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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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Which field of computing looks more promising, the biological computers that have been springing up or quantum computers? [closed]

Hello this is my first question on this site so thank you for taking a look. Anyways as an aspiring electrical engineer I try my best to keep up with the current trends. Such that I’ve noticed two ...
Arthuro Cisneros's user avatar
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How can a quantum gate be constructed to evaluate a blackbox function (as in the Deutsch's algorithm)?

The standard Deutsch's algorithm uses a control-U gate, which takes the control qubit state $x$ as input to get the output from a black-box function $f(x)$. And the output modifies the quantum state ...
Yuan John Jiang's user avatar
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Photonic classical and quantum computing

How different is the hardware used for classical and quantum photonic computers? Are there any proposals on hybrid platforms? I'm curious why I never heard of both within the same context, except for ...
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Why the unit of quantum information for anyons systems should be the qubit?

I'm starting to learn more about anyons systems. I took a read on this article which is an introduction to topological quantum computing, and also took a look in other places like forums and some ...
Lucas Sievers's user avatar
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Is this an alternative realization of Deutsch's algorithm? [closed]

The standard Deutsch's algorithm uses two qubits and a control-$U_f$ gate, which transforms the two qubits, $|x\rangle |-\rangle \rightarrow (-1)^{f(x)}\, |x\rangle |-\rangle $. To realize the ...
Yuan John Jiang's user avatar
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Is two degree quantum entanglement possible? And by two degree of entanglement I mean one in space and other in time? [closed]

Whenever we try to entangle two particles, the entanglement lasts for a very short period. If we observe our present universe we can see that all the matters present in our universe are finely ...
PARADOXIAN PARADOX's user avatar
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Appling a Hadamard gate to a qubit in density matrix form

If a Hadamard gate is applied to a qubit that is in the state $|0\rangle$, then it becomes the state $$|+\rangle = \frac{1}{\sqrt{2}}\left(|0\rangle+|1\rangle\right)$$ In density matrix form, this ...
mp12853's user avatar
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Why does the SWAP operator equal a particular integral over the unitary group?

Let $\mathcal{U}(d)$ be the group of $d$-dimensional unitary matrices and $P_{21}$ be the swap operator ($P_{21}$ operators on a tensor product of Hilbert spaces $\mathcal{H}_A$ and $\mathcal{H}_B$ ...
Maple's user avatar
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Parallel State Machines as an Analog To Quantum Computing

I recently got into Michael Sipser's Introduction to the Theory of Computation. I've only done software a couple years professionally, but in that time have spent a lot of time thinking about state ...
Connor Latham's user avatar
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What is the oracle in every quantum algorithm? [closed]

I hope this question won't be closed. There is a machine called oracle which appears in a lot of algorithms of quantum computing, such as Deutsch's algorithm and QFT period-finding. This oracle ...
tangyao's user avatar
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Bit structure in Adiabatic Quantum Computing [closed]

https://arxiv.org/pdf/quant-ph/0007071.pdf In Chapter 6 in the paper "A Numerical Study of the Performance of a Quantum Adiabatic Evolution Algorithm for Satisfiability”, by Edward Farhi, Jeffrey ...
BetaY's user avatar
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Displacement operator and phase space

I read a paper on open quantum system, it's about non-Markovian process with memory effects. They describe a generic model of two qubits interacting with correlated multimode field. They describe the ...
Yohay Halfon's user avatar
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1 answer
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Generator of two-qubit quantum gate

I would like to know how to derive the explicit form of the GENERATOR of a general two-qubit gate (also here), e.g., controlled-rotation Y. From the definition: $$\exp(-i\theta G) \ ,$$ I see it is: $$...
yvubinotctyui's user avatar
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Quantum oracle for boolean function

As a math student, I am doing some quantum computing. In the course notes of Ronald de Wolf, he says that any Boolean function $$f:\{0,1\}^n\to\{0,1\}^m$$ can be made into a unitary operation that ...
student's user avatar
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Why does the Lamb-Dicke parameter decrease with the number of ions in an ion chain

I would like to understand (ideally with a nice pedagogic reference), why the fidelity of quantum gates in long ion chain decreases with the number of ions in the chain. In this reference, it is ...
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What is the difference between local operations and global operations for quantum systems?

