Distributed quantum sensing in a continuous variable entangled network (1905.09408v2)

Abstract: Networking plays a ubiquitous role in quantum technology. It is an integral part of quantum communication and has significant potential for upscaling quantum computer technologies that are otherwise not scalable. Recently, it was realized that sensing of multiple spatially distributed parameters may also benefit from an entangled quantum network. Here we experimentally demonstrate how sensing of an averaged phase shift among four distributed nodes benefits from an entangled quantum network. Using a four-mode entangled continuous variable (CV) state, we demonstrate deterministic quantum phase sensing with a precision beyond what is attainable with separable probes. The techniques behind this result can have direct applications in a number of primitives ranging from biological imaging to quantum networks of atomic clocks.

• The paper demonstrates that a single squeezed continuous variable state distributed across four nodes improves phase resolution from 9.06° to 5.66°.
• It compares the separable and entangled approaches, with the entangled network achieving Heisenberg scaling in phase estimation.
• The study employs an optical parametric oscillator and homodyne detection, charting a path for scalable, quantum-enhanced sensing applications.

Overview of Distributed Quantum Sensing in a Continuous Variable Entangled Network

The paper entitled "Distributed quantum sensing in a continuous variable entangled network" presents an experimental paper on quantum-enhanced distributed sensing through the utilization of continuous variable (CV) entanglement. The research demonstrates an advancement in phase sensing precision by implementing a four-node network using a single squeezed CV state, achieving a sensitivity surpassing that of separate, unentangled probes.

The core focus of this paper is the enhanced estimation of average phase shifts across spatially distributed nodes. This enhancement is achieved through the implementation of quadrature squeezing and entanglement, fundamentally leveraging quantum mechanical properties to exceed classical limits.

Theoretical Analysis

The paper provides a theoretical framework examining two different distributed sensing setups: a separable approach and an entangled network approach. The separable approach utilizes independent squeezed quantum states at each node, whereas the entangled approach employs a single squeezed state distributed across all nodes. Key outcomes include the entangled system achieving Heisenberg scaling for phase estimation in terms of both the number of modes and the number of photons, illustrating an enhancement in sensitivity proportional to $1/M$ in modes and $1/N$ per mode, respectively. In contrast, the separable setup follows the classical $1/\sqrt{M}$ scaling.

Experimental Design

The experimental setup is thoroughly described, involving a network designed to measure phase shifts at four nodes. A beam-splitter network is employed to distribute a single displaced squeezed state, realized through an optical parametric oscillator (OPO), across the nodes. The optical modes are meticulously measured using homodyne detectors which provide the necessary sensitivity enhancement due to mode-mode entanglement.

Results and Implications

The research successfully demonstrates empirically the anticipated sensitivity gains theoretically predicted. For instance, traditional coherent state probing corresponds to a standard quantum limit resolvability of $9.06^\circ$, whereas the entangled approach achieves phase resolutions down to $5.66^\circ$. This improvement exemplifies tangible gains superior to classical techniques, which are restricted by photon number limitations due to potential sample damage or heating.

Moreover, the experimental results affirm the feasibility of scalable quantum networks facilitating enhanced sensing capabilities in realistic settings. Such advancements hold considerable potential for application areas ranging from molecular tracking and clock synchronization to quantum-enhanced metrology across multiple domains.

Future Prospects

The paper paves the way for further exploration into improved distributed sensing networks beyond current bounds. Future research could focus on increasing the number of nodes and reducing losses in transmission channels, directly translating to improvements in sensitivity. Additionally, transitioning from Gaussian measurements to those involving non-Gaussian features may continue to push the limits. Expanding this framework to other configurations in quantum information applications, like entangled atomic clocks, aligns with ongoing developments towards refined quantum network capabilities.

In summary, the work encapsulates a compelling demonstration of distributed phase sensing via quantum entanglement, projecting a pathway for sophisticated deployment in practical quantum technologies.