Entanglement view
-----------------

Starting from server version 1.0.26, Quantag Studio provides an **Entanglement** section in the *Variables* panel during circuit debugging.

After executing a step (``F10``) or stopping at a breakpoint, two variable groups are shown:

* **Quantum State** – amplitudes of the current quantum state
* **Entanglement** – quantitative entanglement information

.. image:: images/vars_entag.png
   :alt: Entanglement view
   :width: 100%
   :align: center

What is shown in Entanglement
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

For each qubit ``qi``, the entanglement section shows a value::

   S_qi

where ``S_qi`` is the von Neumann entropy of qubit ``qi`` with respect to the rest of the system.

Interpretation:

* ``S_qi = 0.0`` – the qubit is not entangled (separable state)
* ``0.0 < S_qi < 1.0`` – the qubit is partially entangled
* ``S_qi ≈ 1.0`` – the qubit is maximally entangled with the rest of the system

The following summary values may also be displayed::

   S_max   – maximum entanglement among all qubits
   S_avg   – average entanglement over all qubits

Notes
~~~~~

* Entanglement values are updated only when execution stops (step or breakpoint).
* Entanglement is computed from the current quantum state before measurement.
* After a measurement operation, entanglement typically collapses to zero.
* Controlled gates (for example ``cx``) create entanglement only if the control qubit is in superposition.

Example
~~~~~~~

The following OpenQASM code does not create entanglement::

   h q[0];
   cx q[1], q[0];

The following code creates entanglement::

   h q[0];
   cx q[0], q[1];