# Questions tagged [decoherence]

Quantum decoherence is the loss of quantum coherence. As long as there exists a definite phase relation between different states, the system is said to be coherent. This coherence is a fundamental property of quantum mechanics and is necessary for the functioning of quantum computers. (Wikipedia)

88 questions
Filter by
Sorted by
Tagged with
1 vote
51 views

### What is the density matrix of a GHZ state when onle a qubit is in a decoherence channel?

Suppose Alice, Bob and Rob share a GHZ state. Now consider Rob's qubit is in a bit-flip channel. How to obtain the density matrix in this senario? Also i would be glad to get some articles adrresing ...
• 741
31 views

### Depolarizing channel on GHZ-state

Consider a GHZ-state $|\psi\rangle =\frac{1}{\sqrt{2}}(|0\rangle^{n}+|1\rangle^n)$, and consider a depolarizing channel that maps a density matrix $$\rho\to(1-\lambda)\rho + \frac{\lambda}{2^d}I.$$ ...
• 1,517
1 vote
43 views

### Why does Lidar define a decoherence-free subspace as "a subspace that undergoes purely unitary evolution"

In this paper from 2014, Lidar defines: "an open system which undergoes purely unitary evolution (possibly only in a subspace of its Hilbert space) is said to be decoherence-free." Why is ...
• 121
1 vote
37 views

• 221
119 views

### How to estimate the qubit frequency from a two-frequency Ramsey experiment?

In a two-frequency Ramsey experiment, we obtain two Ramsey sequences from $f_d-\delta$ and $f_d+\delta$, where $f_d$ is the current drive frequency and $\delta$ is some detuning. We then estimate ...
• 153
83 views

### What is the explicit form of $T_1$ decay channel?

I see $T_1$ error mentioned in many experimental papers (for example, page $10$ of this paper mentions such a decay.) How do I model $T_1$ decay theoretically? More concretely, here's my question. Say ...
• 1,363
217 views

### What are currently the best reported qubit error rates per gate operation?

Based on some investigation, I have found the best qubit coherence timescales, in superconducting architectures, to be the order of milliseconds. https://journals.aps.org/prb/abstract/10.1103/PhysRevB....
• 173
370 views

### How to recover the original density matrix from the output of amplitude damping channel?

For amplitude damping, we have the below expression $$\xi(\rho)=E_0\rho{E_0}^\dagger + E_1\rho{E_1}^\dagger.$$ How can I perform a matrix inverse operation on $\xi(\rho)$ at the receiver to get back ...
259 views

### De-coherence and Wigners friend?

So in the thought experiment Wigners friend the paradox is ultimately due to a difference of descriptions of density matrices. If the physical variable that is measured of the spin system is denoted ...
43 views

### How and how much do fabrication defects affect superconducting qubit coherence time?

I used simulations to calculate T1 time of a transmon qubit, but the result is much longer than the lab data. I suspect it was because simulation does not consider fabrication defects. May I ask if ...
• 75
1 vote
24 views

### Why does SQcircuits give 0 as the quasiparticle decoherence rates for a transmon qubit?

I am a high schooler new to quantum computing and for my project I use SQcircuits to simulate the T1 of a transmon. However, looking through some literature, it seems that quasiparticle noise is ...
658 views

### What is the charge noise in the context of superconducting qubits?

I read from some papers that the charge noise is one of the factors that cause decoherence of qubits. May I know what charge noise is and how exactly the charge fluctuation affects the qubit frequency?...
• 75
1 vote
70 views