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
14
Nov
2018
Realization of directional amplification in a microwave optomechanical device
Directional transmission or amplification of microwave signals is indispensable in various applications involving sensitive measurements. In this work we show in experiment how to use
a generic cavity optomechanical setup to non-reciprocally amplify microwave signals above 3 GHz in one direction by 9 decibels, and simultaneously attenuate the transmission in the opposite direction by 21 decibels. We use a device including two on-chip superconducting resonators and two metallic drumhead mechanical oscillators. Application of four microwave pump tone frequencies allows for designing constructive or destructive interference for a signal tone depending on the propagation direction. The device can also be configured as an isolator with a lossless nonreciprocal transmission and 18 dB of isolation.
09
Nov
2018
Creation of superposition of arbitrary states encoded in two three-dimensional cavities
The principle of superposition is a key ingredient for quantum mechanics. A recent work (M. Oszmaniec et al., Phys. Rev. Lett. 116, 110403 (2016)) has shown that a quantum adder that
deterministically generates a superposition of two unknown states is forbidden. Here we propose a probabilistic approach for creating a superposition state of two arbitrary states encoded in two three-dimensional cavities. Our implementation is based on a three-level superconducting transmon qubit dispersively coupled to two cavities. Numerical simulations show that high-fidelity generation of the superposition of two coherent states is feasible with current circuit QED technology. Our method also works for other physical systems such as other types of superconducting qubits, natural atoms, quantum dots, and nitrogen-vacancy (NV) centers.
06
Nov
2018
Symmetric Traveling Wave Parametric Amplifier
We developed and experimentally tested a Symmetric Traveling Wave Parametric Amplifier (STWPA) based on Three-Wave Mixing, using the new concept of a Symmetric rf- SQUID. This allows
to fully control the second and third order nonlinearities of the STWPA by applying external currents. In this way, the optimal bias point can be reached, taking into account both phase mismatch and pump depletion minimization. The structure was tested at 4.2K, showing a 4GHz bandwidth and a maximum estimated gain of 17dB. STWPA showed also great flexibility, allowing up-down conversion mixer operations and rf-controlled switch.
Genuine 12-qubit entanglement on a superconducting quantum processor
We report the preparation and verification of a genuine 12-qubit entanglement in a superconducting processor. The processor that we designed and fabricated has qubits lying on a 1D
chain with relaxation times ranging from 29.6 to 54.6 μs. The fidelity of the 12-qubit entanglement was measured to be above 0.5544±0.0025, exceeding the genuine multipartite entanglement threshold by 21 standard deviations. Our entangling circuit to generate linear cluster states is depth-invariant in the number of qubits and uses single- and double-qubit gates instead of collective interactions. Our results are a substantial step towards large-scale random circuit sampling and scalable measurement-based quantum computing.
02
Nov
2018
Linear response theory of Josephson junction arrays in a microwave cavity
Motivated by recent experiments on Josephson junction arrays in microwave cavities, we construct a quantum phase model and calculate the susceptibility of this model in linear response.
Both charge and vortex degrees of freedom are considered, as well as circuits containing either Josephson junctions or coherent quantum phase slip elements. The effects of decoherence are considered via a Lindblad master equation.
30
Okt
2018
Sideband cooling of nearly degenerate micromechanical oscillators in a multimode optomechanical system
Multimode optomechanical systems are an emerging platform for studying fundamental aspects of matter near the quantum ground state and are useful in sensitive sensing and measurement
applications. We study optomechanical cooling in a system where two nearly degenerate mechanical oscillators are coupled to a single microwave cavity. Due to an optically mediated coupling the two oscillators hybridize into a bright mode with strong optomechanical cooling rate and a dark mode nearly decoupled from the system. We find that at high coupling, sideband cooling of the dark mode is strongly suppressed. Our results are relevant to novel optomechanical systems where multiple closely-spaced modes are intrinsically present.
Variational Quantum Gate Optimization
We propose a gate optimization method, which we call variational quantum gate optimization (VQGO). VQGO is a method to construct a target multi-qubit gate by optimizing a parametrized
quantum circuit which consists of tunable single-qubit gates with high fidelities and fixed multi-qubit gates with limited controlabilities. As an example, we apply the proposed scheme to the models relevant to superconducting qubit systems. We show in numerical simulations that the high-fidelity CNOT gate can be constructed with VQGO using cross-resonance gates with finite crosstalk. We also demonstrate that fast and a high-fidelity four-qubit syndrome extraction can be implemented with simultaneous cross-resonance drives even in the presence of non-commutative crosstalk. VQGO gives a pathway for designing efficient gate operations for quantum computers.
26
Okt
2018
Computational modeling of decay and hybridization in superconducting circuits
We present a circuit theoretic technique for computing the complex frequencies and eigenmodes of superconducting circuits with radiative loss. We show that the transmon loss rates obtained
by our method agree with the established approximation C/Re[Y] away from resonance and do not diverge near resonance. Additionally, we demonstrate that a system of two resonators coupled to a common Purcell filter exhibits a pattern of hybridization that cannot be explained by a hierarchy of couplings and detunings. This is due to the significant influence of radiative loss on the mode couplings. As a result, it is necessary to include radiative boundary conditions even in simulations of coherent dynamics. Our simulation technique is useful for designing complex circuit quantum electrodynamic systems.
25
Okt
2018
The high-coherence fluxonium qubit
We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfying T2 > 100 microseconds (T2 > 300 microseconds in one
device). Moreover, given the state of the art values of the surface loss tangent and the 1/f flux noise amplitude, coherence can be further improved beyond 1 millisecond. Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit — over 100 here — must be minimized for best qubit coherence. We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.
23
Okt
2018
Simple preparation of Bell and GHZ states using ultrastrong-coupling circuit QED
The ability to entangle quantum systems is crucial for many applications in quantum technology, including quantum communication and quantum computing. Here, we propose a new, simple,
and versatile setup for deterministically creating Bell and Greenberger-Horne-Zeilinger (GHZ) states between photons of different frequencies in a two-step protocol. The setup consists of a quantum bit (qubit) coupled ultrastrongly to three photonic resonator modes. The only operations needed in our protocol are to put the qubit in a superposition state, and then tune its frequency in and out of resonance with sums of the resonator-mode frequencies. By choosing which frequency we tune the qubit to, we select which entangled state we create. We show that our protocol can be implemented with high fidelity using feasible experimental parameters in state-of-the-art circuit quantum electrodynamics. One possible application of our setup is as a node distributing entanglement in a quantum network.