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Physical Review A

State reconstruction for a collection of two-level systems

G. S. Agarwal

Published Jan 1, 1998• 63 citations
Publisher

Abstract

A method for reconstructing the quantum state of a system of atoms or spins is proposed. The $Q$ distribution is derived from the measured population in the ground state of a system that has interacted with an external field. Using multipole operators the $Q$ function is inverted to derive the density matrix.

DOI
10.1103/physreva.57.671

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A method for reconstructing the quantum state of a system of atoms or spins is proposed. The QQQ distribution is derived from the measured population in the ground state of a system that has interacted with an external field. Using multipole operators the QQQ function is inverted to derive the density matrix.
The density matrix of a two-level system (spin, atom) is usually determined by measuring the three noncommuting components of the Pauli vector. This density matrix can also be obtained via the measurement data of two commuting variables, using a single apparatus. This is done by coupling the two-level system to a mode of radiation field, where the atom-field interaction is described with the Jaynes-Cummings model. The mode starts its evolution from a known coherent state. The unknown initial state of the atom is found by measuring two commuting observables: the population difference of the atom and the photon number of the field. We discuss the advantages of this setup and its possible applications.
We realize on an atom-chip, a practical, experimentally undemanding, tomographic reconstruction algorithm relying on the time–resolved measurements of the atomic population distribution among atomic internal states. More specifically, we estimate both the state density matrix, as well as the dephasing noise present in our system, by assuming complete knowledge of the Hamiltonian evolution. The proposed scheme is based on routinely performed measurements and established experimental procedures, hence providing a simplified methodology for quantum technological applications.
Summary form only given. Determining the unknown state of a quantum system from measurements of observables, a question first posed by Pauli, is a nontrivial task, which does not always permit a solution. The few examples where this inverse problem was experimentally solved include light fields, molecular vibrations, the motion of trapped ions and free atomic waves. We describe a method to reconstruct the complete density matrix of a generally mixed spin state of trapped neutral atoms. Compared to spin- 1/2 systems, the reconstruction of the spin density matrix for a large angular momentum is a much more challenging task. In our system with cesium atoms in the hyperfine ground state, a reconstruction of the 9/spl times/9 density matrix requires 80 real numbers to be experimentally determined. This can be achieved by measuring the 2F+1=9 populations along 4F+1=17 carefully chosen orientations of a Stern-Gerlach apparatus. A numerical simulation of the reconstruction shows this method to be robust against expected experimental uncertainties. We report on an experiment in progress to implement this measurement protocol in a standard laser cooling and trapping arrangement of neutral cesium atoms.
Summary form only given, as follows. The subject of state reconstruction has recently become a major field of study in quantum optics [see Special issue on Quantum state preparation and measurement, J. Mod Opt., 11/12, (1997), edited by W.P. Schleich and M.G. Raymer]. The quasiprobability functions and density matrices have been measured experimentally for single-mode running fields using the homodyne scheme. However, most of the earlier reconstruction schemes are for single-mode fields. In this presentation the author is interested in reconstruction of a two-mode entangled cavity field.
The concept of a quantum state represents one of the most fundamental pillars of the paradigm of quantum theory.'3 Contrary to its mathematical elegance and convenience in calculations, the physical interpretation of a quantum state is not so transparent. The problem is that the quantum state (described either by a state vector, or density operator or a phase-space probability density distribution) does not have a well defined objective status, i.e. a state vector is not an objective property of a particle. According to Peres (see,1 p. 374): "There is no physical evidence whatsoever that every physical system has at every instant a well defined state. .. In strict interpretation of quantum theory these mathematical symbols [i.e., state vectorsi represent statistical information enabling us to compute the probabilities of occurrence of specific events." Once this point of view is adopted then it becomes clear that any "measurement" or a reconstruction of a density operator (or its mathematical equivalent) can be understood exclusively as an expression of our knowledge about the quantum mechanical state based on a certain set of measured data. To be more specific, any quantum-mechanical reconstruction scheme is nothing more than an a posteriori estimation of the density operator of a quantum-mechanical (microscopic) system based on data obtained with the help of a macroscopic measurement apparatus.3 The quality of the reconstruction depends on the "quality" of the measured data and the efficiency of the reconstruction procedure with the help of which the data analysis is performed. In particular, we can specify three different situations. Firstly, when all system observables are precisely measured. In this case the complete reconstruction of an initially unknown state can be performed (we will call this the reconstruction on the complete observation level) . Secondly, when just part of the system observables is precisely measured then one cannot perform a complete reconstruction of the measured state. Nevertheless, the reconstructed density operator still uniquely determines mean values of the measured observables (we will denote this scheme as reconstruction on incomplete observation levels) . Finally, when measurement does not provide us with sufficient information to specify the exact mean values (or probability distributions) but only the frequencies of appearances of eigenstates of the measured observables, then one can perform an estimation (e.g. reconstruction based on quantum Bayesian inference) which is the "best" with respect to the given measured data and the a priori knowledge about the state of the measured system.
Optical bistability in a four-level inverted Y-type atom system inside an optical ring cavity is investigated numerically. Atomic density matrix equations are derived by the density matrix theory. The effect of the Spontaneously Generated Coherence( SGC) on threshold of the optical bistability is studied.When the probing field and controlling field are in two-photon resonance,the atoms are trapped on the two lower states,probing field is transparent and there is no bistability in the system. But when the probing field and coupling field or coupling field and controlling field are in two-photon resonance,there is the optical bistability in the system. Under condition of the stable state,SGC can make the interference destructive from two transition channels. Population distribution on the higher levels is enhanced,transparency window becomes narrow,and the threshold of the bistability becomes large.