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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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3answers
678 views

Quantum Error-Correction and Quantum Gravity

In one of L. Susskind's talks he said something along the line of "some people [including me] think that quantum error-correction is of importance [in quantum gravity or qft in curved spacetime]". He ...
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2answers
120 views

Is quantum annealing of any use?

I've read up on quantum annealing, and particularly on the way it is done by companies like D-wave. I think I understand how it works and how it harnesses tunneling to find global optimal in a given ...
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1answer
151 views

Qubits in quantum computing [closed]

In my picture of the quantum computers, the memory would deal with matter in superposition. The matters actual state is said be precisely unknown and you cannot cleary say it is in this state or ...
2
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3answers
280 views

Where is a classical computer better than a quantum one? [closed]

Where is a classical computer better than a quantum computer? Is there any known domain where classical algorithms always beat quantum ones, say, both in terms of time and space complexity? If yes,...
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3answers
698 views

Superposition of qubits in quantum computer

I was looking for a quantum physics section but I did not find any, so I decided to post my question here. I've been reading articles and watching some videos on quantum computing these last few ...
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1answer
145 views

Quantum information is necessarily related to quantum computing?

I've received an offer to work my master's thesis on an approach to black holes with quantum information. In that setting, before accepting, I'm trying to understand what this is all about really, ...
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0answers
264 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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1answer
146 views

Quantum computer: Hadamard-Gates and Qubits

I'm learning about the quantum computer basics and got confused about qubits and the hadamard-gate. What I understood: A qubit can (naturally) be in the states $\lvert 0 \rangle$, $\lvert 1 \rangle$ ...
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1answer
442 views

What is charge qubit, flux qubit and phase qubit?

What are the differences among charge qubit, flux qubit and phase qubit?
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1answer
90 views

A question about relations between twistor, entanglement and light ray

In his book 'The road to reality', R. Penrose wrote: It is possible to regard twistor theory as a continuation of the spin network programme to obtain a relativistic scheme, in which idealized light ...
2
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1answer
122 views

Create a qubit in |0> state with a quantum circuit without invoking measurement

Is there any possible quantum circuit, containing gates like CNOT and single qubit rotations, that can take (let's say) 100 qubits in unknown states and drive one of them towards $|0\rangle$ without ...
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146 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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2answers
91 views

Is there a circuit which certainly distinguishes between $|0\rangle$ and $|+\rangle$ states?

Suppose I have a qubit $| \theta \rangle$ which equals to $| 0 \rangle$ or to $| + \rangle = \frac{(|0 \rangle + |1 \rangle )}{\sqrt{2}}$. Is it possible to build a quantum circuit C: $C(| 0 \rangle ...
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1answer
63 views

Is the equality $H^{\otimes 2} \mathsf{CNOT} H^{\otimes 2} |xy\rangle = \mathsf{CNOT} |yx\rangle$ correct?

Is the following equality correct? $$H^{\otimes 2} \, \mathsf{CNOT} \, H^{\otimes 2} |xy\rangle = \mathsf{CNOT} |yx\rangle$$ The solution says yes. It claims that ...
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1answer
285 views

Why are ytterbium atoms used for quantum computing?

Ytterbium seems to be the favorite for element for trapped ion quantum computing experiments. Why is this?
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1answer
264 views

Maximally mixed state as input of a quantum circuit

Given a Hilbert space $\mathcal{H}_N$, a state $\rho$ is a maximally mixed state if it can be written as $\rho = \frac{1}{N} \sum_{i=1}^N |b_i\rangle \langle b_i |$ for some (any in fact) orthonormal ...
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1answer
255 views

Parity Measurement Using a Single c-Z gate

I have a system with some number of qubits. To make it simple, but non-trivial, let's say that number is 3. I want to know the parity of the system's state (meaning I can assume that each individual ...
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1answer
148 views

Quantum Computing [closed]

Could quantum computing ultimately harness the Higgs boson interactions to artificially provide mass (energy) to particles in order to manipulate them?
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42 views

Superdense coding - What are the individual qubits A and B?

With reference to this PDF discussing Superdense coding, it is mentioned that there are two qubits A and B whose superposition gives the system $\frac{1}{\sqrt{2}}|00\rangle + \frac{1}{\sqrt{2}}|11\...
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388 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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31 views

Uncertainty-Correlated Qubits

How does one make a pair of collections of qubits that naturally obey an uncertainty relation between them For example given collections of qubits $Q^1 = q_1^1, q_2^1 ... q_k^1$ and $Q^2 = q_1^2,q_2^...
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1answer
66 views

Grover Search / Amplitude Amplification vs. real world Quantum Computer

I read the literature about Grover's Algorithm and Amplitude AmplificationI understand why it delivers (in theory) a quadratic speedup. However I somewhat wonder how this translates to a real world ...
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1answer
53 views

Higher Order Observations in Quantum Computing

In the Quantum Gate model one generally is restricted to a set of universal Quantum gates, and an "observation" gate from which they can build circuits that carry out Quantum algorithms and are then "...
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1answer
259 views

How to produce the basis states of a quantum computer?

Suppose you have a one qubit system; the (traditional) basis states are $|0\rangle$ and $|1\rangle$, and any state of the qubit can be described by a linear combination of these two. Now suppose you ...
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106 views

How to write these quantum systems in vector form

I have been given these three quantum states: 1: $\frac{1}{\sqrt8}(|0\rangle +|1\rangle)(|0\rangle +|1\rangle) (|0\rangle +|1\rangle)|11\rangle$ 2: $\frac{1}{2}(|0\rangle +|1\rangle)(|0\rangle +|1\...
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1answer
341 views

Prove $3 + 5 = 8$ using Quantum Fourier Transform

I am taking Quantum Computing lesson in this term in my School. Our teacher told about Quantum Fourier Transform and its implementation. Then, he did want us to show $3+5$ equals to $8$ with Quantum ...
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1answer
957 views

Why is stoquastic hamiltonian sometimes mentioned together with quantum annealing?

