Quantum Computing Stack Exchange Community Digest

Top new questions this week:

Quantum circuits explain algorithms, why didn't classical circuits?

When explaining a quantum algorithm, many revert to 'circuit-speak' by drawing a diagram of how qubits split off into transformations and measurements, however, rarely if not never would someone ...

algorithm classical-computing  
asked by develarist 9 votes
answered by Sanchayan Dutta 11 votes

Can superdense coding be made more efficient?

Superdense encoding allows us to transmit the information of two classical bits using a single qubit with a pre-shared Bell state qubit pair. We can duplicate the construct to transmit $2n$ classical ...

communication superdense-coding  
asked by czwang 5 votes
answered by Craig Gidney 5 votes

On the distribution of the fidelity of a random product state with an arbitrary many-qubit state

Consider an arbitrary $n$-qubit state $\lvert \psi \rangle$. How much do we understand about the probability distribution of the fidelity of $\lvert \psi \rangle$ with a tensor product $\lvert \alpha ...

quantum-state fidelity haar-distribution product-states  
asked by Niel de Beaudrap 5 votes

How to efficiently calculate the inverse of a Kronecker product?

This is a follow-up question to a previous question I had, where the correct answer was to use the Kronecker product. Given, for example, a vector representing two qbits $$\begin{bmatrix}0 \\ 1 \\ 0 ...

algorithm quantum-state matrix-representation  
asked by 1ijk 4 votes
answered by DaftWullie 4 votes

Why is the order reversed on measurement?

Why is the order reversed on measurement? from qiskit import( QuantumCircuit, execute, Aer) from qiskit.visualization import plot_histogram # Use Aer's qasm_simulator simulator = ...

quantum-gate programming qiskit  
asked by Yushan Lu 3 votes
answered by Martin Vesely 1 vote

Moments-based quantum circuit in Cirq

In Cirq, quantum circuit consists of moments, which in turn consists of operations. This is different from Qiskit/OpenQASM, which doesn’t have the concept of moments. What is the design philosophy ...

programming cirq  
asked by Huang Junye 3 votes
answered by Craig Gidney 2 votes

Premise of the proof of the No-Cloning Theorem

I have seen two similar proofs of the no-cloning theorem. They assume (to the contrary) that there exists a unitary operator $U$ such that $U |\psi\rangle |0 \rangle = | \psi \rangle | \psi \rangle$, ...

nielsen-and-chuang no-cloning-theorem  
asked by abrahimladha 3 votes
answered by Craig Gidney 6 votes

Greatest hits from previous weeks:

What's the Difference between T2 and T2*?

$T_2$ generally refers to the measurement of the coherence of the qubit with respect to its dephasing (that's a rotation through the $|0\rangle$ - $|1\rangle$ axis of the Bloch sphere for those of us ...

decoherence solid-state  
asked by psitae 6 votes
answered by Jay Gambetta 4 votes

Quantum algorithm for linear systems of equations (HHL09): Step 1 - Confusion regarding the usage of phase estimation algorithm

I have been trying to get my head around the famous(?) paper Quantum algorithm for linear systems of equations (Harrow, Hassidim & Lloyd, 2009) (more popularly known as the HHL09 algorithm paper) ...

algorithm hhl-algorithm phase-estimation  
asked by Sanchayan Dutta 11 votes
answered by Nelimee 5 votes

Why do optical quantum computers not have to be kept near absolute zero while superconducting quantum computers do?

This is a follow-up question to @heather's answer to the question : Why must quantum computers be kept near absolute zero? What I know: Superconducting quantum computing: It is an implementation of ...

physical-realization optical-quantum-computing architecture superconducting-quantum-computing  
asked by Sanchayan Dutta 19 votes
answered by DanielSank 27 votes

What makes quantum computers so good at computing prime factors?

One of the common claims about quantum computers is their ability to "break" conventional cryptography. This is because conventional cryptography is based on prime factors, something which is ...

speedup classical-computing  
asked by Paul Turner 18 votes
answered by frogeyedpeas 12 votes

What are the models of quantum computation?

It seems that quantum computing is often taken to mean the quantum circuit method of computation, where a register of qubits is acted on by a circuit of quantum gates and measured at the output (and ...

asked by Kiro 37 votes
answered by Niel de Beaudrap 19 votes

Will deep learning neural networks run on quantum computers?

Deep Learning (multiple layers of artificial neural networks used in supervised and unsupervised machine learning tasks) is an incredibly powerful tool for many of the most difficult machine learning ...

algorithm machine-learning neural-network  
asked by Bob Swain 13 votes
answered by frogeyedpeas 8 votes

How does bra-ket notation work?

Quantum algorithms frequently use bra-ket notation in their description. What do all of these brackets and vertical lines mean? For example: $|ψ⟩=α|0⟩+β|1⟩$ While this is arguably a question about ...

asked by Ella Rose 29 votes
answered by jknappen - Reinstate Monica 17 votes

Can you answer these questions?

How to randomly generate unoptimized stabilizer circuits?

I have to test a simulator with a high number of random stabilizer circuits. One way to create random stabilizer circuits is to generate random graph states and convert those to circuits, this, ...

stabilizer-code random-quantum-circuit  
asked by LittleByBlue 1 vote

Atom magnetic moment caused by orbiting electron

In Nielsen and Chuang Quantum Computation and Quantum Information book section 1.5.1 describing the Stern-Garlach experiment, it says: "Hydrogen atoms contain a proton and an orbiting electron. You ...

physical-qubit nielsen-and-chuang  
asked by czwang 1 vote
answered by user185597 0 votes

Entanglement and teleportation in quantum annealing and parallel computation

In classical optimization problems, some objective function's surface serves as a landscape for finding its minimum through minimization algorithms. Instead of your typical U-shaped objective ...

entanglement teleportation annealing optimization  
asked by develarist 1 vote
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