Questions tagged [gate-synthesis]
For questions about finding (short) gate sequences to implement a specific unitary operation, for example decomposing a complicated multi-qubit gate into a sequence of basic gates. It might apply to optimizing circuits with respect to length or depth or finding gate sequences to implement an algorithm.
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questions
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15 views
Help with understanding Unitary Operator of Quantum Optical Fredkin Gate
I'm referring to the textbook "Quantum Computation and Quantum Information" 10th Anniversary Edition by Nielsen and Chuang. Chapter 7.4 has a Box 7.4 which introduces the Quantum Optical Fredkin Gate....
1
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0answers
36 views
How to implement controlled u3 gate from Qiskit using simpler gates?
I am trying to implement the u3 controlled gate (able to rotate the qubit in any specified direction in 3 dimensions if the control is 1, for two qubits) using simpler gates. The simpler gates ...
3
votes
1answer
38 views
control gate with 3 inputs, two control and rotation gate
My question is about if there is any way to represent a circuit that take 3 inputs and applies a rotation gate on the third qubit if the first two qubits is similar (has the same state)?
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1answer
31 views
Efficient implementation of exponential of projector
If I have an $n$ qubit system and a projector $P$ such as $P_0 = \left|0\right>^{\otimes n}\left<0\right|^{\otimes n}$ (as an example) on those qubits, is there an efficient way to implement the ...
7
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1answer
63 views
Sampling random circuits vs Solovay-Kitaev compiler
Suppose I want to obtain a gate sequence representing a particular 1 qubit unitary matrix.
The gate set is represented by a discrete universal set, e.g. Clifford+T gates or $\{T,H\}$ gates.
A well ...
3
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2answers
70 views
Calculate the square root of Euler angles
I am trying to find a nice way to represent the square root of an arbitrary single qubit unitary to implement Lemma 6.1 from this paper
Given the Euler angles: $R_z(a)R_y(b)R_z(c) = \left(R_z\left(a'\...
1
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1answer
39 views
Approximating unitary matrices — restricted gateset
Note: This question is a follow up of Approximating unitary matrices.
The decompositions provided in Approximating unitary matrices are correct and worked for me without problem.
But I am now facing ...
2
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1answer
59 views
How to create an $n$-qubit normally controlled gate?
Suppose I have a quantum gate $U$ and it's a controlled gate. In particular, I have a $2\times 2$ matrix formulation of the gate's action on 2 adjacent qubits.
How can I make this work on an $n$-bit ...
2
votes
1answer
64 views
Using Quantum Fourier Transform in adding two 2-bit numbers
I am trying to use Qiskit to write a code that uses QFT to add 2 numbers. I am referring to this paper:
https://iopscience.iop.org/article/10.1088/1742-6596/735/1/012083
I have a few questions:
1) Is ...
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0answers
22 views
Controlled controlled adder gates involved
Let's say I have a circuit that given in the figure
As we can see that this circuit consists of $2$-Toffoli gates and $4$ C-NOT gates, and to construct this entire circuit using only single qubit ...
6
votes
1answer
77 views
How to decompose a controlled unitary $C(U)$ operation where $U$ is a 2-qubit gate?
In the vein of this question, say I have a 2-qubit unitary gate $U$ which can be represented as a finite sequence of (say) single qubit gates, CNOTs, SWAPs, cXs, cYs and cZs. Now I need to implement a ...
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2answers
152 views
Transform matrix into a combination of simple quantum gates
I am trying to transform this matrix into a combination of quantum gates but I cannot find any such functionality on Qiskit or anywhere else. I have tried to use Quirk but I do not understand it.
$$\...
5
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1answer
125 views
Nielsen and Chuang's proof for 'approximating arbitrary unitary gates is generically hard'
The following statement is found on the page 199 of Nielsen and Chuang's book (10th Anniversary Edition) in the proof for the fact that 'approximating arbitrary unitary gates is generically hard':
...
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59 views
Transferring classical OR gate in a quantum gate
I would be interested to know how to transform the classic OR gate into a quantum gate. I thought a little about myself. The OR gate can also be rewritten as a NAND gate:
So, I have now tried to ...
3
votes
2answers
96 views
Minimal quantum OR circuit
The quantum OR circuit between $|a\rangle$ and $|b\rangle$ can be made out of 1 Toffoli and 2 CNOT gates, 1 ancillary qubit. Is there any other implementation? Or is this the minimal in the sense of ...
2
votes
1answer
40 views
Making a maximally mixed 2-qubit state in the IBM Q
I am trying to make a 2-qubit maximally mixed state $\mathbb{I}/4$ where $\mathbb{I}$ is the identity $4\times 4$ matrix.
I know that, for a maximally mixed 1-qubit state I can use a Hadamard gate, ...
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2answers
324 views
How is it possible to implement unitary operator when its size is exponential in inputs?
A quantum circuit can use any unitary operator. Its matrix is exponential in the number of input bits. In practice how can this ever be possible (aside from operators which are tensor products), i.e. ...
