arXiv subjects

M. F. Sacchi

Publications and source records attributed to M. F. Sacchi.

48 records · Page 3Linked to original sources

arXiv1999

Feedback-assisted measurement of the free mass position under the Standard Quantum Limit

The Standard Quantum Limit (SQL) for the measurement of a free mass position is illustrated, along with two necessary conditions for breaching it. A measurement scheme that overcomes the SQL is engineered. It can be achieved in three-steps: i) a pre-squeezing stage; ii) a standard von Neumann measurement with momentum-position object-probe interaction and iii) a feedback. Advantages and limitations of this scheme are discussed. It is shown that all of the three steps are needed in order to overcome the SQL. In particular, the von Neumann interaction is crucial in getting the right state reduction, whereas other experimentally achievable Hamiltonians, as, for example, the radiation-pressure interaction, lead to state reductions that on the average cannot overcome the SQL.

quant-ph↗

arXiv1999

Tomographic test of Bell's inequality

We present a homodyne detection scheme to verify Bell's inequality on correlated optical beams at the output of a nondegenerate parametric amplifier. Our approach is based on tomographic measurement of the joint detection probabilities, which allows high quantum efficiency at detectors. A self-homodyne scheme is suggested to simplify the experimental set-up.

quant-ph↗

arXiv1999

On the correspondence between classical and quantum measurements on a bosonic field

We study the correspondence between classical and quantum measurements on a harmonic oscillator that describes a one-mode bosonic field. We connect the quantum measurement of an observable of the field with the possibility of amplifying the observable ideally through a quantum amplifier. The ``classical'' measurement corresponds to the joint measurement of the position $q$ and momentum $p$ of the harmonic oscillator, with following evaluation of a function $f$ of the outcome $α=q+ip$. For the electromagnetic field the joint measurement is achieved by a heterodyne detector. The quantum measurement of an observable $\hat O$ is obtained by preamplifying the heterodyne detector through an ideal amplifier of $\hat O$, and rescaling the outcome by the gain $g$. We give a general criterion which states when this preamplified heterodyne detection scheme approaches the ideal quantum measurement of $\hat O$ in the limit of infinite gain. We show that this criterion is satisfied and the ideal measurement is achieved for the case of the photon number operator and for the quadrature. For both operators the method is robust to nonunit quantum efficiency of the heterodyne detector. On the other hand, we show that the preamplified heterodyne detection scheme does not work for arbitrary observable of the field. As a counterexample, we prove that the simple quadratic function of the field $\hat K=i(a^{†2}-a^2)/2$ has no corresponding polynomial function $f(α,\bar α)$---including the obvious choice $f=\hbox{Im}(α^2)$---that allows the measurement of $\hat K$ through the preamplified heterodyne measurement scheme.

quant-ph↗

arXiv1999

State preparation by photon filtering

We propose a setup capable of generating Fock states of a single mode radiation field. The scheme is based on coupling the signal field to a ring cavity through cross-Kerr phase modulation, and on conditional ON-OFF photodetection at the output cavity mode. The same setup allows to prepare selected superpositions of Fock states and entangled two-mode states. Remarkably, the detector's quantum efficiency does not affect the reliability of the state synthesis.

quant-ph↗

arXiv1999

Quantum tomography of the GHZ state

We present a method of generation of the Greenberger-Horne-Zeilinger state involving type II and type I parametric downconversion, and triggering photodetectors. The state generated by the proposed experimental set-up can be reconstructed through multi-mode quantum homodyne tomography. The feasibility of the measurement is studied on the basis of Monte-Carlo simulations.

quant-ph↗

arXiv1999

Generation and Measurement of Nonclassical States by Quantum Fock Filter

We study a novel optical setup which is able to select a specific Fock component from a generic input state. The device allows to synthesize number states and superpositions of few number states, and to measure the photon distribution and the density matrix of a generic input signal.

quant-ph↗

arXiv1999

On the parametric approximation in quantum optics

We perform the exact numerical diagonalization of the Hamiltonians that describe both degenerate and nondegenerate parametric amplifiers, by exploiting the conservation laws pertaining each device. We clarify the conditions under which the parametric approximation holds, showing that the most relevant requirement is the coherence of the pump after the interaction, rather than its undepletion.

quant-ph↗

arXiv1999

Tomographic measurement of nonclassical radiation states

We propose to experimentally test the nonclassicality of quantum states through homodyne tomography. For single-mode states we check violations of inequalities involving the photon-number probability. For two-mode states we test the nonclassicality by reconstructing some suitable number-operator functions. The test can be performed with available quantum efficiency of homodyne detection, by measuring the pertaining quantities on the corresponding noisy states.

quant-ph↗

arXiv1998

On the general problem of quantum phase estimation

The problem of estimating a generic phase-shift experienced by a quantum state is addressed for a generally degenerate phase shift operator. The optimal positive operator-valued measure is derived along with the optimal input state. Two relevant examples are analyzed: i) a multi-mode phase shift operator for multipath interferometry; ii) the two mode heterodyne phase detection.

quant-ph↗

arXiv1997

Optimized quantum-optical communications in the presence of loss

We consider the effect of loss on quantum-optical communication channels. The channel based on direct detection of number states, which for a lossless transmission line would achieve the ultimate quantum channel capacity, is easily degraded by loss. The same holds true for the channel based on homodyne detection of squeezed states, which also is very fragile to loss. On the contrary, the ``classical'' channel based on heterodyne detection of coherent states is loss-invariant. We optimize the a priori probability for the squeezed-state and the number-state channels, taking the effect of loss into account. In the low power regime we achieve a sizeable improvement of the mutual information, and both the squeezed-state and the number-state channels overcome the capacity of the coherent-state channel. In particular, the squeezed-state channel beats the classical channel for total average number of photons $N<8$. However, for sufficiently high power the classical channel always performs as the best one. For the number-state channel we show that with a loss $η\lesssim .6$ the optimized a priori probability departs from the usual thermal-like behavior, and develops gaps of zero probability, with a considerable improvement of the mutual information (up to 70 % of improvement at low power for attenuation $η=.15$).

quant-ph↗

arXiv1997

Trace forms for the generalized Wigner functions

We derive simple formulas connecting the generalized Wigner functions for $s$-ordering with the density matrix, and vice-versa. These formulas proved very useful for quantum mechanical applications, as, for example, for connecting master equations with Fokker-Planck equations, or for evaluating the quantum state from Monte Carlo simulations of Fokker-Planck equations, and finally for studying positivity of the generalized Wigner functions in the complex plane.

quant-ph↗

arXiv1997

Optical von Neumann measurement

We present an optical scheme that realizes the standard von Neumann measurement model, providing an indirect measurement of a quadrature of the field with controllable Gaussian state-reduction. The scheme is made of simple optical elements, as laser sources, beam splitters, and phase sensitive amplifiers, along with a feedback mechanism that uses a Pockels cell. We show that the von Neumann measurement is achieved without the need of working in a ultra-short pulsed regime.

quant-ph↗