When the transition frequency of a qubit is modulated periodically across an avoided crossing along its energy dispersion curve, tunnelling to the excited state – and consequentlyLandau-Zener-St\“uckelberg interference – can occur. The types of modulation studied so far correspond to a continuous evolution of the system along the dispersion curve. Here we introduce a type of modulation called periodic latching, in which the qubit’s free phase evolution is interrupted by sudden switches in the transition frequency. In this case, the conventional Landau-Zener-St\“uckelberg theory becomes inadequate and we develop a novel adiabatic-impulse model for the evolution of the system. We derive the resonance conditions and we identify two regimes: a slow-modulation regime and a fast-modulation regime, in which case the rotating wave approximation (RWA) can be applied to obtain analytical results. The adiabatic-impulse model and the RWA results are compared with those of a full numerical simulation. These theoretical predictions are tested in an experimental setup consisting of a transmon whose flux bias is modulated with a square wave form. A rich spectrum with distinctive features in the slow-modulation and fast-modulation (RWA) regimes is observed and shown to be in very good agreement with the theoretical models. Also, differences with respect to the well known case of sinusoidal modulation are discussed, both theoretically and experimentally.
Low-noise amplification atmicrowave frequencies has become increasingly important for the research related to superconducting qubits and nanoelectromechanical systems. The fundamentallimit of added noise by a phase-preserving amplifier is the standard quantum limit, often expressed as noise temperature Tq=ℏω/2kB. Towards the goal of the quantum limit, we have developed an amplifier based on intrinsic negative resistance of a selectively damped Josephson junction. Here we present measurement results on previously proposed wide-band microwave amplification and discuss the challenges for improvements on the existing designs. We have also studied flux-pumped metamaterial-based parametric amplifiers, whose operating frequency can be widely tuned by external DC-flux, and demonstrate operation at 2ω pumping, in contrast to the typical metamaterial amplifiers pumped via signal lines at ω.
Quantum systems are notoriously difficult to simulate with classical means. Recently the idea of using another quantum system, which is experimentally more controllable, as a simulatorfor the original problem, has gained a significant momentum. Amongst the experimental platforms studied as quantum simulators, superconducting qubits are one of the most promising, due to relative straigthforward scalability, easy design, and integration with standard electronics. Here I review the recent state-of-the art in the field and the prospects for simulating systems ranging from relativistic quantum fields to quantum many-body systems.
Hybrid quantum systems with inherently distinct degrees of freedom play a key
role in many physical phenomena. A strong coupling can make the constituents
loose their individual characterand form entangled states. The properties of
these collective excitations, such as polaritons of light and phonons in
semiconductors, can combine the benefits of each subsystem. In the emerging
field of quantum information control, a promising direction is provided by the
combination between long-lived atomic states and the accessible electrical
degrees of freedom in superconducting cavities and qubits. Here we demonstrate
the possibility to integrate circuit cavity quantum electrodynamics with
phonons. Besides coupling to a microwave cavity, our superconducting transmon
qubit interacts with a resonant phonon mode in a micromechanical resonator,
allowing the combination of long lifetime, strong tunable coupling, and ease of
access. We measure the phonon Stark shift, as well as the splitting of the
transmon qubit spectral line into motional sidebands representing transitions
between electromechanical polaritons formed by phonons and the qubit. In the
time domain, we observe coherent sideband Rabi oscillations between the qubit
states and phonons. This advance may allow for storage of quantum information
in long-lived phonon states, and for investigations of strongly coupled quantum
systems near the classical limit.
Superconducting circuits with Josephson junctions are promising candidates
for developing future quantum technologies. Of particular interest is to use
these circuits to study effectsthat typically occur in complex
condensed-matter systems. Here, we employ a superconducting quantum bit
(qubit), a transmon, to carry out an analog simulation of motional averaging, a
phenomenon initially observed in nuclear magnetic resonance (NMR) spectroscopy.
To realize this effect, the flux bias of the transmon is modulated by a
controllable pseudo-random telegraph noise, resulting in stochastic jumping of
the energy separation between two discrete values. When the jumping is faster
than a dynamical threshold set by the frequency displacement of the levels, the
two separated spectral lines merge into a single narrow-width,
motional-averaged line. With sinusoidal modulation a complex pattern of
additional sidebands is observed. We demonstrate experimentally that the
modulated system remains quantum coherent, with modified transition
frequencies, Rabi couplings, and dephasing rates. These results represent the
first steps towards more advanced quantum simulations using artificial atoms.
We show that two superconducting qubits interacting via a fixed transversal
coupling can be decoupled by appropriately-designed microwave feld excitations
applied to each qubit. Thistechnique is useful for removing the effects of
spurious interactions in a quantum processor. We also simulate the case of a
qubit coupled to a two-level system (TLS) present in the insulating layer of
the Josephson junction of the qubit. Finally, we discuss the qubit-TLS problem
in the context of dispersive measurements, where the qubit is coupled to a
resonator.
Routers, switches, and repeaters are essential components of modern
information-processing systems. Similar devices will be needed in future
superconducting quantum computers. In thiswork we investigate experimentally
the time evolution of Autler-Townes splitting in a superconducting phase qubit
under the application of a control tone resonantly coupled to the second
transition. A three-level model that includes independently determined
parameters for relaxation and dephasing gives excellent agreement with the
experiment. The results demonstrate that the qubit can be used as a ON/OFF
switch with 100 ns operating time-scale for the reflection/transmission of
photons coming from an applied probe microwave tone. The ON state is realized
when the control tone is sufficiently strong to generate an Autler-Townes
doublet, suppressing the absorption of the probe tone photons and resulting in
a maximum of transmission.