Quantum Computing Stack Exchange
Quantum Computing Stack Exchange

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Why do we need a Classical Register for carrying out Quantum Computations?

I've just started to mess about with QISKit on Python and one thing is confusing me a fair bit. Given that we are building Quantum Circuits what is the need for a classical register ? Is it because the collapsed state must be classical ?
Jake Xuereb
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How to pick initial values for Shor's algorithm?

From my understanding Shor's algorithm can be summarised as follows... Task: given a positive integer $N$ find another positive integer $s$ such that $s$ divides $N$. Steps (informal) Choose an integer $a$ uniformly at random from the interval $[1,…
Callum
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What is the name for the matrix of amplitudes for a bipartite state?

I'm not one for names of things, more concepts. However, when I teach, that creates problems as I can't put a name on something for students to be able to go and find more information. In particular, when you have a bipartite…
DaftWullie
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Exponentiating a product of QFT-related operators

Is there a smart way to implement $e^{i\theta\,\Phi\,\rm{QFT} \, \Phi \, \rm{QFT}^\dagger}$, where both $\Phi \propto\sum_j2^jZ_j$ and $\rm{QFT}$ act on the same set of registers? Even an approximate implementation valid for small $\theta$ would be…
mavzolej
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Resources for quantum biology

Are there any books, courses, tutorials, etc. for studying quantum biology? Preferably they provide some introduction/primer on the relevant quantum aspects of the quantum biological systems being described.
TanMath
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Advantage of Hadamard gate over rotation about the X axis for creating superpositions

When I look at most circuits (admittedly small sample as I'm a beginner), the Hadamard gate is used a lot to prepare a superposition from say the $\mid0\rangle$ state. But upon a little reflection, we can prepare a superposition using a…
Ntwali B.
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How do you represent the output of a quantum gate in terms of its basis vectors?

I'm stuck while trying to understand the Hadamard Gate in a more linear algebra understanding. (I understand the algebraic way). This is because I want to program a simulation of a quantum computer. To apply a gate you multiply each ket by the…
Vitulus
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Why focus on T gates and not some other single qubit rotation R making Clifford + R universal?

Background: In many error correction codes in particular the surface code, the Clifford operations generated by the S,H and CNOT are transversal for quantum computation (meaning that these logical gates can be implemented in easily with very low…
Frederik Ravn Klausen
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Number of gates required to approximate arbitrary unitaries

If I understand correctly, there must exist unitary operations that can be approximated to a distance $\epsilon$ only by an exponential number of quantum gates and no less. However, by the Solovay-Kitaev theorem, any arbitrary unitary operation in…
BlackHat18
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The process for transferring qubits between locations

I understand that right now qubits are physical entities in a Quantum Computer and I am playing around on the IBM Quantum Computer as well as the Q# language and dipping my toes into the Quantum world for the first time. I have read a lot of the…
Leigh Griffin
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What is the stabilizer rank of the W state?

The $ n $ qubit $ W $ state is defined here https://en.wikipedia.org/wiki/W_state The stabilizer rank of a quantum state $|\psi\rangle$ is the minimal $r$ such that \begin{equation} |{\psi}\rangle = \sum_{j=1}^{r} c_j |φ_{j}\rangle. \end{equation}…
Ian Gershon Teixeira
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Are non-secret-based quantum money mini-schemes susceptable to Jogenfors' "reuse attack?"

Aaronson and Christiano call public-key or private-key quantum mini-schemes $\mathcal M$ secret-based if a mint works by first uniformly generating a secret random classical strings $r$, and then generating a banknote $\$:=(s_r,\rho_r)$, where $s_r$…
Mark Spinelli
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Query regarding BQP belonging to PP

I found the following proof of BQP belonging to PP (the original document is here). There is a part of the proof that I have trouble understanding. First, the structure is given below. We try to simulate a polynomial-time generated quantum circuit …
BlackHat18
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Practical implementation of Hamiltonian Evolution

Following from this question, I tried to look at the cited article in order to simulate and solve that same problem... without success. Mainly, I still fail to understand how the authors managed to simulate the Hamiltonian evolution through the…
FSic
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Is it possible to realize CNOT gate in 3 dimension?

CNOT gates have been realized for states living in 2-dimensional spaces (qubits). What about higher-dimensional (qudit) states? Can CNOT gates be defined in such case? In particular, is this possible for three-dimensional states, for example, using…
Goat
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