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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182 views

Why do flux qubits have to be micrometer-sized?

Flux qubits are made using micrometer sized Josephson junctions. They exploit superconducting properties to create and interfere with the magnetic flux between them. My question is that I've seen ...
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63 views

Structure theorems of Bravyi-Vyalyi and zero conditional mutual information

A fundamental result in quantum information is that of Bravyi and Vyalyi (Lemma 8, https://arxiv.org/abs/quant-ph/0308021). Result 1: It states that for commuting hermitian operators $H_{ab}\otimes ...
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829 views

Is it possible to construct a quantum “computer” using laser light similar to the double-slit experiment?

Is it possible to construct an arrangement of optical devices (lasers, mirrors, slits, splitters) such that the construction could carry out a single quantum "computation"? I understand that such a ...
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132 views

The physical picture of uncle Hamiltonian?

Uncle Hamiltonian was built to show the complex relationship between MPS (Matrix Product State) states and Hamiltonians, which claims that for a block injective MPS state, we can build a local ...
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84 views

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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How can blackholes be fast information scramblers?

I noticed that there was already a post discussing the fast scrambling property of black holes. But it seems no satisfactory answer was given. As mentioned by L. Susskind et. al, the fast scrambling ...
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158 views

What's the physical meaning of the eigenvalues of the spin-flipped density matrix?

In the computation of the entanglement of formation(EoF) of a 2 qubits mixed state, $\rho$, according to Wooters, we need to compute the concurrence of the state by computing the eigen values $\{\...
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1k views

Smallest number of quantum gates to simulate other gates?

What is the smallest number of Fredkin gates needed to simulate a Toffoli gate? What is the smallest number of Toffoli gates needed to simulate a Fredkin gate? Where the Toffoli's gate is the CCNOT ...
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120 views

What can quantum adiabatic computation provably accomplish?

Let's say I have a quantum adiabatic computer in a black-box that works perfectly, doesn't suffer from decoherence/noise problems, etc. Are there any proven bounds for an adiabatic algorithm that ...
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365 views

What is a “fiduciary” quantum state?

In Giovanetti et al.'s paper "Quantum Random Access Memory" (arXiv:0708.1879) they state: If the qutrit is initially in the $|wait\rangle$ state, the unitary swaps the state of the qubit in the two ...
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Adiabatic turning on of coupling constant in simulation of $\phi^4$ theories in the JLP algorithm

In the Quantum Algorithms for Quantum Field Theories by Jordan, Lee and Preskill they have devised an efficient algorithm to simulate $\phi^4$ theories. Given by the Lagrangian density $$\mathcal{L}(\...
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237 views

Eigenvalue of the adiabatic Hamiltonian of Farhi's three qubit 2-SAT problem

I was trying to reproduce example 3.3 of Quantum Computation by Adiabatic Evolution by Edward Farhi et. al. This is an adiabatic algorithm to solve an instance of three qubits 2-SAT problem. I think ...
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118 views

Different forms of the Adiabatic Theorem

I've been reading several papers on Adiabatic Quantum Computation, and I am confused by the form in which the adiabatic approximation condition is presented. For example (quant-ph/0001106 eq 2.8) \...
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102 views

Physical consequences of non-trivial quantum states homology

The set of quantum states of a finite dimensional system is a complex projective space, whose homology groups are non-trivial http://en.wikipedia.org/wiki/Complex_projective_space#Homology. Has this ...
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148 views

What is three-photon interference?

Whilst reading this paper on a quantum processor that performs a type of matrix computation, I came across the concept of 'three-photon interference'. A quick Google search shows that this process is ...
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242 views

Studying Feynman articles nowadays

I'm curious to know if it's useful to study Feynman article "Quantum Mechanical Computer" nowadays. I'm a computer scientist, and i don't know any of the literature in quantum computers. Since long ...
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95 views

Can quantum computing solve the curse of dimensionality?

Edited version The curse of dimensionality is ubiquitous in machine learning (ML) modeling, stochastic control and reinforcement learning, arising in a probabilistic sense, with strong connections to ...
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70 views

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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121 views

Transmons and cooper pair box qubits: two islands or a single island and a reservoir

In scientific literature, one typically describes Cooper pair boxes as a small superconducting island coupled to a superconducting reservoir (say, a large ground plane of superconducting metal, or a ...
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65 views

Can adiabatic quantum computing be faster than Grover's algorithm?

It has been proven that adiabatic quantum computing is equivalent to "standard", or gate-model quantum computing. Adiabatic computing, however, shows promises for optimisation problems, where the ...
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Existing microwave trapped ion quantum computer?

In the recent paper Blueprint for a microwave trapped ion quantum computer, the authors envision a scalable design for a quantum computer, built from smaller ones implemented on silicon wafers, "using ...
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94 views

formula for equivalence of number of qubits vs classical computational capability

there are many pop sci articles and press releases wrt quantum computing these days and some may tend to stretch precision in description. one particular area that bothers me at the moment: some ...
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145 views

Why isn't the surface code limited to 2 qubits?

Reading about surface codes I see that N+1 data qubits and N measurement qubits produce only 2 logical qubits. Does this mean that I can only do single qubit computation with such a surface code? If ...
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77 views

What is the output of a quantum simulator?

