41 research outputs found
Fock-state view of weak-value measurements and implementation with photons and atomic ensembles
Weak measurements in combination with post-selection can give rise to a
striking amplification effect (related to a large "weak value"). We show that
this effect can be understood by viewing the initial state of the pointer as
the ground state of a fictional harmonic oscillator, helping us to clarify the
transition from the weak-value regime to conventional dark-port interferometry.
We then describe how to implement fully quantum weak-value measurements
combining photons and atomic ensembles.Comment: 4 pages, 1 figur
Heralded amplification for precision measurements with spin ensembles
We propose a simple heralded amplification scheme for small rotations of the
collective spin of an ensemble of particles. Our protocol makes use of two
basic primitives for quantum memories, namely partial mapping of light onto an
ensemble, and conversion of a collective spin excitation into light. The
proposed scheme should be realizable with current technology, with potential
applications to atomic clocks and magnetometry.Comment: 3 pages, 1 figur
Generation of a squeezed state of an oscillator by stroboscopic back-action-evading measurement
Continuous observation on an oscillator is known to result in quantum
back-action which limits the knowledge acquired by the measurement. A careful
balance between the information obtained and the back-action disturbance leads
to a limit known as the standard quantum limit. The means to surpass this limit
by modulating the measurement strength with the period proportional to half
period of the oscillation has been proposed decades ago (Braginskii et al 1978
JETP Lett. 27 276; Thorne et al 1978 Phys. Rev. Lett. 40 667; Braginskii et al
1980 Science 209 547). Such modulated or stroboscopic observation leading to a
squeezed state of one quadrature of the oscillator motion with the quantum
noise below that of the zero-point fluctuations has been a long-standing goal.
Here, we report on the generation of a quadrature-squeezed state of an
oscillator by stroboscopic back-action evading measurement. The oscillator is
the collective spin of an atomic ensemble precessing in magnetic field. It is
initially prepared in nearly the ground state with an average thermal occupancy
number . The oscillator is coupled to the optical mode of a
cavity, and the cavity output field detected with polarization homodyning
serves as the meter. A back-action-evading measurement is performed by
stroboscopically modulating the intensity of the light field at twice the
Larmor frequency, resulting in a squeezed state conditioned on the
light-polarization measurement with dB noise reduction below the
zero-point fluctuations for the measured quadrature. The demonstrated squeezing
holds promise for metrological advantage in quantum sensing
Magnetic resonance imaging with optical preamplification and detection
Magnetic resonance (MR) imaging relies on conventional electronics that is
increasingly challenged by the push for stronger magnetic fields and higher
channel count. These problems can be avoided by utilizing optical technologies.
As a replacement for the standard low-noise preamplifier, we have implemented a
new transduction principle that upconverts an MR signal to the optical domain
and imaged a phantom in a clinical 3 T scanner with signal-to-noise comparable
to classical induction detection.Comment: 6 pages, 4 figure
Narrowband frequency tunable light source of continuous quadrature entanglement
We report the observation of non-classical quantum correlations of continuous
light variables from a novel type of source. It is a frequency non-degenerate
optical parametric oscillator below threshold, where signal and idler fields
are separated by 740MHz corresponding to two free spectrum ranges of the
parametric oscillator cavity. The degree of entanglement observed, - 3.8 dB, is
the highest to-date for a narrowband tunable source suitable for atomic quantum
memory and other applications in atomic physics. Finally we use the latter to
visualize the Einstein-Podolsky-Rosen paradox.Comment: 11 pages, 9 figures, LaTe
Experimental long-lived entanglement of two macroscopic objects
Entanglement is considered to be one of the most profound features of quantum
mechanics. An entangled state of a system consisting of two subsystems cannot
be described as a product of the quantum states of the two subsystems. In this
sense the entangled system is considered inseparable and nonlocal. It is
generally believed that entanglement manifests itself mostly in systems
consisting of a small number of microscopic particles. Here we demonstrate
experimentally the entanglement of two objects, each consisting of about 10^12
atoms. Entanglement is generated via interaction of the two objects - more
precisely, two gas samples of cesium atoms - with a pulse of light, which
performs a non-local Bell measurement on collective spins of the samples. The
entangled spin state can be maintained for 0.5 millisecond. Besides being of
fundamental interest, the robust, long-lived entanglement of material objects
demonstrated here is expected to be useful in quantum information processing,
including teleportation of quantum states of matter and quantum memory.Comment: Submitted to Nature, June 9, 2001, 11 pages, 3 figures. Contents
changed following referees' suggestion