Clemens Müller

Observation of directly interacting coherent two-level systems in an amorphous material

Parasitic two-level tunnelling systems originating from structural material defects affect the functionality of various microfabricated devices by acting as a source of noise. In particular, superconducting quantum bits may be sensitive to even single defects when these reside in the tunnel barrier of the qubit’s Josephson junctions, and this can be exploited to observe and manipulate the quantum states of individual tunnelling systems. Here, we detect and fully characterize a system of two strongly interacting defects using a novel technique for high-resolution spectroscopy. Mutual defect coupling has been conjectured to explain various anomalies of glasses, and was recently suggested as the origin of low-frequency noise in superconducting devices. Our study provides conclusive evidence of defect interactions with full access to the individual constituents, demonstrating the potential of superconducting qubits for studying material defects. All our observations are consistent with the assumption that defects are generated by atomic tunnelling.

Measuring the temperature dependence of individual two-level systems by direct coherent control

We demonstrate a new method to directly manipulate the state of individual two-level systems (TLSs) in phase qubits. It allows one to characterize the coherence properties of TLSs using standard microwave pulse sequences, while the qubit is used only for state readout. We apply this method to measure the temperature dependence of TLS coherence for the first time. The energy relaxation time T1 is found to decrease quadratically with temperature for the two TLSs studied in this work, while their dephasing time measured in Ramsey and spin-echo experiments is found to be T1 limited at all temperatures.

Quantitative evaluation of defect-models in superconducting phase qubits

We use high-precision spectroscopy and detailed theoretical modeling to determine the form of the coupling between a superconducting phase qubit and a two-level defect. Fitting the experimental data with our theoretical model allows us to determine all relevant system parameters. We observe a strong qubit-defect coupling with a nearly vanishing longitudinal component. We quantitatively compare several existing theoretical models for the microscopic origin of two-level defects.

Multiphoton spectroscopy of a hybrid quantum system

We report on experimental multiphoton spectroscopy of a hybrid quantum system consisting of a superconducting phase qubit coherently coupled to an intrinsic two-level system (TLS). We directly probe hybridized states of the combined qubit-TLS system in the strongly interacting regime, where both the qubit-TLS coupling and the driving cannot be considered as weak perturbations. This regime is described by a theoretical model which incorporates anharmonic corrections, multiphoton processes and decoherence. We present a detailed comparison between experiment and theory and find excellent agreement over a wide range of parameters.

Rabi spectroscopy of a qubit-fluctuator system

Superconducting qubits often show signatures of coherent coupling to microscopic two-level fluctuators (TLFs), which manifest themselves as avoided level crossings in spectroscopic data. In this work we study a phase qubit, in which we induce Rabi oscillations by resonant microwave driving. When the qubit is tuned close to the resonance with an individual TLF and the Rabi driving is strong enough (Rabi frequency of order of the qubit-TLF coupling), interesting four-level dynamics are observed. The experimental data show a clear asymmetry between biasing the qubit above or below the fluctuators level splitting. Theoretical analysis indicates that this asymmetry is due to an effective coupling of the TLF to the external microwave field induced by the higher qubit levels.

Quantum Zeno effect in the strong measurement regime of circuit quantum electrodynamics

We observe the quantum Zeno effect -- where the act of measurement slows the rate of quantum state transitions -- in a superconducting qubit using linear circuit quantum electrodynamics readout and a near-quantum-limited following amplifier. Under simultaneous strong measurement and qubit drive, the qubit undergoes a series of quantum jumps between states. These jumps are visible in the experimental measurement record and are analyzed using maximum likelihood estimation to determine qubit transition rates. The observed rates agree with both analytical predictions and numerical simulations. The analysis methods are suitable for processing general noisy random telegraph signals.

Interacting two-level defects as sources of fluctuating high-frequency noise in superconducting circuits

Since the very first experiments, superconducting circuits have suffered from strong coupling to environmental noise, destroying quantum coherence and degrading performance. In state-of-the-art experiments, it is found that the relaxation time of superconducting qubits fluctuates as a function of time. We present measurements of such fluctuations in a 3D-transmon circuit and develop a qualitative model based on interactions within a bath of background two-level systems (TLS) which emerge from defects in the device material. In our model, the time-dependent noise density acting on the qubit emerges from its near-resonant coupling to high-frequency TLS which experience energy fluctuations due to their interaction with thermally fluctuating TLS at low frequencies. We support the model by providing experimental evidence of such energy fluctuations observed in a single TLS in a phase qubit circuit.

Geometric quantum gates with superconducting qubits

We suggest a scheme to implement a universal set of non-Abelian geometric transformations for a single logical qubit composed of three superconducting qubits coupled to a single cavity. The scheme utilizes an adiabatic evolution in a rotating frame induced by the effective tripod Hamiltonian which is achieved by longitudinal driving of the qubits. The proposal is experimentally feasible with the current state of the art and could serve as a first proof of principle for geometric quantum computing.

Relaxation of Josephson qubits due to strong coupling to two-level systems

We investigate the energy relaxation (T1) process of a qubit coupled to a bath of dissipative two-level fluctuators (TLFs). We consider the fluctuators strongly coupled to the qubit both in the limit of spectrally sparse single TLFs as well as in the limit of spectrally dense TLFs. We conclude that the avoided level crossings, usually attributed to very strongly coupled single TLFs, could also be caused by many weakly coupled spectrally dense fluctuators.

Dual-probe decoherence microscopy: probing pockets of coherence in a decohering environment

We study the use of a pair of qubits as a decoherence probe of a nontrivial environment. This dual-probe configuration is modelled by three two- level systems (TLSs), which are coupled in a chain in which the middle system represents anenvironmentalTLS. This TLS resides within the environment of the qubits and therefore its coupling to perturbing fluctuations (i.e. its decoherence) is assumed much stronger than the decoherence acting on the probe qubits. We study the evolution of such a tripartite system including the appearance of a decoherence-free state (dark state) and non-Markovian behaviour. We find that all parameters of this TLS can be obtained from measurements of one of the probe qubits. Furthermore, we show the advantages of two qubits in probing environments and the new dynamics imposed by a TLS that couples to two qubits at once.