A set of very important simple quantum systems is analyzed from the standpoint of the amount of coherent information that is accessible when information channels corresponding to the systems are used. It is shown that for simple quantum models the coherent information can be calculated and used for estimating the potential possibilities of the corresponding quantum channel as a source of physical information in experiments associated with the effects of the coherence of quantum states. The following physical models are studied: a two-level atom in a laser radiation field, an aggregate of two two-level subsystems in a multilevel atom (hydrogen), a system of two two-level atoms in the process of joint quantum-deterministic evolution and under the action of transformations of quantum measurement and quantum duplication, as well as one and two two-level atoms in the process of emission.
The shot noise in coherent control of a quantum system sets an ultimate precision limit in measurement and information processing. It is caused by the fundamental quantum fluctuation of the photon number of a coherent control field and thus is inevitable. By strongly coupling a two-level system to a quantum vacuum field, we propose an effective
N-type four-level system. In such a configuration, the coupling of a conventional
N-type system to a coherent control field is replaced by the quantum vacuum field. As a result, we can achieve a shot-noise-suppressing quantum nonlinearity, due to the vanishing fluctuation of photon number of the quantum vacuum field. The resulting photon blockade effect is only subject to the contribution from the probe field. Thus, it becomes shot-noise suppressing. Our work shows an advantage of a quantum vacuum protocol over a coherent-field protocol in manipulation of quantum systems and presents an opportunity to exploit the quantum vacuum field as a powerful tool in quantum information processing.
Preserving quantum coherence in the presence of environmental noise is one of the principal challenges for quantum technologies. Noise mitigation using spectator qubits (SQs) has recently emerged as a promising approach, enabling indirect probing of the noise without disturbing the data qubit (DQ). However, existing analyses that probe ultimate performance have been restricted to two-state random telegraph process noise, which does not capture more complex noise processes that may arise in the environment. Therefore, we here develop a SQ-based noise mitigation for DQs subject to general multi-level fluctuator noise. We first derive the coherence dynamics of the DQ under such noise, then develop a mitigation scheme in which information about the noise is inferred from sequential SQ measurements and used for phase correction. A memory-efficient heuristic adaptive protocol is proposed to dynamically select the SQ measurement time and angle based on the current noise estimate. Numerical simulations demonstrate that the proposed strategy significantly suppresses decoherence under multi-level noise, achieving performance comparable to that in the two-level case despite the increased complexity of the noise process.