# Tag Info

## Hot answers tagged decoherence

4

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 correction would be easy. Instead, it's the (decay constant) time after which an initial state $|+\rangle$ will evolve into an equal classical probabilistic ...

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T2 is so-called dephasing time. It describes how long a phase of qubit stay intact. In your words, it is time from $|+\rangle= \frac{1}{\sqrt{2}}(|0\rangle + |1\rangle)$ to $|-\rangle= \frac{1}{\sqrt{2}}(|0\rangle - |1\rangle)$, or conversely. Just note that both T1 and T2 are not actually "time from state x to state y" but rather decay constants. ...

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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 a $|0\rangle$($|1\rangle$) and measuring a $|1\rangle$($|0\rangle$), ie., of having an error in your readout single qubit U2 error rate: The average error per ...

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The length of all backend basis gates is available from backend.properties().gate_length. For example properties = backend.properties() id_gate_length_qo = properties.gate_length('id', 0)

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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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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 The link is out of date and the update one is here: 2_relaxation_and_decoherence 2_relaxation_and_decoherence.ipynb It contains lots of figures, perhaps they are what you want.

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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 measured by generating a spin coherence, and refocusing at progressively longer times then measuring a spin echo. In quantum gate language that would be: ...

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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 in the bit string is $k$. The probability of this particular error arising is then $p^k(1-p)^{n-k}$ because there are $k$ sites with an error and $n-k$ sites ...

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What limits the connectedness is ultimately a platform-specific question. For example, in trapped-ion quantum computers, all of the ions are coupled through a collective motional mode such that all qubits are actually "connected", so that the graph of qubit-qubit connections is complete. The more mainstream approach (at least in industry) involves ...

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So basically you want to distinguish the state $| + \rangle \langle + |$ from the dephased state $\frac{1}{2}(| 0 \rangle \langle 0 | + | 1 \rangle \langle 1 | ) = \frac{\mathbb{I}}{2}$. Here's a simple experiment: apply a Hadamard to both states and then measure in the $\sigma_{z}$ basis. For the true superposition'', this transforms it into the state ...

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A comment said that the most common way to encode q information in photons is using their internal degrees of freedom, not using a "there/not there" encoding. When using photons, quantum information can indeed be encoded into an internal degree of freedom; for instance the polarization of the photon. However, there are plenty of other systems where the ...

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You can also see this directly in Qiskit. To see the cals visually you can do: from qiskit import IBMQ from qiskit.visualization import plot_error_map provider = IBMQ.load_account() backend = provider.backends.ibmq_rome plot_error_map(backend)

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Go to the first page in IBM Q web interface, click on a quantum processor you are interested in and finally click on link Download calibration. You will see a table (CSV file) containing decoherence times $T1$, $T2$, read-out, single-qubit $U2$ error rate and CNOT error rate.

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On IBM Q, it is a few tens microseconds. The best value of dephasing time T2 is around 500 microseconds. Have a look here IBM Q website and navigate to "Qubits as physical system" and then to "dephasing T2" to see development of T2 in last twenty years.

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On a quantum computer, there is no point in implementing a partial trace. Just ignore the qubits which you want to trace.

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qiskit.quantum_info has a partial_trace method you can use for this. It takes as input either a Statevector or a DensityMatrix object. Both these classes implement the from_instruction routine that you can use with the QuantumCircuit object you already have. You would have to do something along the lines of: circuit2 = QuantumCircuit(4,3) ... # your ...

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To clear up my own confusion, these DFSs exist for the channel that maps $|0\rangle$ to $|0\rangle$ and $|1\rangle$ to $e^{j\phi}|1\rangle$. This is not the channel discussed in the question.

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Nuclear moments of europium ions have been experimentally measured to have coherence times of over six hours, although there is no currently known way of performing controlled logic gate operations on them.

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Lets start with measuring circuits. With link to user245427 answer, you should construct following circuits in composer followingly: T1 (relaxation time) T1 is constant connected with spontaneous relaxation from state $|1\rangle$ to state $|0\rangle$. So firstly apply $X$ gate on a qubit to change its state from $|0\rangle$ to $|1\rangle$, then apply a few ...

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I think you are mostly correct (although I am not an expert). I think it makes it easier to understand these terms to remember that these terms are used by experimentalits. They refer to things that can be measured, and their connection to theoretical time-constants is not simple. I agree with your definitions for T1 and T2, but I would give a different ...

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In addition to backend.properties() described above, you can go to https://quantum-computing.ibm.com, click a machine, then click "Download Calibrations".

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