Gaining quantum information without destroying coherence is a crucial aspect of quantum information science and quantum computing. One common approach to achieve this is through non-destructive measurements or techniques that preserve the quantum state during information acquisition. Here are a few methods used to gain quantum information while preserving coherence:
Quantum Non-Demolition (QND) Measurements: QND measurements are designed to extract information about one observable (such as photon number) without disturbing the system's coherence. These measurements use interactions that commute with the system's Hamiltonian, allowing the information to be acquired without destroying the quantum state.
Weak Measurements: Weak measurements involve making a very weak interaction with the system being measured. By choosing a weak interaction strength and post-selecting certain measurement outcomes, it is possible to gain partial information about the system while minimizing the disturbance caused to its coherence.
Quantum Error Correction (QEC): QEC is a technique used to protect quantum information from errors due to noise and decoherence. It involves encoding quantum states into larger, redundant quantum states and applying error-detection and error-correction operations. By employing QEC codes, information can be gained indirectly without directly measuring the state of the quantum system, preserving coherence.
Coherent Control Techniques: Coherent control refers to the manipulation of quantum systems using carefully tailored control fields. By precisely shaping the external fields, one can control the evolution of the system and extract information without introducing significant decoherence. Techniques like quantum state tomography and quantum process tomography utilize coherent control to reconstruct the quantum state or operations without destroying coherence.
Quantum Non-Destructive Measurements (QNDMs): QNDMs are designed to extract specific information about a quantum system without changing its state. These measurements exploit the interaction between the system and a probe, enabling the measurement of certain observables without altering the coherence of the system.
It's important to note that while these techniques aim to preserve coherence as much as possible, no measurement or interaction is entirely free from disturbance. However, by utilizing these methods, researchers can minimize the detrimental effects on the quantum system's coherence while still gaining valuable information.