Questions tagged [quantum-information]

Quantum information is the study of the informational content of quantum states. The most common object of study is the "qubit", the information in a two-state quantum system such as spin-1/2 or photon polarization.

Filter by
Sorted by
Tagged with
-1
votes
0answers
39 views

Quantum multiplier [on hold]

I need a design for a 2x2 quantum multiplier using Toffoli gates or 3x3 it is not a problem. I find it difficult building it without need many qubits, and the result becomes wrong for some numbers.
1
vote
0answers
24 views

Strongly continuous dynamical maps

Let's say we have a bipartite system $\rho(0)=\rho_A \otimes \rho_B$ The evolution of system $A$ alone will be described by a dynamical map $\Phi_t$, such as: $\rho_A(t)=\Phi_t(\rho_A(0))$ If ...
0
votes
0answers
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 - ...
0
votes
1answer
69 views

Is the Second Law a consequence of the Many Worlds principle?

I've done a bit of browsing on this subject, and haven't found any papers that directly address this question. Here's the idea: In the Many Worlds View (MWV), there is no loss of information from ...
1
vote
1answer
75 views

Probability distribution of the overlap between two random quantum states in an $n$-dimensional Hilbert space? [on hold]

Let two pure states $|\Psi\rangle$ and $|\Phi\rangle$ be drawn uniformly and independently from an n-dimensional Hilbert space $\mathbb{H}^n$. (see Note). What is the probability distribution of ...
-2
votes
1answer
57 views

Confusion with identity operator

In the above example why the identity matrix ie ∑|x〉〈x|...is taken as ∫|x〉〈x| dx from negetive to positive infinity? or alternatively can someone explain the steps to expand the ket ψ into x basis
0
votes
0answers
26 views

Minkowski space and Rindler spaces, What's relation?

To describe the dynamic of a qubit one can use the Minkowski cordinates, But when that qubit is accelerated with Unruh acceleration we need Rindler cordinate . So, what is UNRUH acceleration ? and ...
0
votes
0answers
28 views

Confusion with mixed state [duplicate]

I have read that mixed state is a collection of pure states ...while a pure sate is a collection ie suoerposition of eigen states is that right?..so it can be thought of as a superposition of ...
0
votes
1answer
36 views

Quantum Key Distribution via Modified Double Slit Experiment

Here's my reasoning: Setup 1: Take a traditional double slit experiment and turn on the photon source. Interference fringes should appear on a detector screen placed opposite the photon emitter, ...
-1
votes
1answer
48 views

Action of ladder operator squared on squeezed coherent states [closed]

What is the action of the ladder operator squared $\hat a^2$ and ${\hat a^\dagger}^2$ on the state $|\psi\rangle=|{a, b}\rangle +|{a, - b}\rangle$ where $|{a, b}\rangle =\hat D(a) \hat S(b) |0\...
2
votes
1answer
61 views

Finding the quadrature variance of a superposition of squeezed coherent states

How do you find the quadrature variance of a state $$\lvert x\rangle =\lvert a,b\rangle +\lvert a,-b\rangle$$ where $\lvert a,b\rangle = D(a) S(b) \lvert 0\rangle$? $\lvert x\rangle$ is a ...
-1
votes
2answers
57 views

Why are entanglement and purity non-linear functions of $\rho$?

Any linear function of the density matrix can be related to a proper observable, but is it not the case of entanglement and purity?
0
votes
0answers
17 views

Saturability problem about the quantum Cramer-Rao bound for the multi-parameter quantum metrology

I was studying the multi-parameter quantum metrology these days. And I was confused about the saturability of the quantum Cramer-Rao bound for the multiparameter problem. If all of the generators are ...
3
votes
1answer
84 views

Can quantum communication break the Shannon–Hartley theorem?

Just a random stupid thought. Quantum entanglement cannot be used to transfer information faster then speed of light. But can it "embed" information until a key is received? For exmample, assume ...
1
vote
1answer
53 views

Is there a way to measure the degree of superposition of a quantum state?

