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27 views

Using entangled qubits to communicate [duplicate]

After reading up on quantum mechanics considering entangled qubits I was asking myself this simple question: If one qubit (A) positioned on earth is entangled with another qubit (B) which is - say - ...
4
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2answers
68 views

Entangled states and separable states

B Two electrons in the same orbital is clearly an entangled quantum state since it is not a tensor product: $$|\psi\rangle=\frac{1}{\sqrt{2}}(|\uparrow\rangle \otimes|\downarrow\rangle-|\downarrow\...
1
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3answers
136 views

Are superposition quantum systems really in two pure states at once?

I know that quantum systems can have properties of waves as well as those of particles. Also, I know that the mathematical description of quantum states is probabilistic, i.e. $$\Psi = \frac{1}{\sqrt{...
2
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0answers
28 views

Can an arbitrary spin state be written uniquely in a Dicke state basis?

Consider a system of e.g. $N=3$ spin-1/2 particles. The state of the system $\vert\psi\rangle$ lives in a Hilbert space of dimension $2^N=8$. Now, consider the collective spin operator $$\mathbf{J} = ...
1
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2answers
59 views

Is it possible to create coherence in a two-level system starting from the maximally mixed state?

I know how to create coherence between two states if I have a pure state. For example if the system is in pure state $|1\rangle = \begin{bmatrix}1\\0 \\\end{bmatrix}$ then density matrix is given by $\...
1
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1answer
56 views

Reduced density matrix of the edge spin-1/2 in AKLT spin chain

I am trying to understand the paper titled, "Entanglement in a Valence-Bond-Solid State" by Fan, Korepin, and Roychowdhury (https://arxiv.org/abs/quant-ph/0406067). I was able to understand the ...
2
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0answers
128 views

Experimental examples of how to realize POVM

Say I have a spin qubit that is one of two states $\vert\psi_1\rangle$ or $\vert\psi_2\rangle$ and $\langle \psi_2 \vert \psi_1 \rangle \neq 0$. To perform an unambiguous state discrimination, we ...
2
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1answer
136 views

What is the state of a single electron in an entanglement?

If we consider a bipartie system that is entangled, $$|\psi^{AB}\rangle=\frac{|{\uparrow_z\downarrow_z}\rangle-|{\downarrow_z\uparrow_z}\rangle}{\sqrt{2}} $$ And we want to know the probability ...
1
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0answers
415 views

How to measure a qubit state?

I'm trying to work out how to do a non-demolition measurement on a qubit using an ancilla photon. I start with an entangled electron and nucleus in the state $$ \frac{1}{\sqrt{2}}(\alpha|\uparrow\...
2
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1answer
763 views

Bloch sphere for 2 and more qubits

if I represent my qubit in the Bloch sphere as $|ψ⟩=a|u⟩+b|v⟩$ where $|ψ⟩=\cos(θ_2)|u⟩+e^{iϕ}\sin(θ_2)|v⟩$, I can see where my qubit is inside the sphere. Now imagine I have 2 qubits or more and ...
-2
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1answer
56 views

Cat State With Largest Number of Particles

I am wondering what is the largest number of particles that went into creating a successful cat-state? This article gives examples of 6 spins and 4 photons. I am wondering if there have been cat-...
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2answers
316 views

Is there something wrong with Bell's inequality? [closed]

Repeating the title because that's what I posted on Reddit and didn't get an answer. But I guess the proper question is: is there something wrong with the experiment? The text over on Reddit: ...
2
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2answers
631 views

General Two-State Systems and Rabi Oscillations

I've been working on this for quite a while now and can't seem to figure it out. I am concerned with the dynamics of a general two-state system, specifically the case of a particle in a constant ...
4
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2answers
2k views

How are quantum qubits implemented? [duplicate]

Its very easy to pull a logic gate or transistor circuit of the internet for example and begin to understand how it represents physical bits, by either having 5V across the circuit or below 3.3V. But ...
1
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1answer
111 views

Effect of the translation operator affected by spin?

I'm reading an introductory review on quantum walks and at some point it incorporates spin into the translation operator in a way that I don't follow. Initially it states that the translation by ...
0
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1answer
97 views

How to prove that quantum computers can't solve problems outside PSPACE quickly?

What are general methods to prove that certain complexity class is not proper superset of PSPACE class, and how to apply them in quantum case (for BQP class, for example).
6
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1answer
305 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
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1answer
55 views

Reason behind choosing the invariant states for an operator which commutes with an adiabatic Hamiltonian

In the section 4.1 of Quantum Computation by Adiabatic Evolution, Farhi et al proposes a quantum adiabatic algorithm to solve the $2$-SAT problem on a ring. To compute the complexity of the algorithm ...
0
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2answers
73 views

What kind of product is $\prod^n_{j=1}\sigma^{(j)}_x$?