First off the bat, the context of this question is philosophical, but please don't run away, since a physicist is required to comment on this one: Philosophers sometimes contemplate a hypothetical ...
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134 views

Optimality of product input state in quantum channel

Let $\mathcal N^{A_1\rightarrow B_1}_1,..,\mathcal N^{A_1\rightarrow B_1}_k$ be a set of valid quantum evolutions with equal input and output dimensions. And let the effect of a channel on a system $\...
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202 views

Quantum annealing computing

What is Quantum Annealing and quantum annealing computing and what are its advantages and disadvantages with respect to quantum circuit quantum computing/computers?
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1answer
232 views

Using fermion-based analog computers to solve NP-hard problems in polynomial time

If the fermion sign problem is an NP-hard problem as it seems to be proved by this work, is it possible to take an NP-hard problem, convert it into an equivalent fermionic state evolution problem, ...
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1answer
798 views

Continued Fraction Algorithm in Shor's Algorithm

I am just trying to make the final link of Shor's algorithm clear. Here $r$ is the order of $x$ modulo $N$. We have a number $\psi$, which for a rational number $\dfrac{s}{r}$ satisfies \begin{...
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22 views

What is a Polariton and why are they useful in unconventional computing?

My lecturer has frequently mentioned the term polariton and a quick search on the Internet lead me to its Wikipedia page (which is not surprising but the page is not particularly helpful) as well as a ...
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38 views

Good resourse for Ising Problem Model

I am student of mathematics, and I will wrote theoretical article about Ising problem in Adiabatic Computation. Do you have a good resource where it is good explaining? In my context I have ...
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45 views

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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165 views

Entanglement fidelity for a single qubit?

So I have been working out problems from the book "Quantum Computing Explained" by David McMahon. On page 166 he defines "Entanglement Fidelity" and says the following: Consider a density operator for ...
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Grover's algorithm: a real life example?

I'm fairly confused about how Grover's algorithm could be used in practice and I'd like to ask help on clarification through an example. Let's assume an $N=8$ element database that contains colors ...
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60 views

How Forces are Represented from Quantum Computation or Entanglement Perspective?

In the fields of Quantum Computation and Entanglement there is content on the structure of quantum systems and artificial quantum systems like qubits and quantum cellular automata. I have even seen ...
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1answer
96 views

Is it possible to build a “DIY” quantum memory?

I am currently working on a home project which is a computer which will be capable of doing basic quantum computations (my first goal is a random-number generator) based on optical devices. Nowadays I ...
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139 views

Status of the discovery of non-abelian anyons and topological quantum computation?

This week Microsoft announced that it will make available the programming language for quantum computer available by the end of this (2017) year. https://news.microsoft.com/features/new-microsoft-...
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184 views

About complexity of quantum states

From quantum computation point of view, the complexity of a quantum state $|\psi\rangle$ can be defined as the the complexity of the quantum circuit that can generate the state $|\psi\rangle$ from an ...
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56 views

What, exactly, made it difficult to implement the Cirac-Zoller controlled-NOT gate when it was first proposed?

The Cirac-Zoller gate is one of the key tools in the manipulation of quantum information on a string of trapped ions, and its proposal was a huge step in the path of quantum computing from abstract ...
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270 views

Interaction free measurement and the Elitzur–Vaidman bomb tester, paradox solved?

I am now reading something about the interaction-free measurement, where the most famous example is the Elitzur–Vaidman bomb tester. I found some discussion/debate about the 'paradox' about it, ...
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390 views

Is there a roadmap for quantum computers?

Right now there seems to be a lot of buzz on quatum computers. I can read success stories everywhere. E.g. IBM claims the first 5 Qubit processor available in the cloud, D-Wave claims a 1024 Qubit ...
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About the paper 'Exponential Communication Complexity Advantage from Quantum Superposition of the Direction of Communication'

In a recent paper 'Exponential Communication Complexity Advantage from Quantum Superposition of the Direction of Communication', the authors claimed that by superpositioning two different trajectories ...
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1answer
101 views

Measuring the speed of Grover's Algorithm

I'm interested in analyzing how long Grover's algorithm takes as the number of qubits increases. I've been simulating it in IBMs quantum computer but when I asked the same question I got this response:...
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227 views

Connection between BQP and Feynman Path Integral

In chapter 10 of Scott Aaronson's book Quantum Computing Since Democritus, he claims that Feynman path integral formulation immediately implies that the complexity class BQP (Bounded-error Quantum ...
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148 views

Toric code vs Shor's code

I study computer science and wanted to get into the field of quantum computing. Right now, I'm trying to tackle the subject of quantum error correcting codes. As is the usual, I've started with Shor's ...
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Can anyone recommend some recent journal articles on the limitations of Quantum Computing

More specifically, I would like some reading material on what the issue is with quantum computers, why can we not yet create them? Are there any journal articles that comprehensively list the reasons ...
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1answer
123 views

Simulation probabilistic classical computation on quantum computers

Given an $N$-level quantum system with computational basis $\{|1> ,|2> , \cdots, |N> \}$, if we perform an unitary operation $U$ on a computational basis state $\{|j> \}$ and then measure ...
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537 views

Limitations of Quantum Simulations on a Classical Computer

My question is on the simulation of a quantum computer on a classical machine. I understand that a classical computer to simulate any quantum algorithm--the problem is that the quantum computer does ...
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1answer
186 views

Quantum Computer Implementation

Hey I recently finished my bachelor in nanotechnology and am rather interested in quantum computation. I've already looked at the recommendation threads for this topic but they tend to be about ...
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58 views

Information retrieval from a database

Consider a database $\cal D$ containing $N$ entries $A_0, A_1, ... A_{N-1}$, which are some fixed and unknown strings of $k$ bits; you can access this database sending a coherent superposition of ...
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Universality of Quantum Operations

Does an analog of the Solovay-Kitaev theorem exist for quantum operations, a generalization of quantum gates that also includes all completely positive maps?