Superconducting qubits already demonstrated potential in emulating coherent back scattering or weak localization (WL) and tunnelling phenomena however, in a real multipath system they have not been verified yet.Here we show how a double-path system can be emulated by a quantum device through construction of multiple scattering centers in closed paths (detune boxes) and tunnel barriers with a large return probability (Pr) of electrons. Incorporation of such arrangements of tunnel barriers can add an extra geometric phase and demonstrate Aharonov-Bohm type oscillations (phi)0 and (phi/2)0 in a ring and a tube, respectively. A combination of inter and intra layer tunnelling in a double-path circuit creates a phase reversal and subsequently weak anti-localization (WAL) effect with a long coherence time. Finally, angle dependence of Pr firmly establishes stability of the two-path circuit which is also associated with a phase reversal due to the inter-path resonance.
Superconducting qubits already demonstrated potential in emulating coherent back scattering or weak localization (WL) and tunnelling phenomena however, in a real multipath system they have not been verified yet.Here we show how a double-path system can be emulated by a quantum device through construction of multiple scattering centers in closed paths (detune boxes) and tunnel barriers with a large return probability (Pr) of electrons. Incorporation of such arrangements of tunnel barriers can add an extra geometric phase and demonstrate Aharonov-Bohm type oscillations (phi)0 and (phi/2)0 in a ring and a tube, respectively. A combination of inter and intra layer tunnelling in a double-path circuit creates a phase reversal and subsequently weak anti-localization (WAL) effect with a long coherence time. Finally, angle dependence of Pr firmly establishes stability of the two-path circuit which is also associated with a phase reversal due to the inter-path resonance.
In condensed matter systems, coherent backscattering and quantum interference in the presence of time-reversal symmetry lead to well-known phenomena, such as weak localization (WL) and universal conductance fluctuations (UCFs). Here we use multipass Landau-Zener transitions at the avoided crossing of a highly coherent superconducting qubit to emulate these phenomena. The average and standard deviations of the qubit transition rate exhibit a dip and peak when the driving waveform is time-reversal symmetric, analogous to WL and UCFs, respectively. The higher coherence of this qubit enabled the realization of both effects, in contrast to the earlier work by Gustavsson et al. [Phys. Rev. Lett. 110, 016603 (2013)], who successfully emulated UCFs, but did not observe WL. This demonstration illustrates the use of nonadiabatic control to implement quantum emulation with superconducting qubits.
In condensed matter systems, coherent backscattering and quantum interference in the presence of time-reversal symmetry lead to well-known phenomena, such as weak localization (WL) and universal conductance fluctuations (UCFs). Here we use multipass Landau-Zener transitions at the avoided crossing of a highly coherent superconducting qubit to emulate these phenomena. The average and standard deviations of the qubit transition rate exhibit a dip and peak when the driving waveform is time-reversal symmetric, analogous to WL and UCFs, respectively. The higher coherence of this qubit enabled the realization of both effects, in contrast to the earlier work by Gustavsson et al. [Phys. Rev. Lett. 110, 016603 (2013)], who successfully emulated UCFs, but did not observe WL. This demonstration illustrates the use of nonadiabatic control to implement quantum emulation with superconducting qubits.
