# Tag Info

Accepted

### How to find the density operator of two joint systems given the density operator of the individual systems?

This is not possible in general, as both density matrices could be the reduced density matrices of several joint density operators, see e.g. here and here. The reduced density matrices determine the ...
Accepted

### Understanding the probability of measurement w.r.t. density matrix

This is not derived, this is the Born rule or the Lüders-von Neumann measurement rule. Either way, it is a fundamental axiom of quantum mechanics that measurements work this way. See e.g. this, this ...

### Are two localized single-photon states always invariant under the particle exchange?

When you write a state like $|H\rangle_a \otimes |V\rangle_b = \hat{a}^{\dagger}_H\hat{b}^{\dagger}_V|\mathrm{vac}\rangle$ you're using the formalism of second quantization. In this formalism the ...

### Are two localized single-photon states always invariant under the particle exchange?

The question is whether the photons are distinguishable in another degree of freedom. If so, then the state must not be complete symmetric. In this case, the photons are distinguished by their ...
Accepted

### Does Quantum Key Distribution (QKD) guarantee perfect secrecy?

In quantum key distribution there is no message in the ordinary sense of the word. That is, no information is conveyed from A to B. Rather, it is a method whereby a random classical bitstring can be ...
Accepted

### How is the measurement of the $Z_0Z_2$ error syndrome realised by these two circuits?

Two points are important to this: The controlled Z gate is symmetric: It is irrelevant which of the qubits you call the "control" and which the "target". So you can put the Z on ...
Accepted

### Gate operations for controlling Cat States

One way to do this approximately is with the displacement operator: $$D(i\beta) = \mathrm{exp}[i\beta(\hat{a}+\hat{a}^{\dagger})]$$ where $\beta$ is assumed to be real. Applying this operator on the ...
1 vote
Accepted

### How do physicist keep entangled particles entangled while moving them/storing them?

When experiments are performed on entangled pairs: Photons are the most common choices for entangling quanta, and Bell tests are common uses of entangled pairs. Generally, the entangled photons are ...
1 vote

1 vote
Accepted

### How is continuous variable quantum key distribution safe against tapping some photons?

In CV-QKD, we send the signal at low power. At that levels, the signal is masked by noise of quantum nature (e.g. Shot Noise i.e. uncertainty principle of the quadratures). This noise protects the ...

Only top scored, non community-wiki answers of a minimum length are eligible