Definition "Stoquastic" Hamiltonians are sign-problem free Hamiltonians. Background: On attending a meeting last year, I heard the term stoquastic hamiltonian in a talk and noticed the term ...
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1answer
271 views

When can a quantum operation be represented as a 'random unitary'?

Consider the quantum operation that measures a qubit in the computational basis. This is clearly an irreversible operation, but it turns out to be exactly the same as the stochastic unitary operation $...
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1answer
212 views

Is it possible to unify optical and quantum computing?

Motivation Nature recently published a very interesting article that could lead to a great advance in modern computing. To sum it up, it opens the gate to an optical computer, something just like and ...
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1answer
288 views

What is the decoherence time scale for the most promising quantum computing implementations

I'm searching for some data about decoherence time scales for qubits in modern quantum computing implementations. The only reference I have is a table in "Quantum computation and quantum information"...
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120 views

What is the purpose of the Quantum Fourier Transform or what does it operation achieve?

As far as I understand it aids with period finding which can help factor large numbers (i.e why it is used in Shor's algorithm). What I want to know is if I have a quantum system and I apply the ...
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1answer
233 views

Is there a way to represent a 3 qubit system using 3 Bloch Spheres?

I am relatively new to the Quantum Computing world and was wondering if representing a 3 qubit system using 3 Bloch Spheres feasible and if so what would the correct way to do it? I understand a ...
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1answer
2k views

Quantum circuit for a $3$-qubit $|W \rangle$ state

Can someone specify a quantum circuit that will deterministically output the $3$-qubit $|W \rangle$ state, if the input to the circuit is $|0,0,0 \rangle$? Or, is there a quantum circuit with a ...
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86 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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1answer
1k views

If a qubit can be a 1 or 0, and will return both - how can we rely on it? [closed]

In essence, a qubit may 'present itself' (upon observation) as a 1 or a 0. My understanding (as inaccurate as it may be) is that the observation of one particular qubit may result in different outputs....
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118 views

How can I calculate this expected value [closed]

I know this is a simple questions but I'm having trouble with this. I have this wave function and I'm trying to calculate the expected value of QS, where Q and S are the operators Here we have a ...
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1answer
283 views

Initial Hamiltonian in Quantum Annealing

Background: The quantum annealing process performed by the D-Wave machines is described as follows: We have a problem Hamiltonian, $H_{P}$, whose ground state encodes the solution to a problem of ...
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1answer
230 views

Rotate quantum state by a given angle

Is there any gates that allow to rotate quantum state by a given angle $\theta$ in XZ plane? I'd like to move from state $$|\psi\rangle = |0\rangle$$ to state $$|\psi\rangle = {1 \over{\sqrt2}} |0\...
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257 views

Adiabatic Quantum Computing: why not just set the system in its problem Hamiltonian $H_{P}$ immediately?

Background: In any adiabatic quantum computer (AQC) algorithm, we solve problems in the following manner: We have an initial Hamiltonian, $H_{0}$, whose ground state is easy to find, and a problem ...
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1answer
135 views

Ground states being superposition states in adiabatic quantum computation

In Adiabatic Quantum Computing, a Hamiltonian $H$ is evolved for time $T$ according to $$H(t) = (1-t/T)H_{0} + (t/T)H_{P} $$ where $H_{0}$ is an initial Hamiltonian, and the ground state of $H_{P}$ ...
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1answer
232 views

What do the results of a quantum fourier transform describe?

The classical discrete fourier transform takes a sequence of values and outputs another sequence of values that describe a set of coefficients for complex sinusoids which can be used to reconstruct (...
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1answer
73 views

Quantum codes with linear distance

I was wondering if there exists any family of quantum codes (encoding any number of logical qubits) with linear distance. I know of: 1 - Families of topological codes with linear distance when we ...
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2answers
186 views

Can a known quantum state be cloned?

According to the No-cloning theorem, it is impossible to create a clone of unknown quantum state. But how to clone a known quantum state $$\lvert\psi\rangle=\alpha\lvert0\rangle + \beta\lvert1\...
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Why not implement quantum circuits on classical computers?

I have somehow lost sight of the big picture in my study of quantum computing. I understand that we still don't know if quantum computers are more powerful than classical ones, in the sense of ...
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2answers
255 views

Measuring a certain qubit in a three qubit system

Assuming I have a state $|\psi\rangle$ that is a superposition of 3 qubits ($|ijk\rangle\ \ \ \ i,j,k\in\{0,1\}$). When it is said that we measure only the first qubit in, say, the y-axis, does that ...
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2answers
2k views

How are quantum qubits implemented? [duplicate]

Its very easy to pull a logic gate or transistor circuit of the internet for example and begin to understand how it represents physical bits, by either having 5V across the circuit or below 3.3V. But ...
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1answer
641 views

Are quantum computers electronic?

I understand how normal logic gates operate and are implemented using digital circuits and electronics but I wad wondering how qubits are physically implemented. In other words is a physical qubit a ...
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91 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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359 views

How are topological insulators connected to quantum computing?

I see lots of references in news articles on how topological insulators can be used to make a working quantum computer but I'm not sure how a protected edge state is going to lead to a quantum ...
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1answer
185 views

Can a density matrix correspond to two different pure states?

I have an equation of the form $|\psi_1><\psi_1| = |\psi_2><\psi_2|$ This is equating two density matrices - also note that such an outer product can be formed on LHS and RHS. I believe ...