When I hear people talk about qubit operations, they usually make a distinction between local and multi-qubit (global) operations. I understand what they mean intuitively (the local operations act on ...
NikNack's user avatar
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Why does this not break the security of quantum teleportation

I am taking an course on quantum mechanics, and we have just encountered the quantum teleportation protocol that allows for the transfer of one qubit from Alice to Bob. I think I have a way for Eve to ...
Jack's user avatar
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Defining the Problem Hamiltonian for Quantum Annealing in Solving the Shortest Path Problem [closed]

I’m currently studying quantum annealing and its application to solving the shortest path problem. However, I’m facing challenges in defining the problem Hamiltonian, whose ground state should encode ...
CBM's user avatar
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How can we have two independent parameters (physically) during Rabi Oscillations happening in Rydberg Atoms?

Rabi Oscillations are explained as electric interaction between atom and EM field, here. This can be true for any two-level system with specific interactions that can lead to the Hamiltonian given in ...
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How to actually implement a global phase gate?

I've been trying to model some single qubit gates via Rabi oscillations for a project. In this we have some Hamiltonian (in this case it's due to an oscillating electromagnetic field) that when ...
rb101's user avatar
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How can the Rabi frequency be complex?

I've been doing some reading and came across a simple implementation of the Hadamard gate using Rabi oscillations of an atom in a laser field. However, the author mentions that it required the Rabi ...
rb101's user avatar
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How to construct the jump operator in the Lindblad Equation for a 2-qubit system?

I am working on a project for my degree to model a CNOT gate and I'm trying to do this with the Lindblad equation. $$ \dot{\rho} = -\dfrac{i}{\hbar} [H, \rho] + \sum\limits_i \gamma _i (L_i \rho L _i ^...
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How to construct a quantum circuit for quantum Fourier transform in a prime dimensional Hilbert space?

Consider a Hilbert space of dimension $p$ where $p$ is a prime number. Quantum Fourier transform (QFT) in this space is defined as $$ |j\rangle \rightarrow \frac{1}{\sqrt{p}} \sum_{k=0}^{p-1}e^{\frac{...
Abu Saleh Musa's user avatar
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3 answers
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Noise in quantum computing

Consider a quantum computer, and one specific qubit $q$ in it. With the computer, we perform a series of operations which manipulate the qubits including $q$. I am interested in the time interval ...
Riemann's user avatar
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Fastest rate of entangling atomic qubits

Many quantum computing systems, most notably trapped ions, propose to use optical heralded entanglement schemes to entangle small 'modules' of qubits that are further apart. Typically one puts two ...
user34722's user avatar
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Black holes as Quantum Computers

I have been hearing lately that Black holes could be Quantum Computers or that some processes in a black hole simulate the operation of Quantum Computers. This is not my field of study, but I am ...
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Equivalence of two entangling operators with respect to local operators

Suppose that $U_1$ and $U_2$ are two (entangling) operators that act on a quantum system consisting of several qubits. Is there any criterion to tell if these two are equivalent up to applying ...
george doultsinos's user avatar
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How do you solve the eigenproblem for the adiabatic Grover algorithm?

The adiabatic version of Grover's algorithm relies on a straightforward solution of the Schrodinger equation when the Hamiltonian is allowed to linearly vary from its initial state to its final state. ...
requiemman's user avatar
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"Jump" operators from Lindblad equation where the external system is measurements

How do we derive the "jump operators" for the Lindblad equation if the external system is measurements? For example in this article for the Bose Hubbard system the Lindblad operators ...
poliroika's user avatar
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Learning contextual data using a variational quantum circuit

This question is about contextuality in quantum mechanics, about non-quantum data also showing contextuality. Definition for the specifics of the question as well as an example of contextuality in ...
TheStressTensor's user avatar
2 votes
0 answers
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Josephson supercurrent measurement in a lab

This may be a very trivial question but I am a theoretician and can't find the answer to this question for the life of me. Is it possible to actually measure the Josephson supercurrent flowing through ...
SlothForeva's user avatar
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Phase accumulation in Rabi driving

When a single-qubit gate is operated by Rabi driving for a time $t$, the resulting operator is equivalent to $ \hat{O} = R_z(\omega t) R_x(\Omega t)$ where $\omega$ is the frequency of the driving ...
jackphen's user avatar
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1 answer
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How are quantum gates encoded in voltage/current waves for superconducting qubits?

From what I understand, the voltage and current waves propagate down the input transmission line, hit the nonlinear oscillator, and then bounce back toward the generator. How are gates such as the X ...
David L's user avatar
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1 answer
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What guarantees a photonic quantum gate to be unitary?

So in my course of quantum computation i came across this question that "What guarantees a quantum gate to be unitary?" i was specially curious about photonic quantum gate. At first i ...
amir moghaddam's user avatar
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Can a hidden variable theory be seen as a map from unitaries to stochastics with the same dimensions?

