# Tag Info

### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### Why is it not a good idea to run Shor's algorithm on a big noisy quantum computer?

Factoring a 2048 bit number with Shor's algorithm involves performing one billion Toffoli gates under superposition (ref). Suppose your superposed Toffoli gate error rate is an amazing 0.01%. Note ...
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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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### 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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### What is the clear definition of coherent versus incoherent errors?

Coherent vs incoherent: as a rule of thumb, coherent is unitary, incoherent is stochastic. The distinction is not entirely clear when your channel is a mixture of these. Thus, it makes sense to ...
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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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### 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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### 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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### Incorporating idling errors while using stim

No, there's no simple built-in way. You have to do it for yourself. This was an intentional design choice, which I will now attempt to justify because I do realize it's inconvenient. Stim has no ...
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### Is it possible to use fake backends to run Qiskit Runtime primitives?

Using primitives with fake backends You can use BackendEstimator to work with fake backends. As primitives implementation, ...
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### Are operators unitary on a real quantum computer?

A unitary operator represents a invertible operation that is fully devoid of noise. Similarly to how pure states represent us having perfect knowledge about the quantum state. In realistic scenarios, ...
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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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### 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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### 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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### 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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### Explicit form for composition of Choi representation quantum channels

Yes, there is a nice way to represent this composition, valid for any linear map, not only unitaries. It is so useful that it has a name, link product. Let $\mathcal E:A\to B$ and $\mathcal F:B\to C$ ...
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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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### 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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### 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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### 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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### 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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