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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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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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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Can we de-decohere an open system?
Can a mixed state become pure due to its interaction with a vast environment? Certainly, a strange proposal, and yet let's take a diagonal matrix representing a mixed state, say $$\begin{pmatrix}
p_{1}...
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Hilbert space of n qubits always has a decoherence free subspace if n is even?
I've heard that a subspace of an $ n $ qubit Hilbert space is called decoherence-free if it is not affected by errors of the form $ g^{\otimes n} $ where $ g \in SU(2) $.
Let $ \textbf{2} $ denote the ...
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Partial decoherence of a general one-photon state
Let $\rho_1$ be the pure one-photon state described by the ket
$$|\psi_1\rangle = \int dk\ A(k)a^\dagger(k)|0\rangle$$
for a complex amplitude function $A(x)$ and an empty ket $|0\rangle$. This state ...
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Ramsey Measurement for Characterising a Qubit
I'm trying to understand the Ramsey measurement used to characterise dephasing effects in qubits.
When operating on resonance, I understand the following quantum gate diagram to describe the $T_2^*$ ...
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How is Quantum Computing expressed in the language of abstract algebra?
I've lately been taking further coursework in abstract algebra, and it has struck me as fairly reminiscent of quantum computing. Of course, Pauli matrices, etc. have relevant roots within abstract ...
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Erasure errors in quantum error correction
Consider an $[n, k, d]$ classical code. This code can correct up to $d-1$ erasures. For example, if we have the code that maps $0\rightarrow 00$ and $1\rightarrow 11$, this code has distance $2$. ...
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Clarification on Definitions of Relaxation Probabilities
In the paper Modelling and Simulating the Noisy Behaviour of Near-term Quantum Computers, when the authors discuss relaxation times ($T_1$), they define two probabilities, $P_{T_1}$ and $p_{\text{...
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Yet another condition for a map to be completely-positive and trace-preserving
Surely, these conditions are all well-defined and well-known (via the Choi, Kraus, and Stinespring presentations). Is the following 'definition' valid? Does it make sense?
"The map is CPTP if, ...
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What is qubit decoherence?
If I have understood correctly, in time a single qubit in superposition will collapse on it's own to the $|0\rangle$ or $|1\rangle$ state, but I thought it only collapsed when measured. How is the ...
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Is there something like error-suppression codes?
My question is a bit on the soft side, and basically asks if there is something between error mitigation and full error correction?
On the one hand, as far as I understand, trying to increase the ...
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How to estimate the losses of a transmon qubit to its flux bias line
For transparency, I also posted this at the physics stackexchange, but it might be more fruitful to discuss here.
Already in the original transmon paper by Koch et al. is it stated that while the ...
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Representing 1 qubit pauli-channels as an averaging effect of random rotations in the bloch sphere. Basic literature?
I am looking into 1 qubit pauli channels, e.g. the dephasing channel
$$\mathcal{E}(\rho)=(1-p)\rho+p\sigma_z\rho\sigma_z.$$ I found out it could be represented as
$$\mathcal{E}(\rho)=\int p(\lambda)\...
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