Syed Abdullah Aljunid

Phase shift of a weak coherent beam induced by a single atom.

We report on a direct measurement of a phase shift on a weak coherent beam by a single 87Rb atom in a Mach-Zehnder interferometer. By strongly focusing the probe mode to the location of the atom, a maximum phase shift of about 1 degree is observed experimentally.

Excitation of a single atom with exponentially rising light pulses

We investigate the interaction between a single atom and optical pulses in a coherent state with a controlled temporal envelope. In a comparison between a rising exponential and a square envelope, we show that the rising exponential envelope leads to a higher excitation probability for fixed low average photon numbers, in accordance with a time-reversed Weisskopf-Wigner model. We characterize the atomic transition dynamics for a wide range of the average photon numbers and are able to saturate the optical transition of a single atom with ≈50 photons in a pulse by a strong focusing technique.

Interaction of light with a single atom in the strong focusing regime

We consider the near-resonant interaction between a single atom and a focused light mode, where the single atom localized at the focus of a lens can scatter a significant fraction of light. Complementary to previous experiments on extinction and phase shift effects of a single atom, here we report on the measurement of coherently backscattered light. The strength of the observed effect suggests combining strong focusing with a cavity to further enhance the field at the location of the atom. This could make scaling up to a network of several atom + cavity nodes more realistic due to significant technical simplification of the atom–light interface. We consider theoretically a nearly concentric cavity, which has a strongly focused optical mode. Simple estimates show that in such a case one can expect a significant single photon Rabi frequency.

Preparation of an exponentially rising optical pulse for efficient excitation of single atoms in free space

We report on a simple method to prepare optical pulses with exponentially rising envelope on the time scale of a few ns. The scheme is based on the exponential transfer function of a fast transistor, which generates an exponentially rising envelope that is transferred first on a radio frequency carrier, and then on a coherent cw laser beam with an electro-optical phase modulator. The temporally shaped sideband is then extracted with an optical resonator and can be used to efficiently excite a single (87)Rb atom.

Interfacing light and single atoms with a lens

We investigate the interaction between a single atom and a light field in the strong focusing regime. Such a configuration is subject to recent experimental work not only with atoms but also molecules and other atom-like systems such as quantum dots. We derive the scattering probability for photons by such a microscopic object modeled by a two-level system, starting with a Gaussian beam as the spatial mode of the light field. The focusing by an ideal lens is modeled by adopting a field with spherical wave fronts compatible with Maxwell equations. Using a semi-classical approach for the atom-field interaction, we predict a scattering probability of photons by a single atom of up to 98% for realistic focusing parameters. Experimental results for different focusing strengths are compared with our theoretical model.

Excitation of a single atom with a temporally shaped light pulses

We investigate the interaction between a single 87Rb atom and optical pulses with a controlled temporal envelope. In our experiment an atom is coupled to the light field by high numerical aperture aspheric lens that focuses a Gaussian beam to an atom and fixes the spatial overlap with a field. The frequency/temporal overlap can be changed by shaping the temporal envelope of an attenuated coherent pulse with fast modulators [3]. The excitation probability Pe is measured by detecting atomic fluorescence with high temporal resolution and normalizing the acquired rates by optical losses (Fig. 1).

Atom-light interface in strong focusing geometry

It was recently shown that a single atom located at the focus of a simple aspheric lens can efficiently scatter photons out of a focused coherent light beam [1, 2], impose a phase shift [3], and partially reflect a probe beam [4]. With our current experimental system, we observe an extinction of 10% and a phase shift of about 1° (Fig. 1).

Vibrational ground state cooling of a neutral atom in a tightly focused optical dipole trap

It was recently shown that a single atom can efficiently scatter photons out of a focused coherent light beam [1, 2, 3]. The scattering probability is strongly dependent on a thermal motion of the atom and can be maximized if the atom is well localized at a focus. To achieve that, we implement a Raman sideband cooling technique that is commonly used in ion traps [4]. Our trap, formed by focused Gaussian light beam at 980nm, has characteristic frequencies of ν<inf>τ</inf> = 55 kHz and ν<inf>l</inf> = 7 kHz corresponding to transverse and longitudinal confinements. A single ⁸⁷Rb atom is loaded into the trap from an optical molasses. Two Raman beams couple the motional states of |F = 2〉 and |F = 1〉 manifolds with a Lamb-Dicke parameter η = 0.084 (Figure 1). The Raman beams are oriented such that momentum transfer occurs only along the strong confinement of the trap with ν<inf>τ</inf> = 55 kHz. The cooling sequence consists of following steps: (1) initial preparation of the atom in |F = 2,m<inf>F</inf> = −2〉 Zeeman state, (2) Raman transfer between the motional states |F = 2,m<inf>F</inf> = −2,N〉 and |F = 1,m<inf>F</inf> = −1,N − 1〉. (3) recycling the atomic population back to |F = 2,m<inf>F</inf> = −2〉 state via an optical pulse resonant to |5P<inf>3/2</inf>, F = 2〉 state thus removing a phonon via spontaneous emission.

Measuring atomic oscillator strengths by single atom spectroscopy

We propose a method for assessing the oscillator strengths of atomic transitions based on single atom spectroscopy. The method is based on a direct measurement of an AC Stark shift of atomic energy levels for the single atom trapped in an optical tweezer. The method is independent on a knowledge of the trapping field at the atom. The results can be applied to obtaining the previously unknown oscillator strengths for dipole transitions involving the first excited state of alkali metals.

Strong interaction between light and a single trapped atom without a cavity

We measured the extinction of a focused light beam by a single 87Rb atom localized in an optical dipole trap and found a value of 7.2% for a focused Gaussian beam resonantly interacting with an atomic two-level system. Various models describing interaction of an atom with a focused light field are compared to explain our experimental result. Our experiment suggests that a strong coupling may be achieved without cavity assistance. This opens new perspectives for a development of an efficient atom-photon interface for Quantum Information processing.