34,700 research outputs found
Operational Entanglement Families of Symmetric Mixed N-Qubit States
We introduce an operational entanglement classification of symmetric mixed
states for an arbitrary number of qubits based on stochastic local operations
assisted with classical communication (SLOCC operations). We define families of
SLOCC entanglement classes successively embedded into each other, we prove that
they are of non-zero measure, and we construct witness operators to distinguish
them. Moreover, we discuss how arbitrary symmetric mixed states can be realized
in the lab via a one-to-one correspondence between well-defined sets of
controllable parameters and the corresponding entanglement families.Comment: 6 pages, 2 figures, published version, Phys. Rev. A, in pres
Thermal entanglement in the nanotubular system Na_2V_3O_7
Macroscopic entanglement witnesses have been put forward recently to reveal
nonlocal quantum correlations between individual constituents of the solid at
nonzero temperatures. Here we apply a recently proposed universal entanglement
witness, the magnetic susceptibility [New J. Phys. {\bf 7}, 258 (2005)] for the
estimation of the critical temperature in the nanotubular system below which thermal entanglement is present. As a result of an
analysis based on the experimental data for dc-magnetic susceptibility, we show
that K, which is approximately three times higher than the
critical temperature corresponding to the bipartite entanglement.Comment: 6 pages, 3 figures, REVTeX
A simple formula for pooling knowledge about a quantum system
When various observers obtain information in an independent fashion about a
classical system, there is a simple rule which allows them to pool their
knowledge, and this requires only the states-of-knowledge of the respective
observers. Here we derive an equivalent quantum formula. While its realm of
applicability is necessarily more limited, it does apply to a large class of
measurements, and we show explicitly for a single qubit that it satisfies the
intuitive notions of what it means to pool knowledge about a quantum system.
This analysis also provides a physical interpretation for the trace of the
product of two density matrices.Comment: 5 pages, Revtex
Entanglement invariant for the double Jaynes-Cummings model
We study entanglement dynamics between four qubits interacting through two
isolated Jaynes-Cummings hamiltonians, via the entanglement measure based on
the wedge product. We compare the results with similar results obtained using
bipartite concurrence resulting in what is referred to as "entanglement sudden
death". We find a natural entanglement invariant under evolution demonstrating
that entanglement sudden death is caused by ignoring (tracing over) some of the
system's degrees of freedom that become entangled through the interaction.Comment: Sec. V has largely been rewritten. An error pertaining to the
entanglement invariant has been corrected and a correct invariant valid for a
much larger set of states have been found, Eq. (25
Upper limits on the luminosity of the progenitor of type Ia supernova SN2014J
We analysed archival data of Chandra pre-explosion observations of the
position of SN2014J in M82. No X-ray source at this position was detected in
the data, and we calculated upper limits on the luminosities of the progenitor.
These upper limits allow us to firmly rule out an unobscured supersoft X-ray
source progenitor with a photospheric radius comparable to the radius of white
dwarf near the Chandrasekhar mass (~1.38 M_sun) and mass accretion rate in the
interval where stable nuclear burning can occur. However, due to a relatively
large hydrogen column density implied by optical observations of the supernova,
we cannot exclude a supersoft source with lower temperatures, kT < 80 eV. We
find that the supernova is located in the centre of a large structure of soft
diffuse emission, about 200 pc across. The mass, ~3x10^4 M_sun and short
cooling time of the gas, tau_cool ~ 8 Myrs, suggest that it is a
supernova-inflated super-bubble, associated with the region of recent star
formation. If SN2014J is indeed located inside the bubble, it likely belongs to
the prompt population of type Ia supernovae, with a delay time as short as ~ 50
Myrs. Finally, we analysed the one existing post-supernova Chandra observation
and placed upper limit of ~ (1-2) 10^37 erg/s on the X-ray luminosity of the
supernova itself.Comment: 8 pages, 6 figure
Universal and deterministic manipulation of the quantum state of harmonic oscillators: a route to unitary gates for Fock State qubits
We present a simple quantum circuit that allows for the universal and
deterministic manipulation of the quantum state of confined harmonic
oscillators. The scheme is based on the selective interactions of the referred
oscillator with an auxiliary three-level system and a classical external
driving source, and enables any unitary operations on Fock states, two-by-two.
One circuit is equivalent to a single qubit unitary logical gate on Fock states
qubits. Sequences of similar protocols allow for complete, deterministic and
state-independent manipulation of the harmonic oscillator quantum state.Comment: 4 pages, 4 figure
Fast Two-Qubit Gates in Semiconductor Quantum Dots using a Photonic Microcavity
Implementations for quantum computing require fast single- and multi-qubit
quantum gate operations. In the case of optically controlled quantum dot qubits
theoretical designs for long-range two- or multi-qubit operations satisfying
all the requirements in quantum computing are not yet available. We have
developed a design for a fast, long-range two-qubit gate mediated by a photonic
microcavity mode using excited states of the quantum dot-cavity system that
addresses these needs. This design does not require identical qubits, it is
compatible with available optically induced single qubit operations, and it
advances opportunities for scalable architectures. We show that the gate
fidelity can exceed 90% in experimentally accessible systems
Charge qubits and limitations of electrostatic quantum gates
We investigate the characteristics of purely electrostatic interactions with
external gates in constructing full single qubit manipulations. The quantum bit
is naturally encoded in the spatial wave function of the electron system.
Single-electron{transistor arrays based on quantum dots or insulating
interfaces typically allow for electrostatic controls where the inter-island
tunneling is considered constant, e.g. determined by the thickness of an
insulating layer. A representative array of 3x3 quantum dots with two mobile
electrons is analyzed using a Hubbard Hamiltonian and a capacitance matrix
formalism. Our study shows that it is easy to realize the first quantum gate
for single qubit operations, but that a second quantum gate only comes at the
cost of compromising the low-energy two-level system needed to encode the
qubit. We use perturbative arguments and the Feshbach formalism to show that
the compromising of the two-level system is a rather general feature for
electrostatically interacting qubits and is not just related to the specific
details of the system chosen. We show further that full implementation requires
tunable tunneling or external magnetic fields.Comment: 7 pages, 5 figures, submitted to PR
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