Sideband Cooling Beyond the Quantum Limit with Squeezed Light

Abstract: Quantum fluctuations of the electromagnetic vacuum impose an observable quantum limit to the lowest temperatures that can be reached with conventional laser cooling techniques. As laser cooling experiments continue to bring massive mechanical systems to unprecedented temperatures, this quantum limit takes on increasingly greater practical importance in the laboratory. Fortunately, vacuum fluctuations are not immutable, and can be "squeezed" through the generation of entangled photon pairs. Here we propose and experimentally demonstrate that squeezed light can be used to sideband cool the motion of a macroscopic mechanical object below the quantum limit. To do so, we first cool a microwave cavity optomechanical system with a coherent state of light to within 15% of this limit. We then cool by more than 2 dB below the quantum limit using a squeezed microwave field generated by a Josephson Parametric Amplifier (JPA). From heterodyne spectroscopy of the mechanical sidebands, we measure a minimum thermal occupancy of 0.19 phonons. With this novel technique, even low frequency mechanical oscillators can in principle be cooled arbitrarily close to the motional ground state, enabling the exploration of quantum physics in larger, more massive systems.

• The paper demonstrates that using squeezed light, rather than conventional coherent states, enables cooling beyond the quantum limit.
• It employs a Josephson Parametric Amplifier to generate squeezed states, achieving a measurable 2 dB reduction in thermal fluctuations.
• The findings reduce the phonon occupancy to 0.19 ± 0.01, paving the way for advanced quantum control in macroscopic mechanical systems.

Sideband Cooling Beyond the Quantum Limit with Squeezed Light

This paper investigates the use of squeezed light for achieving temperatures below the quantum limit in sideband cooling applications involving macroscopic mechanical systems. The quantum limit, typically imposed by vacuum fluctuations, represents the minimal achievable thermal motion when using standard laser cooling techniques. The study addresses this limitation by utilizing squeezed light, which involves the generation of entangled photon pairs, to further reduce mechanical motion's thermal occupancy.

Key Experiment and Results

The authors experimentally demonstrate the feasibility of using squeezed light to sideband cool a microwave cavity optomechanical system. Initially, laser cooling via a coherent state of light is employed, bringing the system to within 15% of the quantum limit. Subsequently, cooling is enhanced beyond the quantum limit by applying a squeezed microwave field, resulting in a remarkable 2 dB reduction below this limit. The experiment utilizes a Josephson Parametric Amplifier (JPA) to generate the necessary squeezed light field.

From the mechanical sidebands' heterodyne spectroscopy, the investigation records a minimized thermal phonon occupancy of $0.19 \pm 0.01$. This represents a significant advancement over traditional methods, offering an innovative approach to achieve colder states in massive mechanical oscillators, including those operating at lower frequencies.

Implications and Future Directions

This research suggests that using squeezed light as a cooling technique is not fundamentally restricted by the existing quantum limit faced in conventional coherent state cooling. By successfully demonstrating sub-quantum-limit cooling, the study opens up prospects for exploring quantum mechanics in larger systems where initial thermal states could be detrimental to the study of quantum phenomena.

The implications of this work are broad, offering novel pathways for improving the thermal noise performance of cavity optomechanical systems extensively used in quantum information and sensing applications. The insights provided by this paper pave the way for potential developments in cooling techniques, especially in the context of non-linear interactions and dissipative couplings, which might enable more efficient control over quantum states in macroscopic systems.

Further, the utilization of squeezed states could bolster other quantum-enhanced protocols in quantum communication and computation sectors, emphasizing the utility beyond just cooling. Future research might explore the scaling of this method to different frequency regimes or its integration into more complex quantum systems and networks.

Conclusion

By demonstrating sideband cooling beyond the quantum limit with squeezed light, this study challenges preconceived constraints in optomechanical cooling and sets the stage for advanced investigations into quantum physics in massive mechanical systems. The results underscore the potential of squeezed states not just in reducing thermal motion but also in driving the next generation of quantum technologies.