815 research outputs found
Elastic properties of thin h-BN films investigated by Brillouin light scattering
Hexagonal BN films have been deposited by rf-magnetron sputtering with simultaneous ion plating. The elastic properties of the films grown on silicon substrates under identical coating conditions have been de-termined by Brillouin light scattering from thermally excited surface phonons. Four of the five independent elastic constants of the deposited material are found to be c11 = 65 GPa, c13 = 7 GPa, c33 = 92 GPa and c44 = 53 GPa exhibiting an elastic anisotropy c11/c33 of 0.7. The Young's modulus determined with load indenta-tion is distinctly larger than the corresponding value taken from Brillouin light scattering. This discrepancy is attributed to the specific morphology of the material with nanocrystallites embedded in an amorphous matrix
RNA secondary structure design
We consider the inverse-folding problem for RNA secondary structures: for a
given (pseudo-knot-free) secondary structure find a sequence that has that
structure as its ground state. If such a sequence exists, the structure is
called designable. We implemented a branch-and-bound algorithm that is able to
do an exhaustive search within the sequence space, i.e., gives an exact answer
whether such a sequence exists. The bound required by the branch-and-bound
algorithm are calculated by a dynamic programming algorithm. We consider
different alphabet sizes and an ensemble of random structures, which we want to
design. We find that for two letters almost none of these structures are
designable. The designability improves for the three-letter case, but still a
significant fraction of structures is undesignable. This changes when we look
at the natural four-letter case with two pairs of complementary bases:
undesignable structures are the exception, although they still exist. Finally,
we also study the relation between designability and the algorithmic complexity
of the branch-and-bound algorithm. Within the ensemble of structures, a high
average degree of undesignability is correlated to a long time to prove that a
given structure is (un-)designable. In the four-letter case, where the
designability is high everywhere, the algorithmic complexity is highest in the
region of naturally occurring RNA.Comment: 11 pages, 10 figure
Electronic Hong-Ou-Mandel interferometer for multi-mode entanglement detection
We show that multi-mode entanglement of electrons in a mesoscopic conductor
can be detected by a measurement of the zero-frequency current correlations in
an electronic Hong-Ou-Mandel interferometer. By this mean, one can further
establish a lower bound to the entanglement of formation of two-electron input
states. Our results extend the work of Burkard and Loss [Phys. Rev. Lett. 91,
087903 (2003)] to many channels and provide a way to test the existence of
entangled states involving both orbital and spin degrees of freedom.Comment: 6 pages. Revised version. Ref. adde
Relaxation and Dephasing in a Flux-qubit
We report detailed measurements of the relaxation and dephasing time in a
flux-qubit measured by a switching DC SQUID. We studied their dependence on the
two important circuit bias parameters: the externally applied magnetic flux and
the bias current through the SQUID in two samples. We demonstrate two
complementary strategies to protect the qubit from these decoherence sources.
