cqom itn fellow final report ldtoth ·...
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Marie Curie ITN cQOM Summary of the Scientific Achievements
Name of Fellow: Laszlo Daniel Toth Principal Investigator: Prof. Tobias Kippenberg Academic / Industrial Institution: Swiss Federal Institute of Technology in Lausanne (EPFL) Start Date of ITN Fellowship: 01.11.2013 End Date of ITN Fellowship: 31.05.2016 Date of Report: 23.06.2016
1. Description of research work
My research is oriented towards establishing and exploiting the optomechanical interaction using superconducting microwave circuits, in which a mechanical element (a suspended membrane forming an electrode of a capacitor) is coupled to an inductor-‐capacitor circuit made by a thin superconductor. In particular, I am interested in realizing microwave optomechanics in a multimode system where multiple microwave modes couple to multiple mechanical modes. The physical realization of optomechanics in the microwave domain has certain advantages over its optical counterpart. First, since microwave photons have much lower energy, the system can sustain a high number of them – therefore enhancing the optomechanical interaction -‐ without causing heating effects detrimental to many quantum experiments. Second, one can tap on the well-‐established technologies related to microwave generation and detection. Third, the circuit architecture is particularly well suited to realize multimode optomechanics, granted that one establishes sufficient control of the relevant mode parameters in the design and fabrication of the superconducting circuit and the mechanical elements. In the past few years, microwave optomechanics has seen a surge and significant milestones have been achieved, as the cooling of a micromechanical membrane to its quantum ground state, quantum state transfer and entanglement generation between a microwave and a mechanical mode and quantum squeezing of mechanical motion among others. All the experiments mentioned above explicitly exploit the regime in which the cold microwave mode is much more dissipative than the mechanical mode, forming a cold dissipative reservoir for the latter. Breaking from this paradigm, recent theoretical work has considered the opposite regime in which the dissipation of the mechanical oscillator dominates and provides a cold dissipative reservoir to the electromagnetic degree of freedom. This novel scenario, coined as the reversed dissipation regime, allows for manipulation of the electromagnetic mode and enables a new class of dissipative interactions. In practice, this hierarchy can only be achieved in a multimode electromechanical system, where one dissipative microwave mode is used to cool and damp the mechanical oscillator and prepare it as a cold bath for the other microwave mode. A particular goal of my work is to achieve this reversed dissipation regime of circuit electromechanics. Once the reversed dissipation regime is achieved, one can use it to amplify microwave signals with minimum added noise – as dictated by quantum mechanics -‐ provided that the mechanical element is close to its quantum ground state. These types of amplifiers are called quantum-‐limited amplifiers and they have become technologically feasible and relevant in the field of
quantum technologies (quantum computing, quantum sensing etc.) in recent years. The first realization and characterization of such a device based on the reversed dissipation regime is also within the scope of my project. Generally, having two light modes coupled to the same mechanical oscillator provides a more convenient way to perform various measurements compared to single-‐mode optomechanics. For example, one can use one electromagnetic mode for regular sideband cooling and the other one to infer the state of the mechanical oscillator without disturbing it (a scheme called out-‐of-‐loop thermometry). More concretely, when the mode is cooled close to the ground state by one of the modes, one can probe on resonance of the other mode and measure the asymmetry in the scattered sidebands. This asymmetry is a direct consequence of quantum mechanics and provides a self-‐calibrated measure of the thermal occupation of the mechanics. Another use of this system is the demonstration of the phenomenon of level attraction or unstable avoided crossing. If two coupled modes cross in frequencies with one of negative energy and the other of positive energy, instead of the usual level repulsion, the levels attract each other and become unstable when degenerate. The optomechanical Hamiltonian with a blue-‐detuned pump gives rise to this exact setting. Here the second mode is useful only for the practical reason that the mechanical and microwave modes should have similar widths for the resulting dispersion to be visible. The auxiliary microwave mode is then required to damp and cool the mechanical oscillator.
2. Goals achieved and/or progress towards them
Microwave optomechanics was a new direction in the group, therefore the first goal was to set up a lab, essentially from scratch, and all the equipment necessary for measurements. At the heart of the measurement setup is a dry dilution refrigerator (BlueFors), capable of achieving ~ 8mK in continuous operation. Initially set up in the beginning of 2014, various aspects have been improved (including the design of the sample holder, characterization and filtering of the measurement lines, noise calibration capabilites) and the laboratory has been fully renovated and upgraded during the period of 1.11.2015 – 1.05.2016.
The clean room process for fabricating superconducting circuits with a mechanically compliant vacuum-‐gap capacitor was developed by me in the Micro-‐ and Nanofabrication Facility (CMi) at EPFL. The process consists of 4 lithography layers and 3 deposition layers and produces electromechanical LC resonators with Q-‐factors close to 105. An example of a fabricated chip and devices is shown on Figure 1. To achieve the reversed dissipation regime, the capability of producing high quality factors is not enough; one needs to engineer two microwave modes with substantially different external coupling factors (but both coupled to the same mechanical element). We have achieved this condition by utilizing the (symmetric and antisymmetric) normal modes of an initially degenerate coupled hybrid system, requiring careful initial simulations and circuit design (see Figure 2. A). We have reached the reversed dissipation regime by damping the mechanical oscillator to Γeff/2π>6κ using the auxiliary (symmetric, bright) microwave mode (κ is the energy dissipation rate of the dark, antisymmetric mode).
