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

22
Jan
2019

# Observation and stabilization of photonic Fock states in a hot radio-frequency resonator

Detecting weak radio-frequency electromagnetic fields plays a crucial role in wide range of fields, from radio astronomy to nuclear magnetic resonance imaging. In quantum mechanics,

the ultimate limit of a weak field is a single-photon. Detecting and manipulating single-photons at megahertz frequencies presents a challenge as, even at cryogenic temperatures, thermal fluctuations are significant. Here, we use a gigahertz superconducting qubit to directly observe the quantization of a megahertz radio-frequency electromagnetic field. Using the qubit, we achieve quantum control over thermal photons, cooling to the ground-state and stabilizing photonic Fock states. Releasing the resonator from our control, we directly observe its re-thermalization dynamics with the bath with nanosecond resolution. Extending circuit quantum electrodynamics to a new regime, we enable the exploration of thermodynamics at the quantum scale and allow interfacing quantum circuits with megahertz systems such as spin systems or macroscopic mechanical oscillators.

# Multiplexed readout of four qubits in 3D cQED architecture using broadband JPA

We propose and demonstrate a frequency-multiplexed readout scheme in 3D cQED architecture. We use four transmon qubits coupled to individual rectangular cavities which are aperture-coupled

to a common rectangular waveguide feedline. A coaxial to waveguide transformer at the other end of the feedline allows one to launch and collect the multiplexed signal. The reflected readout signal is amplified by an impedance engineered broadband parametric amplifier with 380 MHz of bandwidth. This provides us high fidelity single-shot readout of multiple qubits using compact microwave circuitry, an efficient way for scaling up to more qubits in 3D cQED.

# Accelerating adiabatic protocols for entangling two qubits in circuit QED

We introduce a method to speed up adiabatic protocols for creating entanglement between two qubits dispersively coupled to a transmission line, while keeping fidelities high and maintaining

robustness to control errors. The method takes genuinely adiabatic sweeps, ranging from a simple Landau-Zener drive to boundary cancellation methods and local adiabatic drivings, and adds fast oscillations to speed up the protocol while cancelling unwanted transitions. We compare our protocol with existing adiabatic methods in a state-of-the-art parameter range and show substantial gains. Numerical simulations underline that this strategy is efficient also beyond the rotating-wave approximation, and that the method is robust against random biases in the control parameters.

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.