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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 ...
Silly Goose's user avatar
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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}...
Loading - 146 Complete's user avatar
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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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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 ...
joy Jaganath's user avatar
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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 ...
More Anonymous's user avatar
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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 ...
hlepsneder HCI's user avatar
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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 this paper on page 2. I am reproducing it here, $...
chetan waghmare's user avatar
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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 ...
Hans's user avatar
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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 ...
XL _At_Here_There's user avatar
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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, ...
XL _At_Here_There's user avatar
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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 ...
viking92's user avatar
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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 ...
Josu Etxezarreta Martinez's user avatar
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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 ...
Hao Nguyen's user avatar
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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 ...
Ron Cohen's user avatar
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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: ...
Will Yang's user avatar
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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 ...
Anna Naden's user avatar
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137 views

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 ...
TurbPhys's user avatar
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1 answer
124 views

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 ...
Josu Etxezarreta Martinez's user avatar
4 votes
0 answers
148 views

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 ...
Josu Etxezarreta Martinez's user avatar
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156 views

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 ^...
Atta khan's user avatar
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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 ...
peachnuts's user avatar
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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 ...
KAJ226's user avatar
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4 votes
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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 ...
Quantum Guy 123's user avatar
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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 ...
Blubsiwubsi's user avatar
2 votes
1 answer
89 views

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 ...
ZR-'s user avatar
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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 ...
LOC's user avatar
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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.
Muhammad Saqib's user avatar
2 votes
1 answer
216 views

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 ...
Dina Abdelhadi's user avatar
2 votes
1 answer
108 views

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 ...
JoJo258OrAmI's user avatar
1 vote
1 answer
544 views

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 ...
MusashiK's user avatar