# All Questions

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### Why am I getting this error- "ValueError: x and y must be the same size"? If anyone knows kindly give any hint or clue to rectify it

def get_qc_for_n_qubit_GHZ_state(n): qc = QuantumCircuit(n) qc.h(0) for i in range(n-1): qc.cx(i, i+1) return qc n = 100 qc = get_qc_for_n_qubit_GHZ_state(n) qc.draw(output = 'mpl') from qiskit....
1 vote
7 views

### Is a molecular Hamiltonian defined uniquely? or Where do differences in pennylane Hamiltonians come from?

For a fixed fermion-qubit mapping, basis, active space, and geometry, is the electronic structure Hamiltonian uniquely defined? I would think so, since for a given basis the one- and two-body ...
• 11
1 vote
16 views

### Effect of partial measurement on the fidelity of states

Suppose we have two pure states $|\phi_1 \rangle = \sum_i \alpha_{i,1}|i\rangle| \chi_{i,1}\rangle, |\phi_2 \rangle = \sum_i \alpha_{i,2} |i\rangle| \chi_{i,2}\rangle$ and perform a partial ...
• 11
25 views

### Open-source code for Lattice Surgery in STIM

I'm looking for open-source code that implements lattice surgery merging and splitting between two rotated surface code logical qubits in STIM (for d=3 or above). Whilst there are some excellent ...
41 views

### Fault-tolerant CNOT gates

The controlled-not gate $$CNOT = \begin{bmatrix}1 & 0 & 0 & 0 \\ 0 & 1 & 0 & 0 \\0 & 0 & 0 & 1 \\0 & 0 & 1 & 0 \end{bmatrix}$$ is important in many ...
• 4,572
25 views

### Heisenberg picture for a quantum measurement where the outcome is recorded

I asked a related question here. From the answer by Mateus, we see that one cannot describe a quantum measurement (where the measurement outcome is recorded) as a quantum map. That is because the ...
1 vote
47 views

### What is the quantum map that describes a measurement where the measurement outcome is known?

My question is the other case of this question. Suppose I do a measurement in the $Z$ basis with outcomes $\lambda_0, \lambda_1$ but I don't record the outcome. In this case, I can describe what ...
13 views

• 751
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### Confusion on the Quantum Minimum Finding Algorithm

I am a new beginner in quantum algorithms. Recently, I was trying to learn the quantum minimum finding algorithm appeared in here. I am struggling to fully understand the operations in the algorithm. ...
38 views

### Dephasing noise and error correction

It is my impression that $Z$ errors are the dominant source of noise in many platforms used for quantum computing. For example, Ultrahigh Error Threshold for Surface Codes with Biased Noise says ...
• 4,572
1 vote
55 views

### Super-dense coding by adding quantum correlations between classical bits

Consider the following non-local game between Alice, Bob (who are spatially separated but share a maximally entangled state) and a referee: Referee samples two question bits "$x,y$" and ...
34 views

### How are ancilla qubits physically prepared and measured?

As I understand the general procedure of quantum computing: We prepare our quantum states depending on what calculation we want to do (ex: creating a spin superposition of entangled states by driving ...
21 views

### Qiskit runtime backen cannot state_preperation

I am trying to run a circuit on hardware, for that I need to initialise the state. ...
37 views

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• 63
51 views

### How does one identify Pauli logicals in a hyperbolic surface code?

I understand how the Pauli X and Z logicals on the planar surface code work - they are stringlike operators between either the rough boundaries (in case of the Z logical) or between smooth boundaries (...
• 31
1 vote
53 views

### How to interpret the product of POVM elements?

Suppose $m\not=n$, and fix a state $|\psi\rangle$. If $\\{P_k\\}$ defines a PVM, then we have $\langle \psi|P_n P_m|\psi \rangle = 0$. Physically, this makes sense, as one cannot obtain two different ...
• 518
31 views

### Finding stabilizers from a T distillation code

Consider $T$ state distillation as per Figure 14 in this paper. I am trying to figure out what the stabilizers are. I thought of trying to evolve them starting from e.g. $S = XI\cdots I$ acting on the ...
• 101
282 views

### In exercise 8.23 of Nielsen and Chuang why is the quantum operation no longer trace-preserving?

My question is about this exercise from Nielsen & Chuang: For context, $\mathcal{E}_{AD}$ is the quantum operation for amplitude damping on a single physical qubit, with operation elements given ...
• 23
41 views

1 vote
30 views

### Extracting syndromes by measuring gauge operators in subsystem codes

In subsystem codes, it is well-known that we can extract syndromes by measuring some gauge operators, instead of measuring stabilizers. For example, if a stabilizer $s$ is a product of gauge operators ...
• 61
60 views

### Beyond polynomial speedups in diagonal entries estimation

Consider the task of estimating the diagonal entries of the power of a real sparse matrix A with $N = 2^n$ rows: Input: $A \in \mathbb{R}^{N \times N}$ with $A = A^\top$ and $s$-sparse and row-...
• 763
19 views

### Process Tomography and transpiler not running and eventually crashing kernel

I am running a Quantum Process Tomography on a circuit with a custom pulse added to a calibation for a CNOT gate. Upon running the circuit, running it on the backend takes forever and eventually ends ...
71 views

### Why do we use Z-gates in the following figure?

This is a picture of a 'T-gate gadget' and a proof for a circuit from “Quantum homomorphic encryption for circuits with low T-gate complexity” by Broadbent, Jeffrey (2015) (arXiv). My question is why ...
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• 871
65 views