able to Store Any Data at all?

So if qubits can have more than two states, and according to this video, https://www.youtube.com/watch?v=T2DXrs0OpHU you don't know what you get until you actually "open the box", if its all ...
0
votes
1answer
85 views

Can a system be PERFECTLY simulated by a quantum (and classical) computer? [closed]

This is a thought experiment, and as such will assume some crazy things. Let's say I decide to perfectly simulate my university as it is right now. I use a magic machine to instantly scan the entire ...
-1
votes
1answer
44 views

C2(U) Unitary Matrix Representation

Above is my work for the unitary matrix representation of a C2(U) gate. However, this does not agree with a link I found online for what it should look like. Below my work, I have written the ...
0
votes
1answer
92 views

Unitarity of quantum evolution

In this paper by Charles Bennett, he says on page 25, I understand why U(XOR) gives the result it does but why is that a consequence of its unitary property? Thanks
0
votes
0answers
33 views

How many states does a qubit have? [closed]

There is something I am trying to understand about qubits. If a qubit can exist in an infinite (is that even correct?) number of states prior to the measurement, then why can it only be measured in ...
2
votes
1answer
136 views

How do Quantum Computers work? [closed]

I have a decent knowledge of quantum mechanics, undergraduate level, but would like a description of how a quantum computer works and why it is important that superpositions of states, ie coherence, ...
2
votes
3answers
215 views

What's so confusing about quantum physics? [closed]

Okay, just right off the bat, I want to make it clear that this is a stupid question. Check out this graph, from SMBC: Ignore the 'political discourse' part and replace it with 'talking about ...
0
votes
1answer
62 views

What are some of the differences between (the fields of) quantum computation and quantum information theory? [closed]

I have just started self-learning quantum information theory, and have a sub-trivial question: what is the difference between that field of study and quantum computation? I have some understanding ...
1
vote
0answers
47 views

Electron jump into a transistor to read qubit state

Prof. Andrea Morello, a quantum physicist, in one of its quantum computer models uses the spin of the outermost electron in a phosphorus atom to represent a qubit. This atom is embedded to a transitor....
0
votes
2answers
38 views

Looking for an example of an operator that will not converge on maximally mixed state

Based on my former questions and the kind help from the answers (especially Norbert Schuch), it seems that the maximally mixed state is not that special as I thought. But still I have to admit that it ...
1
vote
1answer
61 views

Holonomic and Topological Quantum Computing [closed]

In topological quantum computation, anyons are braided in spacetime, performing non-trivial evolutions of some degenerate groundstate. In holonomic quantum computation, the system is braided in ...
1
vote
1answer
73 views

How to solve such a maximal entropy oriented optimization problem?

I am considering such a problem: (1) Given a finite dimensional composite system AB whose initial state is a product state of A and B so that $\rho_{AB}=\rho_A\otimes \rho_B$. (2) Assuming AB ...
1
vote
0answers
39 views

Finding the dependence of dephasing time on flux noise in Transmon Qubits

Before introducing some relevant concepts I'll begin by stating my question. What I am looking for is the connection between the noise the flux through the SQUID in my transmon qubit (related to the ...
1
vote
0answers
27 views

Probability of obtaining the desired state in quantum linear regression

I was reading today the paper "Pattern classification with linear regression on a quantum computer" where the authors calculate in a final step the probability of success to measure the last qubit in ...
5
votes
0answers
63 views

Adiabatic turning on of coupling constant in simulation of $\phi^4$ theories in the JLP algorithm

In the Quantum Algorithms for Quantum Field Theories by Jordan, Lee and Preskill they have devised an efficient algorithm to simulate $\phi^4$ theories. Given by the Lagrangian density $$\mathcal{L}(\...
1
vote
0answers
66 views

A question about ultimate computation capability

If we consider the physical limit of computational capability of a system with average energy $E$, then (1) According to Margolus and Levitin, a quantum system with average energy $E$ takes time at ...
1
vote
1answer
68 views

Understanding amplitude amplification in quantum computing

In short, what is the essence of amplitude amplification type of techniques that appear in quantum computing? More precisely, my main questions are, relating them more to Grover's search which ...
2
votes
1answer
68 views

Quantum teleportation of atomic states that involve energy [duplicate]

With quantum computing the concern is with information, and when quantum teleportation appears, the states being transferred are often spin states. However, there is an energy difference, even if ...
3
votes
1answer
59 views

Simulation probabilistic classical computation on quantum computers

Given an $N$-level quantum system with computational basis $\{|1> ,|2> , \cdots, |N> \}$, if we perform an unitary operation $U$ on a computational basis state $\{|j> \}$ and then measure ...
1
vote
0answers
27 views

Temperature influence in Optical Quantum Computing

I've recently been looking into the (perhaps a bit outdated but still very interesting) field of linear optical quantum computing. Using photons as information carriers, and using objects like mirrors,...
1
vote
0answers
23 views

Efficient ways to solve complicated kinematics problems

I'm curious about the range of approaches one can take to perform motion/kinematic simulations on many 3D objects. Currently I am familiar with the iterative process of doing such a simulation, that ...
1
vote
0answers
58 views

How does one arrive at the two qubit matrix that represents a gate? [closed]

The two qubit CNOT gate is represented by a 2x2 matrix. However, it is also represented pictorally. How does one arrive at that matrix representation? I am not looking for "take a diagonal matrix and ...