Research

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Superconducting quantum circuits are composed of structures in which you can design and engineer their key properties in the fabrication process. From these designs, it is possible to have measurement parameters that can be in situ tuned right during experiment. These properties have made superconducting circuits to be one of the most promising candidates for realizing quantum computer in solid state architecture, have allowed for the exploration of quantum optical phenomena on a chip and have facilitated the implementation of a quantum control of nanomechanical degrees of freedom. Our group research continues to focus on superconducting nanodevices, which both use and probe mysterious principles of quantum physics.

Superconducting devices are made with the use of a variety of techniques available in modern nanofabrication facilities. Microwave transmission lines and resonators are made of Al and Nb metal films (Fig.1a) with established fabrication recipes and are highly reproducible. The most essential element within superconducting quantum circuits (see Fig. 1b,c) are Josephson junctions (Fig. 1d), of which the fabrication has also become well established and consists of having its pattern defined by electron beam lithography and successive deposition and oxidation of thin Al films. These superconducting circuits have two key features: very low dissipation thanks to superconductivity and a very high non-linearity of a Josephson junction – properties not found in any other electronic materials.

Superconducting circuits are operated at frequencies of a few GHz and can be cooled to their ground state with a dilution cryostat. Working with microwaves allows us to take advantage of the abundance of commercial electronics available in microwave frequency domain. Arbitrary wave generators and microwave sources are used to create high precision voltage and current controls of superconducting devices at the nanosecond timescale. Concurrently, commercial low noise amplifiers, together with newly developed on-chip quantum amplifiers, are used for the detection of electro-magnetic fields at the single photon level. A widely tunable geometry, reliable fabrication, advanced control and detection place superconducting circuits at the forefront of fundamental and applied research initiatives.

In the following sections you may find our research directions which will hopefully allow us to define the next generation of nanoelectronics based on the synergy of superconductive materials and quantum mechanics.

Jerger M., Vasselin Z. and Fedorov A., 2017
arXiv:1706.05829

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.