Jerger M., Vasselin Z. and Fedorov A., 2017

We present a technique to measure the transfer function of a control line using a qubit as a vector network analyzer. Our method requires coupling the line under test to the the longitudinal component of the Hamiltonian of the qubit and the ability to induce Rabi oscillations through simultaneous driving of the transversal component. We used this technique to characterize the 'flux' control of a superconducting Transmon qubit in the range of 8 to 400\,MHz. Our method can be used for the qubit 'flux' line calibration to increase the fidelity of entangling gates for the quantum processor. The qubit can be also used as a microscopic probe of the electro-magnetic fields on a chip. 


The ability to determine whether a multi-level quantum system is in a certain state while preserving quantum coherence between all orthorgonal states is necessary to realize binary-outcome compatible measurements which are, in turn, a prerequisite for testing the contextuality of quantum mechanics. In this paper, we use a three-level superconducting system (a qutrit) coupled to a microwave cavity to explore different regimes of quantum measurement. In particular, we engineer the dispersive shifts of the cavity frequency to be identical for the first and second excited states of the qutrit which allows us to realize a strong projective binary-outcome measurement onto its ground state with a fidelity of 94.3%. Complemented with standard microwave control and low-noise parametric amplification, this scheme can be used to create sets of compatible measurements to reveal the contextual nature of superconducting circuits. 

Woolley M. J. et al, 2016
Phys. Rev. B, 93, pp. 224518

Resonant coupling of a Quartz nanoresonator to a superconducting qubit has provided the first observation of a macroscopic mechanical degree of freedom in its quantum ground state. However, unlike nano-oscillators, macroscopic quartz oscillators such as Bulk Acoustic Wave (BAW) oscillators offer exceptional mechanical properties including large effective masses, high frequencies, and extremely high quality factors. This makes them an attractive platform not only for the pursuit of quantum optics experiments with phonons, but also for tests of the limits of quantum mechanics itself gravitational wave detection, and tests of Lorentz symmetry. On other hand, their large geometric size makes coupling to them challenging. Most critically, large unavoidable stray capacitance between the BAW oscillator electrodes reduces the amplitude of the oscillating voltage, and the corresponding coupling strength to electrical circuits becomes impractically small. Our theoretical paper studies the scheme which can bust coupling to a such a resonator to achieve and detect the ground state of its mechanical degree of freedom.

Reshitnyk Y., Jerger M. and Fedorov A., 2016
EPJ Quantum Technol., 3, 1, pp. 1-6

Three-dimensional (3D) microwave cavities with embedded superconducting quantum bits (qubits), provide a popular and versatile platform for quantum information processing and hybrid quantum systems. A current issue that has arisen is that 3D superconducting cavities do not permit magnetic field control of qubits embedded in these cavities. In contrast, microwave cavities made of normal metals can be transparent to magnetic fields, but experience a much lower quality factor, which negates many of the advantages of the 3D architecture. Here we presented measurements of a device that bridges a gap between these two types of cavities with magnetic field control and an order of magnitude higher quality factor compared to all previously tested copper cavities. An added benefit to that our hybrid cavity possesses is that it also provides an improved thermal link to the sample that superconducting cavities alone cannot provide. A large improvement in quality factor and magnetic field control makes this 3D hybrid cavity an attractive new platform for circuit quantum electrodynamics experiments.

Jerger M. et al, 2016
Nature Communications, 7, pp. 12930

Contextuality is one of the most fundamental properties of quantum mechanics, distinguishing it from classical physics without a need for nonlocality or entanglement. It is also a critical resource for exponential speedup in universal surface-code quantum computing. Our result is the first experiment violating a noncontextuality inequality with an indivisible system where entanglement cannot be defined which also addresses all known major loopholes, such as the detection, compatibility and individual-existence loopholes. Violating noncontextuality with superconducting circuits, a leading candidate for implementing surface-code quantum computing, comprises an important conceptual milestone in demonstrating their suitability for quantum technological applications.


Kochetov Bogdan and Fedorov Arkady, 2015
Phys. Rev. B, 92, pp. 224304

The paper theoretically studies effects of the higher non linearities originated from a Josephson junction on the Josephson parametric amplifier (JPA). The results help to understand the gain saturation and reveal some of the effects which were not identified in the previous theoretical studies. Some interesting stipulations on the possible new regimes of the JPA are also deiscussed.

We discuss a similarity between resonant oscillations in two nonlinear systems; namely, a chain 
of coupled Duffing oscillators and a bilayer fish-scale metamaterial. In such systems two


Macha Pascal et al, 2014
Nature Communications, 5, pp. 5146

Implementation of a quantum metamaterial using superconducting qubits


van Loo A. F. et al, 2013
Science, 342, 6165, pp. 1494-1496

We use two superconducting qubits in an open one-dimensional transmission line to study unprecedently strong photon-mediated interactions. Also at

Lalumière Kevin et al, 2013
Physical Review A, 88, 4

We study the collective effects that emerge in waveguide quantum electrodynamics where several (artificial) atoms are coupled to a one-dimensional superconducting transmission line.