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1answer
88 views
Implementing these $N×N$ matrices on $\log N$ qubits
Consider $n$ qubits and the $N=2^n$ states that I label
\begin{equation}
|k \rangle = \sum_{i=0}^{n-1} 2^i q_i,
\end{equation}
i.e. $|q_{n-1}\cdots q_0 \rangle \rightarrow |k\rangle$, where $q_j \in \...
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0answers
29 views
How are multi-qubit gates extended into larger registers? [duplicate]
Implementing a single-qubit gate in a multi-qubit register is relatively easy. For example, this gate:
This is equivalent to $I \otimes H \otimes I$. If the $H$ gate was on the first bit, it would be ...
5
votes
2answers
155 views
Implement Fredkin gate with square root of swap
I would like to implement a Fredkin gate based on square root of swap and one-qubit gates. In particular, I was hoping to find the exact gate named "?" in this circuit:
In addition, I want to avoid ...
4
votes
1answer
74 views
Rewrite circuit with measurements with unitaries
In quantum physics, because of the no-cloning theorem, lots of classical proofs of cryptographic problems cannot be turned into quantum proofs (rewinding is usually not possible quantumly). A dream ...
2
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1answer
64 views
Optimise implementation of a quantum algorithm when an input is fixed
I need to implement a quantum comparator that, given a quantum register $a$ and a real number $b$ known at generation time (i.e. when the quantum circuit is generated), set a qubit $r$ to the boolean ...
2
votes
1answer
146 views
Building an N-qubit Controlled S Gate
I've been beating my head against this problem for three days now and I just can't seem to crack it. To construct an N-qubit controlled Unitary gate, I can do something like this (note I'm using ...
2
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1answer
92 views
If CNOTs and single qubit gates are universal then why do we need to prove that controlled U operations can be composed by them as well?
In the book by Chuang and Nielsen they prove that controlled U operations can be made out of CNOTs and single qubit gates. But then they go on to prove that they are universal by showing that every n ...
2
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1answer
326 views
Decompose a general two-qubit gate into general controlled-qubit gates
We often seek to decompose multi-qubit unitaries into single-qubit rotations and controlled-rotations, minimising the latter or restricting to gates like CNOTs.
I'm interested in expressing a general ...
2
votes
1answer
104 views
How to implement a $\frac{\theta}{2}$ rotation from $\theta$ rotation?
Is there a way to create a rotation gate which has half the angle of some implementable gate?
I am looking to implement a gate on Quirk which allows for standard time-dependent rotations
$$R_x(\...
3
votes
0answers
95 views
Decomposition of any 2-level matrix into single qubit and CNOT gates
I saw an example which takes a 2 level matrix. Which is a $8\times8$ matrix that acts non trivially only on 2 levels of only states $|000\rangle$ and $|111\rangle$. The way they do it is by using a ...
4
votes
2answers
509 views
Composing the CNOT gate as a tensor product of two level matrices
I don't understand, why is the control not gate used so often. As far as I understand it, if you apply two 2 level operations on two qubits then you get a 4 x 4 matrix by the tensor product. So how ...
1
vote
1answer
82 views
Proof that $2^n \times 2^n$ operator be decomposed in terms of $2 \times 2$ operators
What is the proof that any $2^n\times 2^n$ quantum operator can be expressed in terms of the tensor product of $n$ number of $2\times 2$ quantum operators acting on a single qubit space each?
5
votes
3answers
515 views
Is there any method of adding two operators in a circuit?
I am trying to reconstruct the time evolution of a Hamiltonian on the quantum computing simulator, quirk. Ideally I would like to generalise this to any simulator. The unitary matrix is
$$U(t)=e^{-...
4
votes
3answers
103 views
Hadamard gate as a product of $R_x$, $R_z$ and a phase
I am having problems with this task.
Since the Hadamard gate rotates a state $180°$ about the $\hat{n} = \frac{\hat{x} + \hat{z}}{\sqrt{2}}$ axis, I imagine the solution can be found the following ...
1
vote
1answer
66 views
Creating a time dependent custom gate in Quirk
I have created a $16\times 16$ unitary operator using a Hamiltonian by finding its exponential
$$U=\exp(-iH\delta t)$$
If I set $\delta t=1$ then I can take this matrix and input it into quirk using ...
1
vote
1answer
122 views
Gate sequence for exponential of product of Pauli Z operators
I want to compile $$\exp(-i \theta \sigma_i^z \sigma_j^z)$$ down to a gate sequence of single qubit rotations and CNOTs. How do I do this? What is the general procedure for compiling a unitary $U$ to ...
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3answers
638 views
Approximating unitary matrices
I currently have 2 unitary matrices that I want to approximate to a good precision with the fewer quantum gates possible.
In my case the two matrices are:
$$G = \frac{-1}{\sqrt{2}}\begin{pmatrix} i &...
5
votes
4answers
648 views
How do I build a gate from a matrix on Qiskit?
