77,346 research outputs found
Optimal state estimation for cavity optomechanical systems
We demonstrate optimal state estimation for a cavity optomechanical system
through Kalman filtering. By taking into account nontrivial experimental noise
sources, such as colored laser noise and spurious mechanical modes, we
implement a realistic state-space model. This allows us to obtain the
conditional system state, i.e., conditioned on previous measurements, with
minimal least-square estimation error. We apply this method for estimating the
mechanical state, as well as optomechanical correlations both in the weak and
strong coupling regime. The application of the Kalman filter is an important
next step for achieving real-time optimal (classical and quantum) control of
cavity optomechanical systems.Comment: replaced with published version, 5+12 page
Practical Advantages of Almost-Balanced-Weak-Values Metrological Techniques
Precision measurements of ultra-small linear velocities of one of the mirrors
in a Michelson interferometer are performed using two different weak-values
techniques. We show that the technique of Almost-Balanced Weak Values (ABWV)
offers practical advantages over the technique of Weak-Value Amplification
(WVA), resulting in larger signal-to-noise ratios and the possibility of longer
integration times due to robustness to slow drifts. As an example of the
performance of the ABWV protocol we report a velocity sensitivity of 60 fm/s
after 40 hours of integration time. The sensitivity of the Doppler shift due to
the moving mirror is of 150 nHz
Weak value amplification: a view from quantum estimation theory that highlights what it is and what isn't
Weak value amplification (WVA) is a concept that has been extensively used in
a myriad of applications with the aim of rendering measurable tiny changes of a
variable of interest. In spite of this, there is still an on-going debate about
its true nature and whether is really needed for achieving high sensitivity.
Here we aim at solving the puzzle, using some basic concepts from quantum
estimation theory, highlighting what the use of the WVA concept can offer and
what it can not. While WVA cannot be used to go beyond some fundamental
sensitivity limits that arise from considering the full nature of the quantum
states, WVA can notwithstanding enhance the sensitivity of real detection
schemes that are limited by many other things apart from the quantum nature of
the states involved, i.e. technical noise. Importantly, it can do that in a
straightforward and easily accessible manner.Comment: 2 pages, 5 figure
Generating the local oscillator "locally" in continuous-variable quantum key distribution based on coherent detection
Continuous-variable quantum key distribution (CV-QKD) protocols based on
coherent detection have been studied extensively in both theory and experiment.
In all the existing implementations of CV-QKD, both the quantum signal and the
local oscillator (LO) are generated from the same laser and propagate through
the insecure quantum channel. This arrangement may open security loopholes and
also limit the potential applications of CV-QKD. In this paper, we propose and
demonstrate a pilot-aided feedforward data recovery scheme which enables
reliable coherent detection using a "locally" generated LO. Using two
independent commercial laser sources and a spool of 25 km optical fiber, we
construct a coherent communication system. The variance of the phase noise
introduced by the proposed scheme is measured to be 0.04 (rad^2), which is
small enough to enable secure key distribution. This technology also opens the
door for other quantum communication protocols, such as the recently proposed
measurement-device-independent (MDI) CV-QKD where independent light sources are
employed by different users.Comment: 11 pages, 10 figure
Quantum sensing
"Quantum sensing" describes the use of a quantum system, quantum properties
or quantum phenomena to perform a measurement of a physical quantity.
Historical examples of quantum sensors include magnetometers based on
superconducting quantum interference devices and atomic vapors, or atomic
clocks. More recently, quantum sensing has become a distinct and rapidly
growing branch of research within the area of quantum science and technology,
with the most common platforms being spin qubits, trapped ions and flux qubits.
The field is expected to provide new opportunities - especially with regard to
high sensitivity and precision - in applied physics and other areas of science.
In this review, we provide an introduction to the basic principles, methods and
concepts of quantum sensing from the viewpoint of the interested
experimentalist.Comment: 45 pages, 13 figures. Submitted to Rev. Mod. Phy
Efficient Direct Detection of M-PAM Sequences with Implicit CSI Acquisition for The FSO System
Compared to on-off keying (OOK), M-ary pulse amplitude modulation (M-PAM,
M>2) is more spectrally efficient. However, to detect M-PAM signals reliably,
the requirement of accurate channel state information is more stringent.
Previously, for OOK systems, we have developed a receiver that requires few
pilot symbols and can jointly detect the data sequence and estimate the unknown
channel gain implicitly. In this paper, using the same approach, we extend our
previous work and derive a generalized receiver for M-PAM systems. A
Viterbi-type trellis-search algorithm coupled with a selective-store strategy
is adopted, resulting in a low implementation complexity and a low memory
requirement. Therefore, the receiver is efficient in terms of energy, spectra,
implementation complexity and memory. Using theoretical analysis, we show that
its error performance approaches that of maximum likelihood detection with
perfect knowledge of the channel gain, as the observation window length
increases. Also, simulation results are presented to justify the theoretical
analysis.Comment: 6 page
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