A Quantum Superwitness

Quantum entanglement is one of the most counterintuitive features of quantum mechanics and at the heart of ground-breaking applications such as quantum computation. Yet, determining whether a large quantum system is entangled or not has proven extremely challenging.

A team of researchers from the University of Queensland has now developed and demonstrated a new technique to witness the presence of quantum entanglement more efficiently than any other method.

Farid Shahandeh, a PhD candidate at the ARC Centre of Excellence for Quantum Computation and Communication technology and the lead author of the study, explained that the new method can improve every existing entanglement detection protocol without additional measurements, which will allow researchers to work with much larger quantum systems than previously possible.

Dr Martin Ringbauer from the ARC Centre of Excellence for Engineered Quantum Systems said: “The situation is much like a court case: if you have a photo of the suspect, or in the quantum case, if you know the full quantum state, then it is easy to identify the suspect or determined whether the state is entangled.”

In most cases, however, such complete information is hard to come by and expensive to process: reconstructing the full quantum state requires an impractically large number of measurements. One would thus have to rely on witness information.

A good witness is able to correctly identify the convict in many situations. However, in order to ensure that the witness never falsely accuses an innocent suspect, they will never be able to catch all convicts.

The same happens for quantum states: in order to make sure that the witness never mistakes an unentangled state for an entangled one, we have to accept that it will sometimes miss entangled states. The witness that identifies the largest number of convicts (entangled states) without accusing an innocent suspect is called optimal.

In practice, the witness may have several pieces of information and when presented with a group of suspects, they will use this information to make an informed decision whether each suspect is guilty. The same happens in standard entanglement witnessing, where a number of measurements are combined to decide whether a given state is entangled or whether no decision can be made.

The new method developed at the University of Queensland is based on a more inclusive use of the available information. The witness would first use the information to narrow down the set of candidate states or suspects and then combine the information in the usual way to make a decision on the remaining suspects. This makes the task much easier, since the witness has to judge a much smaller group of potential suspects.

This more efficient method allows for the detection of many more entangled states than standard approaches, without the need for additional measurements.

Mr Shahandeh noted, “By making the witnessing process more efficient, the new method enables the use of entanglement witnessing for much larger quantum systems.”

“The new method can also use simpler information than required by previous witnesses and can thus detect entanglement where standard witnesses could not.”

Read the paper here.