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Final Report Summary - CAVITYMICROSCOPE (A Cavity-based Microscope for Quantum Gases)

Nicolas Spethmann, Technische Universität Kaiserslautern,

Project context and objectives. Advances in understanding and controlling quantum systems promise to revolutionize science and technology in the near future. This is also reflected by the recent announcement of the research flagship ’Quantum Technologies’ of the European Commission, and corresponding national initiatives. Here, a crucial aspect is the quantum measurement process, inevitably influencing the object under study. This backaction of the measurement is a fundamental aspect of quantum mechanics, limiting both fundamental research and applications in quantum physics. This is the challenge that ’CavityMicroscope’ addresses with a novel microscope, based on sensitive detection of atoms evolving within a high-finesse optical cavity. Facilitated by the Marie-Curie International Outgoing Fellowship, the project benefited from the synergy of the cutting-edge expertise at the outgoing institution of University of California, Berkeley (UCB) with the world-leading research performed at the homegoing institution of Technical University Kaiserslautern (UNIKL).

Cold atom experiments routinely deliver samples with excellent control over both external and internal degrees of freedom. In CavityMicroscope, we combine such quantum systems with quantum-limited sensing facilitated by a high-finesse optical cavity, opening the initially well isolated quantum system to the environment in a controlled manner. In this way, CavityMicroscope realizes a ground-breaking, versatile quantum system under quantum-limited measurement and control. Here, CavityMicroscope focused onto two main applications, the study of dynamical properties of interacting quantum objects and impurities, and the realization of tailored quantum systems, as for instance feedback in the quantum regime.

Work performed and main results. A complex quantum system, as required for future applications of quantum technology, can be constructed by coupling simple elements. However, the mediatad interaction may be compromised by measurement backaction, limiting its usefulness for quantum engineering. To shed light onto this important challenge, we developed the capabilities to detect and manipulate the collective mechanical motion of atomic clouds at the quantum limit. We created a prototypic system composed of two distinghuishable, collective oscillators, as illustrated in Fig. 1. We demonstrated the time-resolved transfer of excitation between these initally isolated oscillators via the cavity light field. Here, we observed the noise increase dictated by measurement backaction, and correlations in this noise, proving quantum-limited performance. Our results point to the potential, and also the challenges, of detecting and coupling quantum objects with quantum light, representing one of the main requirements for future quantum devices.

A complementary non-destructive and sensitive probe for quantum gases is represented by a single atom impurity immersed in the quantum gas under observation. At the homegoing institution (UNIKL), the methods developed within CavityMicroscope were combined with and applied to this line of research. We established techniques previously not available at UNIKL, as for instance high-finesse optical cavities. This facilitated to apply the novel approach generated by the synergy of experience from the outgoing institution with knowledge at the homegoing institution to the research dealing with impurities. In particular, expanding the realization of sideband cooling inside a cavity demonstrated in [2], we implemented a related technique (Raman sideband cooling) to cool single neutral atoms, before immersing them into a quantum gas of a different atomic species. This development facilitates a higher level of control crucial to realize novel physical scenarios. We observed, for instance, a strong signature of impurity-quantum gas interaction with high spatial resolution [3]. These results demonstrate the non-destructive single atom probing of a quantum gas, paving the way to create and study fascinating hybrid quantum systems, relevant for future quantum technology.

The spin degree of freedom of atoms inherently shows quantum features directly applicable to open questions of quantum engineering. In order to exploit this in CavityMicroscope, we realized coupling of the internal degree of freedom of atoms to the cavity light, illustrated in Fig. 2. We thereby expanded the core abilities of CavityMicroscope to the quantum-limited monitoring and sensitive, real-time measurement of the spin of ultracold atoms, paving the way for novel, previously unaccessible scenarios. In particular, we demonstrate the autonomous feedback stabilization of a quantum system near its groundstate despite the presence of measurement backaction. This demonstration shows a way to control and stabilize open quantum systems, which is an unresolved challenge in state-of-the-art quantum engineering. The realization of this novel scenario further offers unique resources for quantum engineering. We demonstrate, for instance, a high-energy groundstate spin oscillator, that can be thought of as a ’negative temperature’ quantum object. Combining this with a more conventional, positive temperature oscillator facilitates to realize systems immune to measurement backaction, relevant for future quantum devices and, in particular, sensors in the quantum regime.

A salient aspect of recent experimental quantum physics research in general and of CavityMicroscope in particular is represented by improving detection methods to reach the limits of quantum mechanics in order to unlock the potential of quantum technology. Beyond this challenging experimental effort, developing measurement strategies of a novel type can provide a substantial improvement for the observation of quantum systems. We proposed such an advanced measurement scheme that goes beyond existing techniques [4]. Here, the general idea is to harness the correlations imprinted by the dynamics of the quantum system onto the light field thus taking the effects of measurement backaction into account. This scheme of ’synodyne detection’ is capable of surpassing the quantum limits of detection. These techniques are applicable to a broad range of quantum systems, and therefore might turn out to be beneficial for a number of applications from quantum technology. For instance, synodyne detection is capable to achieve sensitivity beyond the standard quantum limit of force sensing, and the concepts are transferrable to a wide range of sensors in the quantum regime.

Conclusions and socio-economic impact. While CavityMicroscope focused on fundamental research, the addressed challenges and questions of quantum engineering will have a high practical impact on society and economy in the near future with high probability. Classical engineering, which forms the basis of our modern world, will probably soon be complemented by engineering and technology in the quantum regime. Both classical and quantum engineering crucially rely on measurement. Therefore, the understanding and designing of measurement and feedback in the quantum regime, where system dynamics are significantly influenced by quantum measurement backaction, will be one of the main challenges in unlocking the potential of quantum devices for future quantum technology. However, although predicted several decades ago, it has only recently become possible to perform experiments in this regime, and CavityMicroscope is one of the few examples reaching this capability.

With the advance of technology, an increasing number of important systems both from practice and research will inevitably operate in the regime governed by effects of quantum mechanics and, in particular, the quantum backaction of the measurement. It is therefore crucial to develop strategies and technologies that can overcome the limits imposed by quantum mechanics, and facilitate to exploit novel possibilities beyond classical physics. CavityMicroscope addressed this challenge and pushed the boundaries of quantum physics research further, thereby creating excellent starting points for future quantum physics research. CavityMicroscope, and the ground-breaking research it enabled and triggered, will therefore have a high impact for science, society and economy in the near future.


[1] Nicolas Spethmann, Jonathan Kohler, Sydney Schreppler, Lukas Buchmann, and Dan M. Stamper-Kurn. Cavity-mediated coupling of mechanical oscillators limited by quantum back-action. Nature Physics, 12(1):27–31, January 2016.
[2] Jonathan Kohler, Nicolas Spethmann, Sydney Schreppler, and Dan M. Stamper-Kurn. Cavity-assisted measurement and coherent control of collective atomic spin oscillators. Phys. Rev. Lett., 118:063604, 2017.
[3] In preaparation.
[4] L.F. Buchmann, S. Schreppler, J. Kohler, N. Spethmann, and D.M. Stamper-Kurn. Complex squeezing and force measurement beyond the standard quantum limit. Phys. Rev. Lett., 117:030801, 2016.

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