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Many-body effects in hybrid quantum systems

Periodic Reporting for period 1 - MEHYB (Many-body effects in hybrid quantum systems)

Reporting period: 2015-04-01 to 2017-03-31

The development of novel technologies, which are based on fundamental principles of quantum mechanics like superpositions or entanglement, is currently very actively pursued. Not only researchers, but also more and more companies devote considerable efforts to the realization of quantum computers, secure quantum communication networks, or quantum simulators for material science and chemistry applications. This development is pursed in parallel with a variety of different physical systems including, for example, single photons, trapped atoms and ions, defects centers in solids or superconducting quantum circuits. The interdisciplinary field of hybrid quantum systems aims at the integration of different optical, atomic and solid-state systems to harness their combined functionalities in an optimal way. For example, optical photons are excellent information carriers, while electronic spins and superconducting circuits are ideally suited for storing and processing quantum information, respectively. However, these systems do not naturally interact with each other and therefore new schemes for artificially interfacing quantum systems of different types must be explored.

In the current project we have theoretically analyzed a set of hybrid quantum systems involving superconducting circuits, electronic spins in solids and tiny mechanical resonators. The original objective was to investigate, how the combined functionalities of these system can be used for enhancing magnetometry applications and for realizing new types of quantum simulators for unconventional many-body interactions. In this context we have specifically investigated new efficient schemes for coupling a single electronic spin to the quantized motion of a mechanical nanoresonator. This interface constitutes a basic building block for hybrid quantum systems, where, for example, quantum information is stored in the spin-quantum memory, while the mechanical system is used as an interconnection to superconducting circuits or optical photons. We further showed, how superconducting circuits can be used to simulate light-matter interactions in the so-called ultrastrong coupling regime, which is not accessible with real atoms and photons. In this work we discovered a new quantum many-body effect, which was overlooked in related studies before and can be used as a natural entanglement resource. Finally, we found an unexpected noise-evasion mechanism for quantum communication schemes, which enables a faithful transmission of quantum information through noisy channels. This mechanism makes quantum communication over electric microwave channels possible, where otherwise the weak quantum signal would be washed out by an unavoidable background of thermal microwave photons.

In conclusion, in this project several important results for the further development of hybrid quantum systems and quantum technologies based on superconducting quantum circuits have been obtained. This concerns, in particular, our quantum communication protocol for noisy channels, which enables a completely approach for intra-city quantum networks based on microwave technology only.
"Over the full duration of the project we have performed theoretical work on hybrid quantum systems involving superconducting circuits, electronic spins in solids and nanomechanical resonators, we have analyzed the properties of superconducting circuit QED systems in the ultrastrong coupling regime and we have investigated a new quantum communication protocol for sending quantum states through noisy channels. The results of these works have been published in high-impact physics journals like Physical Review Letters or Physical Review X and presented at international conferences. We have also performed additional work on spin-squeezing effects in solid-state spin ensembles and cold atom settings, as well as studies of the Dicke phase transition in the presence of strong dipole-dipole interactions. The results of those projects are still preliminary and have not been published yet.

In summary the main outcomes of the project are as follows:

1) In close collaboration with our colleagues in China and Japan, we have proposed and analyzed two different schemes for efficiently interfacing a single electronic spin with a macroscopic mechanical resonator. In the first study, which was published in Physical Review Applied [Phys. Rev. Applied 4, 044003, (2015)], we analyzed a hybrid device, where a microscale diamond beam with a single embedded spin qubit is coupled to a superconducting microwave cavity. In the second study published in Physical Review Letters [Phys. Rev. Lett. 117, 015502, (2016)] and selected as an Editor's suggestion, we showed, how spin qubits in diamond can be efficiently coupled to a suspended carbon nanotube via DC currents

2) We have analyzed the ground state of a superconducting circuit QED system in the ultrastrong coupling regime. In this work we could clarify a longstanding debate about the existence or non-existence of a superradiant phase and identified a new type of entanglement mechanism. This work has been published in Physical Review A [Phys. Rev. A 94, 033850, (2016)].

3) We have proposed and analyzed a general scheme for transferring quantum information through a noisy channel. Specifically, we described the application of this principle for the implementation of intra-city quantum networks based on microwave technologies only. This work was published in Physical Review X [Phys. Rev. X 7, 011035, (2017)] and highlighted by a parallel popular science article in the journal ""Physics"". For this work a press release was made, which appeared on the webpage of the TU Wien and in the science section of several Austrian and international newspapers and websites.

Open access versions of all the publications are available on the preprint server arXiv.
The findings of this project go clearly beyond the current scientific state of the art in the field: The proposed hybrid systems make a strong spin-mechanics interface possible, while compared the previous schemes the substantially reduce the experimental complexity. Our work on circuit QED systems in the ultrastrong coupling regime not only clarified a long-standing debate about the exact ground state of this system. It also revealed many unexpected properties of this system, which were unknown before, and show that the physics of ultrastrongly coupled cavity QED systems is just the opposite of what is commonly assumed. Finally, we have shown for the first time, how quantum information can be transferred through a quantum channel in the limit of large noise. This result opens up a completely new perspective on microwave-based quantum communication schemes, which were technically not feasible before.

The expected impact of this project is primary on the level of basic research, where it will contribute to a better understanding superconducting quantum circuits and a further advancement of hybrid quantum systems. On the long run, i.e. when quantum communication and quantum information processing schemes become available for the general public, this research can also have a direct impact our society. In particular, our work on intracity quantum networks based on microwave photons might have a strong influence on future quantum communication strategies.
Q-function of qubits in the paper Phys. Rev. A 94, 033850 (2016)
Schematic of the nanotube-NV hybrid quantum system in the paper Phys. Rev. Lett. 117, 015502 (2016)
Image in the news, which reports our work in the paper Phys. Rev. X 7, 011035 (2017)