Atomic ensembles have an advantage over single atoms in quantum information processing because they are more readily achievable and they interact more strongly with light by a factor of sqrt(N) where N is the number of atoms. In order to make use of the ensemble as a qubit, the excitations have to be controlled such that the only states that interact with the light field are

|0> = |aaaaaa...a>
|1> = (|caaaaa...a> + |acaaaa...a> + |aacaaa...a> + ...)/sqrt(N)

in which a is the ground state of an individual atom and c is the excited state. State |1>, as can be seen, is where only one atom is excited among the whole ensemble; and it is an entangled state as well. However, excitation to other states is possible because the energy spacing between the states is equal. Light shift imbalance induced blockade (LSIB) is a method that manipulates the light shift on the ensemble states such that |0> and |1> become resonant and the higher order states become highly detuned.

Upon further study aided by numerical simulations of state evolutions, however, we found that ensemble states are simply product states of individual atoms. Individual excitation to higher level states using classical light cannot be blocked, and therefore cannot be blocked in product states either. There needs to be interaction between atoms so that the ensemble states are no longer separable, and we achieve this by using Rydberg interaction. Thus, we restore LSIB. Notice that the excitation to solely |1> will produce entanglement in the ensemble, and the read beam that probes this state will produce a single photon.

In reality, we use a lambda scheme, and introduce the Rydberg interaction only on the c state, which is the second ground state. This induces an asymmetric light shift that will lead to light shifting the upper levels more readily than the lower levels.

We plan on demonstrating LSIB with atoms trapped in a dipole trap, and using Hansbury-Brown and Twiss coincidence detection scheme using a beam splitter and two single photon detectors.

Here is the presentation from DAMOP 2013: [See slides]

Lab setup:

Target identification and tracking is the key to many defense and civilian applications. Optical correlators provides simple technique for fast verification and identification of data. To overcome the restrictions of classical correlator, we propose a position, rotation and scale invariant (PRSI) hybrid optoelectronic correlator using SLMs, detectors and VLSI chips, incorporating the Polar Mellin transform to realize an Automatic Target Recognization system that can potentially offer high-speed target recognition.

[See slides]

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