Quantum entanglement is a phenomenon in which two or more particles become correlated in such a way that their quantum states are interdependent. Observing quantum entanglement typically involves measuring the properties of the entangled particles. However, some measurement techniques can indeed disturb or destroy the entanglement.
There are several ways to observe quantum entanglement without destroying the entangled particles. Here are a few commonly used techniques:
Bell Inequality Test: One way to observe entanglement is by performing a Bell inequality test. This test involves measuring the properties of the entangled particles separately and comparing the results to a certain mathematical inequality called a Bell inequality. Violation of the Bell inequality indicates the presence of entanglement between the particles.
Quantum State Tomography: Quantum state tomography is a technique used to reconstruct the quantum state of a system. By performing measurements on multiple copies of the entangled particles, it is possible to reconstruct the overall quantum state without directly measuring the entangled particles themselves. This approach provides information about the entanglement without destroying it.
Quantum Correlation Measurements: Instead of directly measuring the properties of the entangled particles, one can measure the correlations between them. This can be done by measuring the observables that commute with the entangled particles' total angular momentum or other conserved quantities. These measurements indirectly reveal the entanglement without disturbing it.
Weak Measurements: Weak measurements are a type of measurement technique that extracts partial information about a quantum system while minimizing the disturbance to its state. By employing weak measurements, it is possible to observe entanglement without completely destroying it.
It's important to note that no measurement technique is completely free from disturbance. However, these methods aim to minimize the disturbance to the entangled particles and extract as much information about their entanglement as possible.