126 research outputs found
Lyapunov Control on Quantum Open System in Decoherence-free Subspaces
A scheme to drive and manipulate a finite-dimensional quantum system in the
decoherence-free subspaces(DFS) by Lyapunov control is proposed. Control fields
are established by Lyapunov function. This proposal can drive the open quantum
system into the DFS and manipulate it to any desired eigenstate of the free
Hamiltonian. An example which consists of a four-level system with three
long-lived states driven by two lasers is presented to exemplify the scheme. We
have performed numerical simulations for the dynamics of the four-level system,
which show that the scheme works good.Comment: 5 pages, 6 figure
Continuous Generation and Stabilization of Mesoscopic Field Superposition States in a Quantum Circuit
While dissipation is widely considered as being harmful for quantum
coherence, it can, when properly engineered, lead to the stabilization of
non-trivial pure quantum states. We propose a scheme for continuous generation
and stabilization of Schr\"{o}dinger cat states in a cavity using dissipation
engineering. We first generate non-classical photon states with definite parity
by means of a two-photon drive and dissipation, and then stabilize these
transient states against single-photon decay. The single-photon stabilization
is autonomous, and is implemented through a second engineered bath, which
exploits the photon number dependent frequency-splitting due to Kerr
interactions in the strongly dispersive regime of circuit QED. Starting with
the Hamiltonian of the baths plus cavity, we derive an effective model of only
the cavity photon states along with analytic expressions for relevant physical
quantities, such as the stabilization rate. The deterministic generation of
such cat states is one of the key ingredients in performing universal quantum
computation.Comment: 9 pages, 6 figure
Back and forth nudging for quantum state reconstruction
International audienceWe propose an estimation method allowing to identify the initial state of a quantum system based on the continuous weak measurement of a certain physical observable over a fixed interval of time. The algorithm is based on the back- and-forth nudging method consisting in iterative application of Luenberger observers for the time-forward and time-backward dynamics. A clever change of variables unveils the needed symmetry in the observer design leading to the decrease of a certain distance (in an appropriate metric) between the estimator and the main system, both in forward and backward directions
Observing quantum state diffusion by heterodyne detection of fluorescence
A qubit can relax by fluorescence, which prompts the release of a photon into
its electromagnetic environment. By counting the emitted photons, discrete
quantum jumps of the qubit state can be observed. The succession of states
occupied by the qubit in a single experiment, its quantum trajectory, depends
in fact on the kind of detector. How are the quantum trajectories modified if
one measures continuously the amplitude of the fluorescence field instead?
Using a superconducting parametric amplifier, we have performed heterodyne
detection of the fluorescence of a superconducting qubit. For each realization
of the measurement record, we can reconstruct a different quantum trajectory
for the qubit. The observed evolution obeys quantum state diffusion, which is
characteristic of quantum measurements subject to zero point fluctuations.
Independent projective measurements of the qubit at various times provide a
quantitative validation of the reconstructed trajectories. By exploring the
statistics of quantum trajectories, we demonstrate that the qubit states span a
deterministic surface in the Bloch sphere at each time in the evolution.
Additionally, we show that when monitoring fluorescence, coherent
superpositions are generated during the decay from excited to ground state.
Counterintuitively, measuring light emitted during relaxation can give rise to
trajectories with increased excitation probability.Comment: Supplementary material can be found in the ancillary sectio
Mathematical modelling of phenotypic selection with solid tumours
We present a space- and phenotype-structured model of selection dynamics between cancer cells within a solid tumour. In the framework of this model, we combine formal analyses with numerical simulations to investigate in silico the role played by the spatial distribution of oxygen and therapeutic agents in mediating phenotypic selection of cancer cells. Numerical simulations are performed on the 3D geometry of an in vivo human hepatic tumour, which was imaged using computerised tomography. Our modelling extends our previous work in the area through the inclusion of multiple therapeutic agents, one that is cytostatic, whilst the other is cytotoxic. In agreement with our previous work, the results show that spatial inhomogeneities in oxygen and therapeutic agent concentrations, which emerge spontaneously in solid tumours, can promote the creation of distinct local niches and lead to the selection of different phenotypic variants within the same tumour. A novel conclusion we infer from the simulations and analysis is that, for the same total dose, therapeutic protocols based on a combination of cytotoxic and cytostatic agents can be more effective than therapeutic protocols relying solely on cytotoxic agents in reducing the number of viable cancer cells
Dynamically enhancing qubit-oscillator interactions with anti-squeezing
The interaction strength of an oscillator to a qubit grows with the
oscillator's vacuum field fluctuations. The well known degenerate parametric
oscillator has revived interest in the regime of strongly detuned squeezing,
where its eigenstates are squeezed Fock states. Owing to these amplified field
fluctuations, it was recently proposed that squeezing this oscillator would
dynamically boost its coupling to a qubit. In a superconducting circuit
experiment, we observe a two-fold increase in the dispersive interaction
between a qubit and an oscillator at 5.5 dB of squeezing, demonstrating in-situ
dynamical control of qubit-oscillator interactions. This work initiates the
experimental coupling of oscillators of squeezed photons to qubits, and
cautiously motivates their dissemination in experimental platforms seeking
enhanced interactions.Comment: 21 pages, 15 figure
Multifunctional Theranostic Graphene Oxide Nanoflakes as MR Imaging Agents with Enhanced Photothermal and Radiosensitizing Properties
The integration of multiple therapeutic and diagnostic functions into a single nanoplatform for image-guided cancer therapy has been an emerging trend in nanomedicine. We show here that multifunctional theranostic nanostructures consisting of superparamagnetic iron oxide (SPIO) and gold nanoparticles (AuNPs) scaffolded within graphene oxide nanoflakes (GO-SPIO-Au NFs) can be used for dual photo/radiotherapy by virtue of the near-infrared (NIR) absorbance of GO for photothermal therapy (PTT) and the Z element radiosensitization of AuNPs for enhanced radiation therapy (RT). At the same time, this nanoplatform can also be detected by magnetic resonance (MR) imaging because of the presence of SPIO NPs. Using a mouse carcinoma model, GO-SPIO-Au NF-mediated combined PTT/RT exhibited a 1.85-fold and 1.44-fold higher therapeutic efficacy compared to either NF-mediated PTT or RT alone, respectively, resulting in a complete eradication of tumors. As a sensitive multifunctional theranostic platform, GO-SPIO-Au NFs appear to be a promising nanomaterial for enhanced cancer imaging and therapy. © 2021 American Chemical Society
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