Questions tagged [quantum-state]
Questions about or related to quantum states. Consider using the density-matrix tag when relevant.
1,382
questions
1
vote
1
answer
119
views
What is the value of a control qubit as a Controlled-X Gate or Tofolli Gate parameter?
For example in a Controlled-X or Tofolli gate, qubits can be assigned as control bits. But before the measurement ; what are the values of these qubits in the gate tensor since they can not be 1 or 0 ?...
- 33
1
vote
1
answer
100
views
Quantum gate count minimization
In classical digital logic, we have K-map and Quine–McCluskey algorithm for minimizing boolean equation, thereby reducing the number of logic gates needed. Is there such a thing in quantum computing?.
...
- 33
2
votes
0
answers
95
views
Add a negative phase to selected states
I have $n$ qubits in superposition. I want to add a negative phase to all states with more than $m$ $1$s, e.g. for $n=3$ qubits and $m=1$ I want to get:
$\frac{1}{\sqrt{N}}(|000\rangle + |001\rangle + ...
- 21
0
votes
2
answers
68
views
Why after breaking the entanglement does the phase disk in the second qbit not match the result?
I am new in IBM composer quantum and I have a doubt.
I have the next circuit
After reinitializing q0 and reading it. (breaking entanglement) The phase disk for qubit q1 says that 100% is 0, but the ...
0
votes
3
answers
210
views
What is size of the state space?
I'm taking my first Quantum Computing MOOC and the image below is from one of the very first slides:
My question is, what is the state space?
I've a guess that it is the set of all possible results ...
- 101
1
vote
0
answers
110
views
Qutip: Mesolve gives different and weird results with different fock state numbers
I have been trying to simulate the average number of particles at 3 sites of coupled harmonic oscillators.
I have used the code from the below tutorial:
https://notebook.community/ajgpitch/qutip-...
- 11
0
votes
1
answer
48
views
In a state $\alpha \vert HL \rangle + \beta \vert VR \rangle$ can a particle be measured in the $\{|L\rangle,|R\rangle\}$ basis?
Given a pair of entangled particles:
$\alpha \vert HL \rangle + \beta \vert VR \rangle$
where:
$\alpha^2 + \beta^2=1$
$H$ and $V$ are orthonormal vectors.
$L$ and $R$ are orthogonal vectors (but not ...
- 135
0
votes
1
answer
177
views
What is the solution to Nielsen and Chuang Exercise 2.65?
Nielsen&Chuang Exercise2.65: Express the states (|0 + |1)/
√
2 and (|0−|1)/
√
2 in a basis in
which they are not the same up to a relative phase shift.
Consider an orthnormal basis :$\begin{cases}...
- 3
0
votes
2
answers
58
views
It is possible to obtain amplitudes from superposition with certain probability?
In many papers and books on quantum theory it is written that it is not possible to obtain amplitudes exactly when we have a qubit in superposition. In a course we said that actually it is possible, ...
1
vote
3
answers
207
views
Is there any way to always output |1> if the measurement has a probability to output 1?
Let's say I have an oracle that outputs 1 on a specific input (there can be multiple inputs x so that f(x)=1), else it outputs 0.
If there is no such input so that f(x)=1, the quantum state of the ...
2
votes
1
answer
83
views
Set relative phase of qubit to zero without measurement?
Is there a quantum way to set the unknown relative phase of a qubit (assumed in a pure state) to zero, without measurement? The relative phase is not known, otherwise I would subtract it using a phase ...
- 55
2
votes
1
answer
109
views
A doubt regarding Mach-Zehnder Interferometer functioning
I am tying to understand the working of Mach-Zehnder Interferometer in the physical sense (how it behaves and why it does so etc.). The source I am referring to is this: (https://www.cs.princeton.edu/...
- 135
1
vote
1
answer
66
views
A doubt regarding probabilities of a entangled states?
Given an state of entangled particle pair in Horizontal-Vertical basis ($H,V$):
$ \psi_{hybrid}= (1/\sqrt2) (HH+VV)$
Let us define a new Right-Left basis ($R,L$) as follows:
$L=(1/\sqrt2)(H-iV)$
$R=(...
- 135
2
votes
1
answer
124
views
Why are qubits represent as unit vectors in an Hilbert space?
Mathematically, our qubits (pure quantum states) can be represented as a normalized vector (of length 1) in a complex Hilbert space.
is it because of the sum of squares of vertical and horizontal ...
- 439
3
votes
1
answer
301
views
How to calculate the average fidelity of Pauli error channel
This question is related to this and this.
I am working on Qiskit to design QEC schemes.
My model works with Pauli errors.
I would like to give to my Pauli error channel probabilities $p_x,p_y,p_z$ ...
- 1,300
1
vote
0
answers
47
views
Grover Diffuser for states with amplitude value 0?
Is ist possible to do the grover amplification while excluding states with amplitude values of zero?
I was testing this with the classic grover and two diffrent superpositions.
In the first I used the ...