I am wondering if there is a way to calculate the amount of superposition that a quantum state is in. For example, if I have a $2$-qubit quantum system, with basis $\mathcal{B} = \{|00\rangle, |01\...
0
votes
0answers
42 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 ...
0
votes
0answers
13 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
39 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 ...
2
votes
2answers
50 views

Do the pure states in the decomposition of a density operator need to be orthonormal to each other?

So, I was studying quantum computation using the book Nielsen and Chuang and it stated a theorem known as "Spectral Decomposition theorem" $$A=\sum _{i}\lambda _{i} | i \rangle \langle i|$$ I infer ...
0
votes
0answers
9 views

How to obtain a unitary time evolution that leads to the Lindblad equation on tracing out?

It is known that given the time evolution of an open quantum system by Kraus Operators, one can rewrite it as a unitary time evolution of a bigger system. That is, given Kraus operators on the Hilbert ...
2
votes
2answers
91 views

Is it possible that the speed of sound in some strange quantum material is faster than the speed of light in vacuum [closed]

If we believe the spacetime structure(including the limit of speed) could emerge from some vacuum structure(such as string-net condensation),then, is it possible that the speed of mode of excitation ...
6
votes
0answers
145 views

Kraus operators for two interacting harmonic oscillators: Problem with the calculation (Ex. 8.21 of Nielsen-Chuang)

I'm working with Exercise 8.21 of the Nielsen-Chuang book on quantum information. It illustrates the amplitude-damping quantum channel by the interaction between two harmonic oscillators (the first ...
1
vote
0answers
31 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 |...
2
votes
1answer
36 views

Transformation acting only on one of two qubits

Suppose you have a non-polarizing beamsplitter (NPBS) with 2 outputs: A and B. You place a polarizing beamsplitter (PBS) at the output A, and want to follow the evolution of the quantum state after ...
2
votes
1answer
66 views

Is a partial trace cyclic?

I want to know if a partial trace keeps the cyclic property of the trace. The partial trace is defined as $$ tr_B: \mathcal{B}_1(\mathcal{H}_A\otimes \mathcal{H}_B) \longrightarrow \mathcal{B}_1(\...
0
votes
1answer
34 views

Random quantum walk related to quantum computing

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 ...
0
votes
0answers
24 views

Specialized Journals for Quantum Information

I recently wrote a short, fairly mathematical paper on the subject of quantum information and quantum mechanics. Although the results in the paper are not big results, I have not seen them before and ...
4
votes
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\...
2
votes
0answers
28 views

Expanding a density matrix in terms of operators

In Lukasz's paper: https://arxiv.org/pdf/0909.2654.pdf He writes "consider a density matrix ρ, written as a polynomial of the 2N Majoranas cj in such a way that each cj occurs to the power 0 or 1 in ...
0
votes
0answers
43 views

Direct Derivation of Kraus Operator from Interaction Hamiltonian

For the dynamics of open quantum systems, the Kraus operators $K_\kappa$ can be derived from the unitary orbit $U(t)\rho U(t)^\dagger$ for $\rho=\rho_S\otimes\rho_E$ of the composite system given by ...
1
vote
0answers
19 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 \...
2
votes
1answer
56 views

Necessary and sufficient conditions for a pure state

I've seen some claims that idempotency ($\rho^2=\rho$) is necessary and sufficient to guarantee the existence of some state $\psi$ such that $\rho=|\psi\rangle\langle\psi|$, as well as claims on the ...
0
votes
0answers
47 views

Topological phases and quantum information

I am concerned about the theorem saying that there is no topological order in 1d. According to the seminal paper https://arxiv.org/pdf/1008.3745.pdf, there are no non-trivial topological phases in 1d (...
0
votes
1answer
42 views

tensor products of exponential annihilation operators

I'm hoping to use this stackexchange to solve a gate decomposition problem, but I am stumped, so hoping for help. These definitions come from the Xanadu Strawberry Fields whitepaper on Table VI and ...
2
votes
0answers
19 views

Quantum Fisher information for Gaussian states without eigenproblem?

