129 research outputs found
Motional frequency shifts of trapped ions in the Lamb-Dicke regime
First order Doppler effects are usually ignored in laser driven trapped ions
when the recoil frequency is much smaller than the trapping frequency
(Lamb-Dicke regime). This means that the central, carrier excitation band is
supposed to be unaffected by vibronic transitions in which the vibrational
number changes. While this is strictly true in the Lamb-Dicke limit (infinitely
tight confinement), the vibronic transitions do play a role in the Lamb-Dicke
regime. In this paper we quantify the asymptotic behaviour of their effect with
respect to the Lamb-Dicke parameter. In particular, we give analytical
expressions for the frequency shift, ``pulling'' or ``pushing'', produced in
the carrier absorption band by the vibronic transitions both for Rabi and
Ramsey schemes. This shift is shown to be independent of the initial
vibrational state.Comment: 9 pages, 6 figure
Quantum coherence and population trapping in three-photon processes
The spectroscopic properties of a single, tightly trapped atom are studied,
when the electronic levels are coupled by three laser fields in an -shaped
configuration of levels, whereby a -type level system is weakly
coupled to a metastable state. We show that depending on the laser frequencies
the response can be tuned from coherent population trapping at two-photon
resonance to novel behaviour at three photon resonance, where the metastable
state can get almost unit occupation in a wide range of parameters. For certain
parameter regimes the system switches spontaneously between dissipative and
coherent dynamics over long time scales
Laser cooling with electromagnetically induced transparency: Application to trapped samples of ions or neutral atoms
A novel method of ground state laser cooling of trapped atoms utilizes the
absorption profile of a three (or multi-) level system which is tailored by a
quantum interference. With cooling rates comparable to conventional sideband
cooling, lower final temperatures may be achieved. The method was
experimentally implemented to cool a single Ca ion to its vibrational
ground state. Since a broad band of vibrational frequencies can be cooled
simultaneously, the technique will be particularly useful for the cooling of
larger ion strings, thereby being of great practical importance for
initializing a quantum register based on trapped ions. We also discuss its
application to different level schemes and for ground state cooling of neutral
atoms trapped by a far detuned standing wave laser field.Comment: 9 pages, 13 figures, submitted to Appl Phys B 200
Single cold atom as efficient stationary source of EPR-entangled light
The Stokes and anti-Stokes components of the spectrum of resonance fluorescence of a single trapped atom, which originate from the mechanical coupling between the scattered photons and the quantized motion of the atomic center of mass, exhibit quantum correlations which are of two-mode-squeezing type. We study and demonstrate the build-up of such correlations in a specific setup, which is experimentally accessible, and where the atom acts as efficient and continuous source of EPR-entangled, two-mode squeezed light
Entanglement of distant atoms by projective measurement: The role of detection efficiency
We assess proposals for entangling two distant atoms by measurement of
emitted photons, analyzing how their performance depends on the photon
detection efficiency. We consider schemes based on measurement of one or two
photons and compare them in terms of the probability to obtain the detection
event and of the conditional fidelity with which the desired entangled state is
created. Based on an unravelling of the master equation, we quantify the
parameter regimes in which one or the other scheme is more efficient, including
the possible combination of the one-photon scheme with state purification. In
general, protocols based on one-photon detection are more efficient in set-ups
characterized by low photon detection efficiency, while at larger values
two-photon protocols are preferable. We give numerical examples based on
current experiments.Comment: 12 pages, 6 figure
Experiments towards quantum information with trapped Calcium ions
Ground state cooling and coherent manipulation of ions in an rf-(Paul) trap
is the prerequisite for quantum information experiments with trapped ions. With
resolved sideband cooling on the optical S1/2 - D5/2 quadrupole transition we
have cooled one and two 40Ca+ ions to the ground state of vibration with up to
99.9% probability. With a novel cooling scheme utilizing electromagnetically
induced transparency on the S1/2 - P1/2 manifold we have achieved simultaneous
ground state cooling of two motional sidebands 1.7 MHz apart. Starting from the
motional ground state we have demonstrated coherent quantum state manipulation
on the S1/2 - D5/2 quadrupole transition at 729 nm. Up to 30 Rabi oscillations
within 1.4 ms have been observed in the motional ground state and in the n=1
Fock state. In the linear quadrupole rf-trap with 700 kHz trap frequency along
the symmetry axis (2 MHz in radial direction) the minimum ion spacing is more
than 5 micron for up to 4 ions. We are able to cool two ions to the ground
state in the trap and individually address the ions with laser pulses through a
special optical addressing channel.Comment: Proceedings of the ICAP 2000, Firenz
A patient-specific study of type-B aortic dissection: evaluation of true-false lumen blood exchange
Background: Aortic dissection is a severe pathological condition in which blood penetrates between layers of the aortic wall and creates a duplicate channel – the false lumen. This considerable change on the aortic morphology alters hemodynamic features dramatically and, in the case of rupture, induces markedly high rates of morbidity and mortality.
