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). Also consider posting your question to the dedicated [quantum computing stack exchange](https://quantumcomputing.stackexchange.com/).
925 questions
3
votes
0
answers
23
views
Why do random quantum circuits produce entanglement?
There's a lot of work done on random quantum circuits, and very often the story is that you start from a completely unentangled state, then random unitary gates act on it, and you generically end up ...
-1
votes
0
answers
176
views
Is the geometric derivation of computational threshold $λ_c = 1/√2$ from Schmidt circle topology mathematically sound?
I'm investigating whether quantum state manifold geometry predicts computational transitions in quantum systems. Starting from the Schmidt decomposition of bipartite pure states:
|Ψ⟩ = λ₁|s₁⟩⊗|e₁⟩ + λ₂...
- 15
0
votes
2
answers
91
views
Is qubit collapse in quantum computing related to decoherence, and how can probabilistic outcomes yield definite answers?
While studying Grover’s algorithm for quantum search, I came across an explanation that a qubit is represented as a state vector in superposition, with amplitudes corresponding to the probabilities of ...
-1
votes
1
answer
121
views
Is the boat analogy for quantum computing accurate, and if so, what can that tell us about its potential practical applications? [closed]
The boat analogy for quantum computing says that to call a quantum computer a better computer is like calling a boat a better car. The use cases for a boat and a car are too different for any ...
0
votes
1
answer
133
views
Proof for the No-cloning theorem
So I was going through the proof for the no-cloning theorem, in quantum mechanics, which states that no arbitrary state can be cloned. The proof as mentioned on Wikipedia starts with $| \phi \rangle| ...
1
vote
1
answer
101
views
If there are time-dependent coefficients on the left side of the equation, and a wave function on the right side of the equation, is that okay?
The state of a quantum system at time $t$ is given by the orthonormal base states $k$ and $j$, with probability amplitudes:
\begin{equation}
C_k(t_0 + \Delta t) \: = \: \langle{k} \vert \psi(t_0) \...
- 11
3
votes
1
answer
131
views
Physical Means to State-Dependent Solutions to Schrödinger Equations? [closed]
Disclaimer: I am a mathematician/computer scientist interested in quantum computers.
Recently, I started reading about quantum computing, and I read that one uses unitary matrices since they ...
- 133
-1
votes
2
answers
130
views
Is there anything in quantum theory which permits to evaluate maximum time required for a measurement? [closed]
As far as I know, there is nothing in quantum theory or postulates that somehow places an upper bound on a measurement time. Since time is not really a physical observable in quantum theory, it is not ...
2
votes
1
answer
103
views
Are there really zero Cooper pairs in the ground state of the Cooper Pair Box?
Trying to get an understanding of superconducting charge qubits. The literature I've read seems to indicate that the energy levels of the qubit correspond to the number of Cooper pairs on the ...
- 151
1
vote
0
answers
60
views
I need help to understand strong symmetry Lindbad equation
I am in the process of constructing a Lindbad equation within strong symmetry regime for a two-level two qubit system.
$$
\dot\rho \;=\; -\frac{i}{\hbar}[H,\rho]
\;+\;\gamma\,\mathcal D_{\rm coll}[\...
- 11
0
votes
2
answers
147
views
What's the difference between manipulating qubits and the Stern-Gerlach experiment?
In quantum computing, you can send qubits through one-qubit or two-qubit quantum logic gates in order to "rotate their Bloch spheres" (so to say) or entangle them. This can be done without ...
- 603
0
votes
1
answer
159
views
A question in Lecture notes on Quantum Computation by John Preskill [closed]
I have been going through the 2nd chapter in the lecture notes on Quantum Computation by Prof. John Preskill. In section 2.2.1 on spin-1/2 particles, the author briefly discusses the theory of ...
-12
votes
2
answers
225
views
Warehouse of cats in boxes as a quantum computer? [closed]
Settings aside morality for a hypothetical question:
Wouldn't a warehouse filled with boxes, filled with cats, some sensors and deadly, radioactive isotope triggered contraptions, be a valid array of ...
- 77
2
votes
0
answers
63
views
Does the use of trusted components align with the strict definition of device-independent quantum key distribution?
I am a novice quantum cryptography enthusiast writing on the topic for a short dissertation, so please forgive me if I make any incorrect assumptions or ask ignorant questions.
I recently came across ...
- 29
-2
votes
2
answers
320
views
If quantum physics is fundamentally random then how was Microsoft able to line atoms precisely to make a chip? [closed]
Sparked by this video with Microsoft's quantum chip: https://www.youtube.com/shorts/9SBVZr3lbS0
The video mentions how to get the array to work each atom had to be lined up exactly and that it took 17 ...