- 11
1
vote
1
answer
283
views
Append a controlled Initialize in Qiskit
I would like to create a controlled initialize, I have the following code:
...
3
votes
1
answer
56
views
Implementing quantum swap operation such that $|x\rangle|f(x)\rangle$ changes to $|x\rangle|f(x+1)\rangle$
Suppose that a quantum state $|\psi\rangle$ in question is $|\psi\rangle \propto \sum_x |x\rangle|f(x)\rangle$.
I want to implement a quantum swap operation such that computational basis $|x\rangle|f(...
- 33
1
vote
2
answers
151
views
Why can two atoms can be entangled?
As a qubit is a two-state (or two-level) quantum-mechanical system, one of the simplest quantum systems displaying the peculiarity of quantum mechanics. Examples include the spin of the electron in ...
- 439
6
votes
3
answers
2k
views
Is effective quantum cloning possible, given that any classical function can be implemented as a quantum circuit?
As in Compiling a classical function to a quantum circuit in practice, as far as my understanding goes, it is known that any classical function can be implemented as a quantum circuit. So given $f(x)=...
- 407
3
votes
2
answers
409
views
Can EPR pairs be established with only classical communication?
EPR pairs are used in superdense coding and quantum teleportation, but these protocols assume that Alice & Bob "share" half of an entangled quantum state.
How does this "sharing&...
2
votes
0
answers
69
views
A question on random quantum states and the uniform distribution
Consider an $n$ qubit Haar random quantum state $|\psi\rangle$. Consider a distribution $\mathcal{D}_1$ over $n$ bit strings defined as
$$
p_x = |\langle x| \psi \rangle|^{2},
$$
for $x \in \{0, 1\}^{...
- 315
1
vote
2
answers
122
views
How to initialize a $n$ qubit system in this specific state
Basically, I have $2n$ qubits and I want to initialize them in the state $\frac{|\psi\rangle}{\lVert |\psi\rangle \rVert}$ where
$$|\psi \rangle = \sum_{\mathrm{w}\in \{0,1\}^n}|\mathrm{w}\mathrm{w}\...
2
votes
0
answers
62
views
Qubit Resource Estimates for 2D Ising Model
I recently came across this paper where the 1d Transverse Field Ising Model (TFIM) with $n$ spins was simulated on a quantum computer. The estimated resources were $n^2$ for the number of gates and ...
- 121
1
vote
1
answer
170
views
qml.probs with another computational basis state
I am working with pennyLane library in Python and I have a circuit that involves several operations to 10 qubits. I want to measure the occupation probability of a target state.
The function qml.probs(...
0
votes
0
answers
102
views
Gradient registry has no entry for: TfqSimulateState
I am trying to calculate and optimize a loss with TFQ and cirq. Using tfq.layers.State()
to work with statevector instead of its observables. But when I try to fit o get its gradient the following ...
- 41
0
votes
1
answer
97
views
1
vote
2
answers
104
views
Basis for permutation invariant states
It is known that the maximally entangled qubit states form a basis (the Bell basis). Let $\Phi$ be the canonical maximally entangled state i.e.
$$\Phi = \left(\frac{\vert 00\rangle + \vert 11\rangle}{\...
- 2,367
0
votes
0
answers
51
views
Performing QN-SPSA on hybrid quantum-classical neural networks
I had a question on the TorchConnector module in qiksit_machine_learning (based on this tutorial). If I'm not wrong, is ...
- 61
3
votes
2
answers
101
views
When we say that 4 qubits can represent $2^4$ binary bit sequences, how do we iterate to the desired bit sequence?
Being new to QC, always heard that $n$ qubits can represent $2^n$ unique combinations of equivalent binary bits at the same time. We can also say that if we have $n$ data lines, we can pass $2^n$ ...
- 133
2
votes
0
answers
51
views
Alternative measurements cost
Most of QKD protocols depends on performing measurements on different basis:
Some performs the measurements on two basis (standard basis (Z) and the Hadamard basis (X)).
some performs the ...
- 21
0
votes
1
answer
211
views
How to get statevector of qubits after running quantum circuits on IBMQ real hardware to calculate the fidelity of all qubits individually?
Is it possible to get the statevector while running quantum circuits on IBMQ real hardware? If getting the statevector is not possible can I still calculate the fidelity of a qubit somehow?. What I ...
- 33
2
votes
2
answers
355
views
What is the actual Hilbert space of a $N$-qubit system?
This question seems slightly naive. The Hilbert pace of any 2-level quantum system is given by the Bloch sphere and the algebra of observables arises from $SU(2)$, the Lie group generated by the three ...
- 565
0
votes
1
answer
213
views
How does a Hadamard gate impact the initial/previous values of a Qubit? [closed]
I've been studying Quantum Computing and one thing that intrigued me is:
given a qubit q1 with an initial value x, when I apply a Hadamard gate on it, then it goes to superposition, so the probability ...
