Atomic tunneling Systems and fluctuating Spins interacting with superconducting Qubits

Low-frequency excess flux noise strongly impairs the performance of superconducting quantum devices (SQDs) such as SQUIDs and Qubits. It is, for example, the dominating mechanism causing decoherence in flux or phase Qubits and makes SQUID based measurements of low-frequency signals rather challenging. Its spectral shape $A/{f^{\alpha}}$ is characterized by the amplitude $A$ and the noise exponent $\alpha$. Experimentally, it was shown that both parameters are correlated and the noise exponent $\alpha$ turns out to be temperature-dependent. Furthermore, recent experiments strongly hint for surface adsorbates such as molecular oxygen as an origin of low-frequency excess flux noise. Theory suggests that magnetic moments inside the material might give a considerable contribution or that the noise properties depend on the material the device is fabricated of. But even though more and more information on the origin and physical properties of this excess flux noise are gathered, a lot of open questions remain such as whether additional sources of low-frequency excess flux noise exist. In this contribution, we show indications for a correlation between the noise amplitude of dc-SQUIDs and the dc-magnetization of materials used for device fabrication. This suggests that low-frequency excess flux noise is to some extend caused by the conditions of the fabrication process. We further present a SQUID setup which allows for temperature-dependent cross-correlation measurements of the magnetic flux noise of a sample SQUID. Finally, we discuss the scaling of low-frequency excess flux noise of simple washer SQUIDs with device inductance, showing that the energy sensitivity rather than the magnetic flux noise is the more appropriate figure of merit for describing low-frequency excess flux noise.

his work is concerned with Al/Al-oxide(AlOx)/Al-layer systems which are important for Josephson-junction-based superconducting devices such as quantum bits. The device performance is limited by noise, which has been to a large degree assigned to the presence and properties of two-level tunneling systems in the amorphous AlOx-tunnel barrier. This has motivated our study on the optimization of the structural properties of Al/AlOx/Al-layer systems and the nature of the atomic tunneling systems contained in these layers. The structures were fabricated in a high-vacuum electron-beam deposition system. Analysis was performed by transmission electron microscopy combined with electron energy loss spectroscopy for composition determination of the AlOx-tunnel barrier. We show that the fabrication conditions have a strong impact on the nanostructural and nanochemical properties of the Al/AlOx/Al-layer system. Pretreatment of Si(111) substrates and deposition conditions were optimized to obtain quasi-epitaxial Al growth resulting in a low grain boundary density of the lower Al electrode as a prerequisite for AlOx-tunnel barriers with homogeneous thickness. Depending on the oxidation conditions, the composition of the tunnel barrier varies between AlO0.9 and AlO1.3. Low-temperature dielectric measurements show that the properties of two-level tunneling systems depend on the nanostructure and composition of the AlOx-tunnel barrier.

The low temperature dissipation in nanomechanical resonators is dominated by low energy excitations, i.e., intrinsic defects in glassy materials that are modeled as TLSs (tunneling two level systems). Here we present NEMS for studying individual TLSs. Using top-down nanofabrication techniques, we made 4 microns long,300 nm wide and 100 nm thick high-stress SiN doubly-clamped beam resonators with 30 nm of aluminum on top. We will first characterize the mechanical properties of the devices using the magnetomotive technique. Then the devices, with fundamental frequency around 70 MHz, will be cooled to 1 mK, so that they are in their quantum ground state. The motion of the beams will be detected by capacitively coupling the NEMS to a superconducting microwave cavity, allowing us to probe the dynamics of individual TLSs inside the glass beam.

We fabricated a superconducting Josephson phase qubit device by a NbN/AlN-based technology. Clear Rabi oscillations were observed. The oscillations decayed after a mean time was about of 3-5 ns. We performed systematic studies of the qubit energy relaxation time TD on different physical conditions such as different microwave pulse duration and microwave power. The qubit relaxation time TD was found to depend on the duration of the applied microwave pulse reaching values up to 80 ns for long pulses. We described this phenomenon in terms of the Purcell effect induced by interaction with weakly dissipative two-level systems, which remain unsaturated at low microwave powers and thus reduce the qubit life time.

