# \#entanglement **URL:** https://itensor.discourse.group/tag/entanglement/6.md [Latest](https://itensor.discourse.group/latest.md) · [Categories](https://itensor.discourse.group/categories.md) · [Tags](https://itensor.discourse.group/tags.md) --- ## [Question About MPO Compression Algorithms in ITensor](https://itensor.discourse.group/t/question-about-mpo-compression-algorithms-in-itensor/2613)
**Author:** [@kevinh](https://itensor.discourse.group/u/kevinh)\ **Replies:** 1\ **Last updated:** [May 8, 2026, 9:38pm UTC](https://itensor.discourse.group/t/question-about-mpo-compression-algorithms-in-itensor/2613 "2026-05-08T21:38:01Z")
Dear ITensor team, I am currently studying MPO compression algorithms, in particular the arXiv paper by Parker, Cao, and Zaletel, Local Matrix Product Operators: Canonical Form, Compression, & Control Theory (arXiv:1909… --- ## [Slow convergence of spin 1 AF Heisenberg chain](https://itensor.discourse.group/t/slow-convergence-of-spin-1-af-heisenberg-chain/2579)
**Author:** [@Minlo](https://itensor.discourse.group/u/Minlo)\ **Replies:** 2\ **Last updated:** [February 5, 2026, 3:19am UTC](https://itensor.discourse.group/t/slow-convergence-of-spin-1-af-heisenberg-chain/2579 "2026-02-05T03:19:30Z")
Dear friend, I’m testing the sample code given in the documentation. Simply run a DMRG sweep for a spin one antiferromagnetic Heisenberg chain. Strangely, this takes me a huge number of sweeps to get converged. Here is… --- ## [Computing Renyi entropy of a sub-system in the middle of a 1D chain](https://itensor.discourse.group/t/computing-renyi-entropy-of-a-sub-system-in-the-middle-of-a-1d-chain/2523)
**Author:** [@tamoghnaRay](https://itensor.discourse.group/u/tamoghnaRay)\ **Replies:** 3\ **Last updated:** [November 7, 2025, 2:57pm UTC](https://itensor.discourse.group/t/computing-renyi-entropy-of-a-sub-system-in-the-middle-of-a-1d-chain/2523 "2025-11-07T14:57:27Z")
Hi, I have a spin- 1/2 XXZ chain of length L. I have a sub-system of length L\_S, in the middle of the chain. Say L = 10 and the system goes from site 4 to 7 (L\_S = 4). I want to compute S\_n = \\frac{1}{1-n}\\log\[\\text{Tr} … --- ## [Initialize state for a quantum circuit](https://itensor.discourse.group/t/initialize-state-for-a-quantum-circuit/2282)
**Author:** [@Shachar](https://itensor.discourse.group/u/Shachar)\ **Replies:** 1\ **Last updated:** [March 5, 2025, 8:15am UTC](https://itensor.discourse.group/t/initialize-state-for-a-quantum-circuit/2282 "2025-03-05T08:15:14Z")
Hi, I’m new to ITensors and the entire field of MPS and tensor networks, so I might be missing something fundamental. I’m trying to write code that simulates quantum gates, but when I attempt to initialize an entangled… --- ## [Implementing efficient tensor contraction algorithm](https://itensor.discourse.group/t/implementing-efficient-tensor-contraction-algorithm/1570)
**Author:** [@Brian-Khor](https://itensor.discourse.group/u/Brian-Khor)\ **Replies:** 3\ **Last updated:** [February 16, 2025, 6:12pm UTC](https://itensor.discourse.group/t/implementing-efficient-tensor-contraction-algorithm/1570 "2025-02-16T18:12:04Z")
Hi Miles, Matt and other admins, I recent wrote a paper about calculating the entanglement asymmetry in a generic MPS scenario (without integrability, interacting spin-1/2 Ising Hamiltonian, in this paper ( https://arxi… --- ## [Introducing ITensorEntropyTools.jl](https://itensor.discourse.group/t/introducing-itensorentropytools-jl/2192)
