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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210 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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1answer
886 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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177 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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80 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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1answer
879 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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195 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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139 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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79 views

How can time crystals be useful in qRAM design?

A time crystal is a phase of a matter which is ordered in time, similar to classical crystals which are ordered spatially. In other words, the structure of a time crystal is ever-changing but with ...
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100 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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91 views

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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2answers
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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166 views

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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242 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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124 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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105 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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151 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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245 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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1answer
1k 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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60 views

Can classical spin-glass problems be solved exactly in a more efficient way than bringing them to the quantum level?

It is known that classical spin-glass problems are NP-hard, just like their quantum counterparts. Does this truly mean that finding their Ground-State is equally hard in both cases? The most ...
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77 views

What is the maximum amount of computation that can be performed in the future lifespan of the universe?

The ultimate question I have been trying to answer is the maximum universal population allowed by physical limits. Making some transhumanist assumptions I am perfectly happy with, I am equating this ...
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1answer
89 views

How to experimentally perform a POVM measurement?

Reading through Nielsen and Chuang, I came across the following example. Consider a POVM containing three elements, $$ E_1 \equiv \frac{\sqrt{2}}{1 + \sqrt{2}} |1⟩⟨1|,\\ E_2 \equiv \frac{\sqrt{2}}...
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212 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 ...
3
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1answer
91 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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225 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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72 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 ...
3
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1answer
217 views

Is every fusion category with unitary $S$ a unitary modular tensor category (UMTC)?

A tensor category whose solutions to the pentagon/hexagon equations are unitary and whose braiding is nondegenerate is called a unitary modular tensor category (UMTC). When trying to find UMTCs, ...
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90 views

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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96 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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149 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 ...
3
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1answer
241 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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135 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 $\...
3
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1answer
290 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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35 views

How practical is the Landauer limit to actual computing (including distant future)?

Landauer's Principle is often presented as a fundamental limit of efficiency for classical computing. It states that in order to erase one bit of information, at least the following amount of heat has ...
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Has anyone come across the relationship between Higher order confinement wave-functions and corresponding Zeeman Interaction?

I'm studying a hole quantum dot in Germanium. The problem is described in the Luttinger Spin-3/2 basis and takes Fock-Darwin states as solutions in the Quantum Dot plane(xy plane). Furthermore, I ...
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43 views

Are electrons in a semiconductor quantum dot is a real electron or just a quasiparticle with a well-defined spin?

It seems there is a consensus that in the gate-defined quantum dot exists isolated electron with well-defined spin. And I understand that by tuning the voltages applied to the 2DEG at quantum well, ...
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54 views

Why does the no-cloning theorem imply the monogamy of entanglement?

I have seen papers where they mentioned that the monogamy of entanglement can be seen as a consequence of the no-cloning theorem. Could somebody sketch this out? N.B.: I have seen the argument that ...
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1answer
60 views

Shor's algorithm entanglement verification

I would like to ask whether the entanglement verification is necessary in Shor's algorithm In the paper, Nature Photon 6, 773–776 (2012), they mentioned that they tried to factorize 21 to avoid the ...
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1answer
29 views

Does uncopying an information requires a copy of the same program in this scheme? If so, how?

In the paper Information is Physical by Rolf Landauer (reference), it is claimed that Uncopying is not equivalent to erasure and, just like copying, can be done with a dissipation per step ...
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How is deterministic state transfer related to entanglement generation between two qubits?

From a quantum information theoretical point of view, how is deterministic state transfer related to entanglement generation between two qubits? Or, more specifically, how can you exploit the former ...
2
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1answer
84 views

How Do I Calculate the Score of a Bell's Game

I am reading "Quantum Chance" by Nicolas Gisin. I would like to know how to calculate the Bell Game outcome given in the following section of the book. BACKGROUND The Bell Game begins with a pair ...
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62 views

On the quantum computer implementation of FFT

In Fig. 1 of the paper Exact Ising model simulation on a quantum computer, it appears a circuit to implement a Bogoliubov and (discrete) fast Fourier transform (FFT) over 4 qubits in order to ...
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30 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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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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29 views

Why do quantum memories have to be so cold?

I understand that currently, atomic qubits, or just atomic ensembles are kept near absolute zero, in order to try to create an ideal gas structure. For higher temperatures the individual ions will ...
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195 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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119 views

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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61 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
105 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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161 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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223 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 ...