# Tag Info

## Hot answers tagged quantum-optics

Accepted

### How are two photons entangled in the Xanadu computer?

QML researcher at Xanadu here. Our X-series chip produce entangled states by squeezing light and then combining it at beam splitters: those 'cables' are waveguides in a chip, which when they are close ...
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### How do we restrict to a limited number of dimensions, say 3 for qutrits, while using OAM states of light?

As for any platform, one has to choose a suitable $d$-dimensional "computational" subspace. Suitability depends on your application, but generally it means that one should be able to perform ...
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### Shor's Code: Understanding how it satisfies Knill Laflamme Theorem

Strictly, what you have to calculate is that for all $i$ and $j$ $$\langle 0_L|U_iU_j|1_L\rangle=0$$ and $$\langle 0_L|U_iU_j|0_L\rangle=\langle 1_L|U_iU_j|1_L\rangle.$$ (I've ignored the ...
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### What are spin-coherent states?

These are also known as SU(2)-coherent states; one original reference is https://doi.org/10.1103/PhysRevA.6.2211. In a spin system, with states labeled by the eigenvalue of the total angular momentum ...
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### How to write the covariance matrix of a quantum gaussian state as a function of photon numbers?

Recall that $a=(q+ip)/\sqrt{2}$ in some dimensionless units (Weedbrook might change the units because I think they like $\hbar=2$; I'm using $[q,p]=i$ and $[a,a^\dagger]=1$). We can thus find the ...
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### Nielsen and Chuang: Solving equation of motion for amplitude damping

Think about how the density matrix is represented with the Bloch vector: $$\tilde{\rho} = \frac{1}{2}(I + \vec{n} \cdot \vec{\sigma})$$ Then \frac{\mathrm{d}\tilde{\rho}}{\mathrm{d}t} = \dot{\vec{n}}...
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### Does the entanglement have the conventional meaning in optical interferometers?

Entanglement is entanglement, there's just one definition of it (well, there's more complex variations like genuine multipartite entanglement etc, but that's not relevant here). However, (bipartite) ...
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### What are the measurement operators $F_k$ corresponding to a homodyne measurement?

One thing that I feel is worth noting is that there are two ways to understand what "measurement" means in this context. On the one hand, you have "measurements" in the sense of ...
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### Why do we use 'modes' in quantum optics?

Modes are governed by eigenfunctions, I agree. In quantum optics, we need more than just eigenfunctions to describe a state of light: we need to know how many photons have properties corresponding to ...
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