# 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)

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### How quantum laws hold on macroscopic level in classical computers?

Disclaimer: I am not sure about whether my question is suitable for here, but I will ask it anyway. I am making research about quantum cryptography, so I start reading a book. In one paragraph, it is ...
1 vote
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### Repetition code applied to decoherence-free subspaces

The repetition code (figure below) can be used to detect bit-flip errors. Such a code is useful for studying, but it can't be used in more general scenarios where also phase errors occur. I was ...
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### DEJMPS and EXPEDIENT distillation protocols implementation in qiskit

I am new to Qiskit and distillation protocols. I am wondering if there is a tutorial or a link to the implementation of the DEJMPS and EXPEDIENT distillation protocols in Qiskit. What I want to ...
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### Dephasing noise and error correction

It is my impression that $Z$ errors are the dominant source of noise in many platforms used for quantum computing. For example, Ultrahigh Error Threshold for Surface Codes with Biased Noise says ...
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1 vote
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### 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 ...
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### 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 $$\rho\to(1-\lambda)\rho + \frac{\lambda}{2^d}I.$$ ...
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1 vote
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### 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 ...
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### 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 ...
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### 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 ...
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### 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....
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
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### 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?...
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