**Author:** [@ryanlevy](https://itensor.discourse.group/u/ryanlevy)\ **Replies:** 0\ **Last updated:** [January 6, 2025, 2:01pm UTC](https://itensor.discourse.group/t/introducing-itensorentropytools-jl/2192 "2025-01-06T14:01:39Z")
Introducing a new 3rd party package to compute entanglement entropy ITensorEntropyTools.jl: Features The idea behind ITensorEntropyTools.jl is to automate some common calculations/tools: Entanglement Entropy calcula… --- ## [Best Method to Generate Haar Random States for a Spin-1/2 Chain?](https://itensor.discourse.group/t/best-method-to-generate-haar-random-states-for-a-spin-1-2-chain/2135)
**Author:** [@ktrdcp](https://itensor.discourse.group/u/ktrdcp)\ **Replies:** 1\ **Last updated:** [November 25, 2024, 1:19pm UTC](https://itensor.discourse.group/t/best-method-to-generate-haar-random-states-for-a-spin-1-2-chain/2135 "2024-11-25T13:19:50Z")
I am trying to generate a Haar random state for a spin-1/2 chain of length 3. My goal is to sample a truly Haar random pure state in the Hilbert space of the chain. I have considered using randomMPS with a sufficiently … --- ## [Entanglement entropy](https://itensor.discourse.group/t/entanglement-entropy/54)
**Author:** [@itensor\_questions](https://itensor.discourse.group/u/itensor_questions)\ **Replies:** 6\ **Last updated:** [November 9, 2024, 8:23pm UTC](https://itensor.discourse.group/t/entanglement-entropy/54 "2024-11-09T20:23:28Z")
This is a previously asked question reposted here to make searching our new forum more useful. ITensor user jacopovit asked: How we can calculate the entanglement entropy of an half chain with Julia? --- ## [Extracting Singular Values of an entanglement cut within a Quantum Number Block](https://itensor.discourse.group/t/extracting-singular-values-of-an-entanglement-cut-within-a-quantum-number-block/2084)
**Author:** [@amoghanakru](https://itensor.discourse.group/u/amoghanakru)\ **Replies:** 1\ **Last updated:** [October 24, 2024, 1:55pm UTC](https://itensor.discourse.group/t/extracting-singular-values-of-an-entanglement-cut-within-a-quantum-number-block/2084 "2024-10-24T13:55:51Z")
Hi, I want to ask a question that is sort of a follow-up/continuation of this question: How to realize momentum Entanglement spectrum in ITensor.jl - #3 by kevinh. The goal of that question (which is my goal as well) is … --- ## [Site ordering for MPS](https://itensor.discourse.group/t/site-ordering-for-mps/2068)
**Author:** [@John-J-Tanner](https://itensor.discourse.group/u/John-J-Tanner)\ **Replies:** 3\ **Last updated:** [October 18, 2024, 5:48pm UTC](https://itensor.discourse.group/t/site-ordering-for-mps/2068 "2024-10-18T17:48:03Z")
Hi there, Julia and ITensor version info: julia\> versioninfo() Julia Version 1.10.2 Commit bd47eca2c8a (2024-03-01 10:14 UTC) Build Info: Official https://julialang.org/ release Platform Info: OS: macOS (arm64-appl… --- ## [How to correctly compute two-site reduced density matrices of fermionic MPS?](https://itensor.discourse.group/t/how-to-correctly-compute-two-site-reduced-density-matrices-of-fermionic-mps/1929)
**Author:** [@wangfh5](https://itensor.discourse.group/u/wangfh5)\ **Replies:** 2\ **Last updated:** [August 29, 2024, 2:10am UTC](https://itensor.discourse.group/t/how-to-correctly-compute-two-site-reduced-density-matrices-of-fermionic-mps/1929 "2024-08-29T02:10:07Z")
Hi everyone, I am new to this amazing package and working on extracting two-site reduced density matrices from MPS. With the help of Is this a correct way to calculate one-site and two-site entanglement spectrum and en… --- ## [Two-site reduced density matrix](https://itensor.discourse.group/t/two-site-reduced-density-matrix/719)
