No Arabic abstract
We study the dynamics of a pair of atoms, resonantly interacting with a single mode cavity, in the situation where the atoms enter the cavity with a time delay between them. Using time dependent coupling functions to represent the spatial profile of the mode, we considered the adiabatic limit of the system. Although the time evolution is mostly adiabatic, energy crossings play an important role in the system dynamics. Following from this, entanglement, and a procedure for cavity state teleportation are considered. We examine the behaviour of the system when we introduce decoherence, a finite detuning, and potential asymmetries in the coupling profiles of the atoms.
We analyse the problem of a single mode field interacting with a pair of two level atoms. The atoms enter and exit the cavity at different times. Instead of using constant coupling, we use time dependent couplings which represent the spatial dependence of the mode. Although the system evolution is adiabatic for most of the time, a previously unstudied energy crossing plays a key role in the system dynamics when the atoms have a time delay. We show that conditional atom-cavity entanglement can be generated, while for large photon numbers the entangled system has a behaviour which can be mapped onto the single atom Jaynes-Cummings model. Exploring the main features of this system we propose simple and fairly robust methods for entangling atoms independently of the cavity, for quantum state mapping, and for implementing SWAP and C-NOT gates with atomic qubits.
Generalized quantum measurements are an important extension of projective or von Neumann measurements, in that they can be used to describe any measurement that can be implemented on a quantum system. We describe how to realize two non-standard quantum measurements using cavity quantum electrodynamics (QED). The first measurement optimally and unabmiguously distinguishes between two non-orthogonal quantum states. The second example is a measurement that demonstrates superadditive quantum coding gain. The experimental tools used are single-atom unitary operations effected by Ramsey pulses and two-atom Tavis-Cummings interactions. We show how the superadditive quantum coding gain is affected by errors in the field-ionisation detection of atoms, and that even with rather high levels of experimental imperfections, a reasonable amount of superadditivity can still be seen. To date, these types of measurement have only been realized on photons. It would be of great interest to have realizations using other physical systems. This is for fundamental reasons, but also since quantum coding gain in general increases with code word length, and a realization using atoms could be more easily scaled than existing realizations using photons.
We propose a cavity-QED-based scheme of generating entanglement between atoms. The scheme is scalable to an arbitrary number of atoms, and can be used to generate a variety of multipartite entangled states such as the Greenberger-Horne-Zeilinger, W, and cluster states. Furthermore, with a role switching of atoms with photons, the scheme can be used to generate entanglement between cavity fields. We also introduce a scheme that can generate an arbitrary multipartite field graph state.
Ultracold $^{87}$Rb atoms are delivered into a high-finesse optical micro-cavity using a translating optical lattice trap and detected via the cavity field. The atoms are loaded into an optical lattice from a magneto-optic trap (MOT) and transported 1.5 cm into the cavity. Our cavity satisfies the strong-coupling requirements for a single intracavity atom, thus permitting real-time observation of single atoms transported into the cavity. This transport scheme enables us to vary the number of intracavity atoms from 1 to $>$100 corresponding to a maximum atomic cooperativity parameter of 5400, the highest value ever achieved in an atom--cavity system. When many atoms are loaded into the cavity, optical bistability is directly measured in real-time cavity transmission.
We investigate theoretically quantum entanglement of light with the collective spin polarization of a cold atomic ensemble in cavity-assisted Raman schemes. Previous works concentrated mostly on the bad cavity limit where the signals are much longer than the cavity field lifetime. In view of atomic relaxation and other imperfections, there may arise a need to speed-up the light-atoms interface operation. By increasing the cavity field lifetime, one can achieve better light-matter coupling and entanglement. In our work, we consider the non-adiabatic effects that become important beyond the bad cavity limit in both low-photon and continuous variables regime. We find classical control field time profiles that allow one to retrieve from the cavity an output quantized signal of a predefined time shape and duration, which is optimal for the homodyne detection, optical mixing or further manipulation. This is done for a wide range of the signal duration as compared to the cavity field lifetime. We discuss an optimal cavity-atomic ensemble matching in terms of the cavity field lifetime which allows one to apply less intense control field and to minimize a variety of non-linear effects, such as AC light shifts, four-wave mixing, etc, which may be potentially harmful to an experiment.