A branch of atomic, molecular, and optical physics dealing with how individual quanta of light, known as photons, interact with atoms and molecules. It includes the study of the particle-like properties of photons.
Questions tagged [quantum-optics]
49 questions
8
votes
1 answer
What are spin-coherent states?
Trying to understand the paper; https://arxiv.org/pdf/1702.02577.pdf and ran into "spin-coherent" states.
I wonder those are.
John Parker
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6
votes
1 answer
Shor's Code: Understanding how it satisfies Knill Laflamme Theorem
I'm new to Quantum Error Correction, and I have a question on Shor's Code.
If we have a protected subspace, $V \subset \mathbf{C}^2\otimes \cdots \otimes \mathbf{C}^2$
$V=\operatorname{span}\{|0_{l}\rangle, |1_{L}\rangle.$ We also consider Pauli…
Anon
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6
votes
1 answer
Why is a state Gaussian if and only if its covariance matrix satisfies $\boldsymbol\sigma+i\boldsymbol\Omega\ge0$?
Let $\rho$ be a Gaussian state, described by the $2N\times 2N$ covariance matrix $\newcommand{\bs}[1]{{\boldsymbol{#1}}}\bs\sigma$. Denote with $\bs\Omega$ the $N$-mode symplectic form associated with the space:
$$\bs\Omega\equiv…
glS
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5
votes
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Finding the ground state of a Gaussian Hamiltonian
It is well known (arXiv) in continuous variable (CV) quantum information processing that an $N$-mode CV system whose dynamics are entirely Gaussian can be simulated efficiently with a classical computer as the dynamics can be simulated by computing…
atman
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5
votes
1 answer
Trace distance between thermal states
The $M$-mode thermal state in quantum optics can be written in Fock space in the diagonal form
$$ \rho = \bigotimes_{j=1}^M\sum_{n_j=0}^\infty P(n_j|\mu_0^{(j)})|n_j\rangle\langle n_j|,$$ given the Bose-Einstein probability distribution…
Noobgrammer
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4
votes
1 answer
Does the covariance matrix of $\rho_1-\rho_2$ have a simple expression in terms of the individual covariance matrices?
Suppose $\rho_1$ and $\rho_2$ are two Gaussian states. The trace distance of a Gaussian state with covariance matrix $V$ can be computed as the sum of the eigenvalues of $i\Omega V$, so to find the distance of two Gaussian states we need the…
Noobgrammer
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4
votes
1 answer
How do the Type I and Type II fusions work?
I am going through the original paper on Type I and Type II fusions. But I cannot understand how this is actually implmented. They say they start with
$$\frac{1}{2} |H,H\rangle+|V,V\rangle \otimes |H,H\rangle+|V,V\rangle$$
Then they perform a PBS…
GaussStrife
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4
votes
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What are the measurement operators $F_k$ corresponding to a homodyne measurement?
By definition, a measurement is characterized by a set of positive-semidefinite matrices $\{F_k\}$ satisfying the completeness relation $\sum_k F_k = \textbf{I}$. I am interested in knowing how does the Homodyne measurement fit into this…
User101
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4
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What is an electro-optic switch?
I found an implementation of a CNOT in photonic QC and I just couldn't find any info on a component named "electro-optic switch"
What does it do exactly?
How does it work?
The implementation in question is this:…
Mimo Pakou
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4
votes
3 answers
Why does joint ground state not change under action of beam splitting unitary operator?
How can one show that $\hat{U}|00\rangle=|00\rangle$ where $\hat{U}=e^{-igt(\hat{a}^\dagger_2\hat{a}_1+\hat{a}^\dagger_1\hat{a}_2)}$ and $|00\rangle$ is the unique joint zero eigenstate of the annihilation operators $\hat{a}_1$ and $\hat{a}_2$, i.e.…
heromano
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3
votes
1 answer
What are the antisymmetric terms in $\sigma_{mn}$ in the expression for the Fisher information?
Given a parameter-dependent density operator $\hat\rho^\lambda$ and its spectral decomposition $\{\rho_m^\lambda, |\psi_n^\lambda\rangle\}$, Eq. $(17)$ from this review shows that one can compute its quantum Fisher information (QFI)…
Bentanglement
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3
votes
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How are two photons entangled in the Xanadu computer?
I was recently reading about how Xanadu made a scalable, room-temperature-operating quantum computer using photons as qubits (see this link). It said that in the logic gates, it entangles two photons at some point, and in the video it just looks…
Blue Herring
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3
votes
1 answer
How is measurement performed on a stream of polarized photons?
Is it possible to use a simple laser to measure qubits in optical quantum computers, or is a single-photon emitter absolutely necessary? If you can do it with a simple laser, how would you go about measuring it since you wouldn't have to measure a…
Mimo Pakou
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3
votes
1 answer
Relation between Wigner quasi-probability distribution and statistical second-moment
Is there any relation between the Wigner quasi-probability distribution function $W$ and the statistical second-moment (also known as covariance matrix) of a density matrix of a continuous variable state, such as Gaussian state?
Kianoosh.kargar
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2
votes
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How to simulate a beam splitter in QuTiP?
I'm trying to mix two states on a beam splitter whose unitary operator is
$$\hat{U}=e^{i\theta \left( \hat{a}^†\hat{b} + \hat{a}\hat{b}^† \right)},$$
where $\hat{a}$, $\hat{b}$ are annihilation operators in mode A and B and $\cos^2{\theta}=T$ and T…
Jakub Kouřil
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