**Author:** [@nticea](https://itensor.discourse.group/u/nticea)\ **Replies:** 8\ **Last updated:** [August 11, 2024, 6:33am UTC](https://itensor.discourse.group/t/two-site-reduced-density-matrix/719 "2024-08-11T06:33:43Z")
Hi! Hope this finds you well. At the high level, I am trying to do the following: Starting with an MPO of length L, choose two sites, A and B. These sites can be located anywhere in the MPO; as such, they can be separa… --- ## [Entanglement entropy of any subsystem](https://itensor.discourse.group/t/entanglement-entropy-of-any-subsystem/1598)
**Author:** [@yu252](https://itensor.discourse.group/u/yu252)\ **Replies:** 1\ **Last updated:** [March 29, 2024, 10:44am UTC](https://itensor.discourse.group/t/entanglement-entropy-of-any-subsystem/1598 "2024-03-29T10:44:13Z")
Hello All, I have a question regarding calculating the entanglement entropy (EE) of a subregion. Let’s say I want to calculate EE of subsytstem A for case A and B shown below. For case A, this calculation of EE is … --- ## [entanglement entropy in tdvp calculation](https://itensor.discourse.group/t/entanglement-entropy-in-tdvp-calculation/1594)
**Author:** [@javahedi](https://itensor.discourse.group/u/javahedi)\ **Replies:** 2\ **Last updated:** [March 28, 2024, 1:57pm UTC](https://itensor.discourse.group/t/entanglement-entropy-in-tdvp-calculation/1594 "2024-03-28T13:57:47Z")
Dear iTensorJulia Community, I am encountering an issue with the attached code snippet, which I borrowed from the TDVP example and extended with a function to calculate entanglement entropy. However, upon running the co… --- ## [MPS Entanglement Entropy](https://itensor.discourse.group/t/mps-entanglement-entropy/1545)
**Author:** [@amello](https://itensor.discourse.group/u/amello)\ **Replies:** 1\ **Last updated:** [March 14, 2024, 11:00am UTC](https://itensor.discourse.group/t/mps-entanglement-entropy/1545 "2024-03-14T11:00:04Z")
Hi, I am quite new to using ITensors and Julia in general, so my question may sound a bit naive. I am sorry about this. I am trying to write a code that computes the entanglement entropy of an MPS for a certain bond b be… --- ## [Computing Central Charge from Entanglement Entropy](https://itensor.discourse.group/t/computing-central-charge-from-entanglement-entropy/1507)
**Author:** [@qrao](https://itensor.discourse.group/u/qrao)\ **Replies:** 3\ **Last updated:** [February 28, 2024, 1:14pm UTC](https://itensor.discourse.group/t/computing-central-charge-from-entanglement-entropy/1507 "2024-02-28T13:14:44Z")
Hi, I’m new to Julia and ITensor so please forgive me if I’m missing a very obvious error in my code. I’m trying to compute the central charge of the tricritical Ising model at a critical point from the entanglement ent… --- ## [Retrieve bipartite reduced density matrix](https://itensor.discourse.group/t/retrieve-bipartite-reduced-density-matrix/940)
**Author:** [@Brian-Khor](https://itensor.discourse.group/u/Brian-Khor)\ **Replies:** 6\ **Last updated:** [February 20, 2024, 9:00pm UTC](https://itensor.discourse.group/t/retrieve-bipartite-reduced-density-matrix/940 "2024-02-20T21:00:07Z")
Hi Miles and Matt (and any other admin), In the typical calculation of von Neumann (or Renyi) entropy, one gauge the center and then perform SVD around the gauge center (hence bipartition the chain into left/right bipar… --- ## [MPS representation used in ITensor and use of orthogonalize!](https://itensor.discourse.group/t/mps-representation-used-in-itensor-and-use-of-orthogonalize/1333)
**Author:** [@tamoghnaRay](https://itensor.discourse.group/u/tamoghnaRay)\ **Replies:** 4\ **Last updated:** [December 5, 2023, 4:16pm UTC](https://itensor.discourse.group/t/mps-representation-used-in-itensor-and-use-of-orthogonalize/1333 "2023-12-05T16:16:21Z")
Hi, I am using ITensor in Julia and came across this function called orthogonalize! which shifts the orthogonality center of the MPS to a particular site. I am trying to understand what this means and how using this one… --- ## [Entanglement of mixed hilbert space: How trace odd/even sites](https://itensor.discourse.group/t/entanglement-of-mixed-hilbert-space-how-trace-odd-even-sites/1298)
