# Questions tagged [density-matrix]

For questions about density matrices and related concepts and ideas, e.g. procedures for computing properties of quantum states from their density matrices.

55 questions with no upvoted or accepted answers
Filter by
Sorted by
Tagged with
133 views

### Query on Reduced Graph States

Reduced graph states are characterized as follows (from page 46 of this paper): Let $A \subseteq V$ be a subset of vertices of a graph $G = (V,E)$ and $B = V\setminus A$ the complement of $A$ in $V$. ...
• 911
109 views

### Tripartite quantum marginal problem

Consider a tripartite quantum system with the three subsystems labeled $A, B,$ and $C$. Now take two states $\rho_{AB}$ on the joint system $AB$ and $\rho_{BC}$ on the joint system $BC$. Under what ...
• 1,006
94 views

### Pure state ensembles achieving the Holevo $\chi$-quantity with at most $d^2$ states

Theorem 8.10 in Chapter 8 of Theory of Quantum Information asserts that the Holevo capacity of a quantum channel (between density operators on $\mathbb{C}^d$) can be achieved by an ensemble consisting ...
• 2,111
81 views

### Reduced Density Matrix Equation of Motion to describe an Ellipse

Given a pure two qubit state $|\psi_{AB}\rangle$. If we trace out system $B$, the remaining density matrix $\rho_A = Tr_B|\psi_{AB}\rangle\langle\psi_{AB}|$, can be represented as a point lying ...
• 668
61 views

### What proportions of certain sets of PPT-two-retrit states are bound entangled or separable?

For two particular (twelve-and thirteen-dimensional) sets of two-retrit states (corresponding to 9 x 9 density matrices with real off-diagonal entries), I have been able to calculate the Hilbert-...
47 views

### What is the minimum number of separable states (not necessarily pure) needed to decompose arbitrary separable states?

For a bipartite separable quantum state $\rho$ acting on Hilbert space $H\otimes H'$ with $\dim H=D$ and $\dim H'=D'$, what is the minimum number of separable state needed for a decomposition? That ...
64 views

### Distribution of density operators under Stochastic Master Equation

Stochastic master equations (SME) are used in studies of open quantum systems. The general form of an SME is: \begin{align} \tag{1} d\tilde{\sigma}(t) = - i [H, \tilde{\sigma}(t) ]dt + \frac{1}{2}\...
• 2,704
293 views

### Reduced density matrix of a Haar random state and its Schmidt decomposition

Consider a Haar random quantum state $|\psi\rangle$. Note that $$\rho =\mathbb{E}[|\psi\rangle\langle \psi|] = \frac{\mathbb{I}_{n}}{2^{n}}.$$ $\mathbb{I}_n$ is the identity operator on $n$ qubits. ...
• 1,373
260 views

### Is there a way to write down the eigenstates of this two-qubit density matrix?

I am considering the density matrix which represents an arbitrary state for a pair of qubits. When written out in terms of the Pauli operators, this is as follows (certain terms vanish for another ...
• 141
62 views

• 327
143 views

• 348
1 vote
50 views

1 vote
43 views

### How are $\theta, \phi$ and $\lambda$ for the U3 gate derived in Abhijith et al. 2018?

I am looking to implement Quantum PCA from this paper (page 62). They have their code on Github. I have gone through the paper multiple times but failing to understand how they got numbers (for theta, ...
• 135
1 vote
194 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-...
1 vote
63 views

### Is $\rho | \psi \rangle$ invariant in the Wigners friend thought experiment?

Background Let's say I have a gas of $N$ particles where I cannot distinguish between the particles at a temperature $T$. Its density matrix is given by $\rho$. Note, if my friend happens to measure ...
1 vote
278 views

### QuTiP VS RK45: Which one gives the correct results for time-dependent systems?

I am writing a code for a quantum thermal machine which includes both coherent and dissipative time evolutions in its different stages of operation. However, evolving the system with "mesolve&...
• 111
1 vote
345 views

### What is the relation between density matrices and phase-space probability distributions?

According to its tag description, a density matrix is the quantum-mechanical analogue to a phase-space probability measure (probability distribution of position and momentum) in classical statistical ...
• 985
1 vote
319 views

### Numerical methods for finding an eigen basis of a degenerate Liouvillian

I'm trying to find the steady-state of a master equation, $$\dot{\rho}(t) = \mathcal{L}\rho(t),\tag{1}\label{1}$$ In the form where we vectorise the density matrix and matrixify (??) the Liouvillian ...
• 11
13 views

### Alice and Bob play a Multi-Qubit game

Well I am quite new to this so excuse me if the question is absurd Alice and Bob each can "measure" variables A and B respectively: Alice can use $a_1$ and $a_2$ methods of measurement while ...
38 views

### Tests of entanglement between one and many qubits

I have a 5-qubit state $|\psi \rangle$, which has a physical interpretation that the middle qubit is the "impurity" with spin $s_{imp} = 1/2$. The rest 4 qubits highlight the presence of ...
16 views

### Particle number expectation value in QuTip

I am learning now to use QuTiP by going through their documentation site. I am trying to understand what does the argument - particle number expectation value in thermal density matrix do? How does it ...
26 views

### Qiskit: Density matrix's dimension is too large

My circuit is with 18 qubits. I want to find density matrix by calling DensityMatrix.from_instruction(qc) but then I get an error "Maximum supported dimension for an ndarray is 32, found 36"....
• 137
29 views

### Better optimization of bounds on sums of Pauli strings?

I'm trying to bound a quantity $||\sum_i \alpha_i P_i ||$ where the $P_i$ are arbitrary Pauli strings, $||.||$ is the operator norm (max eigenvalue) and $\alpha_i$ are arbitrary real coefficients. If ...
10 views

### How to analyze a system in nonthermal equilibrium?

In quantum information theory, density matrix is one of the main resource for analyzing a system. I know in general how to obtain density matrix of a system but there is a case that still i dont know ...
• 761
88 views

• 518
32 views

### how to find a quantum gate matrix from RHO before and RHO after evolve

to evolve a 4x4 density matrix I use this method: rhoafter = np.dot(np.dot(gate,rhobefore),np.conjugate(gate.T)) And I want to find the gate from rhobefore and ...
• 285