Andreas Angerer

Smooth Optimal Quantum Control for Robust Solid-State Spin Magnetometry.

We experimentally demonstrate a simple yet versatile optimal quantum control technique that achieves tailored robustness against qubit inhomogeneities and control errors while requiring minimal bandwidth. We apply the technique to nitrogen-vacancy (NV) centers in diamond and verify its performance using quantum process tomography. In a wide-field NV center magnetometry scenario, we achieve a homogeneous sensitivity across a 33% drop in control amplitude, and we improve the sensitivity by up to 2 orders of magnitude for a normalized detuning as large as 40%, achieving a value of 20  nT Hz(-1/2) μm(3/2) in sensitivity times square root volume.

Collective strong coupling with homogeneous Rabi frequencies using a 3D lumped element microwave resonator

We design and implement 3D-lumped element microwave cavities that spatially focus magnetic fields to a small mode volume. They allow coherent and uniform coupling to electron spins hosted by nitrogen vacancy centers in diamond. We achieve large homogeneous single spin coupling rates, with an enhancement of more than one order of magnitude compared to standard 3D cavities with a fundamental resonance at 3 GHz. Finite element simulations confirm that the magnetic field distribution is homogeneous throughout the entire sample volume, with a root mean square deviation of 1.54%. With a sample containing 1017 nitrogen vacancy electron spins, we achieve a collective coupling strength of Ω = 12 MHz, a cooperativity factor C = 27, and clearly enter the strong coupling regime. This allows to interface a macroscopic spin ensemble with microwave circuits, and the homogeneous Rabi frequency paves the way to manipulate the full ensemble population in a coherent way.

Coherent Coupling of Remote Spin Ensembles via a Cavity Bus

We report coherent coupling between two macroscopically separated nitrogen-vacancy electron spin ensembles in a cavity quantum electrodynamics system. The coherent interaction between the distant ensembles is directly detected in the cavity transmission spectrum by observing bright and dark collective multiensemble states and an increase of the coupling strength to the cavity mode. Additionally, in the dispersive limit we show transverse ensemble-ensemble coupling via virtual photons.

Solid-state electron spin lifetime limited by phononic vacuum modes

Longitudinal relaxation is the process by which an excited spin ensemble decays into its thermal equilibrium with the environment. In solid-state spin systems, relaxation into the phonon bath usually dominates over the coupling to the electromagnetic vacuum1–9. In the quantum limit, the spin lifetime is determined by phononic vacuum fluctuations10. However, this limit was not observed in previous studies due to thermal phonon contributions11–13 or phonon-bottleneck processes10, 14,15. Here we use a dispersive detection scheme16,17 based on cavity quantum electrodynamics18–21 to observe this quantum limit of spin relaxation of the negatively charged nitrogen vacancy (NV−) centre22 in diamond. Diamond possesses high thermal conductivity even at low temperatures23, which eliminates phonon-bottleneck processes. We observe exceptionally long longitudinal relaxation times T1 of up to 8 h. To understand the fundamental mechanism of spin–phonon coupling in this system we develop a theoretical model and calculate the relaxation time ab initio. The calculations confirm that the low phononic density of states at the NV− transition frequency enables the spin polarization to survive over macroscopic timescales.A systematic investigation of the spin relaxation in nitrogen vacancy centres in diamonds, induced by phononic vacuum modes at low temperature, reveals an upper limit of eight hours.

