57 research outputs found
Dynamics of a qubit while simultaneously monitoring its relaxation and dephasing
Decoherence originates from the leakage of quantum information into external
degrees of freedom. For a qubit the two main decoherence channels are
relaxation and dephasing. Here, we report an experiment on a superconducting
qubit where we retrieve part of the lost information in both of these channels.
We demonstrate that raw averaging the corresponding measurement records
provides a full quantum tomography of the qubit state where all three
components of the effective spin-1/2 are simultaneously measured. From single
realizations of the experiment, it is possible to infer the quantum
trajectories followed by the qubit state conditioned on relaxation and/or
dephasing channels. The incompatibility between these quantum measurements of
the qubit leads to observable consequences in the statistics of quantum states.
The high level of controllability of superconducting circuits enables us to
explore many regimes from the Zeno effect to underdamped Rabi oscillations
depending on the relative strengths of driving, dephasing and relaxation.Comment: Supplemental videos can be found at
http://physinfo.fr/publications/Ficheux1710.html and supplemental information
can be found as an ancillary file on arxi
Quantum system characterization with limited resources
The construction and operation of large scale quantum information devices
presents a grand challenge. A major issue is the effective control of coherent
evolution, which requires accurate knowledge of the system dynamics that may
vary from device to device. We review strategies for obtaining such knowledge
from minimal initial resources and in an efficient manner, and apply these to
the problem of characterization of a qubit embedded into a larger state
manifold, made tractable by exploiting prior structural knowledge. We also
investigate adaptive sampling for estimation of multiple parameters
Planar multilayer circuit quantum electrodynamics
Experimental quantum information processing with superconducting circuits is
rapidly advancing, driven by innovation in two classes of devices, one
involving planar micro-fabricated (2D) resonators, and the other involving
machined three-dimensional (3D) cavities. We demonstrate that circuit quantum
electrodynamics can be implemented in a multilayer superconducting structure
that combines 2D and 3D advantages. We employ standard micro-fabrication
techniques to pattern each layer, and rely on a vacuum gap between the layers
to store the electromagnetic energy. Planar qubits are lithographically defined
as an aperture in a conducting boundary of the resonators. We demonstrate the
aperture concept by implementing an integrated, two cavity-modes, one
transmon-qubit system
Single-photon Resolved Cross-Kerr Interaction for Autonomous Stabilization of Photon-number States
Quantum states can be stabilized in the presence of intrinsic and
environmental losses by either applying active feedback conditioned on an
ancillary system or through reservoir engineering. Reservoir engineering
maintains a desired quantum state through a combination of drives and designed
entropy evacuation. We propose and implement a quantum reservoir engineering
protocol that stabilizes Fock states in a microwave cavity. This protocol is
realized with a circuit quantum electrodynamics platform where a Josephson
junction provides direct, nonlinear coupling between two superconducting
waveguide cavities. The nonlinear coupling results in a single photon resolved
cross-Kerr effect between the two cavities enabling a photon number dependent
coupling to a lossy environment. The quantum state of the microwave cavity is
discussed in terms of a net polarization and is analyzed by a measurement of
its steady state Wigner function.Comment: 8 pages, 6 figure
Robust concurrent remote entanglement between two superconducting qubits
Entangling two remote quantum systems which never interact directly is an
essential primitive in quantum information science and forms the basis for the
modular architecture of quantum computing. When protocols to generate these
remote entangled pairs rely on using traveling single photon states as carriers
of quantum information, they can be made robust to photon losses, unlike
schemes that rely on continuous variable states. However, efficiently detecting
single photons is challenging in the domain of superconducting quantum circuits
because of the low energy of microwave quanta. Here, we report the realization
of a robust form of concurrent remote entanglement based on a novel microwave
photon detector implemented in the superconducting circuit quantum
electrodynamics (cQED) platform of quantum information. Remote entangled pairs
with a fidelity of are generated at Hz. Our experiment
opens the way for the implementation of the modular architecture of quantum
computation with superconducting qubits.Comment: Main paper: 7 pages, 4 figures; Appendices: 14 pages, 9 figure
Stabilizing a Bell state of two superconducting qubits by dissipation engineering
We propose a dissipation engineering scheme that prepares and protects a
maximally entangled state of a pair of superconducting qubits. This is done by
off-resonantly coupling the two qubits to a low-Q cavity mode playing the role
of a dissipative reservoir. We engineer this coupling by applying six
continuous-wave microwave drives with appropriate frequencies. The two qubits
need not be identical. We show that our approach does not require any
fine-tuning of the parameters and requires only that certain ratios between
them be large. With currently achievable coherence times, simulations indicate
that a Bell state can be maintained over arbitrary long times with fidelities
above 94%. Such performance leads to a significant violation of Bell's
inequality (CHSH correlation larger than 2.6) for arbitrary long times.Comment: 5 pages, 4 figure
Stabilizing entanglement autonomously between two superconducting qubits
Quantum error-correction codes would protect an arbitrary state of a
multi-qubit register against decoherence-induced errors, but their
implementation is an outstanding challenge for the development of large-scale
quantum computers. A first step is to stabilize a non-equilibrium state of a
simple quantum system such as a qubit or a cavity mode in the presence of
decoherence. Several groups have recently accomplished this goal using
measurement-based feedback schemes. A next step is to prepare and stabilize a
state of a composite system. Here we demonstrate the stabilization of an
entangled Bell state of a quantum register of two superconducting qubits for an
arbitrary time. Our result is achieved by an autonomous feedback scheme which
combines continuous drives along with a specifically engineered coupling
between the two-qubit register and a dissipative reservoir. Similar autonomous
feedback techniques have recently been used for qubit reset and the
stabilization of a single qubit state, as well as for creating and stabilizing
states of multipartite quantum systems. Unlike conventional, measurement-based
schemes, an autonomous approach counter-intuitively uses engineered dissipation
to fight decoherence, obviating the need for a complicated external feedback
loop to correct errors, simplifying implementation. Instead the feedback loop
is built into the Hamiltonian such that the steady state of the system in the
presence of drives and dissipation is a Bell state, an essential building-block
state for quantum information processing. Such autonomous schemes, broadly
applicable to a variety of physical systems as demonstrated by a concurrent
publication with trapped ion qubits, will be an essential tool for the
implementation of quantum-error correction.Comment: 39 pages, 7 figure
Adiabatic passage and ensemble control of quantum systems
This paper considers population transfer between eigenstates of a finite
quantum ladder controlled by a classical electric field. Using an appropriate
change of variables, we show that this setting can be set in the framework of
adiabatic passage, which is known to facilitate ensemble control of quantum
systems. Building on this insight, we present a mathematical proof of
robustness for a control protocol -- chirped pulse -- practiced by
experimentalists to drive an ensemble of quantum systems from the ground state
to the most excited state. We then propose new adiabatic control protocols
using a single chirped and amplitude shaped pulse, to robustly perform any
permutation of eigenstate populations, on an ensemble of systems with badly
known coupling strengths. Such adiabatic control protocols are illustrated by
simulations achieving all 24 permutations for a 4-level ladder
- …