In the section 4.1 of Quantum Computation by Adiabatic Evolution, Farhi et al proposes a quantum adiabatic algorithm to solve the $2$-SAT problem on a ring. The adiabatic Hamiltonian is defined as $$...
2
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1answer
234 views

Why does $\prod^n_{j=1}\sigma^{(j)}_x$ commute with this adiabatic Hamiltonian? [closed]

In the section 4.1 of Quantum Computation by Adiabatic Evolution, Farhi et al proposes a quantum adiabatic algorithm to solve the $2$-SAT problem on a ring. The adiabatic Hamiltonian is defined as $$...
1
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0answers
1k views

Collective angular momentum , Dicke states and indistinguishable particles

During course of quantum mechanics we dealt with addition of angular momenta. If we have two particles with spin $j_1$ and $j_2$ we can introduce total spin operator: $$\mathbf{J} = \mathbf{j}^{(1)} +...
4
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0answers
94 views

How long does it take to a local perturbation to propagate along a quantum system?

Imagine to have a one-dimensional system in its ground state, and to apply a local perturbation at one edge of the system. How does the system evolve after being perturbed? More specifically, how ...
1
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1answer
540 views

Quantum entanglement and information transfer

I know it was stated many times that information transfer using entanglement is not possible and I am most probably wrong but I would be glad if you can at least point me to where I am mistaken. If ...
2
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2answers
530 views

How many parameters are needed to specify a quantum state?

We have a spin state \begin{align} \ |{\Psi}\rangle=a_u|{U}\rangle+a_d|{D}\rangle \end{align} where $|U\rangle$ and $|D\rangle$ are $up$ and $down$ basis vectors, and $a_u$,$a_d$ are their complex ...
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2answers
318 views

Stern-Gerlach: Why is it probability 1/2 for cooked up silver atoms?

Why does cooking up silver atoms in an oven give them equal probability spin up and spin down?
-1
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1answer
63 views

Question regarding entanglement

Could the paradoxical nature of entanglement be the result of a change in the nature of quantum properties, or an incomplete understanding of such, instead of non-locality?
2
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3answers
958 views

Spin in magnetic field and eigenvalues

We have some arbitrary quantum state, lets say $$\vert\Psi\rangle=\alpha_{1}\vert\uparrow\rangle+\alpha_{2}\vert\downarrow\rangle= \begin{pmatrix} \alpha_{1} \\ \alpha_{2} \\ \end{pmatrix}$$. And ...
3
votes
5answers
927 views

Physical interpretation of applying a unitary operator to a state

When we apply one of the Pauli matrices $\sigma_y$ on one of its eigen-vectors $| \odot \rangle$, what does the eigen-value tell us about $| \odot \rangle$? Is this considered a measurement of $| \...
2
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2answers
983 views

Writing down an entanglement in bra-ket notation

I have a relatively complex (for me anyhow) situation which I would like to capture in bra-ket notation, rather than in words. However, I've not been able to find a source which will help me ...
0
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1answer
194 views

Geometric Interpretation of Rotated basis of Hamiltonian and collective Dicke states

Suppose I start with a basis of states for a two spin-1/2 particle system, namely, $\{\left|\uparrow\uparrow\right\rangle, \left|\downarrow\downarrow\right\rangle, \left|\uparrow\downarrow\right\...
2
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0answers
80 views

Adiabatic evolution for initial Hamiltonian on Hadamard basis and problem Hamiltonian as diagonal

This is spawned from a comment at the answer to one of my previous questions. Someone suggested to me that claiming the following statement might be NP-hard. Could anyone please help me to figure out ...
1
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1answer
98 views

Confusion about a lemma on the time constraint of an adiabatic evolution (arXiv:quant-ph/0604077)

I am going through the paper Quantum adiabatic evolutions that can't be used to design efficient algorithms by Zhaohui Wei and Mingsheng Ying. On the second page they prove a lemma. The statement goes ...
2
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0answers
81 views

Spin Transition Energies

I am reading a paper: http://arxiv.org/ftp/arxiv/papers/1305/1305.2445.pdf On p. 22, the following Hamiltonian is given: $$ H = \mu_B g \mathbf{B} \cdot \mathbf{S} + D(S_Z^2+\frac{1}{3}S(S+1)) + E(...
1
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1answer
966 views

Ising spin vs Pauli spin matrices

Are Ising spins scalar or operators? I am not a condensed matter physicist hence having some confusion. I have learnt about Ising models from adiabatic quantum algorithm papers. For example this ...
1
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0answers
76 views

Experimental realization of Quantum Teleportation of Spin, not polarization, not ions or atoms

I've looked everywhere in databases my school provides, to google searches, to the questions asked in physics forums, and here. As I understand, the original QT (quantum teleportation) protocol ...
18
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2answers
212 views

What Shannon channel capacity bound is associated to two coupled spins?

The question asked is: What is the Shannon channel capacity $C$ that is naturally associated to the two-spin quantum Hamiltonian $H = \boldsymbol{L\cdot S}$? This question arises with a view ...
2
votes
3answers
564 views

Is it possible to bind an entangled electron to the outer shell of an atom or just a proton?

Say we start with an entangled electron--positron pair and we separate them. I want to take the entangled electron and bind it to a proton or the outer shell of an atom. Is it possible to do this ...
1
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2answers
253 views

fermions and quantum gates

Say that I have 2 qubits - 2 spin half fermions. my initial condition is $|00\rangle$ in the spin-wave function and some anti-symmetrical spacial wave function. I'm wondering about what happens when ...
4
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3answers
2k views

The complementary variable to the qubit and spin-1/2

The qubit is a big topic of quantum information theory. A qubit is a single quantum bit. Physical examples of qubits include the spin-1/2 of an electron, for example, see page 39 of Preskill: http://...