Quantum Computing Stack Exchange
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Self reducibility of QCMA problems

Self reducibility is when search version of the problems in a language reduce to decision versions of the same problems. NP-complete problems are self reducible. Are QCMA complete problems self reducible?
BlackHat18
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How to construct a circuit to perform this operation? Is there a general way of getting a circuit from a matrix?

I want to build a circuit that performs the following operation: $$ U_f = \left(\begin{array}{cccccccccc} 1 & 0 & 0 & \dots & \dots & \dots & \dots & \dots & \dots & 0 \\ 0 & 1 & 0 & & & & & & & \vdots\\ 0 & 0 & 1 & & & & & & & \vdots \\ \vdots & &…
SrJaimito
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How to draw Qiskit's HHL algorithm as a circuit?

Qiskit Aqua has a module that implements the HHL algorithm: https://qiskit.org/documentation/stable/0.32/stubs/qiskit.aqua.algorithms.HHL.html How does one draw the implemented circuit? *** EDIT *** I'm working with an example from the github. I…
user727041
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Are three POVM measurements on a single qubit physically realizable?

In Nielsen and Chuang Quantum Computation and Quantum Information book section 2.2.6, a POVM of three elements are used to measure a single qubit in order to know for sure whether the state is $|0\rangle$ or $|+\rangle$ if the first two measurement…
czwang
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How to define a quantum channel for the partial trace?

I understand that the partial trace is a linear map, a completely positive map and a trace-preserving map. However, I have no idea how to define a quantum channel with the partial trace because partial trace depends on an index $i$(i.e. $i$ of…
John
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What is the location of the Qiskit config file?

I'm trying to follow the instructions here https://qiskit.org/documentation/getting_started.html. It says The default backend for qiskit.visualization.circuit_drawer() or QuantumCircuit.draw() is the text backend. However, depending on your …
user1936752
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How do quantum states collapse to the right answer?

I am a computer engineer and have recently started understanding quantum computers. I am still struggling to wrap my head around some concepts. One such query is the following - It is said that during the computation the qubits exist in the…
dhiraj suvarna
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How to parallelly run a quantum circuit on IBMQ?

I am trying to implement the following circuit and my code is The above circuit has size 5 and depth 3. My question is, how can I run the circuit parallelly. I mean, the gate circuit.cx(qr[0], qr[1]) and gate circuit.cx(qr[2], qr[3]) will be run…
wei zi
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Prove that $\|p^{\otimes n} - q^{\otimes n}\| \leq n \|p-q\|$ for density operators $p,q$

I've been trying to figure this out for a while and I'm totally lost. My goal is to show that for two density operators $p$, $q$, that $$||p^{\otimes n} - q^{\otimes n}|| \leq n ||p-q||$$ So far I have constructed a weak inductive hypothesis over…
Confused grad student
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What does W stand for in the W entangled state?

For the $|W_3\rangle=\frac{1}{\sqrt{3}}(|001\rangle+|010\rangle+|100\rangle)$, what does W stand for? Does it refer to an author name? Anyone knows a reference? Thanks
czwang
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How to check if a two-qubit gate is entangling?

I would like to know if there's an analog for Schmidt rank that can tell me if a two-qubit unitary is entangling? Suppose I have a parametrized two-qubit unitary $U^{(2)}(\theta)$. I would like to know a test to tell the degree of entanglement this…
forky40
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Shor's Code: Understanding how it satisfies Knill Laflamme Theorem

I'm new to Quantum Error Correction, and I have a question on Shor's Code. If we have a protected subspace, $V \subset \mathbf{C}^2\otimes \cdots \otimes \mathbf{C}^2$ $V=\operatorname{span}\{|0_{l}\rangle, |1_{L}\rangle.$ We also consider Pauli…
Anon
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Developing quantum circuits for specific quantum chemistry configurations

I am interested in learning more about the following: would it be possible for me to simulate a molecule consisting of copper ions through a quantum circuit? And if so, can that circuit allow me to measure the decoherence time of that molecule? I…
Enrique Segura
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How to prepare a specific initial state of three qubits?

I would like to prepare the following initial state for variational quantum algorithms: $$ \sin\theta_1 \sin\theta_2 \sin\theta_3 |000\rangle + \sin\theta_1 \sin\theta_2 \cos\theta_3 |001\rangle + \sin\theta_1 \cos\theta_2 |010\rangle + \cos\theta_1…
Ashy
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Do we know anything about the computational complexity of the exchange-correlation functional?

Density functional theory is based on the Hohenberg-Kohn (HK) theorems and aims to compute the ground-state many-body wavefunction of a physical material and/or molecules. To put it simply, the HK theorems show that there is a unique one-to-one…
Dr. T. Q. Bit
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