Figure 1. An example a fabricated device with a multimode superconducting circuit (Al on sapphire). A) Photograph of the 6.5 mm x 9.5 mm chip. B) optical micrograph of the device. The mechanically compliant vacuum-‐gap capacitor is in the middle whose bottom plate is split and connected to two inductors. C) Scanning electron micrograph of the vacuum-‐gap capacitor. D) Topography of the capacitor obtained by optical profilometry at room temperature.
Figure 2. Reaching the required hierarchy of dissipation rates for the microwave modes for the reversed dissipation regime. A) Optical micrograph of the device. B) Simplified measurement scheme. Multiple sources are combined and sent down the dilution refrigirator through various stages of attenuation. The signal interacts with the chip and is measured after a HEMT amplifier. D) Response of the device featuring the hierarchy in the dissipation rates of the two modes.
Using this hierarchy, we then demonstrated the “mechanical spring effect”, i.e. the modification of the susceptibility of a microwave mode using the dissipative mechanical element through the optomechanical interaction. Effectively, this provides a way to tailor the internal dissipation rate of the cavity and when this turns negative, there is net gain introduced and the system works as a low-‐noise amplifier. We have characterized this amplifier, found that, although in a narrow bandwidth, it can beat the commercial HEMT amplifiers in terms of added noise and currently are working on fully understanding and improving its noise properties.
In the longer run, beyond offering the manipulation of microwave fields, such a dissipative reservoir for microwave light, when coupled to multiple cavity modes, forms the basis of microwave entanglement schemes and dissipative quantum phase transitions. Equally important, combining such reservoir-‐mediated interaction with coherent dynamics allows for the realization of recently predicted non-‐reciprocal devices, which would extend the available toolbox of quantum-‐limited microwave manipulation techniques. We have also optimized our device and measured level attraction in an optomechanical system. These measurements are currently being analysed and a full theoretical model taking into account experimental details is being set up.
3. Training received (complementary/soft skills, ITN workshops attended)
I have attended the following workshops during my ITN fellowship: 7-‐9 October 2013 Theory of cavity optomechanics (Erlangen) 10 October 2013 Taking an idea to a product (Munich) 2-‐4 April 2014 Laser Stabilization and high-‐sensitivity displacement sensing (Paris) 21-‐23 July 2014 Finite element modeling (Lausanne) 1-‐5 February 2015 cQOM Diavolezza workshop 30 Nov – 1 Dec 2015 Taking a research idea to a product (IBM Zurich) 31 Jan – 4 Feb 2016 cQOM Diavolezza workshop 17-‐19 May 2016 From photonics research to the CMOS-‐fab (Ghent)
4. Participation and role in dissemination and outreach activities
I have participated in various outreach activities throughout my ITN fellowship. In particular:
1. I participated in setting up and presenting at our booth at the Nuit de la Science in Geneva (July 2014). We showcased three custom made photonics-‐related experiments. Nuit de la Science is a multi-‐day public event organized by the Museum d`histoire naturelle in Geneva every two years, attracting thousands of people from all age groups. The booth was visited by hundreds of people and we were engaged in discussions with the public for two entire days. I acknowledge the great help of Clément Javerzac-‐Galy and Nicolas Piro.
2. I have been involved in the “Journée des classes” in June 2015, where we showcased fun experiments related to the fundamental aspects of light such as light scattering, diffraction, polarization etc. Journée des classes is a full day event at EPFL where primary and high-‐school students visit labs and participate in interactive experiments. I acknowledge support from Clément Javerzac-‐Galy for masterminding our participation in the event and obtaining distributable materials.
3. Together with ESR Talitha Weiss, I have managed the project to update the article on “cavity optomechanics” on Wikipedia, which is the popular web-‐based, multilingual free-‐content encyclopedia project with over 375 million unique visitors a month. The project was supported by ESRs Koppany Kormoczy, Nenad Kralj, Liu Qiu and Ryan Schilling and I corresponded with the original Wikipedia contributor to the topic over the course of the project. Currently the page receives over 30 unique visitors a day on average, contributing significantly to provide information to the general public about the central topic of our research.
5. List of conferences attended
Apart from the ITN workshops listed above, I have attended the following conferences and invited seminars: 9-‐14 March 2014 Gordon Research Conference (Ventura, USA) poster presented 7-‐9 January 2015 QSIT NCCR Meeting (Arosa, Switzerland) poster presented 7-‐12 June 2015 QSIT conference (Monte Verita, Switzerland) poster presented 19 June 2015 Center for Nanoscale Systems (Harvard, USA) invited talk 4 January 2016 Wigner Research Center (Budapest, Hungary) invited seminar talk 6-‐11 March 2016 Gordon Research Conference (Ventura, USA) poster presented
6. Publications (with links) L. D. Toth, N. Bernier, A. Nunnenkamp, E. Glushkov, A. K. Feofanov, T. J. Kippenberg: Engineered dissipative reservoir for microwave light. E-‐print: http://arxiv.org/abs/1602.05180 C. Javerzac-‐Galy, K. Plekhanov, N. Bernier, L. D. Toth, A. K. Feofanov, T. J. Kippenberg: On-‐chip integrated microwave-‐to-‐optical quantum coherent converter. E-‐print: http://arxiv.org/abs/1512.06442