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27 votes

What's the difference between T1 and T2?

Slight correction to Martin Vesely's answer: $T_2$ is not the (decay constant) time after which an initial state $|+\rangle$ will necessarily switch to the state $|-\rangle$. If it were, then error ...
tparker's user avatar
  • 2,791
22 votes
Accepted

What's the difference between T1 and T2?

T2 is so-called dephasing time. It describes how long the phase of a qubit stays intact. In your words, it is time from $|+\rangle= \frac{1}{\sqrt{2}}(|0\rangle + |1\rangle)$ to $|-\rangle= \frac{1}{\...
Martin Vesely's user avatar
13 votes
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What's the Difference between T2 and T2*?

The naming started in NMR and it has become the difference between the following two experiments. Experiment one: Prepare the qubit in a superposition state (apply a H gate) and vary the wait time ...
Jay Gambetta's user avatar
10 votes
Accepted

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

Decoherence is the very general term which, more or less, is anything resulting in a loss of purity during the evolution of a system. Sometimes, when people are being a bit non-specific, they might ...
DaftWullie's user avatar
  • 58.7k
9 votes
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Is there a classical limit to quantum computing?

In simpler terms your question is: if noise/decoherence keeps entering the computation, how can a big computation possibly survive? The key concept you're missing is quantum error correction, which ...
Craig Gidney's user avatar
  • 37.6k
8 votes
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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 short: "coherence"! It's the crucial difference between quantum and classical. $\rho=|0\rangle\langle 0|+|1\rangle\langle 1|$ is just a statistical mixture, and behaves like a classical coin - any ...
DaftWullie's user avatar
  • 58.7k
7 votes
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Quantum simulation of environment-assisted quantum walks in photosynthetic energy transfer

One major idea there seems to be that the "environment" (quantum decoherence) assists or optimizes the transport of a signal The idea that photosynthetic systems are doing a Grover search or ...
user1271772 No more free time's user avatar
6 votes
Accepted

Are qutrits more robust to decoherence?

To simplify things a bit, let's take a single qubit and a single qutrit for comparison. First, the amplitude damping channel (giving e.g. emission of a photon) for a qubit is $\mathcal E\left(\rho\...
Mithrandir24601's user avatar
  • 3,696
6 votes

Are qutrits more robust to decoherence?

The statement in Wikipedia is very generic, and only cites this paper as a reference. Quoting from the abstract of the paper: We demonstrate that decoherence of many-spin systems can drastically ...
glS's user avatar
  • 25.3k
6 votes

Does the quantum coherence in the FMO complex have any significance to quantum computing (on a biological substrate)?

I agree with most of what you've written in the first paragraph, though I would say that at roughly the same time (only 1 month apart!) as the Rebentrost et al. paper you mentioned, a very similar ...
user1271772 No more free time's user avatar
6 votes

What is the maximum separation between two entangled qubits that has been achieved experimentally?

I believe the current record is held by the Jian-Wei Pan group in China, who are able to generate entanglement via a satellite. The journal article is here, while there's plenty of media coverage that ...
DaftWullie's user avatar
  • 58.7k
6 votes
Accepted

IBM Q calibration parameters

frequency (GHz): The frequency(energy) associated with the transition between the qubit's ground state ($|0\rangle$) and first excited state ($|1\rangle$). readout error: The probability of preparing ...
ThomasAlexander's user avatar
6 votes
Accepted

$T_2>2T_1$ qubits on the ibm_washington quantum processor

Good catch! This is a result of the T1 and T2 properties of the qubits being estimated in separate measurement batches. What was happening is that a qubit fluctuation such as a TLS would cause a low ...
ThomasAlexander's user avatar
5 votes

What is the difference between $\vert 0 \rangle + \vert 1 \rangle$ and $\vert 0 \rangle \langle 0 \vert + \vert 1 \rangle \langle 1 \vert$?

There are multiple ways to mathematically express the state of a quantum system. One is to write it as a linear combination of basis states, as either a vector or a matrix, as you have here. This is ...
James Wootton's user avatar
5 votes

Passive improving of nanodiamond surfaces for NV centers?

