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20 votes

Is error correction necessary?

That doesn't scale well. After a moderately long calculation you're basically left with the maximally mixed state or whatever fixed point your noise has. To scale to arbitrary long calculations you ...
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13 votes
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What exactly is meant by "noise" in the following context?

As an addition to Nat's answer, it's worth mentioning that 'noise' is a specific concept1 in quantum computing. This answer will use Preskill's lecture notes as a basis. In essence, noise is indeed ...
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12 votes

Is error correction necessary?

If the error rate were low enough, you could run a computation a hundred times and take the most common answer. For instance, this would work if the error rate were low enough that the expected number ...
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12 votes
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How good is basic_device_noise_model() simulating the noise in the quantum computer?

There is no specific paper for this, though information on the model can be found in the Qiskit Aer API documentation and is based on the research of IBMQ quantum computing group. As examples you can ...
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11 votes

What exactly is meant by "noise" in the following context?

Unfortunately for analog computation it turns out that when realistic assumptions about the presence of noise in analog computers are made, their power disappears in all known instances; they cannot ...
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10 votes
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How does approximating gates via universal gates scale with the length of the computation?

Throughout this answer, the norm of a matrix $A$, $\left\lVert A\right\rVert$ will be taken to be the spectral norm of $A$ (that is, the largest singular value of $A$). The solovay-Kitaev theorem ...
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  • 3,397
8 votes

Which quantum error correction code has the highest threshold (as proven at the time of writing this)?

As far as I’m aware, the surface code is still regarded as the best. With an assumption of all elements failing with equal probability (and doing so in a certain way) it has a threshold of around 1%. ...
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7 votes
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Degradable channels and their quantum capacity

A channel $\Phi$ is said to be degradable if there exists another channel $\Xi$ such that $\Xi\Phi$ is complementary to $\Phi$. The idea here is as follows. Suppose $\Phi$ is a channel and $\Psi$ is ...
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  • 4,413
7 votes

How can noise on a device be simulated using measured noise parameters?

This can be done using the 'Aer' component of Qiskit. The properties information can be turned into a noise model using ...
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7 votes
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How do quantum computers prevent "quantum noise"?

How do we prevent quantum noise in a quantum computer? Well, technically the answer is (at least for most systems): we use ridiculously low temperatures (much colder than space), we shield everything ...
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6 votes

Which quantum error correction code has the highest threshold (as proven at the time of writing this)?

I believe that the Centre for Engineered Quantum Systems, School of Physics, The University of Sydney and the Center for Theoretical Physics, Massachusetts Institute of Technology use of a tensor ...
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  • 519
6 votes

Depolarizing channel implementation on IBM Q

There are several ways that you could realise the depolarising map $ \mathcal N_p(\rho) = (1\!-\!p)\:\!\rho + p \!\!\:\cdot\!\tfrac{1}{2}\mathbf 1$ map on a quantum computer — including an ...
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6 votes
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What kind of errors does the master equation in the Lindblad form describe, continuous errors or discrete errors?

The errors that are described by the Master equation are continuous errors. The action of error correction is to discretize those errors. For example, dephasing noise can be described by the Master ...
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6 votes
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Why attenuator and not filters for QC driving line

(1) Both filters and attenuators are used Let me just start by saying that non-attenuating filters have not been completely ruled out by people working in the design of cold quantum computers. I will ...
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5 votes
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Is it true that observing a quantum state will end the superposition of states? How can I not observe?

I'm going to go for an intuitive answer here, as requested. Let's s go in steps: Your input is (often?) classical, so up to that point we're good. Then you start doing quantum operations and achieve, ...
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5 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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  • 621
5 votes

How to not optimize the quantum gates in a qiskit circuit when running it in the real device?

Optimization level 0 does not perform 1 qubit gate optimization and it will send 2 X gates (well 2 U3 gates after it unrolls to the basis set). You can see the passes optimization level 0 runs here: ...
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5 votes
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Why should we use density matrices to simulate quantum systems with noise?

Noise effects introduce classical uncertainty in what the underlying state is. A mixed state is a statistical ensemble of several quantum states $|\psi_i\rangle$ (not necessarily orthogonal), with ...
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5 votes

Is spontaneous excitation possible?

As a first note: the (uncontrolled) transition of $|1\rangle$ to $|0\rangle$ is generally not referred to as dephasing but as relaxation. The noise-process that involves (spontaneous) relaxation is ...
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  • 4,630
5 votes
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How can I fit an unknown quantum channel?

The 2-norm difference typically isn't particularly physical. So no, this is most likely not the right distance. What you want from a physical point of view is a distance measure which measures the ...
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5 votes

Proven quantum advantage (in the algorithmic sense) without error correction (for specific algo, or noise models)

A photon based quantum computer named Jiuzhang in 2020 performed gaussian boson sampling (GBS) (Link to paper) of 76 photons.Researchers are interested in quantum algorithm for Boson sampling using ...
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4 votes

How to add noise to existing gates in Cirq?

One simple way to do it is by defining a composite gate, like this: ...
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4 votes
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How to selectively apply noise in Qiskit simulations?

Yes you can: When you build a noise model the noise is defined with respect to the name or label of gate objects, so you can use the labelto create multiple versions of a single gate but reference ...
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  • 951
4 votes
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How are Rigetti and IBM QX device parameters related to Kraus operators?

For amplitude damping, $\gamma$ is something like $e^{-\Delta t/T_1}$ where $\Delta t$ is how long the Kraus operator is supposed to act. But be very careful, Kraus evolution assumes your system has ...
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  • 346
4 votes
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Exponential Growth of Noise in Quantum Computers

According to so-called threshold theorem, it is possible to get rid of errors in quantum computation with arbitrary precision. However, there is an assumption that you have enough qubits. To ...
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4 votes
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Depolarization of density operator with zeros in diagonal

Quantum channels are foremost, linear operators. So given a basis for the Hilbert-Schmidt operator space (for example the states $\{|0\rangle\langle 0|,|0\rangle\langle 1|,|1\rangle\langle 0|,|1\...
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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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4 votes
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Determining whether there exists an equivalent set of unitary Kraus operators

As Adam Zalcman has stated in his answer, channels whose Kraus operators are proportional to unitary operators are called mixed-unitary channels (or, alternatively, random unitary channels). Every ...
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  • 4,413
4 votes

How are eavesdroppers detected when using BB84 in the presence of noise?

The standard noisy approach is not to try to determine the presence of an eavesdropper as such, but to create a final key where, even if there is an eavesdropper, you can still be confident that the ...
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