23 research outputs found
Magnon squeezing in an antiferromagnet: reducing the spin noise below the standard quantum limit
At absolute zero temperature, thermal noise vanishes when a physical system
is in its ground state, but quantum noise remains as a fundamental limit to the
accuracy of experimental measurements. Such a limitation, however, can be
mitigated by the formation of squeezed states. Quantum mechanically, a squeezed
state is a time-varying superposition of states for which the noise of a
particular observable is reduced below that of the ground state at certain
times. Quantum squeezing has been achieved for a variety of systems, including
the electromagnetic field, atomic vibrations in solids and molecules, and
atomic spins, but not so far for magnetic systems. Here we report on an
experimental demonstration of spin wave (i.e., magnon) squeezing. Our method
uses femtosecond optical pulses to generate correlations involving pairs of
magnons in an antiferromagnetic insulator, MnF2. These correlations lead to
quantum squeezing in which the fluctuations of the magnetization of a
crystallographic unit cell vary periodically in time and are reduced below that
of the ground state quantum noise. The mechanism responsible for this squeezing
is stimulated second order Raman scattering by magnon pairs. Such squeezed
states have important ramifications in the emerging fields of spintronics and
quantum computing involving magnetic spin states or the spin-orbit coupling
mechanism
High-speed tunable photonic crystal fiber-based femtosecond soliton source without dispersion pre-compensation
We present a high-speed wavelength tunable photonic crystal fiber-based
source capable of generating tunable femtosecond solitons in the infrared
region. Through measurements and numerical simulation, we show that both the
pulsewidth and the spectral width of the output pulses remain nearly constant
over the entire tuning range from 860 to 1160 nm. This remarkable behavior is
observed even when pump pulses are heavily chirped (7400 fs^2), which allows to
avoid bulky compensation optics, or the use of another fiber, for dispersion
compensation usually required by the tuning device.Comment: 8 pages, 11 figure
Control of spin dynamics with laser pulses: Generation of entangled states of donor-bound electrons in a Cd1-xMnxTe quantum well
A quantum-mechanical many-particle system may exhibit non-local behavior in
that measurements performed on one of the particles can affect a second one
that is far apart. These so-called entangled states are crucial for the
implementation of quantum information protocols and gates for quantum
computation. Here, we use ultrafast optical pulses and coherent pump-probe
techniques to create and control spin entangled states in an ensemble of up to
three non-interacting electrons bound to donors in a Cd1-xMnxTe quantum well.
Our method, relying on the exchange interaction between optically-excited
excitons and the paramagnetic impurities, can in principle be applied to
entangle an arbitrarily large number of electrons. A microscopic theory of
impulsive stimulated Raman scattering and a model for multi-spin entanglement
are presented. The signature of entanglement is the observation of overtones of
donor spin-flips in the differential reflectivity of the probe pulse. Results
are shown for resonant excitation of localized excitons below the gap, and
above the gap where the signatures of entanglement are significantly enhanced.
Data is also presented on the generation of coherent excitations of
antiferromagnetically-coupled manganese pairs, folded acoustic phonons, exciton
Zeeman beats and entanglement involving two Mn2+ ions.Comment: Long version of quant-ph/020619
Photon counting statistics using a digital oscilloscope
We present a photon counting experiment designed for an undergraduate physics laboratory. The statistics of the number of photons of a pseudo thermal light source is studied in two limiting cases: well above and well below the coherence time, giving Poisson and Bose-Einstein distributions, respectively. We show that using a digital oscilloscope the experiment can be done in a reasonable time, without need of counting boards. The use of the oscilloscope has the additional advantage of allowing the storage of the data for further processing. Hence, using the same set of data, the analysis of the statistics of the occurrence of n photons as a function of the time windows adds important evidence to determine accurately the nature of the light source. The stochastic nature of the detection phenomena adds an additional value to this type of experiments, since the student is forced to a thorough visit through data processing and statistics
Ultrafast optical generation of coherent phonons in CdTe1-xSex quantum dots
We report on the impulsive generation of coherent optical phonons in
CdTe0.68Se0.32 nanocrystallites embedded in a glass matrix. Pump probe
experiments using femtosecond laser pulses were performed by tuning the laser
central energy to resonate with the absorption edge of the nanocrystals. We
identify two longitudinal optical phonons, one longitudinal acoustic phonon and
a fourth mode of a mixed longitudinal-transverse nature. The amplitude of the
optical phonons as a function of the laser central energy exhibits a resonance
that is well described by a model based on impulsive stimulated Raman
scattering. The phases of the coherent phonons reveal coupling between
different modes. At low power density excitations, the frequency of the optical
coherent phonons deviates from values obtained from spontaneous Raman
scattering. This behavior is ascribed to the presence of electronic impurity
states which modify the nanocrystal dielectric function and, thereby, the
frequency of the infrared-active phonons
All Optical Implementation of Multi-Spin Entanglement in a Semiconductor Quantum Well
We use ultrafast optical pulses and coherent techniques to create spin
entangled states of non-interacting electrons bound to donors (at least three)
and at least two Mn2+ ions in a CdTe quantum well. Our method, relying on the
exchange interaction between localized excitons and paramagnetic impurities,
can in principle be applied to entangle a large number of spins.Comment: 17 pages, 3 figure
Magnon squeezing in antiferromagnetic MnF2 and FeF2
Quantum squeezing of a collective spin-wave excitation or magnon using femtosecond optical pulses has been used to generate correlated pairs of spins with equal and opposite wave vectors in antiferromagnetic MnF\u2082 and FeF\u2082. in the squeezed state, the fluctuations of the magnetization of a crystallographic unit cell vary periodically in time and are reduced below that of the ground state quantum noise. the results obtained, including their temperature dependence for FeF\u2082, are compared with earlier spontaneous Raman scattering measurements. The squeezing effect is observed to be at least one order of magnitude stronger in FeF\u2082 than in MnF\u2082.NRC publication: Ye