I'm creating a gate for a project and need to test if it has the same results as the original circuit in a simulator, how do I build this gate on Qiskit? It's a 3 qubit gate, 8x8 matrix:
$$
\frac{1}{...
4
votes
1answer
153 views
Construction of ${R_n(\theta)}$ using only the Hadamard and ${\pi/8}$ gates
In the "Quantum Computation and Quantum Information 10th Anniversary textbook by Nielsen & Chuang", they claim that Eqn(4.75) is a rotation about the axis along the direction
( ${cos(\pi/8)}$, ${...
3
votes
2answers
198 views
Implementing gate with two parameters using Qiskit in Python
I am trying to implement the HHL algorithm (for solving $Ax=b$). I am assuming $A$ to be unitary and Hermitian so that I can find the Hamiltonian simulation for it easily.
For any $A$ to be Hermitian ...
6
votes
2answers
407 views
How to obtain Y rotation with only X and Z rotations gates?
Let's say you have a system with which you can perform arbitrary rotations around the X and Z axis. How would you then be able to use these rotations to obtain an arbitrary rotation around the Y axis?
...
5
votes
2answers
290 views
Measuring the Hamiltonian in the VQE
I am trying to implement VQE in pyQuil and am dumbfounded by how to measure the expectation value of a general Hamiltonian on $\mathbb{C}^{2^n}$ i.e. determine $\langle\psi , H \psi\rangle$ on a ...
5
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2answers
92 views
Clarification needed: “Simulation” of $e^{-iHt}$ and its time complexity
On page 3 here it is mentioned that:
However, building on prior works [32, 36, 38] recently it has been
shown in [39] that to simulate $e^{−iHt}$ for an $s$-sparse Hamiltonian
requires only $\...
3
votes
0answers
70 views
Circuit to construct a $n$-qubit state which is a superposition of states with only a single qubit being $\lvert1\rangle$ [duplicate]
So the question came up in a book I am working through. Given a circuit with $n$ qubits, construct a state with only $n$ possible measurement results, each of which has only $1$ of $n$ qubits as $1$, ...
2
votes
1answer
132 views
How to construct a quantum gate producing 1 if r divides x, 0 otherwise?
If you have two registers in the state $\frac{1}{2^{n/2}} \sum_{x = 0}^{2^{n/2} - 1} |x\rangle |0\rangle$, how could you construct a gate that produces a superposition of states $|x\rangle|1\rangle$ ...
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2answers
105 views
How does the stated Pauli decomposition for $\operatorname{CP\cdot A\cdot CP}$ arise?
I'm having a bit of trouble understand @DaftWullie's answer here.
I understood that the $4\times 4$ matrix $A$
$$ \frac{1}{4} \left[\begin{matrix}
15 & 9 & 5 & -3 \\
9 & 15 & 3 &...
6
votes
2answers
368 views
Simulation vs Construction of Fredkin gate with Toffoli gates
I'm working my way through the book "Quantum computation and quantum information" by Nielsen and Chuang. (EDIT: the 10th anniversary edition).
On chapter 3 (talking about reversibility of the ...
7
votes
2answers
165 views
Is there a general method to implement a 'greater than' quantum circuit?
I am interesting in finding a circuit to implement the operation $f(x) > y$ for an arbitrary value of $y$. Below is the circuit I would like to build:
I use the first three qubits to encode $|x⟩$, ...
2
votes
1answer
122 views
What quantum gate is XNOR equivalent to?
The standard way to implement a reversible XOR gate is by means of a controlled-NOT gate or CNOT; this is the "standard quantum XOR operation". Physics.Stackexchange
Is there a "standard quantum XNOR ...
5
votes
1answer
107 views
How can I decompose a gate into $\{\mathrm{CNOT}, \mathrm{H}, \mathrm{P}(\theta)\}$?
I am working with the set $\{\mathrm{CNOT}, \mathrm{H}, \mathrm{P}(\theta)\}$
where $\mathrm{H}$ is the Hadamard gate, and $\mathrm{P}(\theta)$ is the phase gate with angle $\theta$.
I want to build ...
11
votes
3answers
649 views
How to derive the CNOT matrix for a 3-qbit system where the control & target qbits are not adjacent?
In a three-qbit system, it's easy to derive the CNOT operator when the control & target qbits are adjacent in significance - you just tensor the 2-bit CNOT operator with the identity matrix in the ...
5
votes
1answer
355 views
Decomposition of an arbitrary 1-qubit gate into a specific gateset
Any 1-qubit special gate can be decomposed into a sequence of rotation gates ($R_z$, $R_y$ and $R_z$). This allows us to have the general 1-qubit special gate in matrix form:
$$
U\left(\theta,\phi,\...
7
votes
1answer
464 views
Decomposition of arbitrary 2 qubit operator
As you know, universal quantum computing is the ability to construct a circuit from a finite set of operations that can approximate to arbitrary accuracy any unitary operation.
There also exist some ...
