Optical preparation and measurement of atomic coherence at gigahertz bandwidth

Abstract

We detail a method for the preparation of atomic coherence in a high density atomic medium, utilising a coherent preparation scheme of gigahertz bandwidth pulses. A numerical simulation of the preparation scheme is developed, and its efficiency in preparing coherent states is found to be close to unity at the entrance to the medium. The coherence is then measured non-invasively with a probe field.Controlling the absorptive and dispersive properties of high density alkali metal vapours has allowed the realisation of storage of light [1], quantum memory [2] and entanglement of macroscopic systems [3]. The preparation of atomic coherence continues to draw a lot of attention [4][5][6], with potential applications in quantum computation and quantum information protocols [7]. The control of quantum states required in quantum logic operations is often achieved via the application of optical fields to atomic systems, for example, in the implementation of qubit rotations for logic gates [8]. In this instance the qubit takes the form of an atomic coherence state, but usage of single photons as 'flying qubits' is also an area of great interest since transmitting information via light is common place in telecommunications [9]. Often both atomic and photonic qubits are utilised and hence the requirement of reliable transfer of quantum states between photons and atoms, which is the principle behind optical quantum memory [10,11]. The phenomenon of photon echoes [12,13] has been used to store and retrieve arbitrary single-photon wave packets [14] along with the related gradient echo memory [15,16]. The storage and retrieval of photons at high speeds (gigahertz bandwidth) for optical quantum memory [17,18] is advantageous for quantum computation, which calls for high-rate operations such as fast quantum gates based on Rydberg atoms [19,20].Most quantum computation schemes require a few working states amongst which interactions are mediated via optical control fields, and typically require the system to be prepared initially in a pure state. Preparation schemes based on spontaneous emission, such as Coherent population trapping (CPT) [21], take many excited state lifetimes for a medium to reach its final state, which substantially limits the bandwidth of the operation. Also, they begin to fail at increasingly high density, as photons scatter from the pumping field and continue to interact with the sample [22]. For these reasons, transfer via an off-resonant coherent mechanism is preferred, in which there is a certain amount of freedom to choose the final atomic state of the atom, and can be completed over timescales faster than those necessary for incoherent pumping. Stimulated Raman adiabatic passage (STI-RAP) is one such technique for the highly efficient transfer of population between two non-degenerate metastable states, facilitated via a stimulated two-photon transition involving an unstable intermediate state [23]. The technique has been further expanded with the generalisation of the three levels into degenerate manifolds...

Light propagation through atomic vapours

“…Under these conditions STIRAP is also possible [171,172]. Finally, there exist certain techniques which have applications in both the coherent preparation and non-invasive probing of quantum states in atomic vapour [173][174][175][176].…”

Coherence creation in an optically thick medium by matched propagation of a chirped-laser-pulse pair

“…The above mentioned works generally concentrate on AC in single atoms and neglect the back-action of the atoms on the laser fields along with other propagation effects, such as interaction of the laser pulses with each other. The latter effects, however, have to be taken into account when AC is performed in an optically thick medium [17,[28][29][30]. In this case, preparation of the atoms of a medium in coherent superposition of the quantum states may significantly modify its optical properties leading to very interesting and important propagation effects.…”