- 141
2
votes
0
answers
89
views
Speed-up of Hamiltonian Simulations for Kitaev Chain
To my understanding, quantum computers offer value for simulating Hamiltonian systems by using methods such as Quantum Phase Estimation or Suzuki-Trotterization.
However, this is only the case at some ...
- 21
1
vote
1
answer
249
views
Could quantum computers still work if Einstein was right about hidden variables? [closed]
Could quantum computers still work if Einstein was right about hidden variables? To put it another way, did we need John Bell and Bell's theorem to prove that quantum computers are feasible?
- 209
3
votes
1
answer
411
views
CNOT gate appears to violate non-cloning theorem: is my understanding right?
I'm just starting to get into quantum computing.
We are discussing an apparent violation of the non-cloning-theorem by a CNOT:
If $y=|0\rangle$ it seems at a first glance that we could make a copy of ...
- 1,553
3
votes
1
answer
110
views
Operational meaning of spectral radius of quantum channels
It is known from Chap.6 of Wolf's lecture notes that CPTP maps or quantum channels have spectral radius 1. That is the absolute value of the eigenvalues is at most 1, corresponding to the asymptotic ...
- 205
-1
votes
1
answer
198
views
Is von Neumann entropy used in the design of any quantum algorithm or protocol?
von Neumann entropy is defined and discussed in just about every quantum information book. But how is it used in the design of quantum computing algorithms and key distribution protocols?
For ...
- 419
1
vote
0
answers
79
views
Discrepancy Between Stabilizer Formalism and State Vector Analysis in XY Measurements
When we have two zero qubits, we apply an X-axis $-\pi/2$ rotation to the control qubit to prepare it in the $|+i\rangle$ state. Then, we apply a CNOT gate:
$$\frac{1}{\sqrt{2}} (|00\rangle + i|11\...
- 11
-1
votes
2
answers
148
views
Isolated system, closed system, reversible, unitary, Landauer limit, how do they relate to noise in quantum computing?
Sections 2.2.1&2 of Nielson and Chuang's book list two postulates, one is about "any isolated physical system" and the other about "a closed quantum system." Isn't an isolated ...
- 419
0
votes
1
answer
92
views
Physical Geometric Implementation of Stabiliser Properties: Can Multi-Layer Containment Architectures Embody Stabiliser Generator Functions
Background:
Recent work exploring the role of geometric containment in quantum state preservation (Angelidi et al. 2024; Arab 2024) has led me to consider connections with stabiliser formalism. Of ...
- 1
1
vote
1
answer
210
views
Quantum computers: how are Qbits and gates wired together?
I think I don't have to be ashamed not to understand quantum computing, even after several attempts. So I try to break it down into small pieces.
What I (think to) understand, is that there is ...
- 1,212
-2
votes
3
answers
297
views
Why are qubits assumed fundamentally noiseless?
It seems that noise and error correction are afterthoughts of quantum information technologies. With other information technologies, we always consider them bounded by thermal noise. Why thermal noise ...
- 419
5
votes
2
answers
564
views
Can one check whether a qubit "equals" another with a single copy?
Is it possible to test whether a certain qubit ${\mid}\psi\rangle$ equals, say, ${\mid}\psi_0\rangle = a{\mid}0\rangle + b{\mid}1\rangle$; i.e., can one get a boolean out of ${\mid}\psi\rangle \...
- 1,415
1
vote
1
answer
89
views
How to find stabilizer generators of a subsystem from a known set of generators?
$\newcommand{\ket}[1]{\left|#1\right>}$
I am working on a problem and would appreciate some help. I'm working with a multi-qubit state defined by a set of stabilizer generators, ie $\ket{\psi} \in ...
- 55
1
vote
2
answers
283
views
Go a step further and get the both results of $f(x)$ in Deutsch algorithm
The Deutsch-Josza algorithm gives the result in one operation of the type of a $(0,1) \mapsto (0,1)$ function $f(x)$, i.e. if $f(x)$, is constant or balanced. With the parallel computing power of a ...
- 419
2
votes
0
answers
67
views
Can a quantum computer simulate a classical field theory efficiently?
I know there’s a large speed-up in simulating quantum field theories, but was wondering if quantum computers are able to simulate a classical field theory as efficiently as they can simulate a quantum ...
0
votes
1
answer
125
views
Should one learn the math of quantum computing from a "pure" perspective, or is quantum computing texts "good enough"? [closed]
Math for quantum computing includes advanced linear algebra, functional analysis, group and representation theory, probability theory, and more.