I have worked with NVs in nanodiamonds a little bit, and you are totally right, surface characteristics have a huge influence on how far we can push them. There are definitely multiple groups working ...
Dr. Sarah Kaiser's user avatar
5 votes

State of the art gate speeds and decoherence times

I guess your best shot would be to look for experimental comparisons like this one on Arxiv. But I am not aware of a tracking. I do not think we can consider having a "state of the art" in this ...
cnada's user avatar
  • 4,754
5 votes
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What is the maximum separation between two entangled qubits that has been achieved experimentally?

Photons travel fast, and there's often the option to transfer their entanglement to solid state. Of course, the advantage of transferring entanglement to a solid state qubit is that one is able to ...
agaitaarino's user avatar
  • 3,827
5 votes
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How can quantum decoherence be managed?

The quantum circuit model describes a quantum computer as a closed quantum system and assumes that there is a system which executes the circuit but is completely isolated from the rest of the universe....
Sanchayan Dutta's user avatar
5 votes
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Can quantum error correction work on any type of channel?

For any quantum error correcting code, it is possible to construct a channel which introduces errors that the code cannot correct. However, the key point is that such channels are highly adversarial ...
Adam Zalcman's user avatar
  • 22.9k
5 votes

How to formulate Dynamical Decoupling passes in Qiskit to improve result upon circuit execution

Partial answer After discussing this with someone and going back through the API documentation on dynamical decoupling, it was clear on how these numbers were chosen. They corresponds to the gate ...
KAJ226's user avatar
  • 13.9k
5 votes

How to recover the original density matrix from the output of amplitude damping channel?

Inverting a quantum channel Kraus representation does not make it convenient to find the inverse. However, quantum channels are linear maps, so we can represent them as matrices which can be inverted ...
Adam Zalcman's user avatar
  • 22.9k
4 votes

Can one interrogate black boxes for quantum coherence?

Why not input one half of a maximally entangled state as the input to the black box (so that half has the same dimension as the input dimension)? Then you could test your favourite measure, such as ...
DaftWullie's user avatar
  • 58.7k
4 votes
Accepted

How to write a post-measurement state, if we don't know the measurement result?

Suppose you have a state $\rho$, and a random process that changes this to a state $\rho_j$ with probability $p_j$. If you know what the value of $j$ is, your knowledge of the resulting state will be ...
James Wootton's user avatar
4 votes
Accepted

Dephasing in graph states

Imgine that there is a bit string $\vec{k}\in\{0,1\}^n$. We use this to specify sites (bit value 1) where an error has occurred, and sites (bit value 0) where no error has occurred. The number of 1s ...
DaftWullie's user avatar
  • 58.7k
4 votes

Measuring T1 and T2 constants on IBM Q

I cannot give you a complete answer(I am not too familiar with the IBM quantum tools) however I might be able to give you a few hints from a NMR/EPR perspective. In magnetic resonance T2 is commonly ...
user245427's user avatar
4 votes

Qubit dephasing times

As you described, $T_2^*$ defines the decay time of coherence in a Ramsey experiment, while $T_2^{(echo)}$ and more generally $T_2^{(DD)}$ run a similar experiment but with intermediate refocusing ...
Chris E's user avatar
  • 1,428
4 votes

Fault-tolerant code and the error rate

There is no contradiction. The threshold theorem requires that the error rate, i.e. error probability per gate (or per unit of time), be below a threshold. On the other hand, the exponential decay ...
Adam Zalcman's user avatar
  • 22.9k
4 votes
Accepted

What is the explicit form of $T_1$ decay channel?

Relaxation time $T_1$ describes the strength of amplitude damping$^1$ by specifying the mean lifetime$^2$ of the $|1\rangle$ state. More precisely, $\mathcal{N}$ has a Kraus representation $$ E_0=\...
Adam Zalcman's user avatar
  • 22.9k
4 votes
Accepted

What is qubit decoherence?

When we measure a qubit in superposition, we interact with the system, causing it to collapse into either $|0\rangle$ or $|1\rangle$, with certain probabilities determined by the coefficients in the ...
banercat's user avatar
  • 763

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