15 votes
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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}{\...
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14 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 ...
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10 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 ...
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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 ...
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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 ...
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6 votes
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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\...
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  • 3,477
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 ...
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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 ...
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6 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 ...
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6 votes
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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 ...
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6 votes
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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 ...
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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 ...
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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 ...
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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 ...
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5 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 ...
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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....
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5 votes
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$T_2>2T_1$ 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 ...
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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 ...
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4 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 ...
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  • 3,657
4 votes

State of the art gate speeds and decoherence times

You could also look at the following webpage: https://quantumcomputingreport.com/scorecards/qubit-quality/ where they provide recent (I'm not sure how often they update this scores) values for gate ...
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4 votes
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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 ...
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4 votes
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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 ...
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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 ...
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4 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 ...
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3 votes

How is the decoherence rate connected to the error rate?

You will need to know how long it takes for each gate of the circuit to be performed. Then the decoherence error rate is simply $$e^{t_{gate}/t_{decoherence}}$$
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3 votes

Graphing the decoherence of qubits being used to perform an algorithm

Exactly, it's a little bit difficult to find related examples except matthew mentioned. The link is out of date and the update one is here: 2_relaxation_and_decoherence 2_relaxation_and_decoherence....
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3 votes

Clarifying the relationship of the probabilities of depolarizing channel and relaxation/dephasing times

Josu, You might be mis-understanding something. Your equation for the Pauli channel says that when $t\rightarrow \infty$, all operators ($X,Y,Z,I$) have an equal probability (1/4) of transforming the ...
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  • 151
3 votes

What's the Difference between T2 and T2*?

From Chapter 15 of NII's quantum information lecture series on "Fundamentals of Noise processes" (link here): An applied DC field $H_0$ is not completely uniform in all space points. If many spin ...
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  • 1,576
3 votes

How to store qubits while preserving Heisenberg's uncertainty principle?

Your question revolves implicitly around the concept of quantum decoherence and how to protect real-world implementations of qubits from it for a long time. This is an incredibly general problem, and ...
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