Developing such technologies will require the ability to freely generate a multi-photon quantum entangled state, and then to efficiently identify what kind of entangled state is present.
If available, an entangled measurement can identify the entangled state with a one-shot approach.
Such a measurement for the Greenberger-Horne-Zeilinger—GHZ—entangled quantum state has been realized, but for the W state, the other representative entangled multi-photon state, it has been neither proposed nor discovered experimentally.
It could also lead to new quantum communication protocols, the transfer of multi-photon quantum entangled states, and new methods for measurement-based quantum computing.
In the future, the team aims to apply their method to a larger-scale, more general multi-photon quantum entangled state, and plans to develop on-chip photonic quantum circuits for entangled measurements.
One example of the difference between classical and quantum physics is the idea of quantum entanglement. This fundamental feature of quantum mechanics, which refers to a scenario in which it is impossible to characterize the physics of each photon independently, goes against the traditional expectation that each particle should have a reality of its own, which greatly alarmed Einstein.
Gaining an understanding of this concept’s potential is crucial to the development of potent new quantum technologies.
Freely creating a multi-photon quantum entangled state and effectively determining the type of entangled state present are prerequisites for the development of such technologies. Conventional quantum tomography, a popular technique for state estimation, presents a serious data collection challenge because the number of measurements needed increases exponentially with the number of photons.
One-shot identification of the entangled state can be achieved with an entangled measurement, if it is available. Although such a measurement has been achieved for the Greenberger-Horne-Zeilinger—GHZ—entangled quantum state, it has not been proposed or experimentally found for the W state, the other representative entangled multi-photon state.
A group of scientists from Kyoto University and Hiroshima University were inspired to tackle this problem, and they eventually succeeded in creating a novel entangled measurement technique to determine the W state. Science Advances is the journal where the paper was published.
“With true experimental demonstration for 3-photon W states, we have finally obtained the entangled measurement for the W state as well, more than 25 years after the initial proposal concerning the entangled measurement for GHZ states,” says corresponding author Shigeki Takeuchi.
A photonic quantum circuit that performs quantum Fourier transformation for the W state of any number of photons was used to theoretically propose a method to create an entangled measurement. The team concentrated on the properties of the cyclic shift symmetry of the W state.
Using high-stability optical quantum circuits, they developed a device to illustrate the suggested approach for three photons. This allowed the device to function steadily for a considerable amount of time without active control.
In order to show that the device can differentiate between various three-photon W states, each of which corresponds to a distinct non-classical correlation between the three input photons, the team inserted three single photons into the device in the proper polarization states.
The researchers were able to assess the entangled measurement’s fidelity, which is equivalent to the likelihood of getting the right answer for an input that is pure W-state.
Quantum teleportation, or the transfer of quantum information, is made possible by this accomplishment. The transfer of multi-photon quantum entangled states, new quantum communication protocols, and new techniques for measurement-based quantum computing could all result from it.
Takeuchi states that “deepening our understanding of basic concepts is crucial to coming up with innovative ideas in order to accelerate the research and development of quantum technologies.”.
The team intends to create on-chip photonic quantum circuits for entangled measurements and hopes to expand the application of their technique to a more general, larger-scale multi-photon quantum entangled state in the future.