Methods: In this study, we establish a patient-specific computational model and simulate the pulsatile blood flow within the dissected aorta. The k-ω SST turbulence model is employed to represent the flow and finite volume method is applied for numerical solutions. Our emphasis is on flow exchange between true and false lumen during the cardiac cycle and on quantifying the flow across specific passages. Loading distributions including pressure and wall shear stress have also been investigated and results of direct simulations are compared with solutions employing appropriate turbulence models.
Results: Our results indicate that (i) high velocities occur at the periphery of the entries; (ii) for the case studied, approximately 40% of the blood flow passes the false lumen during a heartbeat cycle; (iii) higher pressures are found at the outer wall of the dissection, which may induce further dilation of the pseudo-lumen; (iv) highest wall shear stresses occur around the entries, perhaps indicating the vulnerability of this region to further splitting; and (v) laminar simulations with adequately fine mesh resolutions, especially refined near the walls, can capture similar flow patterns to the (coarser mesh) turbulent results, although the absolute magnitudes computed are in general smaller.
Conclusions: The patient-specific model of aortic dissection provides detailed flow information of blood transport within the true and false lumen and quantifies the loading distributions over the aorta and dissection walls. This contributes to evaluating potential thrombotic behavior in the false lumen and is pivotal in guiding endovascular intervention. Moreover, as a computational study, mesh requirements to successfully evaluate the hemodynamic parameters have been proposed
Shot-noise limited monitoring and phase locking of the motion of a single trapped ion
We perform high-resolution real-time read-out of the motion of a single
trapped and laser-cooled Ba ion. By using an interferometric setup we
demonstrate shot-noise limited measurement of thermal oscillations with
resolution of 4 times the standard quantum limit. We apply the real-time
monitoring for phase control of the ion motion through a feedback loop,
suppressing the photon recoil-induced phase diffusion. Due to the spectral
narrowing in phase-locked mode, the coherent ion oscillation is measured with
resolution of about 0.3 times the standard quantum limit
A diode laser stabilization scheme for 40Ca+ single ion spectroscopy
We present a scheme for stabilizing multiple lasers at wavelengths between
795 and 866 nm to the same atomic reference line. A reference laser at 852 nm
is stabilized to the Cs D2 line using a Doppler-free frequency modulation
technique. Through transfer cavities, four lasers are stabilized to the
relevant atomic transitions in 40Ca+. The rms linewidth of a transfer-locked
laser is measured to be 123 kHz with respect to an independent atomic
reference, the Rb D1 line. This stability is confirmed by the comparison of an
excitation spectrum of a single 40Ca+ ion to an eight-level Bloch equation
model. The measured Allan variance of 10^(-22) at 10 s demonstrates a high
degree of stability for time scales up to 100 s.Comment: 8 pages, 11 figure
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