Quantum Computing Stack Exchange
Quantum Computing Stack Exchange

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How do quantum computers prevent "quantum noise"?

On the Wikipedia page for Shor's algorithm, it is stated that Shor's algorithm is not currently feasible to use to factor RSA-sized numbers, because a quantum computer has not been built with enough qubits due to things such as quantum noise. How do…
ack
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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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How to get state vector from QasmSimulator in Qiskit

I'm currently able to get state vector in Qiskit using state_vector backend using something like : backend = Aer.get_backend('statevector_simulator') job = execute(qc, backend=backend, shots=1, memory=True) job_result =…
user12910
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Impact of ordering Hamiltonian terms for Trotterization

In Trotterization, the typical Hamiltonian considered is: $$ H = \sum_{p, q} h_{pq} a^{\dagger}_p a_q + \sum_{p, q, r, s} a^{\dagger}_p a^{\dagger}_q a_r a_s $$ Which is then converted into a sequence of gates by the Jordan Wigner…
C. Kang
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Consequences of $MIP^\ast=RE$ Regarding Quantum Algorithms

The (pending-peer review) proof of $MIP^\ast=RE$ in this pre-print has been hailed as a significant breakthrough. The significance of this result is addressed by Henry Yuen (one of the authors) in this blog post. Scott Aaronson also lists some of…
Jonathan Trousdale
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Preparing any superposition of fixed Hamming weight states

There exists a nice way of preparing any superposition (with real amplitudes — this is the case I'm interested in) of states $\{\ldots0001\rangle,\,|\ldots0010\rangle,\,|\ldots0100\rangle,\ldots\}$, etc. This can be achieved with an $O(\log_2…
mavzolej
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How to explain in layman’s terms the significance of the difference of qubits of the D-Wave and IBM QX?

So, @AndrewO mentioned recently that he has had 'encounters' with people wondering why D-Wave has a lot more qubits than IBM. Of course, this comparison is faulty, since the IBM and D-Wave's machine may both exploit quantum effects to a certain…
Discrete lizard
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Hadamard Overlap Test

I am trying to understand a test called Hadamard Overlap Test, which consists of a destructive swap test (section IV of swap test and Hong-Ou-Mandel effect are equivalent) right after a Hadamard test. The circuit is from the Variational Quantum…
Enrico
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Are there any experimental groups currently measuring quantum contextuality?

I work as a theorist with my current research interests in "quantum contextuality". For those perusing the question, this is essentially a generalization of non-locality where we can show a quantum system does not admit a hidden-variable model. So…
Emily Tyhurst
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Decomposing Hamiltonian into qubit model representation

One of the main applications of VQE is its application to find the approximation to the ground state energy (smallest eigenvalue of the Hamiltonian) for a particular molecule through an iterative method. To be able to do this, we first need to write…
KAJ226
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How can I see, without math, the action of a gate in matrix form?

Suppose we have the Fredkin gate with $$ F= \left( {\begin{array}{cc} 1 & 0 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 1 & 0 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1 & 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 & 1 & 0 & 0 &…
Bidon
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How to prepare mixed states on a quantum computer?

I am a little bit confused by density matrix notation in quantum algorithms. While I am pretty confident with working with pure states, I never had the need to work with algorithm using density matrices. I am aware we can have/create a quantum…
Nicholas Sathripa
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Logical qubit initialization for the surface code

I am reading Fowler et al's paper on the surface code.. I do not understand how to initialize a logical qubit in an arbitrary state with the surface code. I do understand how to initialize the qubit in logical $|{0}\rangle$ and $|{1}\rangle$, but…
Peter-Jan
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Changing one qubit causes changes in another independent qubit on ibmqx2 in IBM Q Experience

When I run this program on ibmqx2 (the backend matters) in IBM Q Experience, the results are interesting. Essentially, the program measures the error rate on $|q_0\rangle$ after fiddling with $|q_1\rangle$. OPENQASM 2.0; include "qelib1.inc"; gate…
balios
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What are the "nice" properties of the diamond norm and why is it used?

I have heard about the diamond norm, and from what I understood it is a "nice" tool to quantify quality of quantum gates in the NISQ era. I would like to know a little more before going in detail in the litterature. My question are the…
Marco Fellous-Asiani
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