One consists in biasing the qubit so that its resonance frequency is stationary
with respect to the control parameters ({\it optimal point}) ; the second
consists in {\it decoupling} the qubit from current noise by chosing a proper
bias current through the SQUID. At the decoupled optimal point, we measured
long spin-echo decay times of up to .Comment: 4 pages, 4 figures, submitted to Phys. Rev. Letter
Dephasing of a superconducting qubit induced by photon noise
We have studied the dephasing of a superconducting flux-qubit coupled to a
DC-SQUID based oscillator. By varying the bias conditions of both circuits we
were able to tune their effective coupling strength. This allowed us to measure
the effect of such a controllable and well-characterized environment on the
qubit coherence. We can quantitatively account for our data with a simple model
in which thermal fluctuations of the photon number in the oscillator are the
limiting factor. In particular, we observe a strong reduction of the dephasing
rate whenever the coupling is tuned to zero. At the optimal point we find a
large spin-echo decay time of .Comment: New version of earlier paper arXiv/0507290 after in-depth rewritin
Electromagnetically induced transparency in superconducting quantum circuits : Effects of decoherence, tunneling and multi-level cross-talk
We explore theoretically electromagnetically-induced transparency (EIT) in a
superconducting quantum circuit (SQC). The system is a persistent-current flux
qubit biased in a configuration. Previously [Phys. Rev. Lett. 93,
087003 (2004)], we showed that an ideally-prepared EIT system provides a
sensitive means to probe decoherence. Here, we extend this work by exploring
the effects of imperfect dark-state preparation and specific decoherence
mechanisms (population loss via tunneling, pure dephasing, and incoherent
population exchange). We find an initial, rapid population loss from the
system for an imperfectly prepared dark state. This is followed by a
slower population loss due to both the detuning of the microwave fields from
the EIT resonance and the existing decoherence mechanisms. We find analytic
expressions for the slow loss rate, with coefficients that depend on the
particular decoherence mechanisms, thereby providing a means to probe,
identify, and quantify various sources of decoherence with EIT. We go beyond
the rotating wave approximation to consider how strong microwave fields can
induce additional off-resonant transitions in the SQC, and we show how these
effects can be mitigated by compensation of the resulting AC Stark shifts
Non-additivity of decoherence rates in superconducting qubits
We show that the relaxation and decoherence rates 1/T_1 and 1/T_2 of a qubit
coupled to several noise sources are in general not additive, i.e., that the
total rates are not the sums of the rates due to each individual noise source.
To demonstrate this, we calculate the relaxation and pure dephasing rates 1/T_1
and 1/T_\phi of a superconducting (SC) flux qubit in the Born-Markov
approximation in the presence of several circuit impedances Z_i using network
graph theory and determine their deviation from additivity (the mixing term).
We find that there is no mixing term in 1/T_\phi and that the mixing terms in
1/T_1 and 1/T_2 can be positive or negative, leading to reduced or enhanced
relaxation and decoherence times T_1 and T_2. The mixing term due to the
circuit inductance L at the qubit transition frequency \omega_{01} is generally
of second order in \omega_{01}L/Z_i, but of third order if all impedances Z_i
are pure resistances. We calculate T_{1,2} for an example of a SC flux qubit
coupled to two impedances.Comment: 5 pages, 2 figure
Universal Quantum Computation with the Exchange Interaction
Experimental implementations of quantum computer architectures are now being
investigated in many different physical settings. The full set of requirements
that must be met to make quantum computing a reality in the laboratory [1] is
daunting, involving capabilities well beyond the present state of the art. In
this report we develop a significant simplification of these requirements that
can be applied in many recent solid-state approaches, using quantum dots [2],
and using donor-atom nuclear spins [3] or electron spins [4]. In these
approaches, the basic two-qubit quantum gate is generated by a tunable
Heisenberg interaction (the Hamiltonian is between spins and ), while the one-qubit gates require the control
of a local Zeeman field. Compared to the Heisenberg operation, the one-qubit
operations are significantly slower and require substantially greater materials
and device complexity, which may also contribute to increasing the decoherence
rate. Here we introduce an explicit scheme in which the Heisenberg interaction
alone suffices to exactly implement any quantum computer circuit, at a price of
a factor of three in additional qubits and about a factor of ten in additional
two-qubit operations. Even at this cost, the ability to eliminate the
complexity of one-qubit operations should accelerate progress towards these
solid-state implementations of quantum computation.Comment: revtex, 2 figures, this version appeared in Natur
Coherence-Preserving Quantum Bits
Real quantum systems couple to their environment and lose their intrinsic
quantum nature through the process known as decoherence. Here we present a
method for minimizing decoherence by making it energetically unfavorable. We
present a Hamiltonian made up solely of two-body interactions between four
two-level systems (qubits) which has a two-fold degenerate ground state. This
degenerate ground state has the property that any decoherence process acting on
an individual physical qubit must supply energy from the bath to the system.
Quantum information can be encoded into the degeneracy of the ground state and
such coherence-preserving qubits will then be robust to local decoherence at
low bath temperatures. We show how this quantum information can be universally
manipulated and indicate how this approach may be applied to a quantum dot
quantum computer.Comment: 5 pages, 1 figur
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