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Questions tagged [noise]

For questions about different types of noise a quantum computer (or qubits) may experience; how this affects the outcome of the computation; how to reduce a specific type of noise on a specific implementation of quantum computer; or how to simulate noise.

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Degradable channels and their quantum capacity

Note: I'm reposting this question as it was deleted by the original author, so that we do not lose out on the existing answer there, by Prof. Watrous. Further answers are obviously welcome. I have ...
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0answers
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Implementing a depolarizing channel for 2 qubits on IBM Q

I am trying to use IBM Q to perform the following depolarizing channel on a state of 2 qubits $\rho=|\psi \rangle \langle \psi |$: $$\rho \to (1-\lambda)\rho + \frac{\lambda}{4}I$$ This is within ...
5
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1answer
101 views

Quantum teleportation with “noisy” entangled state

This is actually an exercise from Preskill (chapter 4, new version 4.4). So they are asking about the fidelity of teleporting a random pure quantum state from Bob to Alice, who both have one qubit of ...
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0answers
35 views

Application of improved compatibility

Note: Cross-posted on Physics SE. It's a standard piece of quantum information theory that noise can be helpful in augmenting compatibility of quantum observables. For example given a qubit state $\...
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0answers
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What noise channel that is not coming from the control can lead to XY qubit rotations?

In a semiclassical treatment, the noise Hamiltonian of a quantum system can be treated as a summation over its noise channels in the following way: $$\hat{H}_{\text{noise}}=\sum_{k}\beta_{k}\left(t\...
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46 views

What are reasonable noise/dephasing parameters for QuEST to emulate state-of-the-art quantum computers?

The QuEST code allows for simulation of quantum computers taking dephasing and depolarizing noise into account via one and two qubit dephasing and depolarization. What are realistic parameters for ...
3
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0answers
46 views

What do you specify when you physically apply a unitary?

In the Environment and Quantum Operations in Nielsen and Chuang, section 8.2.2, they say that when you apply a unitary on a state, you expect the output as the just the state transformed by the ...
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Does self-induced collapse limit the size of a quantum computer?

This question is related to this question, but I'm specifically interested in self-induced collapse. Suppose that the size of a quantum system alone somehow leads to a projection of the state (a "...
4
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1answer
140 views

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

The real devices used as backends for Qiskit have calibration data that quantifies errors ($T_1$ and $T_2$, gate fidelities, etc). This can be accessed by, for example ...
6
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1answer
132 views

How are Rigetti and IBM QX device parameters related to Kraus operators?

Rigetti reports the following parameters: (https://www.rigetti.com/qpu) T1, T2* times 1-qubit gate fidelity (F1q) 2-qubit gate fidelity (F2q) and, read-out fidelity (Fro) IBM QX reports the ...
6
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1answer
199 views

Depolarizing channel implementation on IBM Q

Given a single qubit in the computational basis, $|\psi\rangle =\alpha |0\rangle + \beta|1\rangle$, the density matrix is $\rho=|\psi\rangle\langle\psi|=\begin{pmatrix} \alpha^2 & \alpha \beta^*\\ ...
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1answer
50 views

Do avoided crossings / CTs /ZEFOZs optimize quantum fidelity in practice?

CTs / ZEFOZs: Energy level structures that include avoided crossings at accessible energies tend to be resilient to noise and therefore present high coherence times, at least in the case of spin ...
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0answers
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Threshold and practical requirements for initial state preparation?

At the beginning of a quantum computational process we generally want to start in a perfectly known initial state, and evolve from there. This cannot be done perfectly, for fundamental reasons, but I ...
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2answers
201 views

Is it true that observing a quantum state will end the superposition of states? How can I not observe?

I have no background in quantum physics, and no understanding of most formulas used in this context. I'm not looking for an in depth answer, i'd just like to vaguely understand the concept. The way i ...
8
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2answers
406 views

How do quantum computers prevent “quantum noise”?

On the Wikipedia page for Shor's algorithm, it is stated that Shor's algorithm is not currently feasible to use to factor RSA-sized numbers, because a quantum computer has not been built with enough ...
7
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2answers
101 views

Does Barkhausen noise affect the measurement of magnetic particle based qubits?

Q1: I've tried to find out if Barkhausen noise affects the measurement of spin-wave excitations in magnetic particle material based qubits. I prefer implementations such as those described in "...
12
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1answer
125 views

How does approximating gates via universal gates scale with the length of the computation?

I understand that there is a constructive proof that arbitrary gates can be approximated by a finite universal gate set, which is the Solovay–Kitaev Theorem. However, the approximation introduces an ...
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5answers
605 views

Is error correction necessary?

Why do you need error correction? My understanding is that error correction removes errors from noise, but noise should average itself out. To make clear what I'm asking, why can't you, instead of ...
19
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3answers
342 views

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

Which quantum error correction code currently holds the record in terms of the highest threshold for fault-tolerance? I know that the surface code is pretty good ($\approx10^{-2}$?), but finding exact ...
13
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3answers
317 views

What exactly is meant by “noise” in the following context?

The strengthened version of the Church-Turing thesis states that: Any algorithmic process can be simulated efficiently using a Turing machine. Now, on page 5 (chapter 1), the book Quantum ...
7
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0answers
231 views

How to make qubits more stable towards noise? [closed]

In this paper and this paper, the "Noise Stability of Qubits" (the stability of qubits to external noise) has been discussed. In the first one, Gil Kalai states that it is difficult to create a ...