I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
17
Jan
2019
Role of the quasi-particles in an electric circuit with Josephson junctions
Josephson junctions provide highly non-linear impedances at the root of many applications such as quantum limited parametric amplifiers or superconducting qubits. These junctions are
often described by a sinusoidal relation I=Icsinφ which relates the current I to the integral over time of the voltage across the junction φ. This relation properly captures the contribution of the superconducting condensate but not the quasi-particles that appear when the system is driven out-of-equilibrium. Here, we construct a unifying framework that includes a microscopic description of the junction (full fledged treatment of the time-dependent Bogoliubov-De-Gennes equation) in presence of a classical electronic circuit. Our approach generalizes the standard Resistor-Capacitor-Josephson model (RCJ) to arbitrary junctions (including e.g. multi-terminal geometries and/or junctions that embed topological or magnetic elements) and classical circuits. We apply our technique to two situations. First, a RC circuit connected to single channel Josephson junction that exhibits Multiple Andreev Reflection (MAR) phenomena. We show that the theory properly describes both MAR and the hysteresis loops due to the electromagnetic environment. We show that out-of-equilibrium, the current-phase relation of the junction becomes strongly distorted from the simple sinusoidal form. Second, we embed the junction into a RLC circuit and show that the out-of-equilibrium non-sinusoidal current phase relation leads to a strong change of the shape of the resonance.
Observation of topological magnon insulator states in a superconducting circuit
Searching topological states of matter in tunable artificial systems has recently become a rapidly growing field of research. Meanwhile, significant experimental progresses on observing
topological phenomena have been made in superconducting circuits. However, topological insulator states have not yet been reported in this system. Here, for the first time, we experimentally realize a spin version of the Su-Schrieffer-Heeger model and observe the topological magnon insulator states in a superconducting qubit chain, which manifest both topological invariants and topological edge states. Based on simply monitoring the time evolution of a singlequbit excitation in the chain, we demonstrate that the topological winding numbers and the topological magnon edge and soliton states can all be directly observed. Our work thus opens a new avenue to use controllable qubit chain system to explore novel topological states of matter and also offers exciting possibilities for topologically protected quantum information processing.
16
Jan
2019
Correlating decoherence in transmon qubits: Low frequency noise by single fluctuators
We report on long-term measurements of a highly coherent, non-tunable transmon qubit, revealing low-frequency burst noise in coherence times and transition frequency. We achieve this
through a simultaneous measurement of the qubits relaxation and dephasing rate as well as its resonance frequency and an analysis of their correlations. These yield information about the microscopic origin of the intrinsic decoherence mechanisms in Josephson qubits. Our data is consistent with a small number of microscopic two-level systems located at the edges of the superconducting film, which is further confirmed by a spectral noise analysis.
14
Jan
2019
Decoherence benchmarking of superconducting qubits
We benchmark the decoherence of superconducting qubits to examine the temporal stability of energy-relaxation and dephasing. By collecting statistics during measurements spanning multiple
days, we find the mean parameters T1 = 49 μs and T∗2= 95 μs, however, both of these quantities fluctuate explaining the need for frequent re-calibration in qubit setups. Our main finding is that fluctuations in qubit relaxation are local to the qubit and are caused by instabilities of near-resonant two-level-systems (TLS). Through statistical analysis, we determine switching rates of these TLS and observe the coherent coupling between an individual TLS and a transmon qubit. Finally, we find evidence that the qubit’s frequency stability is limited by capacitance noise. Importantly, this produces a 0.8 ms limit on the pure dephasing which we also observe. Collectively, these findings raise the need for performing qubit metrology to examine the reproducibility of qubit parameters, where these fluctuations could affect qubit gate fidelity.
20
Dez
2018
Towards long-distance quantum networks with superconducting processors and optical links
We design a quantum repeater architecture, necessary for long distance quantum networks, using the recently proposed microwave cat state qubits, formed and manipulated via interaction
between a superconducting nonlinear element and a microwave cavity. These qubits are especially attractive for repeaters because in addition to serving as excellent computational units with deterministic gate operations, they also have coherence times long enough to deal with the unavoidable propagation delays. Since microwave photons are too low in energy to be able to carry quantum information over long distances, as an intermediate step, we expand on a recently proposed microwave to optical transduction protocol using excited states of a rare-earth ion (Er3+) doped crystal. To enhance the entanglement distribution rate, we propose to use spectral multiplexing by employing an array of cavities at each node. We compare our achievable rates with direct transmission and with a popular ensemble-based repeater approach and show that ours could be higher in appropriate regimes, even in the presence of realistic imperfections and noise, while maintaining reasonably high fidelities of the final state. In the short term, our work could be directly useful for secure quantum communication, whereas in the long term, we can envision a large scale distributed quantum computing network built on our architecture.
