Two qubit entanglement preservation through the addition of qubits

Naghi Behzadi, Bahram Ahansaz, Esfandyar Faizi

Yu-Chen Hou, Guo-Feng Zhang, Yan Chen +1 more

Shih-Hsuan Chen, Meng-Lok Ng, Che‐Ming Li

Preserving entanglement is a crucial dynamical process for entanglement-based quantum computation and quantum-information processes, such as one-way quantum computing and quantum key distribution. However, the problem of quantifying the ability of an experimental process to preserve two-qubit entanglement in experimentally feasible ways is not well understood. Accordingly, herein, we consider the use of two measures, namely composition and robustness, for quantitatively characterizing the ability of a process to preserve entanglement, referred to henceforth as entanglement preservability. A fidelity benchmark is additionally derived to identify the ability of a process to preserve entanglement. We show that the measures and introduced benchmark are experimentally feasible and require only local measurements on single qubits and preparations of separable states. Moreover, they are applicable to all physical processes that can be described using the general theory of quantum operations, e.g., qubit dynamics in photonic and superconducting systems. Our method extends the existing tools for analyzing channels, e.g., channel resource theory, to quantify entanglement preservability for non-trace-preserving quantum processes. The results are of significant interest for applications in quantum-information processing in which entanglement preservation is required.

Muhammad Abdullah Mutahar, Adam Zaman Chaudhry

Using an exactly solvable pure dephasing model, we show how entanglement between qubits can be generated via the interaction with a common environment and concurrent application of suitable control pulses. The control pulses are able to effectively remove the detrimental effect of the environment while preserving the indirect interaction between the qubits, thereby leading to the generation of near-perfect entanglement. Furthermore, we also investigate the entanglement dynamics if the qubits are directly interacting; this interaction may even contain a noise term. The present of this additional noise leads to an additional decoherence term. This decoherence term cannot be removed by applying the pulses at the same time to both qubits. Rather, we show that by introducing a time delay between the two pulse sequences, near-perfect entanglement can still be generated via the interaction with the common environment.

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Salwa Shaglel, Satoya Imai

We develop systematic frameworks for characterizing the entanglement properties of two-qubit channels beyond unitary settings. We introduce averaged local-unitary invariants, referred to as moments, obtained from Haar integrals over input states or unitaries. These moments provide computable descriptions of how a quantum channel can create, preserve, or destroy bipartite entanglement. We first show that second-order moments yield criteria for non-entangling and entanglement-breaking channels, which allow us to detect entanglement-creating and entanglement-preserving channels. We then demonstrate that higher-order moments can capture additional information and distinguish channels beyond second-order moments alone. Finally, we show that combinations of moments associated with different channel families improve the discrimination of locally inequivalent two-qubit unitaries.