Ultralong relaxation times in bistable hybrid quantum systems

Amplitude bistability in a solid-state hybrid quantum system shows critical slowing down with ultralong relaxation times. Nonlinear systems, whose outputs are not directly proportional to their inputs, are well known to exhibit many interesting and important phenomena that have profoundly changed our technological landscape over the last 50 years. Recently, the ability to engineer quantum metamaterials through hybridization has allowed us to explore these nonlinear effects in systems with no natural analog. We investigate amplitude bistability, which is one of the most fundamental nonlinear phenomena, in a hybrid system composed of a superconducting resonator inductively coupled to an ensemble of nitrogen-vacancy centers. One of the exciting properties of this spin system is its long spin lifetime, which is many orders of magnitude longer than other relevant time scales of the hybrid system. This allows us to dynamically explore this nonlinear regime of cavity quantum electrodynamics and demonstrate a critical slowing down of the cavity population on the order of several tens of thousands of seconds—a time scale much longer than observed so far for this effect. Our results provide a foundation for future quantum technologies based on nonlinear phenomena.Andreas Angerer§,1, 2, ∗ Stefan Putz§,1, 2, 3 Dmitry O. Krimer, Thomas Astner, 2 Matthias Zens, Ralph Glattauer, Kirill Streltsov, William J. Munro, Kae Nemoto, Stefan Rotter, Jörg Schmiedmayer, and Johannes Majer 2 Vienna Center for Quantum Science and Technology, Atominstitut, TU Wien, Stadionallee 2, 1020 Vienna, Austria Zentrum für Mikround Nanostrukturen, TU Wien, Floragasse 7, 1040 Vienna, Austria Department of Physics, Princeton University, Princeton, New Jersey 08544, USA Institute for Theoretical Physics, TU Wien, Wiedner Hauptstraße 8-10/136, 1040 Vienna, Austria NTT Basic Research Laboratories, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan National Institute of Informatics, 2-1-2 Hitotsubashi, Chiyoda-ku, Tokyo 101-8430, Japan (Dated: July 19, 2018)

Hybrid quantum systems in the microwave regime (Conference Presentation)

The efficient generation, coherent control, manipulation and measurement of quantum states of light and matter is at the core of quantum technologies. Hybrid quantum systems, where one combines the best parts of multiple individual quantum systems together without their weaknesses, are now seen as a way to engineer composite quantum systems with the properties one requires. This would in principle allow one to probe new physical regimes. However, the issue until recently has been that hybridization has not resulted in systems with superior properties. Recently however we [Nature Photonics 11, 3639 (2016)] have shown an increased coherence times in hybrid system is of composed nitrogen-vacancy centers strongly coupled to a superconducting microwave resonator. This demonstration has enabled this kind of hybrid system to enter the regime where quantum nonlinearities are present. We discuss several types of nonlinearity effects that can be naturally explored (bistability and superradiance). Our work paves the way for the creation of spin squeezed states, novel metamaterials, long-lived quantum multimode memories and solid-state microwave frequency combs. Further in the longer term it may enable the exploration of many-body phenomena in new cavity quantum electrodynamics experiments.

Superradiant emission from colour centres in diamond

Andreas Angerer, ∗ Kirill Streltsov, 2 Thomas Astner, Stefan Putz, 3 Hitoshi Sumiya, Shinobu Onoda, Junichi Isoya, William J. Munro, 8 Kae Nemoto, Jörg Schmiedmayer, and Johannes Majer 9 Vienna Center for Quantum Science and Technology, Atominstitut, TU Wien, Stadionallee 2, 1020 Vienna, Austria Present Address: Institute for Theoretical Physics, Universität Ulm, 89069 Ulm, Germany Present Address: Department of Physics, Princeton University, Princeton, New Jersey 08544, USA Sumitomo Electric Industries Ltd., Itami, Hyougo, 664-0016, Japan National Institutes for Quantum and Radiological Science and Technology, 1233 Watanuki, Takasaki, Gunma 370-1292, Japan Research Centre for Knowledge Communities, University of Tsukuba, 1-2 Kasuga, Tsukuba, Ibaraki 305-8550, Japan NTT Basic Research Laboratories, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan National Institute of Informatics, 2-1-2 Hitotsubashi, Chiyoda-ku, Tokyo 101-8430, Japan Wolfgang Pauli Institut, c/o Fak. Mathematik Univ. Wien (Dated: February 21, 2018)

Large-scale quantum technology based on luminescent centers in crystals

Luminescent defects in crystals are prime candidates for the creation of a quantum technology given their superlative spin coherence lifetimes. Here we describe the main features of a quantum technology based on crystalline defects which builds upon the impressive recent achievements in diamond research. The basic features of the architecture and the requirements for its implementation are outlined, together with experimental progress and practical considerations towards its realization.