Quantum Computing Stack Exchange
Quantum Computing Stack Exchange

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How many operations can a quantum computer perform per second?

I want to know what time complexity is considered efficient/inefficient for quantum computers. For this, I need to know how many operations a quantum computer can perform per second. Can anyone tell me how to calculate it and what factors it depends…
Archil Zhvania
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Implementing a CCCNOT gate using only Toffoli gates

A CCCNOT gate is a four-bit reversible gate that flips its fourth bit if and only if the first three bits are all in the state $1$. How would I implement a CCCNOT gate using Toffoli gates? Assume that bits in the workspace start with a particular…
chuster
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Quantum Walk: Why the need of adding "tail" nodes to the root?

As stated in the question, I have found in several papers (e.g. 1, 2) that in order to perform a quantum walk on a given tree it is necessary to add some nodes to the root $r$, say $r^{'}$ and $r^{"}$. Why are they needed? References: Farhi E.,…
FSic
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Why does (almost) every pair of Hamiltonians generate, through repeated commutation, the whole space of Hermitian matrices?

In [1], the problem of simulating a Hamiltonian using repeated applications of a different set of Hamiltonians is discussed. In particular, let $A$ and $B$ be a pair of Hermitian operators, and let $\mathcal L$ be the algebra generated from $A, B$…
glS
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Composing multiple quantum circuits in single quantum program in Qiskit

I was wondering if there is a way to compose a program with multiple quantum circuits without having the register reinitialized at $0$ for each circuit. Specifically, I would like run a second quantum circuit after running the first one, as in this…
asdf
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Rotating about the y- or z-axis of the Bloch sphere

In order to rotate about an axis of the Bloch sphere we ususally use pulses e.g. in trapped ion quantum computing or superconducting qubits. Let's say we have rotation around the x-axis. What do I have to change in order to be able to rotate around…
Quasar
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Implementation of the oracle of Grover's algorithm on IBM Q using three qubits

I am trying to get used to IBM Q by implementing three qubits Grover's algorithm but having difficulty to implement the oracle. Could you show how to do that or suggest some good resources to get used to IBM Q circuit programming? What I want to do…
Bick
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Review paper on depth, qubits and $T$ gates number on Clifford+T decomposition for various "typical" algorithms

My question I am looking for some review paper, or a list of different papers providing concrete numbers about the depth, number of qubits and number of $T$ gates required on the Clifford+T basis for usefull quantum algorithms (quantum Fourier…
Marco Fellous-Asiani
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Grover's algorithm: what to input to Oracle?

I am confused about what to input to Oracle in Grover's algorithm. Don't we need to input what we are looking for and where to find what we are looking for to Oracle, in addition to the superpositioned quantum states? For example, assume we have a…
Bick
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Why is an oracle qubit necessary in Grover's algorithm?

I am a bit confused about the necessity of an oracle qubit in Grover's algorithm. My question is, does it depend on how you implement your oracle whether you need an oracle qubit or not? Or, it there any reason for an oracle qubit? (such as, there…
Bick
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How does the invertibility of a quantum map reflect on its Kraus operators?

Consider a quantum map $\Phi\in\mathrm T(\mathcal X)$, that is, a linear operator $\Phi:\mathrm{Lin}(\mathcal X)\to\mathrm{Lin}(\mathcal X)$ for some finite-dimensional complex vector spaces $\mathcal X$. In the specific case in which $\Phi$ is also…
glS
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AWS Braket Backend for Qiskit?

Is there a working AWS Braket backend provider for Qiskit 0.29 that can be used with the MinimumEigenOptimizer interface? The qiskit-aws-braket-provider package is unfortunately not working.
Nepomuk Hirsch
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How many qubits would be needed to do boson sampling in Qiskit?

In December 2020, there was this claim of quantum advantage/supremacy by a team of UST China using Gaussian boson sampling. Here is the paper and here is an explanatory news article in Nature. To get an idea of how that works, I wanted to try to…
Mauricio
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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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Are there any quantum software startups?

I am aware of the quantum hardware startup Rigetti and I wonder if there are any quantum startups that build software on top of current quantum computer hardware for commercial applications? Related question: Does a complete list of open quantum…
QNerd
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