# Tag Info

### What problems in chemistry or materials science could be solved with 100 fault-tolerant qubits?

With $N$ fault-tolerant qubits we can find the ground state energy of a system of electrons, with $N$ spin orbitals in the basis set that is used to represent the electronic wavefunction. This means ...

### How to prepare an initial state for variational quantum algorithms?

State preparation in general is a common task for beginning quantum algorithms. Most generically, you can prepare your state with Classiq like this. Classiq Function Library in Github ...

### Does subtraction circuit have a class like full or half Carry adder circuit in qiskit? one subtraction circuit code-example in qiskit?

For subtraction as mentioned earlier you have to negate the number, and than use an adder. There are few possible implementations for that (adder can be with QFT or with ripple-carry adder with more ...
• 1,482

### Is it possible to implement an in-place multiplication quantum circuit?

I have modified a quantum-quantum multiplication operator to do what you're looking for. ...
Accepted

### Hamiltonian Simulation: What's the meaning of t in $\exp(iAt)$?

If your eigenvalues of $A$ are $\lambda_n$, then the eigenvalues of $e^{iAt}$ are $e^{i\lambda_n t}$. These values are periodic in $2\pi$, which could lead to some ambiguity in correspondence between ...
• 59.3k
Accepted

### Can non-linear operations be implemented as a circuit on a quantum computer?

If you mean any kind of circuit, or more general quantum operation, that takes as input some $\rho$, and produces output probabilities equal to $\operatorname{tr}(\rho_A^2)$, or more generally a ...
• 25.6k
Accepted

### General number of classical bits: recycles qubits

You can use the unpacking operator: qc = QuantumCircuit(m, w, *[cl_reg[i] for i in range(M)])
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### What is the point of building arithmetic circuits in a quantum computer?

Yes, it is practical! For Grover algorithm - check the following example For risk analysis in finance - once you want to prepare some variables representing some payoff in the market. Check those ...
• 1,482
1 vote
Accepted

### What is a 2way quantum computer (2WQC)?

In my view, the whole concept of 2WQC is built on a misconception of the CPT symmetry of physics. Yes, the laws of nature as we know them are invariant under the combined transformation of charge ...
• 465
1 vote
Accepted

### Problem with understanding advantage of QAOA over bruteforce

You are not iterating over all bit strings, only the ones that have appeared as outcomes to measuring the circuits. The number of bit strings you iterate over is therefore bounded above by the number ...
• 26
1 vote

### Can non-linear operations be implemented as a circuit on a quantum computer?

If you have two copies of $|\psi\rangle$, you simply perform the swap test on the "A" components.
• 59.3k
1 vote

### Existence of FHE scheme that supports evaluation of obfuscated Quantum Functionalities

The interpretation looks correct. A lot of the details for Mahadev's scheme are trivial and skipped over quite quickly and the difficult parts are mainly covered in the paper. The universal $U$ is ...
• 101
1 vote
Accepted

### Verifying block encoding by computing inner products

You can use Hadamard test for this task. Assume that you want to estimate the value of $A_{ij}$, then you can swap $i$ and $j$ columns then use Hadamard test to estimate $\langle j| U_A |j \rangle$. ...
• 10.9k
1 vote
Accepted

### Unable to extract the quantum registers information from qiskit quantum circuit data

You can use QuantumCircuit.find_bit() method: ...
• 10.9k
1 vote
Accepted

### What is the "equivalent" quantum computational complexity class of the classical class APX (or PTAS)?

Marked CW because I'm probably speaking with improper authority. The classes FPTAS, PTAS, and APX are defined with respect to a classical polynomial-time algorithm to give the appropriate ...
1 vote

### What is the point of building arithmetic circuits in a quantum computer?

Generally speaking, you're right that you want to do as much of the arithmetic as you can on classical computers. But in a quantum algorithm, many of the values you need to do arithmetic to will be ...
• 38.6k

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