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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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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How's the depolarization channel applied to one qubit circuits?

Suppose that in all gates used in a one qubit circuit the depolarization channel is applied, i.e. ...
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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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Mixed state vs superposition , experiment test

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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What is the difference between relaxation, dephasing, and decoherence?

Many sources seem to loosely use the terms "relaxation", "dephasing", and "decoherence" interchangeably, while others seem to treat certain of them as special cases of another terms, but I can't find ...
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General Master Equation with Decoherence Query

The following general master equation (from this paper 'Dynamical quantum correlations of Ising models on arbitrary lattice and their resilience to decoherence') describes the various types of ...
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Measuring Coherence Length for T1 and T2 values with IBMQ Experience

So I'm currently working on a little program to measure how long the coherence time of a qubit is. Basically the idea is to initialise a qubit with a X or H gate then stick in a varying amount of ...
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Can decoherence account for suppression of interference in all basis?

(I am importing this question that I asked in Physics stack exchange) Let the system formed by particle(microscopic or macroscopic)- environment coupling be described by $|ϕ\rangle\langleϕ|$ . In ...
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Quantum state where phase information is unknown

I'm trying to obtain a more intuitive understanding of the notion of quantum coherence and how to mathematically represent it. I know that coherence has to do with the interaction of phases between ...
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How is the decoherence rate connected to the error rate?

I'm reading about the threshold theorem, which states that "a quantum computer with a physical error rate below a certain threshold can, through the application of quantum error correction schemes, ...
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Is the theory of decoherence general or specific to the physical implementation?

I’m looking for the theoretical calculations of charge and transmon qubit decoherence specifically, but at a high level I'm wondering how decoherence is approached generally. I’ve seen explanations ...
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Graphing the decoherence of qubits being used to perform an algorithm

How would I be able to graph the decoherence of qubits being used to perform an algorithm? I'm working with the IBMQ Qiskit framework and I'm trying to write a program that allows me to find the ...
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Clarifying the relationship of the probabilities of depolarizing channel and relaxation/dephasing times

I am reading the paper Duality of Quantum and Classical Error Correction Codes: Design Principles & Examples and at the beginning of it, the authors describe how the general Pauli channel is ...
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What noise channel that is not coming from the control can lead to XY qubit rotations?

In a semiclassical treatment, the noise Hamiltonian of a quantum system can be treated as a summation over its noise channels in the following way: $$\hat{H}_{\text{noise}}=\sum_{k}\beta_{k}\left(t\...
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Decoherence in quantum systems always produces $\vert0\rangle$

I was recently asked two questions concerning error in quantum computing: Is it possible for quantum computers to exhibit behavior similar to flip errors in classical computers where a state $\vert0\...
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Measurement of a qubit and storage of the information on a bit

Suppose we have the quantum circuit below with a quantum register of 2 qubits and a classical register of 2 bits. The Hadamard gates and CNOT gate are not important for the question. When we measure a ...
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Does self-induced collapse limit the size of a quantum computer?

This question is related to this question, but I'm specifically interested in self-induced collapse. Suppose that the size of a quantum system alone somehow leads to a projection of the state (a "...
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How to write a post-measurement state, if we don't know the measurement result?

Once a state is measured, but we don't look at the result, is the state now written as a density matrix, that is, the probability that it could land on a measurement operator multiplied by the ...
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7 votes
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Is there a classical limit to quantum computing?

Why are quantum computers scalable? With the subjects of spontaneous collapse models and decoherence in mind, it seems to me that the scalability of quantum computers is something which is not only ...
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State of the art gate speeds and decoherence times

I am interested in the state of the art gate speeds and decoherence times for the qubit types I know are being pursued by companies presently: superconducting qubits, ion trap qubits, photonic qubits....
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Quantum Belief Propagation decoding

I have been reading about a family of quantum error correction codes called Quantum Turbo Codes, which are the quantum analog of the well-known classical Turbo codes. This codes were introduced in ...
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What is the difference between $\vert 0 \rangle + \vert 1 \rangle$ and $\vert 0 \rangle \langle 0 \vert + \vert 1 \rangle \langle 1 \vert$?

In a discussion with Jay Gambetta on the QISKit Slack channel, Jay told me that "T2 is the time that $\vert 0 \rangle + \vert 1 \rangle$ goes to $\vert 0 \rangle \langle 0 \vert + \vert 1 \rangle \...
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Does the quantum coherence in the FMO complex have any significance to quantum computing (on a biological substrate)?

The quantum effects of the FMO complex (photosynthetic light harvesting complex found in green sulfur bacteria) have been well studied as well as the quantum effects in other photosynthetic systems. ...
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