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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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
$$\mathcal{E}(\rho)=\int p(\lambda)\...
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How to estimate the qubit frequency from a two-frequency Ramsey experiment?
In a two-frequency Ramsey experiment, we obtain two Ramsey sequences from $f_d-\delta$ and $f_d+\delta$, where $f_d$ is the current drive frequency and $\delta$ is some detuning. We then estimate ...
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What is the explicit form of $T_1$ decay channel?
I see $T_1$ error mentioned in many experimental papers (for example, page $10$ of this paper mentions such a decay.)
How do I model $T_1$ decay theoretically? More concretely, here's my question. Say ...
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What are currently the best reported qubit error rates per gate operation?
Based on some investigation, I have found the best qubit coherence timescales, in superconducting architectures, to be the order of milliseconds.
https://journals.aps.org/prb/abstract/10.1103/PhysRevB....
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How to recover the original density matrix from the output of amplitude damping channel?
For amplitude damping, we have the below expression
$$\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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Density matrix derivation of an entangled state in amplitude dampling channel
The density matrix for an entangled state when both the qubits decohere with probability D1 and D2 in amplitude damping channel is given as $\rho_d$ in \href{https://www.nature.com/articles/nphys2178}...
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How does qiskit metal calculate T1 time
We are a group of students working on a project about superconducting qubits. We have used Qiskit Metal to do some simulation and got sets of results of Ec, Ej and coherence time T1. However, we are ...
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Thermodynamic Speed Limit to Quantum Computing
There's a lot of mystifying jargon in the field of quantum computation, so I would like to examine some elementary physics to maybe help clarify the assumptions being made.
Is it not true that the ...
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Is the assumption of constant per gate error probability $q$ in quantum computation proved , practical or valid?
The threshold theorem is valid provided that the error rate( per gate or per unit time), be under a constant threshold. But I have not read or known any proof about the constancy of theshold.
Is “the ...
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Fault-tolerant code and the error rate
Threshold theorem assumes that the error rate is bounded above by a constant. However, quantum decoherence is characterized by $$\langle E_{i}(t)|E_{j}(t) \rangle \propto e^{-t/\tau _{d}}$$
see here, ...
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Decoherence graph of $T_1$, $T_2$, $T_2^*$ in IBMQ
I would like to ask you a question because I cannot clearly understand the difference between $T_2$ and $T_2^*$ in the decoherence graph provided by IBM. Let me summarize the questions I have as ...
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Qubit dephasing times
The dephasing time (also known as decoherence time in the literature), $T_2$, speaks about the timescale in which qubits experience dephasing effects. This relates with the decay of the off-diagonal ...
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Why do we need T2 = 2T1, and why is there a factor of 2?
I am trying to understand why we want T2=2T1. After reading several definitions of T1 and T2, I just got really confused about these two constant. Is there a way (mathematically or visually) to ...
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Decompose into completely stabilizer preserving channel in surface codes
In the article "Sampling-based quasiprobability simulation for fault-tolerant quantum error correction on the surface codes under coherent noise" they are talking about decomposing (possibly ...
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Convert Coherent Noise to Clifford Errors with Probability on Surface Codes
Following my question about the equivalence of coherent and no coherent error, in surface codes.
Now I understand, it is not equivalent. I tried to read some articles about it, and I couldn't find a ...
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How to comparing a quantum channel with a unitary?
I have a target unitary that I want to implement and I have a known dynamics that results in a quantum channel. I already have some fidelity measures to characterize the overlap between them. (like: ...
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Simulating a quantum circuit with decoherence and noise
Based on the answers given here and here, it is pretty clear that an arbitrary quantum circuit can be simulated with matrix algebra. The difficulty is that this assume perfect fidelity. I am unsure ...
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How to simulate a quantum walk with decoherence?
I'm trying to reproduce quantum walk with decoherence as shown in figure 3 in V. M. Kendon, Phil. Trans. R. Soc. A, 364, 2006 (quant-ph/0609035).
I am able to reproduce quantum walk without ...
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Raman scattering goes in all directions, so why doesn't it decohere trapped ions?
I'm trying to learn about the physics of ion traps, and my understanding is that an ion can be moved from a ground state to an excited state by stimulation with a laser pulse. More specifically, this ...
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$T_2>2T_1$ qubits on the ibm_washington quantum processor
I have been checking out the parameters of the new ibm_washington processor and I have the following doubt about the calibration data provided by them. Checking out the relaxation and dephasing times ...
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What is the quantum capacity of the combined amplitude and phase damping channel?
