Questions tagged [quantum-computer]

The quantum computing tag is relevant for computing that uses quantum states such as superposition and/or entanglement to locate low energy states as solutions to complex problems (rather than laboriously enumerating and checking solutions as would be done with non-quantum traditional computing).

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54 views

Why do we need Jordan-Wigner transformation when simulating fermionic systems on quantum circuits?

Why do we need to do a Jordan-Wigner transformation when we want to simulate the fermionic systems with a quantum circuit model? I understand that the spin operator $\sigma_x, \sigma_y, \sigma_z$ on ...
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What distinguish a non-interacting two fermions gate from an interacting one?

We start with a Hilbert space consists of two fermions(a and b), which is 4-dimensional. The basis would be $|vec>$,$b^\dagger|vac>$,$a^\dagger|vac>$,$a^\dagger b^\dagger |vac>$. Denote ...
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Can quDit gates be non-unitary

I have derived a quantum quDit gate that is implemented by photon Fock state inteference. It turns out to be non-unitary. I thought I must have made a mistake, so I have checked several times. I know ...
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how to construct a triple-controlled unitary gate?

I try to construct multiply-controlled qubit gates and I did. But I'm not sure how to construct a triple-controlled unitary gate for four qubits, starting from a version that controlled by only one ...
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Inverse covariance matrix for a Gaussian state

I was reading an article about Gaussian Boson Sampling (https://arxiv.org/pdf/1801.07488.pdf) and following some calculation appear an inverse covariance matrix when he defines the following matrix A. ...
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Quantum Gates: What is the mathematical difference between the the phase shift gate and the Pauli Z rotated by pi/2?

I come from computer science, and I am trying to get into quantum computing research. I am going through Microsoft's quantum katas and I'm struggling to grasp some of the mathematical nuances ...
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Understanding Skein relations

I'm just studying some concepts in the topology, but I cannot figure out totally how Skein relations work. in the following I've one example: which tries to relate Reidmeister II to the Skein ...
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Is it possible to know whether an arbitrary system is performing a quantum computation?

Without knowing in advance that a physical system has been configured to do so, is it possible to determine whether that system is preforming, or has just performed a quantum computation? That is, if ...
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Can all CFT state be prepared through scale invariant MERA

It is known that in numeric computation, scale invariant MERA is useful for representing a CFT vacuum state. Is the converse true? i.e. all CFT vacuum state (the quantum state with translation and ...
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What is the simplest circuit on a quantum computer that “proves” superposition/entanglement

I am a Physics Teacher at pre university level. Tunneling, wavefunctions and uncertainty are on the syllabus on a very vague way. Quantum superposition and entanglement are not but tie in well and I ...
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5 qubit quantum error correction property [closed]

Having $$\eqalign{ |v_0⟩&=|00000⟩+|11000⟩+|01100⟩+|00110⟩+ {}\cr &\quad |00011⟩+|10001⟩−|10010⟩-|10100⟩− {}\cr &\quad |01001⟩−|01010⟩−|00101⟩−|11110⟩− {}\cr &\quad |...
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Random quantum walk related to quantum computing

I am an undergraduate doing research on QC/QI. My current topic to learn is continuous-time quantum walks, but first I must learn the random quantum walk. That being said, I was wondering if someone ...
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For Grover's search: How can a quantum computer store the data, and how can it overall be faster than a classical computer

I think Grover's algorithm can be used to find which entry in a list of names contains a given name, say "Fred" in entry number 12345. Each name can be seen as a binary number, so we might expect to ...
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In quantum search algorithm, how to interpret the effect of $U(t)$ as a rotation on the Bloch sphere?

In Nielsen's QCQI, in page 259, it reads, $$U \left ( \Delta t \right ) = \left ( \cos^2 \left ( \frac {\Delta t} 2 \right ) - \sin ^2 \left ( \frac {\Delta t} 2 \right ) \vec \psi \cdot \hat z \...
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How to perform quantum logic gates on plasmons?

in the link provided researches utilize strong two-plasmon absorption in graphene nanoribbons to create a “square-root-of-swap”, could such a device be utilized to perform other quantum logic gates? ...
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Topological superconductors

I have been reading a lot about quantum computation by using topological materials and I could see that the standard approach is to engineer a p-wave superconductor by using a 1-D semiconductor wire ...
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Universe simulation

Is it theoretically possible to simulate the universe? I know that some philosophical theories say that our universe is a computer simulation, but is this feasible? Would it ever be possible for a ...
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What happens to physical qubits when they decohere during quantum error correction?