In Quantum Computing in Hidden Variables, Scott Aaronson defines a hidden variable theory as something that transforms a unitary matrix mapping $\begin{bmatrix}\alpha_1 & \ldots & \alpha_n\end{...
Jun Inoue's user avatar
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2 answers
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Exponential expression of controlled unitary gate

The controlled unitary gate $CU$, for some general unitary gate $U$, is (1): $$CU=|0⟩⟨0|⊗I+|1⟩⟨1|⊗U$$ From the Wikipedia article on the CNOT gate (https://en.wikipedia.org/wiki/Controlled_NOT_gate) a ...
pll04's user avatar
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Notation of 2-qubit quantum gates as "Matrix exponentiated by Matrix"

I am currently studying this paper about Cartan's KAK Decomposition. In the end of the section "Canonical Class Vector" some there is a calculation on how to find the canonical class vector ...
LampeAnTonAus's user avatar
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1 answer
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Substituting qubit for classical memory

I recently came across a paper that argues that we can substitute a qubit for arbitrarily large number of bits in a physical system (https://arxiv.org/abs/quant-ph/0110166). The notion of bit ...
madeel's user avatar
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Understanding the Relationship Between Stochastic Reconfiguration and Natural Gradient in Variational Monte Carlo

I've been delving into variational Monte Carlo methods, particularly in the context of ground state energy minimization for quantum wave function ansatzes. In my studies, I've come across multiple ...
H44S's user avatar
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Is it possible that when measured more than one Cooper pair will have tunneled across the junction?

If I've understood the idea correctly a charge qubit is formed by a superconducting island coupled by a Joseph junction to a superconducting reservoir. The state of the qubit is determined by the ...
Omeglac's user avatar
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1 answer
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Where exactly lies the quantum supremacy?

What makes quantum computers faster in certain problems than normal computers? Does quantum computing means that many solutions are explored simultaneously instead of one at a time due to quantum ...
jim andr's user avatar
2 votes
1 answer
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Time complexity for quantum computing

Is there any systematic way to obtain a quantum algorithm's time complexity knowing the classical time complexity of the same algorithm type running for bits instead of qubits? For example, we know ...
Quantum Fields's user avatar
1 vote
1 answer
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For any unitary $U = e^{iA}$, question about the matrix element $A_{pq}$ of matrix $A$ and its functional derivative [closed]

Any unitary operation $U \in U(N)$ can be represented as $U = e^{iA}$, where $A$ is an arbitrary $N \times N$ Hermitian matrix. In the case of a time-independent Hamiltonian $H$, we have $A = -itH$ (...
Jon Megan's user avatar
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Is photonic computing subject to the Landauer limit?

In what way are they both related in terms of computation? Are photons subject to thermodynamic limits?
Akshay's user avatar
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Exercises on the subject Theory of Quantum Information

I have recently started to study quantum information theory on Nielsen and Chuang book, but in order to understand the theory better I need to solve more problems, there are in this book. Can you ...
1 vote
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How to obtain quantum-state purity and concurrence by measurement on quantum computer?

My whole question is aimed at the implementation on quantum computers utilizing quantum circuits, i.e. we can assume, that I can prepare several "copies" of the same state, without really ...
Eenoku's user avatar
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3 votes
1 answer
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When applying the adiabatic theorem, why doesn't the gap become doubly exponentially small generically?

Suppose we have a parameterised family of Hamiltonians $H(s)$, $s\in [0,1]$, acting on $n$ spins/qubits. When applying the adiabatic theorem, it is well known that if we wish to remain in the ground ...
Hans Schmuber's user avatar
1 vote
1 answer
67 views

How do we measure the probability of outcomes in a quantum computer if qubits collapse when measured?

I understand the idea behind Quantum Computing being that we superpose all possible states of a system of qubits and amplify the probabilities of whatever it is that we want depending on the circuit ...
Omeglac's user avatar
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2 answers
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When is Quantum State Teleportation useful?

I've been working through the Brilliant Course on Quantum Computing, and I've reached the lesson on Quantum Teleportation. After completing the lesson and reading several questions and answers here: ...
Connor's user avatar
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1 answer
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Number of operations on Cbits and Qbits [closed]

What is the maximum number of operations that can be defined on Cbits (like mechanical switches) and Qbits (like quantum states)? If they are different how will the ratio change if we limit ourselves ...
GodessOfPhysics's user avatar

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