21 research outputs found
State leakage during fast decay and control of a superconducting transmon qubit
Superconducting Josephson junction qubits constitute the main current
technology for many applications, including scalable quantum computers and
thermal devices. Theoretical modeling of such systems is usually done within
the two-level approximation. However, accurate theoretical modeling requires
taking into account the influence of the higher excited states without limiting
the system to the two-level qubit subspace. Here, we study the dynamics and
control of a superconducting transmon using the numerically exact stochastic
Liouville-von Neumann equation approach. We focus on the role of state leakage
from the ideal two-level subspace for bath induced decay and single-qubit gate
operations. We find significant short-time state leakage due to the strong
coupling to the bath. We quantify the leakage errors in single-qubit gates and
demonstrate their suppression with DRAG control for a five-level transmon in
the presence of decoherence. Our results predict the limits of accuracy of the
two-level approximation and possible intrinsic constraints in qubit dynamics
and control for an experimentally relevant parameter set
Vibronic spectroscopy of an artificial molecule
With advanced fabrication techniques it is possible to make nanoscale
electronic structures that have discrete energy levels. Such structures are
called artificial atoms because of analogy with true atoms. Examples of such
atoms are quantum dots in semiconductor heterostructures and Josephson-junction
qubits. It is also possible to have artificial atoms interacting with each
other. This is an artificial molecule in the sense that the electronic states
are analogous to the ones in a molecule. In this letter we present a different
type of artificial molecule that, in addition to electronic states, also
includes the analog of nuclear vibrations in a diatomic molecule. Some of the
earlier experiments could be interpreted using this analogy, including qubits
coupled to oscillators and qubits driven by an intense field. In our case the
electronic states of the molecule are represented by a Josephson-junction
qubit, and the nuclear separation corresponds to the magnetic flux in a loop
containing the qubit and an LC oscillator. We probe the vibronic transitions,
where both the electronic and vibrational states change simultaneously, and
find that they are analogous to true molecules. The vibronic transitions could
be used for sideband cooling of the oscillator, and we see damping up to
sidebands of order 10.Comment: 5 pages, 4 figure
Efficient protocol for qubit initialization with a tunable environment
We propose an efficient qubit initialization protocol based on a dissipative environment that can be dynamically adjusted. Here, the
qubit is coupled to a thermal bath through a tunable harmonic oscillator. On-demand initialization is achieved by sweeping the
oscillator rapidly into resonance with the qubit. This resonant coupling with the engineered environment induces fast relaxation to
the ground state of the system, and a consecutive rapid sweep back to off resonance guarantees weak excess dissipation during
quantum computations. We solve the corresponding quantum dynamics using a Markovian master equation for the reduced
density operator of the qubit-bath system. This allows us to optimize the parameters and the initialization protocol for the qubit.
Our analytical calculations show that the ground-state occupation of our system is well protected during the fast sweeps of the
environmental coupling and, consequently, we obtain an estimate for the duration of our protocol by solving the transition rates
between the low-energy eigenstates with the Jacobian diagonalization method. Our results suggest that the current experimental
state of the art for the initialization speed of superconducting qubits at a given fidelity can be considerably improved
Stark effect and generalized Bloch-Siegert shift in a strongly driven two-level system
A superconducting qubit was driven in an ultrastrong fashion by an
oscillatory microwave field, which was created by coupling via the nonlinear
Josephson energy. The observed Stark shifts of the `atomic' levels are so
pronounced that corrections even beyond the lowest-order Bloch-Siegert shift
are needed to properly explain the measurements. The quasienergies of the
dressed two-level system were probed by resonant absorption via a cavity, and
the results are in agreement with a calculation based on the Floquet approach.Comment: 4+ page
Flux-tunable heat sink for quantum electric circuits
© 2018 The Author(s). Superconducting microwave circuits show great potential for practical quantum technological applications such as quantum information processing. However, fast and on-demand initialization of the quantum degrees of freedom in these devices remains a challenge. Here, we experimentally implement a tunable heat sink that is potentially suitable for the initialization of superconducting qubits. Our device consists of two coupled resonators. The first resonator has a high quality factor and a fixed frequency whereas the second resonator is designed to have a low quality factor and a tunable resonance frequency. We engineer the low quality factor using an on-chip resistor and the frequency tunability using a superconducting quantum interference device. When the two resonators are in resonance, the photons in the high-quality resonator can be efficiently dissipated. We show that the corresponding loaded quality factor can be tuned from above 10 5 down to a few thousand at 10 GHz in good quantitative agreement with our theoretical model
Spectroscopy of artificial atoms and molecules
Abstract
Elementary experiments of atomic physics and quantum optics can be reproduced on a circuit board using elements built of superconducting materials. Such systems can show discrete energy levels similar to those of atoms. With respect to their natural cousins, the enhanced controllability of these ‘artificial atoms’ allows the testing of the laws of physics in a novel range of parameters. Also, the study of such systems is important for their proposed use as the quantum bits (qubits) of the foreseen quantum computer.
In this thesis, we have studied an artificial atom coupled with a harmonic oscillator formed by an LC-resonator. At the quantum limit, the interaction between the two can be shown to mimic that of ordinary matter and light. The properties of the system were studied by measuring the reflected signal in a capacitively coupled transmission line. In atomic physics, this has an analogy with the absorption spectrum of electromagnetic radiation. To simulate such measurements, we have derived the corresponding equations of motion using the quantum network theory and the semi-classical approximation. The calculated absorption spectrum shows a good agreement with the experimental data. By extracting the power consumption in different parts of the circuit, we have calculated the energy flow between the atom and the oscillator. It shows that, in a certain parameter range, the absorption spectrum obeys the Franck-Condon principle, and can be interpreted in terms of vibronic transitions of a diatomic molecule.
A coupling with a radiation field shifts the spectral lines of an atom. In our system, the interaction between the atom and the field is nonlinear, and we have shown that a strong monochromatic driving results in energy shifts unforeseen in natural or, even, other artificial atoms. We have used the Floquet method to calculate the quasienergies of the coupled system of atom and field. The oscillator was treated as a small perturbation probing the quasienergies, and the resulting absorption spectrum agrees with the reflection measurement