Recent experiments (Yamamoto et al 2012 Nature Nanotechnology 7 247 ) used the transport of electrons through an Aharonov–Bohm (AB) interferometer and two coupled channels (at both ends of the interferometer) to demonstrate a manipulable flying qubit. Results included in-phase and anti-phase (AB) oscillations of the two outgoing currents as a function of the magnetic flux, for strong and weak inter-channel coupling, respectively. Here we present new experimental results for a three terminal interferometer, with a tunnel coupling between the two outgoing wires. We show that in some limits, this system is an even simpler realization of the 'two-slit' experiment. We also present a simple tight-binding theoretical model which imitates the experimental setup. For weak inter-channel coupling, the AB oscillations in the current which is reflected from the device are very small, and therefore the oscillations in the two outgoing currents must cancel each other, yielding the anti-phase behavior, independent of the length of the coupling regime. Technically, the tight binding equations within the two coupled wires have four solutions for each electronic energy. In the 'anti-phase' region all of these solutions are wave-like, oscillating with the distance along the wires. As the coupling between the wires increases, two of these solutions become evanescent, and their amplitudes decay as the electron moves in the wires. In this regime, the amplitudes of the two remaining 'running' waves are proportional to each other, with a ratio which is practically flux-independent. As a result, the two outgoing currents are proportional to each other, yielding the 'in phase' behavior. For larger coupling all the solutions are evanescent, and the outgoing currents become very small.
We consider the phase-coherent transport of electrons passing through an Aharonov–Bohm ring while interacting with a tunnel charge in a double quantum dot (representing a charge qubit) which couples symmetrically to both arms of the ring. For Aharonov–Bohm flux Φ AB = h /2 e we find that electrons can only be transmitted when they flip the charge qubit's pseudospin parity an odd number of times. The perfect correlations of the dynamics of the pseudospin and individual electronic transmission and reflection events can be used to entangle the charge qubit with an individual passing electron.
Constructing circuits which invoke the tunnelling effect of Josephson junctions in the superconducting regime manifest controllable, quantum properties. The development of a quantum computer employing these phenomena is just one of the many advantages to be gained from building such devices. However, the use of quantum bits for computation is dependent on the ability to operate them in an essentially isolated environment. The phenomenon of decoherence refers to the instability of a quantum state of a system when it interacts with the surrounding environment. Superconducting qubits are sensitive to decoherence mechanisms within the readout leads connecting to the device, and more importantly from the materials from which they are manufactured. Removal or control of these imperfections is required before quantum computers using superconducting circuit architecture can be realised. One identifiable noise source is the so called 'strongly coupled' two-level system (TLS). Comparable resonance frequencies to the qubit; strong coupling strengths and decoherence times long enough to allow coherent oscillations between the qubit and TLS have been experimentally measured. The premise of this thesis is that positional anharmonicity of oxygen atoms arises within the AlOx barrier of the Josephson junction solely due to its amorphous construction. This ansatz allows the existence of various spatial configurations throughout the layer, causing unique TLS properties based solely on atomic positions and rotation in relation to the external electric field. To validate this conjecture, Josepson junction models are constructed using a hybrid ab initio and molecular mechanics approach, with various stoichiometry and density properties to reflect experimental observations of the barrier. The resultant atomic positions provide input conditions through a Voronoi classification scheme to a framework describing an oxygen atom that has the capacity to become spatially delocalised as bonds perturb away from a crystalline structure. A direct digitalisation method is developed for low dimensional descriptions and a Wick-rotated time-dependent Schrödinger equation implementation is used for three dimensional investigations. Calculated properties are compared to many current experimental strongly coupled TLS measurements in phase qubits, which shows the models' capacity to explain how an oxygen atom can generate a large dipole and appropriate ground to first excited state splitting values expected of a TLS by merely migrating from its preferred lattice position.
Quantum computing based on superconducting qubits requires the understanding and control of the materials, device architecture, and operation. However, the materials for the central circuit element, the Josephson junction, have mostly been focused on using the AlO x tunnel barrier. Here, we demonstrate Josephson junctions and superconducting qubits employing two-dimensional materials as the tunnel barrier. We batch-fabricate and design the critical Josephson current of these devices via layer-by-layer stacking N layers of MoS 2 on the large scale. Based on such junctions, MoS 2 transmon qubits are engineered and characterized in a bulk superconducting microwave resonator for the first time. Our work allows Josephson junctions to access the diverse material properties of two-dimensional materials that include a wide range of electrical and magnetic properties, which can be used to study the effects of different material properties in superconducting qubits and to engineer novel quantum circuit elements in the future.