There are plenty of pure math books out there for those,...
0
votes
0
answers
69
views
Can a universe-sized computer predict the universe? [duplicate]
I'm not asking about a simulation and so I don't see this as a duplicate, and I'm not a physicist or anything like that, I have some physics-related questions though.
Current models suggest local ...
-1
votes
1
answer
95
views
Relationship between coupling Strength in Cross-Resonance Gates and Coefficients in Hamiltonian Tomography
In the effective Hamiltonian for the cross-resonance (CR) gate, the interaction term is written as:
$$
\tilde{H}_{\rm eff}^{\rm CR} = - \frac{\Delta_{12}}{2}\sigma_1^z
+ \frac{\Omega(t)}{2} \left(\...
3
votes
0
answers
158
views
How to transform collapse operators in rotating frame?
I want to simulate the evolution of a transmon qubit driven by a classical voltage using qutip. The problem is that the qubit (and the drive) have a frequency of around 5GHz and I typically run ...
- 1,150
0
votes
0
answers
106
views
Are there quantum computing algorithms specialized for machine learning applications?
I've heard that some companies are attempting to utilize quantum computing algorithms for machine learning tasks (ex:protein folding and drug discovery). As far as I know, quantum computing has been ...
6
votes
1
answer
324
views
Is useful Shor factorization really practically feasible?
INTRO TO THE ISSUE. After studying this beautiful math concept many years ago, I recently revisited Shor's algorithm for factoring integers and its implications for RSA encryption. This time, I needed ...
- 601
0
votes
0
answers
79
views
How does Quantum Gate Teleportation Actually Work?
I am trying to understand how quantum GATE teleportation works. Abstractly, I understand it as the ability to do a 2-qubit operation between two qubits that are not located in proximity.
From ...
0
votes
0
answers
67
views
Why the parameters of quantum convolutional neural network (QCNN) is $O(\log N)$ for a $N$-qubits input?
I am currently reading the paper arXiv:1810.03787. The authors claim that QCNN uses only $O(\log N)$ variational parameters, where $N$ is the number of qubits. However, I am having difficulty ...
- 26
1
vote
0
answers
269
views
A variant of Campbell (1897) identity (related to BCH formula)
Campbell's identity (see in https://en.wikipedia.org/wiki/Baker%E2%80%93Campbell%E2%80%93Hausdorff_formula) takes the form:
$$ e^XYe^{-X}\equiv ad_{e^X}Y=e^{ad_X}Y\equiv \sum_{n=0}^\infty\frac{[(X)^n,...
- 11
2
votes
1
answer
137
views
How to Construct Arbitrary Rotations from Discrete Rational Rotations?
One of the most commonly used universal quantum gates sets is the Clifford + T set, containing {CNOT, H, S, T}. However, as S is a $\pi / 2$ rotation about the Z axis of the bloch sphere and as T is a ...
1
vote
0
answers
208
views
Using classical $g^{(1)}$ to calculate quantum $g^{(1)}$
Imagine a free space optical switch with several inputs and outputs. My question is: can we use the first order cross correlation between the complex amplitudes of two classical beams of light at the ...
- 11
0
votes
1
answer
138
views
Conjugacy of $x$ and $p$ for simulation of Schrodinger's equation
In Nielsen-Chuang, page 207, there is this box:
I have some strong doubts about the last passage. The logic they follow is that since $H_0$ is diagonal in momentum basis but not in position basis, ...
- 29
2
votes
1
answer
343
views
Edge Modes and SPT Order
I am trying to understand the notion of edge modes in the context of symmetry protected topological order (SPTO) and its relation (if there is any) with the virtual quantum register that sits at the ...
- 558
4
votes
3
answers
419
views
Why isn't spin-statistics applied in quantum computing?
Why isn't spin-statistics taken into account in quantum computing?
By spin statistics I mean the fact that fermion and boson states must by respectively totally anti-symmetric and symmetric. I say it'...
- 864
5
votes
1
answer
213
views
Questions on Matrix Product States (Tensor Networks)
I just have a few questions on Matrix Product States. I have learnt them from the point of view of performing cuts via Schmidt Decompositions, as seen in Chubb and Bridgeman 2017.
Is the bond ...
- 51
2
votes
2
answers
128
views
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 ...
- 419
1
vote
0
answers
69
views
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 ...
- 3,123
0
votes
1
answer
140
views
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 ...
- 183
1
vote
0
answers
85
views
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 ...
- 419
-2
votes
1
answer
83
views
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 ...
0
votes
1
answer
117
views
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 ...
- 75