## Entanglement, Decoherence and the Quantum/Classical Boundary*

Christine Marie T. Ceblano

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Since 1996 to 1998, great advancement has been made in the entangling experiments, like ions in trap and atoms in high *Q*-cavities which are disparate in techniques and yet similar in a way that they both comprehend a simple situation in which a two-level atom is coupled to a quantized harmonic oscillator. The Hamiltonian of this system, though simple, describes a great variety of interesting situations. With such system, proposals have been made to incorporate the Schrödinger’s cat where its role would be played by an excited harmonic oscillator.

In the ion trap experiment of the National Institute of Standards and Technology (NIST) group in Boulder, Colorado headed by Monroe and Wineland, a single beryllium ion is monitored by using sequence of laser pulses which later on split the ion’s Gaussian wavefunction into two wave packets, correlated to the [eq]\mid +>[/eq] and [eq]\mid ->[/eq] hyperfine states, a Schrödinger’s cat situation, then finally recombine the two hyperfine states.

The experiment was repeated with different values of phase [eq]\varphi[/eq]. When it approaches zero, interference fringes were observed in [eq]\mid +>[/eq] fluorescence signal. This shows the coherent superposition of the ion’s states of motion. The interesting part of the experiment is the fact that the packets remain Gaussian and do not disperse in time, which is somehow useful for decoherence studies.

Another entangling experiment is the atom-cavity performed by Serge Haroche group in Paris where the role of the cat is played by a field oscillator consisting of a few photons stored in high *Q*-cavity. Later on a rubidium atom (quantum mouse) is sent across the cavity probing the cat state to detect the oscillator quantum coherence. After interaction, the field oscillates with two different phases at once, a Schrodinger’s cat situation. By varying the delay time between the two atoms, decoherence is observed and is faster with large phase separation between cavity field components. This is an implication of the fragility of coherences between two macroscopically distinct states. Somehow, this experiment gives us a glimpse from the quantum mechanical (with the cat both dead *and* alive) to the classical realm transition (with the cat is either dead *or* alive).

Overall, both experiments show a statistical distribution of outcomes over many repetitions of experiments. And both can also be useful to various interesting applications like quantum information processing machines and quantum teleportation.

*This is a summary of “ENTANGLEMENT, DECOHERENCE AND THE QUANTUM/CLASSICAL BOUNDARY” by Serge Haroche