DISCRETE QND ENTANGLEMENT-PROTECTION PROTOCOL FOR A PHASE-SHIFTED BELL PAIR
Abstract
We evaluate a discrete entanglement-protection protocol for a Bell two-qubit pair undergoing unitary evolution with a single mid-sequence phase change. The test circuit consists of state preparation, a phase shift that does not affect entanglement and emulates a useful quantum gate, and a final standard readout. We compare two variants over a total evolution time of 16 arbitrary time intervals: in the first variant, the phase operation is bracketed by two blocks of five two-qubit QND measurements without readout, while in the baseline variant these measurements are absent. Performance is quantified by the fidelity F, based on the concurrence metric; we take as the minimally acceptable lower bound, below which hardware noise can no longer be safely compensated by the available specialized tools.
Simulations show that the protected scheme maintains throughout the entire observation window, whereas the unprotected scheme, under free evolution in realistic noise regimes encountered in experiments, quickly drops below this threshold even before the gate is applied. These results demonstrate that repeated projective measurements of the entangled state in the Bell basis, implemented as discrete quantum non-demolition (QND) two-qubit measurements without readout, substantially slow the loss of entanglement despite coupling to a thermal bath. The protocol requires fewer measurements, distinguishing it from high-frequency schemes that rely on the Zeno effect.
The approach is directly relevant for fine-tuning superconducting and trapped-ion processors, for pure-state engineering, and for optimizing protocols that use mixed entangled input states. In particular, in these architectures the protocol naturally integrates with existing QND procedures and phase control, providing a means to curb decoherence. Practically, this implies periodic in-operation measurements every 1–2 arbitrary time steps, as well as the possibility of scaling to multi-qubit modules with improved gate fidelities and simplified calibration.
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