- 189
1
vote
1
answer
78
views
Cirq: Getting The Instances (samples) of a Quantum Circuit with Probabilistic Unitaries or Mixtures
The wavefunction simulator in Cirq uses a Monte Carlo approach to simulate a certain subset of quantum noise channels, namely through probabilistic/stochastic application of unitary gates. These are ...
- 11
0
votes
1
answer
183
views
How to find initial quantum states from the density matrix?
Recently, I came across density matrix. Given a qubit $|\psi \rangle = \alpha |0\rangle + \beta |1\rangle$, we can find its density matrix by computing $\rho \equiv |\psi \rangle \langle \psi |$.
My ...
- 341
3
votes
1
answer
110
views
Can a polynomial-sized superposition of computational basis states be prepared with a polynomial-sized quantum circuit?
Suppose I am working with a class of states which consist of a superposition of $O(\text{poly}(N))$ computational basis states on $N$ qubits. Examples of this would be the subspace of states of fixed ...
- 411
4
votes
1
answer
99
views
In what contexts are different notations used for indicating measurement outcomes?
I have seen a few different notations for denoting measurement outcomes. Does anyone know of which notation is more widely used in various contexts?
For instance, I like referring to this Wikipedia ...
- 1,331
0
votes
1
answer
58
views
Is there a quantum gate that can turn any superposition $|\varphi \rangle $ into a unit column vector $|00\cdots 1\cdots 0\rangle $
that is ${{U}_{f}}|\varphi \rangle =|00\cdots 1\cdots 00\rangle $,where $|00\cdots 1\cdots 0\rangle $ is a random unit vector
eg.$${{U}_{f}}(\sqrt{\frac{1}{3}}|0\rangle +\sqrt{\frac{1}{3}}|1\rangle +\...
- 390
3
votes
1
answer
251
views
Visualize representation of a qubit
I am new to quantum computing and I am learning about qubits.
I recently learnt a qubit can be represented by $\alpha |0\rangle + \beta|1\rangle$ where $\alpha, \beta \in \mathbb{C}$ and $|\alpha|^2 + ...
- 341
1
vote
2
answers
163
views
How to write a code to separate two qubits?
Is it possible to write a code to separate $\left|01\right> $ as $\left|0\right> ,\left|1\right> $ or are there any circuits i can use to get output like that?
- 67
3
votes
1
answer
125
views
Is there a gate sending $|0\rangle^{\otimes n}$ to a state where some amplitudes are zero?
By preparing $n$ qubits in $|0 \rangle ^{\otimes n}$ and then passing through the Hadamard gate, one can obtain a system superposition in $\{0,1 \}^n$, each state with equal probability.
I’m wondering ...
- 143
0
votes
2
answers
176
views
What is the maximally entangled state in Choi matrix?
For a quantum channel $\mathcal{E}$, Choi matrix is defined as follows:
$C(\mathcal{E}) = (\mathcal{E} \otimes I) (\sum_{i}\lvert ii\rangle \sum_j \langle jj\lvert) $, where $\sum_{i}\lvert ii\rangle$ ...
- 455
6
votes
1
answer
297
views
Is 216 qumodes photonic quantum processor equivalent to 216 qubits superconducting quantum processor, in terms of computational power?
Xanadu just launched borealis, 216 qumodes photonic quantum computer, this week.
https://xanadu.ai/blog/beating-classical-computers-with-Borealis
Its number of qubits is very interesting because it ...
0
votes
1
answer
746
views
How to tell if a state is entangled?
I've read that to tell if a 2 qubit state is not entangled e.g. $$αγ|00⟩+αλ|01⟩+βγ|10⟩+βλ|11⟩$$ then multiplying the coefficients of $|00\rangle$ and $|11\rangle$ should equal the same as multiplying ...
1
vote
1
answer
354
views
Can someone explain to me what is T gate and tdg like im five?
I'm newbie to quantum computing, i read about these two gates on IBM quantum computing docs, but I can't seem to understand what is t-gate and tdg or t-dagger, can someone give me a simple explanation ...
- 193
3
votes
2
answers
209
views
what does it mean to "measure a qubit" if measurement is defined on operators
As far as I know "measurement" depends on the two variables : state and operator.
So what exactly does measuring a qubit mean? is it implied that the operator is
the $Z$ operator on that ...
- 1,742
0
votes
1
answer
52
views
Difficulty of creating arbitrary amplitude-encoded quantum states
Let $b \in \mathbb{C}^N$ be some vector, which we want to encode as a quantum state $\sum_{j=0}^{N-1} b_j |j \rangle$.
The problem of achieving this encoding seems to be difficult in many cases, ...
- 275
2
votes
1
answer
614
views
What is the difference between "maximally entangled" and "entangled" states?
when we talk about bell state we say that these states are maximally entangled. so just wanted to understand is there any difference between just entangled and maximally entangled ?
- 439
0
votes
2
answers
52
views
When starting from Qubit State |0>, which operations when applied (from right to left) will result in the qubit state |->
I found this MCQ in a book that my friend gave me. Not entirely sure about this question.
When starting from Qubit State $|0\rangle$, which operations when applied (from right to left) will result in ...
- 43