Quantum capacity for the amplitude damping channel and the pure dephasing channel have closed-form formulas as it can be seen in section 24.7.2 of From Classical to Quantum Shannon Theory. However, I ...
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Coherence measurement for density matrix
I have a density matrix of the form:
$$\rho(t)=\left[
\begin{array}{ccc}
\frac{1}{4}+\frac{1}{12} e^{-\frac{2 \tau ^{2 H+2}}{2 H+2}} & \frac{1}{3} & \frac{1}{4}+\frac{1}{12} e^{-\frac{2 \tau ^...
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Does Randomized Benchmarking characterize decoherence error?
In my understanding, Randomized Benchmarking (RB) generates a sequence of Clifford gates with different lengths and then characterizes the average error. Since RB is not sensitive to SPAM error, it ...
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How to formulate Dynamical Decoupling passes in Qiskit to improve result upon circuit execution
First, let me say that I am not familiar with the idea of Dynamical Decoupling. The goal of this question is to understand how to set up a circuit with dynamical decoupling to improve my hardware ...
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Can quantum error correction work on any type of channel?
It says on wikipedia that quantum error correction can (at best) correct phase flips and bit flips. A popular form of representing a quantum channel is in its Kraus representation (scroll down to ...
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Do the Heisenberg uncertainty principle and quantum decoherence relate in some way?
Up until now I assumed (in simple words) that a qubit collapses because of the heisenberg uncertainty principle, meaning that we can not measure a qubit without changing it state.
But now I've read ...
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How can I understand these two equations about the indirect measurement?
I'm reading an article about environmental monitoring and information transfer. Suppose $S$ represents a quantum system and $E$ is the environment. Assume at time $t=0$ there are no correlations ...
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Implementing Partial Trace in IBMs quantum computer
I am trying to implement the partial trace operation on IBMs quantum computer. I am simulating the depolarising channel with the following code
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Need mathematical Calculation of input(T1,T2,f) and output(Readout Error)
I would like to know about the mathematical relations among T1, T2, frequency, readout error, and single-qubit error?
This screenshot is from the ibmq-16 Melbourne excel data file.
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Finding the irreducible representation of Kraus operators of a dephasing channel
I would like to understand an example of finding a noiseless subsystem of a quantum channel from the irreducible representation of its Kraus operators.
Assume we have $2$ dephasing channels acting on ...
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What determines the topology/connectedness of quantum computers?
Some quantum computers hold their qubits in a 2D array. I've even seen a 1D topology I think.
On IBM Q, you can see which qubits talk to which qubit. I would like to know what determines/limits the ...
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What experiments can distinguish between mixed and pure states?
To distinguish between a coherent and de-cohered stage of the same system
what experiments can provide the answer? The term Experiment is used here in the Bohr-Einstein-debate sense, a realizable ...
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Do photons also suffer from decoherence?
In this Question Why do optical quantum computers not have to be kept near absolute zero while superconducting quantum computers do?
A comment said that the most common way to encode q information in ...
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Readout error in IBM quantum experience processor
Is there a section on the IBM website that shows information about the readout error for the devices of quantum experience? I have only seen the errors for single-qubit (U2) and CNOT gates but nothing ...
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IBM Q calibration parameters
If I download calibration of a quantum processor on IBM Q website I see these parameters:
T1
T2
frequency (GHz)
readout error
single qubit U2 error rate
CNOT error rate
T1 and T2 are relaxation and ...
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How long can a qubit exist without decoherence?
In 2020, practically speaking, how long (time-wise) will a qubit function normally before decoherence kicks in?
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What's the difference between T1 and T2?
I learned that T1 is relaxation time (time from $|1\rangle$ to $|0\rangle$) and T2 is coherence time.
The relaxation is a specific case of decoherence.
What's the difference between them and what's ...
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Rigorous definition of phenomenological master equation for decoherence
I need to have a resource in which phenomenological decoherence model are well defined and explained.
Indeed, from what I read there are three usual timescales considered to model decoherence of a ...
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IBM quantum computers gate delay values
I am doing an state of the art study about the existing quantum computer technologies and one of the parameters of interest is the gate delay value, as it determines the time expected from a gate from ...
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Measuring T1 and T2 constants on IBM Q
We have been asked to measure relaxation and dephasing times T1 and T2 on the IBM Q using the composer only, Qiskit not allowed. I am a bit confused about the way to do so. Can someone explain the ...
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Dephasing in graph states
The Appendix to a recent paper Graph States as a Resource for Quantum Metrology states:
We model an $n$ qubit graph state $G$ undergoing iid dephasing via $$G
\to G^{\text{dephasing}} =
\sum_{\vec{...
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