Fig. 5: Preserving and generating entangled states.

a Return probability for the singlet state of an ST qubit defined on sites 3 and 4 of an L = 6 spin chain. The two remaining ST qubits are initialized in the state \(\left| \uparrow \downarrow \right\rangle\). b Two-qubit probabilities before and after the execution of a two-qubit CZ gate, using the modified DTC protocol (for a chain of length L = 4). The initial state of ST qubit 1 is the triplet \(\left|{T}_{0}\right\rangle\) and of qubit 2 is the singlet. The x coordinate of each point is the total time of all the pulse sequences. The y coordinate of each point is the joint two-qubit probability. The “expected post-CZ state'' is an entangled state of the two ST qubits. The results in both panels are averaged over 4096 realizations.