Given a Gaussian-preserving interaction (including a unitary operation and losses) for a Gaussian input, I want to know if there is an "easy" way to compute the Quantum Fisher information without ...
0
votes
1answer
52 views

What relation exists between Wolfram's model of the Cellular Automata universe and the Holographic Principle and the Bekenstein Bound?

Are computational models of the universe, like Stephen Wolfram's cellular automaton model of the universe (proposed in his book A New Kind of Science), somehow related to the Holographic Principle and ...
1
vote
1answer
57 views

Entanglement of Werner States

Let the Werner state $$\rho_W = W\mid\Psi^-\rangle\langle\Psi^-\mid + \frac{1-W}{4}\mathbb{I},\ W\in [0,1],$$ where $|\Psi^-\rangle=(|01\rangle-|10\rangle)/\sqrt{2}$. I have repeatedly heard that ...
0
votes
1answer
35 views

Does non-orthogonality of joint states follow from non-orthogonality of ALL marginals?

Suppose we have a composite system $AB$ and we are considering two joint (pure) states $$ \psi,\psi^\prime \in \mathcal{H}_{AB} . $$ Let's define the reduced or marginal states of $A$ and $B$ in the ...
3
votes
1answer
56 views

Can decoherence account for suppression of interference in all basis?

Let the system formed by particle(microscopic or macroscopic)- environment coupling be described by $|\phi><\phi| $ . In decoherence approach, to retrieve the classical properties, one focuses ...
1
vote
0answers
18 views

Can we define a parent Hamiltonian for a given Multiscale Entanglement Renormalization Ansatz (MERA) state?

Given a state represented by a MERA (Multiscale Entanglement Renormalization Ansatz), can we define a local Hamiltonian such that the MERA is the exact ground state of such a Hamiltonian? If this is ...
1
vote
1answer
59 views

Can we restore a state of a whole system from its subsystem?

I am thinking about the deletion error correcting codes for quantum information. In classical information theory, there exist some deletion error correcting codes. An easy example is the following ...
1
vote
0answers
29 views

Channel Capacity and the Rank of Channel

Let's say $\Phi$ defined a $d-$dimensional quantum system $I$ is a quantum channel with entanglement-assisted classical capacity no smaller than $\log d$. This means that there exists a bipartite ...
0
votes
1answer
44 views

How to differentiate between quantum states based on phase

Imagine you have a source of photons and a single-photon detector. The source emits photons that can be in any of the following four states: $ |1\rangle $, or $ -|1\rangle $, or $ i|1\rangle $, or $ -...
2
votes
0answers
93 views

Why doesn't Wigner's friend interact with the system?

So I was recently modelling something that turned out to be basically Wigner's friend. I saw there were some differences (in the Wiki page) in how it was modelled: Namely, that Wigner's friend ...
0
votes
1answer
86 views

Does the Choi-Jamiolkowski isomorphism really establish a connection between kinematics and dynamics?

I understand the mathematical construction of the Choi-Jamiolkowski isomorphism aka channel-state duality . It all makes sense formally, yet I still struggle to grasp its physical (or quantum-...
0
votes
0answers
30 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 ...
1
vote
0answers
48 views

Single-particle QM

In learning device-independent QM from [https://arxiv.org/abs/1303.3081], Scarani tells us that all quantum statistics involving one particle can be reproduced with local (hidden) variables. He then ...
2
votes
0answers
197 views

Is this analysis contradictory with the $2$'nd measurment?

Summary and Motivation "The below idea is about making a mathematical statement on system $2$ which induces a measurement on system $1$ while $1+2$ obeys unitary evolution." Basically, I'm modelling ...
1
vote
1answer
34 views

Is there a maximum quantum advantage in sensing?

This is a rather conceptual question. Quantum sensing takes advantage of entanglement (and other quantum properties such as squeezing) to get variances that scale much better than the ones one can ...
3
votes
1answer
52 views

Expansion of Von Neumann entropy for small deviations

Suppose that your quantum system is described by $\sigma = \rho + \delta\rho$, where both $\sigma$ and $\rho$ are density matrices while $\delta\rho$ is "small". The Von Neumann entropy of the system ...