**Author:** [@ElevenBadger](https://itensor.discourse.group/u/ElevenBadger)\ **Replies:** 2\ **Last updated:** [November 20, 2023, 5:07pm UTC](https://itensor.discourse.group/t/entanglement-of-mixed-hilbert-space-how-trace-odd-even-sites/1298 "2023-11-20T17:07:46Z")
Hi, Let’s say I perform a DMRG calculation to a system with a mixed Hilbert space (as in the example of the documentation). How could I trace one species (for example the spin 1/2 sites in the example), and then calcula… --- ## [Entanglement Entropy Bug](https://itensor.discourse.group/t/entanglement-entropy-bug/1296)
**Author:** [@mbj](https://itensor.discourse.group/u/mbj)\ **Replies:** 2\ **Last updated:** [November 20, 2023, 2:09pm UTC](https://itensor.discourse.group/t/entanglement-entropy-bug/1296 "2023-11-20T14:09:35Z")
Hello! I’ve been trying to make use of the entropy\_von\_neumann function provided in the MPS and MPO examples. However, I encountered an error when calculating the von Neumann entropy across a cut which separates the firs… --- ## [Ramping up of bond dimension of a 1D critical system](https://itensor.discourse.group/t/ramping-up-of-bond-dimension-of-a-1d-critical-system/1231)
**Author:** [@Zoldyck](https://itensor.discourse.group/u/Zoldyck)\ **Replies:** 0\ **Last updated:** [October 18, 2023, 4:58pm UTC](https://itensor.discourse.group/t/ramping-up-of-bond-dimension-of-a-1d-critical-system/1231 "2023-10-18T16:58:46Z")
Hi, I am trying to study a short range 1D chain at a critical point. I have two questions. 1.Theoretically the entanglement entropy(S) across the ground state should be a semicircle like graph against Log(l) where l i… --- ## [Time correlations for an MPS](https://itensor.discourse.group/t/time-correlations-for-an-mps/653)
**Author:** [@tsegev](https://itensor.discourse.group/u/tsegev)\ **Replies:** 3\ **Last updated:** [September 4, 2023, 10:43pm UTC](https://itensor.discourse.group/t/time-correlations-for-an-mps/653 "2023-09-04T22:43:21Z")
Hey all, I want to calculate a time dependent autocorrelation function for a spin. \\langle S\_z(t)S\_z(t') \\rangle I saw in the documentation that there is an option to calculate a correlation matrix, but I think it is… --- ## [How to calculate tri-partite entanglement or mutual information](https://itensor.discourse.group/t/how-to-calculate-tri-partite-entanglement-or-mutual-information/1115)
**Author:** [@singhroy.monalisa](https://itensor.discourse.group/u/singhroy.monalisa)\ **Replies:** 1\ **Last updated:** [August 25, 2023, 9:52am UTC](https://itensor.discourse.group/t/how-to-calculate-tri-partite-entanglement-or-mutual-information/1115 "2023-08-25T09:52:02Z")
Hi, I am trying to calculate the tri-partite entanglement entropy or mutual information (I) in a one dimension (1D) spin-1/2 chain, defined as, I (A, B, C) ≡ S(A) + S(B) + S(C) − S(A ∪ B) − S(A ∪ C) − S(B ∪ C) + S(A ∪ … --- ## [Extracting the central charge of a 1d critical quantum system](https://itensor.discourse.group/t/extracting-the-central-charge-of-a-1d-critical-quantum-system/956)
**Author:** [@Sunny](https://itensor.discourse.group/u/Sunny)\ **Replies:** 2\ **Last updated:** [June 23, 2023, 7:02pm UTC](https://itensor.discourse.group/t/extracting-the-central-charge-of-a-1d-critical-quantum-system/956 "2023-06-23T19:02:51Z")
Hi, I basically want to extract the central charges at the different critical regions of a 1 D quantum system. I know of three methods to do this- Using PBC in a finite size chain. Using IDMRG or VUMPS. Using MERA. A… --- ## [General Question about MPO's and the apply function](https://itensor.discourse.group/t/general-question-about-mpos-and-the-apply-function/854)
