Researchers at Shandong University introduced a new way to verify entanglement in quantum networks without trusting detectors. This method works for four users and avoids risks from tampered equipment. "Building a quantum network is only part of the challenge," said He Lu. The team used photonic chips for this test.
Quantum networks are systems that connect different users or devices using principles of quantum mechanics. In the future, these networks could help improve the security of communications.
For entanglement-based quantum networks to work, however, researchers need to be able to verify that particles are entangled and assess the extent of their entanglement. Entanglement is a quantum mechanical effect in which the shared states of distant particles cannot be described as separate or independent.
Researchers at Shandong University recently introduced a new approach for verifying entanglement in a quantum network that does not require trusting the equipment used to analyze and detect photons (i.e., light particles), as most previously developed strategies do.
Their proposed method, outlined in a paper published in Physical Review Letters, was successfully used to verify entanglement in a quantum network with four users.
"Building a quantum network is only part of the challenge," He Lu, senior author of the paper, told Phys.org. "We also need to know whether the entanglement distributed across the network is genuinely there. My interest in this problem dates back to 2014, when I participated in one of the first experimental demonstrations of measurement-device-independent (MDI) entanglement witnessing."
In their earlier work, Lu and his colleagues demonstrated that conventional entanglement verification techniques could be misled by measurements that were tampered with. Specifically, manipulating measurement results could trick conventional tests into detecting entanglement when none was present.
They also tested an approach dubbed MDI entanglement witnessing that overcame this limitation. Despite its potential, this approach required the preparation of additional photons in known states for testing, which complicated its implementation.
"In recent years, our group has been developing broadband entangled-photon sources based on integrated lithium-niobate photonics," said Lu. "Meanwhile, fully connected quantum networks have been advancing rapidly, allowing entanglement to be distributed among multiple users. These developments motivated us to revisit MDI entanglement verification in the context of quantum networks and explore whether it could be implemented more efficiently."
Checking links between users in a quantum network
The main goal of the team's recent study was to develop a new practical approach for reliably verifying and quantifying entanglement across a multiuser quantum network. Most importantly, they wanted this approach to work even without trusting measurement devices or preparing additional photons for testing.
"We built our quantum network using a small photonic chip that generates pairs of entangled photons over a broad range of wavelengths," said Lu. "By distributing entangled photon pairs through different wavelength channels, we connected four users so that every pair could share entanglement. This created six connections in total, each spanning 20 km (12 miles) of optical fiber."
The key idea behind the researchers' measurement method is to use two different properties of the same photon. The researchers used the possible arrival times of photons to carry entanglement and the orientation of the photons' light waves to prepare known states for testing it. Notably, this method still requires these known test states to be prepared reliably.
"By performing a joint measurement on these two properties of each photon, we could verify entanglement without trusting the measurement devices or requiring additional ancillary photons," said Lu.
"We then recorded photon coincidence counts between pairs of users. By analyzing these measurements, we could verify the presence of entanglement in all six connections and establish a lower bound on the amount of entanglement shared between each pair of users, without relying on trusted measurement devices."
In addition to testing entanglement between individual pairs of photons, the team's approach can be applied across an entire quantum network. In initial tests, the researchers successfully used it to confirm entanglement across six links connecting four users in a quantum network, without relying on trusted measurement devices.
"More importantly, our method goes beyond simply determining whether entanglement exists," said Lu. "It also allows us to quantify entanglement by establishing a lower bound on the amount shared between users, using the same measurement data. This provides more information about the quantum resources available in the network."
The team's MDI approach also requires fewer experimental resources. This is because it uses the same photons both to realize entanglement and to verify it.
"In our earlier experiment in 2014, our implementation of MDI entanglement verification required additional ancillary photons, making the experiment technically demanding," said Lu. "In the present work, we eliminate this requirement by encoding trusted quantum inputs in the polarization of the same photons that carry time-bin entanglement."
Verifying entanglement with fewer resources
Lu and his colleagues think that their approach could make the reliable characterization of entanglement more practical for larger quantum networks. In larger fully connected networks, the number of connections between users increases, so minimizing the complexity of the methods used to verify entanglement becomes increasingly important.
"Our next goal is to extend our approach to larger and more complex quantum networks," said Lu. "In particular, we are interested in exploring both high-dimensional and multipartite entanglement. Time-bin encoding offers a natural way to generate high-dimensional quantum states, while multipartite entanglement would allow several users to share a common entangled state, opening new possibilities for quantum communication and networking."
The lithium-niobate source used by the team in their experiments produces photons with a broad range of wavelengths. This allows them to share entanglement via several separate light channels, which could be useful when creating quantum networks with more users and connections.
"As these networks become more complex, developing efficient and reliable methods to verify and quantify entanglement will be increasingly important," added Lu. "Ultimately, we hope to combine broadband integrated photon sources with scalable entanglement characterization methods to support more advanced quantum networks."