Certified Entangled: Physicists Develop a Method to Recover Quantum Entanglement

Researchers refine entanglement certification strategies to recover initial entanglement, challenging the need for complete trust in quantum state sources.

Quantum entanglement, a phenomenon that has perplexed scientists for decades, plays a crucial role in various fields, from quantum communication to quantum computation. However, certifying entanglement in a quantum system has always posed a challenge, as traditional testing methods destroy the entanglement in the process. But now, physicists from the Korea Advanced Institute of Science and Technology (KAIST) have developed a groundbreaking technique that allows for the recovery of entanglement after certification, eliminating the need for complete trust in quantum state sources.

A Mysterious State with a Precise Definition:

Entanglement, a concept within quantum mechanics, refers to the inseparable connection between two or more quantum systems. In an entangled system, the subsystems cannot be seen as independent entities, leading to the famous adage that “the whole is greater than its parts.” Certifying entanglement is crucial for researchers working in the field, as it provides a way to verify the existence of this mysterious state.

Refining Entanglement Certification Strategies:

The KAIST team, led by physicist Hyeon-Jin Kim, focused on refining conventional entanglement certification (EC) strategies to recover entanglement post-certification. Conventionally, there are three EC strategies: witnessing, steering, and Bell nonlocality. Each strategy involves deriving inequalities that, if violated, certify the presence of entanglement. However, these strategies typically result in the complete destruction of the initial entanglement.

The Key to Recovery: Weak Measurement:

To overcome the challenge of recovering entanglement, the researchers introduced weak measurement into the certification process. Weak measurement is a process that probes a quantum system without sharply disturbing its subsystems, allowing them to remain entangled. In contrast, projective measurements, commonly used in conventional EC strategies, completely destroy entanglement.

The KAIST team incorporated a control parameter for the strength of measurement on each subsystem and re-derived the certifying inequality to include these parameters. By iteratively preparing the qubit system in the state to be certified and performing weak measurements, they collected statistics to check for the violation of the certification inequality. Once a violation occurred, indicating the presence of entanglement, they further implemented suitable weak measurements to recover the initial entangled state with some probability.

Lifting the Trust Assumption:

The researchers demonstrated their theoretical proposal using a photonic setup called a Sagnac interferometer. They found that as the measurement strength increased, the reversibility of entanglement decreased, but the certification level remained high. This suggests the existence of a measurement strength “sweet spot” where entanglement could be certified without significant loss and subsequently recovered.

In real-world experiments, trusting the entanglement source to consistently produce the same state becomes challenging. The KAIST team tackled this issue by applying their method to a noisy source that produced a mixture of an entangled and a separable state over time. By employing weak measurements at different time steps, they successfully certified and recovered entanglement from the mixture, eliminating the need for complete trust in the source.


The breakthrough by the KAIST physicists in developing a method to recover entanglement after certification represents a significant advancement in the field of quantum entanglement. By incorporating weak measurements and refining certification strategies, they have challenged the traditional notion that entanglement must be destroyed to be certified. This research opens up new possibilities for harnessing entanglement in practical applications, paving the way for more secure quantum communication and enhanced quantum computing capabilities.