Say you have a scaled up system with 98% fidelity so 2% decoheres. These are physical qubits doing error correction on information encoded on logical qubits. 98% of these entangled qubits will be ...
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Understanding Anyonic Exchange

In the book of "Introduction To Topological Quantum Computation" by Jiannis K. Pachos, in chapter 5, it tries to explain anyonic exchange. In the following, the $m$ and $e$ quasi-particles are ...
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Why can a nitrogen-vacancy center be viewed as a basic unit of a quantum computer?

I was on the Wikipedia page for Nitrogen-Vacancy Center and in the first paragraph the following statement is made: An individual N-V center can be viewed as a basic unit of a quantum computer, and ...
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What does $e^{i\alpha}$ stand for in the general expression for a qubit gate $e^{i\alpha}R_n(\theta)$?

All qubit gates can be written in the form of: $$U = \exp(i\alpha)R_n(\theta).$$ I know $R_n(\theta)$ is a rotation of $\theta$ about an arbitrary axis n in Bloch sphere, but what does $\exp(i\alpha)...
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“Quantum information and quantum computation”: quantum period finding algorithm

The procedure for a quantum period-finding algorithm is described on page 236 of "Quantum Computation and Quantum Information" by Isaac Chuang and Michael Nielsen. In step 3 of the procedure, authors ...
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Fluxes on a finite group $G$

So I've been studying about topological quantum computation and I have a few questions I haven't been able to solve. The first one is why fluxes take values on a finite group $G$? Does it have to do ...
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What quantum volume is needed to represent a single fault-tolerant logical qubit?

The quantum volume metric $V_Q$ is a proposed metric for quantifying and comparing the performance of quantum computers[1]. The quantum volume is defined as $$V_Q = \max_{n<N} \left(\min\left[n, d(...
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Can any 1D critical state be represented by a MERA tensor network?

My understanding of the Multiscale Entanglement Renormalisation Ansatz (MERA) is that it is designed to represent highly entangled, but low complexity states. Is MERA capable of representing high ...
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Why is EIT needed to story memory using rydberg atoms?

I was going through a paper explaining how the write-in and read-out efficiencies can be increased using cold atoms where they used Rydberg atoms, and mentioned that they were probed using EIT(...
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Isn't the Church-Turing-Deutsche (CTD) Principle an empirical question?

The Church-Turing-Deutsche (CTD) Principle is the idea that all physical processes are computable by a quantum computer (i.e. quantum Turing machine). Before I knew this idea had a name, I always ...
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Superoperator cannot increase relative entropy

So I have to show that a superoperator $\$$ cannot increase relative entropy using the monotonicity of relative entropy: $$S(\rho_A || \sigma_A) \leq S(\rho_{AB} || \sigma_{AB}).$$ What I have to ...
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How to identify whether a quantum state is entangled or separable if you can measure it only in one basis?

Suppose you have two qubits and don't know whether they're in an entangled state $ \frac{1}{\sqrt{2}}\left(|0,0\rangle + |1,1\rangle\right)$ or in an equal mixture of states $ |0,0\rangle $ and $ |1,1\...
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Can someone explain how does Gisin apply the No Signaling criterion here?

So given the density operator for the state. He uses the no signaling condition which says that "If we have an entangled state between Alice and Bob and Alice measures the state in any basis pair the ...
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How do I apply no signaling condition for the given Quantum Adder problem

The Unitary transformation for the Imperfect Quantum Adder problem is such: Both the states belong to two Hilbert Spaces and we are trying to add both states to one system using Ancillary qubit. I ...
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How to measure a Bell inequality violation in IBM Q? (Or analytically)

I made some circuit to prepare a 2 qubit state, but I am having trouble understanding how to measure Bell's inequality. I know the inequality is of the form $$|E(a,b)-E(a,b')+E(a',b)+E(a',b')| \leq 2$...
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Doing addition and subtraction with tensor diagrams?