18
Dez
2018
Microwave-to-optics conversion using a mechanical oscillator in its quantum groundstate
Conversion between signals in the microwave and optical domains is of great interest both for classical telecommunication, as well as for connecting future superconducting quantum computers
into a global quantum network. For quantum applications, the conversion has to be both efficient, as well as operate in a regime of minimal added classical noise. While efficient conversion has been demonstrated with several approaches using mechanical transducers, they have so far all operated with a substantial thermal noise background. Here, we overcome this limitation and demonstrate coherent conversion between GHz microwave signals and the optical telecom band with a thermal background of less than one phonon. We use an electro-opto-mechanical device, that couples surface acoustic waves driven by a resonant microwave signal to an optomechanical crystal featuring a 2.7 GHz mechanical mode. By operating at Millikelvin temperatures, we can initialize the mechanical mode in its quantum groundstate, which allows us to perform the transduction process with less than one quantum of added thermal noise. We further verify the preservation of the coherence of the microwave signal throughout the transduction process.
15
Dez
2018
Thermodynamics in Single-Electron Circuits and Superconducting Qubits
Classical and quantum electronic circuits provide ideal platforms to investigate stochastic thermodynamics and they have served as a stepping stone to realize Maxwell’s demons
with highly controllable protocols. In this article we first review the central thermal phenomena in quantum nanostructures. Thermometry and basic refrigeration methods will be described as enabling tools for thermodynamics experiments. Next we discuss the role of information in thermodynamics which leads to the concept of Maxwell’s demon. Various Maxwell’s demons realized in single-electron circuits over the past couple of years will be described. Currently true quantum thermodynamics in superconducting circuits is in focus of attention, and we end the review by discussing the ideas and first experiments in this exciting area of research.
14
Dez
2018
A mesoscopic Hamiltonian for Josephson travelling wave parametric amplifiers
We present the theory describing parametric amplification in a Josephson junction embedded transmission line. We will focus on the process of four-wave mixing under the assumption of
an undepleted pump. However, the approach taken is quite general, such that a different parametric process or the process under different assumptions is easily derived. First the classical theory of the coupled mode equations as presented by O’Brien et al. [Phys. Rev. Lett., 113:157001] is shortly reviewed. Then a derivation of the full quantum theory is given, resulting in a Hamiltonian that describes the process of parametric amplification. We show that the coupled mode equations can be derived from this Hamiltonian in the classical limit and elaborate on the validity of the theory.
11
Dez
2018
Bath engineering of a fluorescing artificial atom with a photonic crystal
A quantum emitter decays due to vacuum fluctuations at its transition frequency. By virtue of the entwined nature of dissipation and fluctuations, this process can be controlled by
engineering the impedance of the environment. We study how the structured vacuum environment of a microwave photonic crystal can be used for bath engineering of a transmon qubit. The photonic crystal is realized by a step-impedance transmission line which suppresses and enhances the quantum spectral density of states akin to a Purcell filter. We demonstrate a bath engineering protocol upon driving an emitter near the photonic band edge that allows dissipation to produce non-trivial steady-states.
08
Dez
2018
Microwave to optical photon conversion via fully concentrated rare-earth ion crystals
Most investigations of rare earth ions in solids for quantum information have used rare earth ion doped crystals. Here we analyse the conversion of quantum information from microwave
photons to optical frequencies using crystals where the rare earth ions, rather than being dopants, are part of the host crystal. The potential of large ion densities and small linewidths makes such systems very attractive in this application. We show that, as well as high efficiency, large bandwidth conversion is possible. In fact, the collective coupling between the rare earth ions and the optical and microwave cavities is large enough that the limitation on the bandwidth of the devices will instead be the spacing between magnon mode modes in the crystal.