Quantum Computing Stack Exchange
Quantum Computing Stack Exchange

Questions tagged [nisq]

For questions related to Noisy Intermediate Scale Quantum (NISQ) computing. NISQ computers cannot be classically simulated in a reasonable time but are not advanced enough to also be error corrected.

Noisy Intermediate Scale Quantum (NISQ) computers have enough qubits to see non-classically-simulable behavior, but not enough for error correction. NISQ algorithms often revolve around sampling very short circuits many times and using classical feedback to choose which circuit to sample next. The cost bottlenecks for NISQ computation are things that introduce noise into the results (e.g. circuit depth and two-qubit gates).

See John Preskill's 2018 paper "Quantum Computing in the NISQ era and beyond".

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What is meant by "Noisy Intermediate-Scale Quantum" (NISQ) technology?

Preskill introduced recently this term, see for example Quantum Computing in the NISQ era and beyond (arXiv). I think the term (and the concept behind it) is of sufficient importance that it deserves to be explained here in a pedagogical manner.…
agaitaarino
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What use has quantum computing been?

Most of us on this site believe that quantum computing will work. However, let's play devil's advocate. Imagine that we suddenly hit some fundamental stumbling block that prevented further development towards a universal quantum computer. Perhaps…
DaftWullie
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How should different quantum computing devices be compared?

In the last years, there has been a spur of demonstrations of devices able to perform proof of principle, small-scale, non-fault-tolerant quantum computation (or Noisy Intermediate-Scale Quantum technologies, how they have been referred to). With…
glS
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Devising "structured initial guesses" for random parametrized quantum circuits to avoid getting stuck in a flat plateau

The recent McClean et al. paper Barren plateaus in quantum neural network training landscapes shows that for a wide class of reasonable parameterized quantum circuits, the probability that the gradient along any reasonable direction is non-zero to…
Daniel Yaacov
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Minimum number of CNOTs for a 4-qubit increment on a planar grid

Recently I've been wondering how high NISQ machines will be able to "count". What I mean by that is, given the most optimized increment circuit you can make, how many times can you physically apply that circuit to qubits in a secret initial state…
Craig Gidney
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What use cases are there for 127 qubit QPUs?

IBM have recently announced their 127 qubit Eagle processor. Other approaches, such as Rydberg arrays, have now 256 qubits, as for example in QuEra's QPU QPU. While these are without a doubt outstanding techical acheivements, I am wondering what is…
Lior
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What are the "nice" properties of the diamond norm and why is it used?

I have heard about the diamond norm, and from what I understood it is a "nice" tool to quantify quality of quantum gates in the NISQ era. I would like to know a little more before going in detail in the litterature. My question are the…
Marco Fellous-Asiani
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Definition of a NISQ device with respect to qubit counts and error rates

How do we define whether a device is a noisy intermediate-scale quantum (NISQ) device with respect to number of qubits and their error rates? Does it make sense to do this? I believe I once saw a definition of a NISQ device as one with on the order…
Greenstick
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Pennylane and Qiskit for quantum machine learning

I'm interested in quantum computing, specifically in “quantum machine learning” (QML). I'm going to start my masters program in computer science and have previous experience in classical machine learning. I'd like to learn quantum mechanics…
Carlos Alfredo Vergara Rojas
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Most efficient way for general state generation

Assume we are given an $n$-qubit system and complex numbers $a_0, \ldots, a_{m-1}$ with $m = 2^n$. Assume further we start with the initial state $|0 \ldots 0\rangle$ and want to make the transformation $$|0 \ldots 0\rangle \rightarrow…
tobias
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Are there practical benefits of using qudits over qubits?

It's clear from foundational research that qudits can provide an enhanced control of the Hilbert space over qubits, and I've encountered references that highlight improved robustness and noise tolerance in quantum protocols such as QKD when using…
banercat
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Does Google's error correction paper invalidate Gil Kalai's arguments?

In his paper "The Argument against Quantum Computers, the Quantum Laws of Nature, and Google’s Supremacy Claims", Gil Kalai argues that quantum advantage will never be reached. For NISQ devices in particular, he argues that for a large variety of…
Tristan Nemoz
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Publicly available samples for quantum circuits and/or simulators

With Yosi Rinott and Tomer Shoham we studied various statistical aspects of samples coming from NISQ computers. My question is about available data consisting of samples from NISQ computers We would be very happy to get samples from runs of quantum…
Gil Kalai
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What's the point of VQE if classical computers can solve for eigenvalues easily?

From a few VQE tutorials online I see that they normally start with something like: VQE is a way of getting a good estimate for the upper bound of the ground state of a quantum system's Hamiltonian. The Hamiltonian is known. Then they proceed to…
Alexander Soare
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Could the Hamiltonian of a 2x2 Rubik's Cube be simulated with a NISQ device?

Consider the four cells on each of the six faces of the 2x2x2 Rubik's cube (the pocket cube). We can construct and simulate a quarter-turn Hamiltonian as below. $^*$ Let $\langle F_1,U_1,R_1\rangle$ be the quarter-turn moves that rotate each of…
Mark Spinelli
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