Quantum Computing Stack Exchange
Quantum Computing Stack Exchange

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Can I remove gates from a QuantumCircuit?

Suppose I want to implement run several circuits one after another, but they are constructed in a similar fashion. I could reinstantiate a QuantumCircuit for each iteration, like this: params = np.linspace(0, 2 * np.pi, num=20) for p in params: …
Alexey Uvarov
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Why it is hard to simulate a quantum device by a classical devices?

I was recently watching a talk by Urmila Mahadev on "Classical Verification of Quantum Computations" (see this). I am not new to quantum computation just have a familiarity with the qubit and some part of quantum mechanics. I even did not get the…
new_bee
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How can we be sure that for every $A$, $A^\dagger A$ has a positive square root?

In the Polar Decomposition section in Nielsen and Chuang (page 78 in the 2002 edition), there is a claim that any matrix $A$ will have a decomposition $UJ$ where $J$ is positive and is equal to $\sqrt{A^\dagger A}$. Firstly, how can we be sure that…
Mahathi Vempati
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Quantum attack on hash functions

The line of questioning is inspired by the pick one trick in Section 4 of the PDF version of the paper Quantum Attacks on Classical Proof Systems - The Hardness of Quantum Rewinding (Ambainis et al., 2014). Slides available here. I don't fully…
user1936752
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Do any educational quantum computing toys or devices exist?

Question inspired by this article from IEEE Spectrum on blocks containing different polarization filters for use in classrooms, and my previous question on representing the three-polarizing-filter experiment in quantum computing terms. Here I want…
ahelwer
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Is there a general method to implement a 'greater than' quantum circuit?

I am interesting in finding a circuit to implement the operation $f(x) > y$ for an arbitrary value of $y$. Below is the circuit I would like to build: I use the first three qubits to encode $|x\rangle $, use the other three qubits to encode $|f(x)…
Max
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Is there a simple rule for the inverse of a Clifford circuit's stabilizer table?

In Improved Simulation of Stabilizer Circuits by Aaronson and Gottesman, it is explained how to compute a table describing which Pauli tensor products the X and Z observable of each qubit get mapped to as a Clifford circuit acts upon them. Here as…
Craig Gidney
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What can tensor networks mean for quantum computing?

I am trying to understand what the importance of tensor networks is (or will/could be) for quantum computing. Does it make sense to study tensor networks deeply and develop them further to help pave the way towards quantum supremacy? If so, how…
dumkar
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Classical Memory enough to store states up to 40 qubits quantum system?

As a part of a discussion with my 'classical' friend, he insisted that making a a state machine for calculating the outcome of quantum computer is possible; so, simply calculate the outcomes of (known) algorithms on supercomputers and store their…
mando
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Is there a Hamiltonian simulation technique implemented somewhere?

I was wondering if there was some code available for Hamiltonian simulation for sparse matrix. And also if they exist, they correspond to a divide and conquer approach or a Quantum walk approach?
cnada
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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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Are quantum states like the W, Bell, GHZ, and Dicke state actually used in quantum computing research?

I recently started studying quantum computing and learned about several well-known quantum states such as the W state, GHZ state, and Dicke state. I noticed that there are also some questions here on Stack Exchange regarding quantum state…
Jayce
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What is the simplest algorithm to demonstrate intuitively quantum speed-up?

What's the simplest algorithm (like Deutsch's algorithm and Grover's Algorithm) for intuitively demonstrating quantum speed-up? And can this algorithm be explained intuitively? Ideally this would be also illustrate clear how quantum interference is…
Steven Sagona
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CNOT Gate on Entangled Qubits

I was trying to generate Greenberger-Horne-Zeilinger (GHZ) state for $N$ states using quantum computing, starting with $|000...000\rangle$ (N times) The proposed solution is to first apply Hadamard Transformation on the first qubit, and then start…
Satvik Golechha
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CSS codes are the only stabilizer codes with transversal CNOT?

Given a stabilizer code $\mathcal{C}$ then $$ \mathcal{C} \text{ is CSS} \iff \text{CNOT} \text{ is transversal}. $$ The forward implication is well known, see for example Transversal logical gate for Stabilizer (or at least Steane code). On the…
Eric Kubischta
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