41 research outputs found
Tripartite interactions between two phase qubits and a resonant cavity
The creation and manipulation of multipartite entangled states is important
for advancements in quantum computation and communication, and for testing our
fundamental understanding of quantum mechanics and precision measurements.
Multipartite entanglement has been achieved by use of various forms of quantum
bits (qubits), such as trapped ions, photons, and atoms passing through
microwave cavities. Quantum systems based on superconducting circuits have been
used to control pair-wise interactions of qubits, either directly, through a
quantum bus, or via controllable coupling. Here, we describe the first
demonstration of coherent interactions of three directly coupled
superconducting quantum systems, two phase qubits and a resonant cavity. We
introduce a simple Bloch-sphere-like representation to help one visualize the
unitary evolution of this tripartite system as it shares a single microwave
photon. With careful control and timing of the initial conditions, this leads
to a protocol for creating a rich variety of entangled states. Experimentally,
we provide evidence for the deterministic evolution from a simple product
state, through a tripartite W-state, into a bipartite Bell-state. These
experiments are another step towards deterministically generating multipartite
entanglement in superconducting systems with more than two qubits
Juxtaposition of system dynamics and agent-based simulation for a case study in immunosenescence
Advances in healthcare and in the quality of life significantly increase human life expectancy. With the aging of populations, new un-faced challenges are brought to science. The human body is naturally selected to be well-functioning until the age of reproduction to keep the species alive. However, as the lifespan extends, unseen problems due to the body deterioration emerge. There are several age-related diseases with no appropriate treatment; therefore, the complex aging phenomena needs further understanding. It is known that immunosenescence is highly correlated to the negative effects of aging. In this work we advocate the use of simulation as a tool to assist the understanding of immune aging phenomena. In particular, we are comparing system dynamics modelling and simulation (SDMS) and agent-based modelling and simulation (ABMS) for the case of age-related depletion of naive T cells in the organism. We address the following research questions: Which simulation approach is more suitable for this problem? Can these approaches be employed interchangeably? Is there any benefit of using one approach compared to the other? Results show that both simulation outcomes closely fit the observed data and existing mathematical model; and the likely contribution of each of the naive T cell repertoire maintenance method can therefore be estimated. The differences observed in the outcomes of both approaches are due to the probabilistic character of ABMS contrasted to SDMS. However, they do not interfere in the overall expected dynamics of the populations. In this case, therefore, they can be employed interchangeably, with SDMS being simpler to implement and taking less computational resources
Gigahertz quantized charge pumping in graphene quantum dots
Single electron pumps are set to revolutionize electrical metrology by
enabling the ampere to be re-defined in terms of the elementary charge of an
electron. Pumps based on lithographically-fixed tunnel barriers in mesoscopic
metallic systems and normal/superconducting hybrid turnstiles can reach very
small error rates, but only at MHz pumping speeds corresponding to small
currents of the order 1 pA. Tunable barrier pumps in semiconductor structures
have been operated at GHz frequencies, but the theoretical treatment of the
error rate is more complex and only approximate predictions are available.
Here, we present a monolithic, fixed barrier single electron pump made entirely
from graphene. We demonstrate pump operation at frequencies up to 1.4 GHz, and
predict the error rate to be as low as 0.01 parts per million at 90 MHz.
Combined with the record-high accuracy of the quantum Hall effect and proximity
induced Josephson junctions, accurate quantized current generation brings an
all-graphene closure of the quantum metrological triangle within reach.
Envisaged applications for graphene charge pumps outside quantum metrology
include single photon generation via electron-hole recombination in
electrostatically doped bilayer graphene reservoirs, and for readout of
spin-based graphene qubits in quantum information processing.Comment: 13 pages, 11 figures, includes supplementary informatio
Quantum ground state and single-phonon control of a mechanical resonator
Quantum mechanics provides a highly accurate description of a wide variety of physical systems. However, a demonstration that quantum mechanics applies equally to macroscopic mechanical systems has been a long-standing challenge, hindered by the difficulty of cooling a mechanical mode to its quantum ground state. The temperatures required are typically far below those attainable with standard cryogenic methods, so significant effort has been devoted to developing alternative cooling techniques. Once in the ground state, quantum-limited measurements must then be demonstrated. Here, using conventional cryogenic refrigeration, we show that we can cool a mechanical mode to its quantum ground state by using a microwave-frequency mechanical oscillator—a ‘quantum drum’—coupled to a quantum bit, which is used to measure the quantum state of the resonator. We further show that we can controllably create single quantum excitations (phonons) in the resonator, thus taking the first steps to complete quantum control of a mechanical system