**Author:** [@tsegev](https://itensor.discourse.group/u/tsegev)\ **Replies:** 1\ **Last updated:** [May 14, 2023, 6:56am UTC](https://itensor.discourse.group/t/general-question-about-mpos-and-the-apply-function/854 "2023-05-14T06:56:21Z")
Hey all! I have some urgent questions about ITensor. The process I am trying to calculate is built from four main part: Building a density matrix. Building a long range interaction gates. Develop the density matrix i… --- ## [Possible error in the Purification example on Github](https://itensor.discourse.group/t/possible-error-in-the-purification-example-on-github/833)
**Author:** [@tsegev](https://itensor.discourse.group/u/tsegev)\ **Replies:** 0\ **Last updated:** [May 3, 2023, 6:26am UTC](https://itensor.discourse.group/t/possible-error-in-the-purification-example-on-github/833 "2023-05-03T06:26:17Z")
Hey, sorry for going back to this topic, but the question seems relevant here. In the example of the Purification the initial maximally mixed state is: rho = MPO(s, "Id") ./ √2 Can it be that there is a mistake ? b… --- ## [About the TDVP function](https://itensor.discourse.group/t/about-the-tdvp-function/752)
**Author:** [@tsegev](https://itensor.discourse.group/u/tsegev)\ **Replies:** 2\ **Last updated:** [April 9, 2023, 2:24pm UTC](https://itensor.discourse.group/t/about-the-tdvp-function/752 "2023-04-09T14:24:43Z")
Hey all, The tdvp function is a function to propagate in time my MPS object. Does this method can applied for an imaginary time approach ? site=siteinds("Qubit",n) psi= randomITensor(site) dt=0.1 psi= tdvp(H,psi,-dt) … --- ## [Overlap between thermal state and imaginary time evolution state](https://itensor.discourse.group/t/overlap-between-thermal-state-and-imaginary-time-evolution-state/735)
**Author:** [@tsegev](https://itensor.discourse.group/u/tsegev)\ **Replies:** 4\ **Last updated:** [April 2, 2023, 12:24pm UTC](https://itensor.discourse.group/t/overlap-between-thermal-state-and-imaginary-time-evolution-state/735 "2023-04-02T12:24:29Z")
Hey all, I am trying to measure the overlap between a thermal state which I created using diagonalization of the Hamiltonian to an imaginary site development state. It seems that no matter what \\delta \\beta is, I will … --- ## [Entanglement Spectrum with momentum](https://itensor.discourse.group/t/entanglement-spectrum-with-momentum/567)
**Author:** [@L\_Zhang](https://itensor.discourse.group/u/L_Zhang)\ **Replies:** 7\ **Last updated:** [February 8, 2023, 3:31pm UTC](https://itensor.discourse.group/t/entanglement-spectrum-with-momentum/567 "2023-02-08T15:31:56Z")
To charactorize the topological property of a certain state, the entanglement in k-space are often calculated. But I haven’t found an detailed instruction about it. How can we get entanglement spectrum seperated with mo… --- ## [MixedSiteSet in TEBD and DMRG](https://itensor.discourse.group/t/mixedsiteset-in-tebd-and-dmrg/538)
**Author:** [@Riku](https://itensor.discourse.group/u/Riku)\ **Replies:** 1\ **Last updated:** [January 22, 2023, 8:25am UTC](https://itensor.discourse.group/t/mixedsiteset-in-tebd-and-dmrg/538 "2023-01-22T08:25:37Z")
Dear ITensor developers and users, Thank you for wonderful and useful library. I do DMRG and want to do TEBD with ITensor in C++ version. In my model, the system consists of the conduction electron subsystem and local … [Next page](https://itensor.discourse.group/tag/entanglement/6.md?match_all_tags=true&page=1&tags%5B%5D=entanglement)