Witnessing Genuine Multipartite Entanglement in Phase Space with Controlled Gaussian Unitaries Scientists Implement Genuine Multipartite Entanglement Witnesses

Genuine multipartite entanglement, a cornerstone of quantum information science, remains challenging to verify in continuous-variable systems, particularly when accessing these quantum degrees of freedom indirectly through qubit readouts. Lin Htoo Zaw from the Centre for Quantum Technologies, National University of Singapore, Jiajie Guo and Shuheng Liu from the State Key Laboratory for Mesoscopic Physics, Peking University, alongside Matteo Fadel and colleagues, now demonstrate methods to witness this entanglement through direct phase-space measurements. The team proposes five practical schemes, utilising controlled Gaussian unitaries, that can detect key multipartite states including Dicke states, W states, and GHZ-type cat states. Importantly, these new witnesses require significantly fewer measurement settings than traditional methods, offering a robust and efficient pathway to confirm genuine multipartite entanglement in a range of experimental platforms such as circuit and cavity electrodynamics, and trapped ion systems.

Witnessing genuine multipartite entanglement in phase space with controlled Gaussian unitaries represents a significant advance in quantum physics. The research team demonstrates the creation and verification of entanglement involving multiple quantum particles, specifically focusing on systems manipulated using Gaussian unitaries. This approach allows for precise control over quantum states and facilitates the observation of entanglement in the continuous variable domain, a crucial step towards advanced quantum technologies. This achievement expands the toolkit for characterizing and harnessing multipartite entanglement, paving the way for applications in quantum communication, computation, and sensing.

Smoothed Wigner Function Detects Non-Classical States

This research details a method for detecting non-classicality in multi-photon states, specifically N00N states, using a smoothed Wigner function. The core idea involves smoothing the Wigner function with a specific kernel, then analyzing the resulting value to determine if the state is non-classical. A negative value indicates a non-classical state, offering a robust and practical way to verify the creation and manipulation of these states, which are crucial for quantum information processing, quantum metrology, and other quantum technologies. N00N states are entangled states of multiple photons, where all photons exist in a superposition of two modes.

The Wigner function is a quasi-probability distribution representing a quantum state in phase space, a powerful tool for analyzing quantum properties. Smoothing the Wigner function with a kernel, such as a Gaussian, reduces noise and makes it easier to detect non-classical features. Researchers focused on the specific case of a three-photon N00N state, calculating the smoothed Wigner function using different kernels, demonstrating that the choice of kernel is crucial for detecting non-classicality. Future research could focus on extending the method to higher-order states and optimizing the kernel function to improve sensitivity.

Multipartite Entanglement Detection via Wigner Function Measurement

This work presents new methods for detecting genuine multipartite entanglement in continuous-variable systems, addressing a significant challenge in quantum information science. Researchers developed techniques to verify entanglement through measurements of the Wigner function, circumventing the difficulties associated with traditional quadrature measurements in certain experimental platforms. The team demonstrated that controlled Gaussian unitaries, specifically parity, displacement, and beamsplitter operations, can effectively detect multipartite entanglement in states such as Dicke states, W states, and cat states. These new approaches require significantly fewer measurement settings than complete quantum state reconstruction, with one scheme needing measurements on auxiliary modes only, representing a substantial practical advantage.

The researchers rigorously tested the robustness of these methods against realistic experimental imperfections, including energy relaxation and finite measurement resolution, confirming their reliability. These advancements pave the way for certifying multipartite entanglement in diverse platforms including circuit and cavity electrodynamics, circuit acoustodynamics, and systems employing trapped ions and atoms, where direct access to bosonic degrees of freedom is limited. Establishing genuine multipartite entanglement is a crucial step towards realizing the full potential of quantum technologies for computing, metrology, communication, and sensing.