Resonance Beating of Light Stored Using Atomic Spinor Polaritons

We investigate the storage of light in atomic rubidium vapor using a multilevel-tripod scheme. In the system, two collective dark polariton modes exist, forming an effective spinor quasiparticle. Storage of light is performed by dynamically reducing the optical group velocity to zero. After releasing the stored pulse, a beating of the two reaccelerated optical modes is monitored. The observed beating signal oscillates at an atomic transition frequency, opening the way to novel quantum limited measurements of atomic resonance frequencies and quantum switches.

Figure 1

Simplified scheme of relevant levels, as used for the theoretical calculations described in the text. The levels $|g_0⟩$ and $|e⟩$ are coupled by one strong control field with the Rabi frequency ${\Omega }_C$ while the two signal fields described by the quantum field operators ${\widehat{E}}_1$ and ${\widehat{E}}_2$ drive transitions between $|g_{-}⟩$ and $|e⟩$ and $|g_{+}⟩$ and $|e⟩$, respectively. The dots are to indicate the ground state populations.

Figure 2

(a) Scheme of the experimental setup. The input quantum state, encoded in a superposition of the two signal fields with optical frequencies ${\omega }_{S1}$ and ${\omega }_{S2}$, is reversibly stored in a rubidium buffer gas sample. The optical control field with frequency ${\omega }_C$ has an orthogonal linear polarization with respect to the signal fields. The signal fields and the control field are spatially overlapped upon entry into the rubidium cell. (b) Full scheme of the rubidium levels addressed by the two signal fields and one control field (solid lines). Also shown are the off-resonant circularly polarized components of the signal beams, i.e., the component with frequency ${\omega }_{S2}$ and ${\sigma }_{+}$-polarization and that with frequency ${\omega }_{S1}$ and ${\sigma }_{-}$-polarization (thin dashed lines).

Figure 3

(a) Total signal beams transmission signal (dc photodiode signal) averaged over 10 scans versus the two-photon detunings ${\delta }_1$ and ${\delta }_2$. A visible small modulation near the line determined by ${\delta }_1+{\delta }_2=0$ is instrumental and attributed to higher order mixing of the signal beams with residual control beam light leaking through our polarizing beam splitter, which near this line of degeneracy also contributes to a signal near dc frequency. (b) Selected part of the photodiode signal after a light storage time of $10\mu \mathrm{s}$ (solid curve) fitted with a sinusoidal curve (dashed line). This modulation arises from the beating between two reaccelerated optical modes and is interpreted as evidence for their relative coherence. The rectangular shape of the pulse form is determined by the gated signal processing of the electronic signal, and the visible spiking at the beginning is attributed to transients of this electronics.

Figure 4

Measured beat frequency shift of the released signal beams, after storage as a function of the applied (transverse) magnetic field. For all shown data points, the atomic sample was irradiated with optical signal fields of the same frequency during the storage procedure.