Physicists have long recognized the untapped potential of photonic graph states in quantum information processing. However, the challenge of creating these states has been a significant barrier. In a groundbreaking development, researchers from The Grainger College of Engineering at the University of Illinois Urbana-Champaign have introduced a novel “emit-then-add” scheme. This approach, detailed in the journal npj Quantum Information, could harness existing hardware to produce highly entangled photon states, paving the way for advanced quantum operations such as measurement-based quantum computing.
Entanglement is crucial for enhancing the speed and security of computational and information systems. Yet, creating large, entangled states with more than two photons is notoriously difficult due to inherent losses in optical systems. These losses mean that most photon sources have a low probability of successfully producing a photon that survives to detection. Consequently, attempts to build large entangled states often result in missing photons, which disrupts the state. Detecting these gaps is a destructive process, preventing any possibility of filling them.
Innovative Approach to Overcome Challenges
To tackle this issue, a team led by Associate Professor of Physics Elizabeth Goldschmidt and Professor of Electrical and Computer Engineering Eric Chitambar adopted a novel perspective. “We’ve known for years that these photonic graph states are incredibly valuable,” Goldschmidt explained. “For this project, we shifted our focus from the most useful end result to what we can achieve with current resources. It took us a long time to realize that destructively measuring the photons would be acceptable in many useful scenarios.”
This shift in approach led to the development of a heralded scheme for creating photonic graph states compatible with state-of-the-art coherent quantum emitters. The innovation lies in the concept of “virtual graph states.” By adding a photon to a virtual graph only after confirming its detection, the primary limitation shifts from photon loss to the coherence of the spin qubits used to emit the photons, which can be significantly longer.
Potential Applications and Future Directions
The Illinois Grainger engineers emphasize that their protocol is fully general if non-destructive photon measurement can be implemented—though this remains beyond the reach of current hardware. As a result, they propose a broad class of protocols that can be effectively implemented using destructively measured photons and virtual graph states. One example-use case involves secure two-party computation through the repeated generation of small graph states.
“There’s something almost counterintuitive about it,” said Max Gold, a graduate student and co-lead author of the paper. “We’re building up these correlations that can only be described by quantum systems across different photons. We have these photons that don’t ever exist at the same time in nature, and something mediating their interactions that’s not the photons themselves. Even though we talk about it as a single state, not all the qubits in the state exist at one time.”
“This could be done on a number of experimental apparatuses around the world,” said Jianlong Lin, a graduate student and co-lead author. “Our method is feasible in practice even for emitter-based platforms with traditionally low photon collection efficiencies such as trapped ions and neutral atoms. It would be one of very few demonstrations of photonic graph states with practical uses.”
Implications for Quantum Computing
If implemented, the researchers’ methodology could revolutionize fields such as measurement-based quantum computing, secure two-party computation, and even quantum sensing. Goldschmidt’s lab is actively working to realize their protocol, with Lin focusing on the experimental side and Gold exploring theoretical applications for graph states using their methodology.
“We’ve created a protocol based on realistic hardware that has at least one use, which is this multi-party computation,” Goldschmidt noted. “A lot of the literature has ignored hardware limitations, and I hope this work encourages others to consider what could be achieved given the real constraints of near-term hardware.”
Elizabeth Goldschmidt is an associate professor in the Department of Physics at Illinois Grainger Engineering. She is affiliated with the Materials Research Laboratory, the NSF QLCI: Hybrid Quantum Architectures and Networks, and serves as the Associate Director of the Illinois Quantum and Information Science and Technology Center. Eric Chitambar, a professor in the Department of Electrical and Computer Engineering, is affiliated with the Illinois Quantum and Information Science and Technology Center, the NSF QLCI: Hybrid Quantum Architectures and Networks, and the Coordinated Science Laboratory.