Tensor diagrams are a beautiful and useful tool for making calculations with tensors, up until you need to contract with the sum or difference of two tensors, at which point it seems to become awful. ...
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Which system for a quantum computer?

We are aware that we use binary system for our present day digital computers as they comprise of two states (0&1). But what system would fit the states of a quantum computer?
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Literature Anyons [duplicate]

In a few months I have to give a talk about Identical Particles in Quantum Physics, which should answer the following questions or explain following concepts: Why are there only Fermions and Bosons ...
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Encoding infinite information in a qubit vs. classical system

In this Quantum Computing article by Michael Nielsen he argues about some of the limitations imposed by quantum measurement. In particular how the amplitude $\alpha$ of a single qubit $\alpha |0> +...
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Intuition behind quantum CNOT gate applied to a pair of electrons [duplicate]

I'm looking for some intuition behind how a cNOT gate works. I think I understand the mathematics; but, I'm having trouble imagining how two electrons would interact to produce the predicted result. ...
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Rydberg Blockade's experimental mechanism

This idea for an experiment is for a high school competition (bl4s). Utilizing a positron beam,can we knock out gas which has been energized to its Rydberg state.I plan to do this to verify when the ...
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Do quantum error correction codes depend on the type of quantum computation (circuit QED/topological/etc)?

So I know that there are different types of quantum computation that can be realized (cQED, topological quantum computation among others), and different error correction codes that exist. Naively I ...
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Can we recover a state on a composite system from a state on the subsystem?

$\renewcommand{\ket}[1]{\left \lvert #1 \right \rangle}$ $\renewcommand{\bra}[1]{\left \langle #1 \right \rvert}$ I'm wondering if we can recover the state of a composite system from the information ...
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Say you run qubit $A = (|0⟩ + |1⟩) / √2$ and qubit $B = |0⟩$ through a CNOT gate. What is the state of qubit $B$ afterwards?

I am new to the weeds of quantum computing and this question is probably pretty elementary. Say you run qubit A = (|0⟩ + |1⟩) / √2 and qubit B = |0⟩ through a CNOT gate. What is the state of qubit B ...
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About superposition of states

In quantum computing, we can always create an arbitrary superposition of states by rotation of $|0\rangle$ state for one qubit. This raises a question: for arbitrary superposition of states, is there ...
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What's wrong with this method of solving NP problems using a quantum computer?

As far as we know, Quantum Computers can not solve arbitrary NP problems in polynomial time. I have what appears to be a solution, but it is obviously to simple to be correct, since otherwise the ...
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Does adiabatic quantum computation require the initial and final ground states to be non-orthogonal?

Background At a recent talk, I was told by the speaker that it is not possible to adiabatically transfer from one ground state $|\psi_0 \rangle$ to another $|\psi_1 \rangle$ if these states are ...
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Do qubits need to be in superposition to be entangled? [closed]

Do qubits need to be in superposition to be entangled? Put another way, can qubits be entangled but not in superposition?
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Is a mixture of Bell states entangled?

Setup: I have a two qubit system $\mathbb{C}^2 \otimes \mathbb{C}^2$ an we consider the Bell states $\newcommand{\ket}[1]{\left|#1\right>}$ $\newcommand{\bra}[1]{\left<#1\right|}$ $\...
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Quantum computing explanation for the quantum Zeno effect

I'm playing around with the IBM Q to demonstrate the qunatum Zeno effect. If we prepare a qubit in the $|0\rangle$ state and apply 5 consecutive $R_y(\pi/5)$ gates, we should end up in state $|1\...
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Faster than light speed transfer of certain event

I am CS/Math person I am not quite sure what it means in physics information cannot be transferred faster than light (FTL). I tried to understand the proof of no transfer theorem but I lacked the ...
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Is the eavesdropper detected by chance in QKD?

So I was starting to familiarise myself with QKD when I came across this video on YouTube breaking down the process (I believe this is the E91 protocol but I could be mistaken) using an animation that ...
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Rank of a density matix

I was just trying to understand the meaning of rank of a density matrix. I came across the following post, which says that the rank of density matrix is the number of non-zero eigenvalues. And for a ...