Experimental realization of multipartite entanglement of 60 modes of a quantum optical frequency comb

Abstract: We report the experimental realization and characterization of one 60-mode copy, and of two 30-mode copies, of a dual-rail quantum-wire cluster state in the quantum optical frequency comb of a bimodally pumped optical parametric oscillator. This is the largest entangled system ever created whose subsystems are all available simultaneously. The entanglement proceeds from the coherent concatenation of a multitude of EPR pairs by a single beam splitter, a procedure which is also a building block for the realization of hypercubic-lattice cluster states for universal quantum computing.

• The paper demonstrates the largest entangled system with 60 simultaneously accessible optical modes, surpassing previous multipartite experiments.
• It employs a dual-rail continuous-variable cluster state approach using a bimodally pumped optical parametric oscillator with PPKTP crystals.
• Experimental validation via variance-based entanglement witnesses and -3.2 dB raw squeezing underscores its potential for scalable quantum computing.

Overview of the Paper on Experimental Realization of Multipartite Entanglement in Quantum Optical Frequency Combs

This paper presents a significant advancement in the experimental realization of multipartite entanglement involving a 60-mode quantum optical frequency comb. The authors Moran Chen, Nicolas C. Menicucci, and Olivier Pfister explore a robust approach using the quantum optical frequency comb of a bimodally pumped optical parametric oscillator (OPO) to create entangled states that could pave the way for universal quantum computing using cluster states.

Background and Motivation

Entanglement is a cornerstone for quantum computing owing to its potential to enable exponential computational speedups as theorized by foundational work from Feynman, Shor, and others. Prior experiments in multipartite entanglement, such as 14-trapped ion systems and photon-based discrete-variable systems, highlighted challenges in achieving scalable entangled systems. Continuous-variable (CV) quantum optics offers a promising alternative due to reduced susceptibility to decoherence and potential scalability.

Contributions and Findings

1. Largest Ever Simultaneous Access Entangled System:

This research successfully demonstrates the largest known entangled system wherein all subsystems (60 modes) are simultaneously accessible. This is a significant jump from previous capabilities, such as the generation of 15 independent cluster states over 60 modes.

2. Experimental Setup and Methodology:

Utilizing a single OPO with periodic poled KTiOPO$_4$ (PPKTP) crystals in a polarization-degenerate configuration, the authors achieved dual-rail CV cluster states. The approach involves coherent concatenation of Einstein-Podolsky-Rosen (EPR) pairs using a single beam splitter, marking a sophisticated engineerable resource for quantum computing.

3. Validation and Measurements:

The study validated multipartite entanglement through measuring variance-based entanglement witnesses and analyzing nullifiers derived from Heisenberg equations. The fidelity of the experimental setup was demonstrated with raw squeezing measurements of -3.2 dB, with corrected levels indicating strong conformance with theoretical predictions.

4. Scalability and Implications:

The methodology demonstrates intrinsic scalability, limited in this experiment by measurement availability rather than generation capability. Theoretically, the process could create in excess of 6,000 entangled modes beyond the current experimental constraints, extending its utility in larger-scale quantum computing applications.

Implications and Future Directions

A. Theoretical Implications:

The availability of a 60-mode entangled state opens new avenues for testing theories on quantum computing, particularly concerning the practicality of CV-based quantum systems. This work corroborates the potential for multi-dimensional lattice states, impossible to achieve in three-dimensional space through other current methods.

B. Practical Utility:

From a practical standpoint, generating hypercubic-lattice cluster states is vital for universal quantum computing applications, including error correction, blind quantum computing, and experimental topological order studies.

C. Future Developments:

The future could see advancements in frequency-doubled pump sources and improved detection systems to extend entangled mode access significantly. Additionally, exploring setups involving multiple identical OPOs could yield even larger cluster states, further steering toward scalable, real-world quantum computing implementations.

Conclusion

This paper's achievements in realizing a 60-mode quantum optical frequency comb entangled system mark a pivotal development in quantum optics and cluster-state quantum computing. By addressing both theoretical considerations and practical methodologies, the authors have laid a foundation that could contribute broadly to quantum technologies. Future work should focus on overcoming current experimental limitations and exploring broader applications within quantum information science.