Quantum two-level systems interacting with the surroundings are ubiquitous in nature. The interaction suppresses quantum coherence and forces the system towards a steady state. Such dissipative processes are captured by the paradigmatic spin-boson model, describing a two-state particle, the “spin”, interacting with an environment formed by harmonic oscillators. A fundamental question to date is to what extent intense coherent driving impacts a strongly dissipative system. Here we investigate experimentally and theoretically a superconducting qubit strongly coupled to an electromagnetic environment and subjected to a coherent drive. This setup realizes the driven Ohmic spin-boson model. We show that the drive reinforces environmental suppression of quantum coherence, and that a coherent-to-incoherent transition can be achieved by tuning the drive amplitude. An out-of-equilibrium detailed balance relation is demonstrated. These results advance fundamental understanding of open quantum systems and bear potential for the design of entangled light-matter states. Nat. Commun. 9, 1403 (2018).

To be presented soon

Read-out of a Majorana Qubit by a Current Measurement, Measurement-induced Dephasing and Relaxation Lupei Qin1,2, Xin-Qi Li2, Alexander Shnirman3, Gerd Schön1 1. Institut für Theoretische Festkörperphysik, KIT, 76131 Karlsruhe 2. Center for Joint Quantum Studies, Physics Dept., Tianjin University, 300072，China 3. Institut für Theorie der Kondensierten Materie, KIT 76131 Karlsruhe We investigate a setup, which allows measuring the state of a Majorana qubit formed by two parallel topological wires coupled by a trivial superconductor. For the measurement this system is coupled to two quantum dots which are coupled to normal metallic leads and also coupled directly, thus providing an interferometric loop. The interference effects in the current distinguish two basis states of the qubit in a QND measurement. We analyze the dephasing and relaxation resulting from the measurement. We consider the charging energy and discuss in detail the state of Majorana box qubit in the charge basis. Because of Coulomb-blockade effects the high-energy states are only virtually excited, but they are necessary for a current to flow. We also analyze the influence of varying gate voltages on the current.

The standard tunneling model for the description of the low temperature properties of disordered solids matches a broad variety of different physical quantities which have all the same microscopic origin – atoms or groups of atoms in metastable configurations, the so-called tunneling systems (TS). In this work we investigate the change of the velocity of sound in metallic glasses which is dominated by the dynamics of these TS. We show that the sound velocity of the superconducting and normal conducting states of the metallic glass can be consistently described by the standard tunneling model without changing the underlying density of states. The influence of the interaction between TS and electrons on the line-width of TS is crucial and explains the differences in the sound velocity between the superconducting and normal states in these materials. Taking into account an additional dephasing of TS further enhances the agreement between prediction and measurement. This agreement holds over the investigated range of frequencies from 1~kHz up to 2~GHz and temperatures from 10~mK to 2~K.

Out of equilibrium quasiparticles have been shown to be a prominent source of both relaxation and dephasing in superconducting quantum bits. Although determining their source continues to be an open question, we can link the quasiparticle dynamics to propagating phonons in the substrate [1]. We demonstrate [2] that surrounding granular aluminum resonators with thin film aluminum islands, which act as phonon traps, increases the internal quality factors of the resonators in the single photon regime, suppresses both noise amplitude and $1/f$ exponent, and reduces the rate of quasiparticle generation events recently observed in granular aluminum [3]. [1] F. Valenti, F. Henriques et int. al. I. M. Pop, arXiv:1810.12341 [2] F. Henriques, F. Valenti et int. al. I. M. Pop, in preparation [3] L. Grünhaupt et int. al. I.M. Pop, PRL 121.117001