Jianwei Lee

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.

Substantial scattering of a weak coherent beam by a single atom

In order to observe substantial interaction between single atoms and photons, cavities with a high finesse can be used to enhance the field at the location of an atom. Alternatively, an attempt can be made to tailor the spatial mode of a photon to closely match the mode corresponding to an electrical dipole transition of an atom by strong focusing [1]. In this paper, we present our experimental work on measurements of extinction and phase shift of a weak coherent beam due to the presence of an atom in the strongly focused part of a Gaussian light beam.

Photon number and timing resolution of a near-infrared continuous-wave source with a transition-edge sensor

Transition-edge sensors (TES) are calorimetrie spectrometers that have near unit efficiency and are photon-number resolving. A recovery time on the order of microseconds, however, limits the number resolving ability and timing accuracy in high photon-flux conditions. This can be addressed by pulsing the light source or discarding overlapping signals, thereby limiting its applicability. A method that works for stationary light sources uses the differentiated TES signal to determine the detection time of overlapping signals and to count photons [1]. However, the low signal-to-noise ratio of TES response when detecting near-infrared photons complicates the direct estimation of their detection times by this method. Here, we present an alternative approach where we use a discriminator inherently robust against noise to coarsely locate pulses in time, the integral of the identified signal regions to count photons, and a separate fit to determine the exact detection instant.

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.