Quantum Computing Stack Exchange
Quantum Computing Stack Exchange

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Can one interrogate black boxes for quantum coherence?

This question is based on a scenario that is partly hypothetical and partly based on the experimental features of molecule-based quantum devices, which often present a quantum evolution and have some potential to be scalable, but are generally…
agaitaarino
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What is the computational complexity of quantum annealing?

Quantum annealing can be thought of as a black box solver that can find approximate solutions to hard optimization problems. For example, D-Wave quantum annealers can approximately solve quadratic unconstrained binary optimization (QUBO) problems,…
Dr. Prasanna Date
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What real quantum computers are available for students to use?

I am aware that IBM, Rigetti, and Google have built some small-scale devices. Which of them are available for access by an undergraduate student? For how long? With how many qubits?
hopefully coherent
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Quantum Amplitude Estimation vs Quantum Phase Estimation

Quick question concerning the probability of success after a phase estimation algorithm vs an amplitude estimation algorithm. Given the calculation on the wikipedia page, the probability of measuring the desired output in a phase estimation…
sheesymcdeezy
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Is there an efficient circuit implementing the unitary $U|x\rangle|0\rangle=|x\rangle\Big(\sqrt{1 - x/2^n}\,|0\rangle+\sqrt{x/2^n}|1\rangle\Big)?$

Given an $n$-qubit register $|x\rangle$, does there exist an efficient circuit implementing unitary operation $U$ such that $$U |x\rangle|0\rangle = |x\rangle\Big(\sqrt{1 - x/2^n}\, |0\rangle + \sqrt{x/2^n}\, |1\rangle\Big)?$$ I've found this…
orlp
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CSS Code in disguise

Suppose I have a quantum error correcting code described by a set of stabilizers. Is there any easy way of checking whether or not it is equivalent to a code constructed using the CSS construction, using local Clifford transformations?
guest-20
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Does quantum control allow to implement any gate?

Using quantum control techniques it is possible to control quantum systems in a wide range of different scenarios (e.g. 0910.2350 and 1406.5260). In particular, it was shown that using these techniques it is possible to implement gates like the…
glS
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How does the Curry-Howard correspondence apply to quantum programs?

In words of Wikipedia, The Curry–Howard correspondence is the observation that two families of seemingly unrelated formalisms—namely, the proof systems on one hand, and the models of computation on the other—are in fact the same kind of…
fr_andres
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What countries are leading this "Global Quantum Computing Race"?

The terms Quantum Computing Race and Global Quantum Computing Race have been used in the press and research communities lately in an effort to describe countries making investments into a "battle" to create the first universal quantum computer.…
Vikram Palakurthi
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How could Majorana particles be used to improve quantum computers?

This recent press release claiming that Improved measurements bring final proof of Majorana particles closer than ever, which summarizes the results of a recent paper in Nature simply entitled "Quantized Majorana conductance" claims that Thanks to…
Discrete lizard
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Where is the parallelism in Deutsch-Jozsa algorithm?

I am newbie on Quantum Computing. Actually I am a software engineer but I want to understand how quantum computers work. So my question may be absurd. Sorry about that. I tried to understand Deutsch-Jozsa algorithm to understand how quantum…
overlord
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Does the oracle in Grover's algorithm need to contain information about the entirety of the database?

Grover's algorithm is often described as a way to search a database in $O(\sqrt{N})$ time. For using it we need an oracle gate that represents some function $f$ such that $f^{-1}(1)$ is the answer. But how do you actually make such a “database…
Norrius
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Intuition behind the construction of an ansatz circuit

I'm learning about the VQE algorithm. When I looked at the declaration in Qiskit I saw you need to pass an ansatz which prepares the state. I looked at some commonly used ansatz functions, e.g. EfficientSU2 of Qiskit, and I saw many of them use…
user14092875
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QUBO, Ising Hamiltonians and VQA

I understand that usually the combinatorial optimisation problems are turned into QUBO, which has a very simple mapping to Ising Hamiltonians. Ising Hamiltonians in turn have the desired properties of being diagonal in computational basis and the…
karolyzz
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Quantum tensor product closer to Kronecker product?

Coming more from a computer science background, I never really studied tensor products, covariant/contravariant tensors etc. So until now, I was seeing the "tensor product" operation mostly as (what appears to be) a Kronecker product between the…
Léo Colisson
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