Jiefei Chen

Discussion and Outlook

We discuss the potential applications of optical precursors. Precursor fields are generated from the linear dispersion effect, such that the precursor fields can be stacked to achieve extremely high transient pulse. Another application in pulse manipulation is stimulated by the precursor generated from phase step-modulation, which may be applied to differential phase shifted key scheme. Communication in dense material is another possible application of precursor. Constructed from far off-resonance spectral components regardless of specific medium, precursor finds further advantage in communication under water.

Optimal storage and retrieval of single-photon waveforms

We report an experimental demonstration of optimal storage and retrieval of heralded single-photon wave packets using electromagnetically induced transparency (EIT) in cold atoms at a high optical depth. We obtain an optimal storage efficiency of (49 ± 3)% for single-photon waveforms with a temporal likeness of 96%. Our result brings the EIT quantum light-matter interface closer to practical quantum information applications.

A dark-line two-dimensional magneto-optical trap of 85Rb atoms with high optical depth

We describe the apparatus of a dark-line two-dimensional (2D) magneto-optical trap (MOT) of (85)Rb cold atoms with high optical depth (OD). Different from the conventional configuration, two (of three) pairs of trapping laser beams in our 2D MOT setup do not follow the symmetry axes of the quadrupole magnetic field: they are aligned with 45° angles to the longitudinal axis. Two orthogonal repumping laser beams have a dark-line volume in the longitudinal axis at their cross over. With a total trapping laser power of 40 mW and repumping laser power of 18 mW, we obtain an atomic OD up to 160 in an electromagnetically induced transparency (EIT) scheme, which corresponds to an atomic-density-length product NL = 2.05 × 10(15) m(-2). In a closed two-state system, the OD can become as large as more than 600. Our 2D MOT configuration allows full optical access of the atoms in its longitudinal direction without interfering with the trapping and repumping laser beams spatially. Moreover, the zero magnetic field along the longitudinal axis allows the cold atoms maintain a long ground-state coherence time without switching off the MOT magnetic field, which makes it possible to operate the MOT at a high repetition rate and a high duty cycle. Our 2D MOT is ideal for atomic-ensemble-based quantum optics applications, such as EIT, entangled photon pair generation, optical quantum memory, and quantum information processing.

Coherence time limit of the biphotons generated in a dense cold atomcloud

Biphotons with narrow bandwidth and long coherence time can enhance light-atom interaction, which leads to strong coupling between photonic and atomic qubits. Such strong coupling is desirable in quantum information processing, quantum storage and communication. In particular, paired photons with a long coherence time over submicroseconds facilitate the direct manipulation of biphoton wavefunction. In this paper, we report the narrow-band biphotons with a coherence time of 2.34 μs generated from spontaneous four-wave mixing (SFWM) in a dense cold atom cloud, in which the anti-Stokes photons go through a narrow electromagnetically-induced transparency (EIT) window. In our knowledge, this is the best record of coherence time for paired photons achieved so far. A number of factors limiting the coherence time are analyzed in detail. We find the EIT coherence plays an essential role in determining the coherence time for paired photons. The EIT dephasing rate is the ultimate limit to the coherence time, and an ultra-long coherence time above ten microseconds is possible by further improvement of the dephasing rate below 100 kHz.

Interfering single photons retreived from collective atomic excitations in two dense cold-atom clouds

We report the Hong–Ou–Mandel (HOM) interference, with visibility of 91%, produced from two independent single photons retrieved from collective atomic excitations in two separate cold-atom clouds with high optical depths of 90. The high visibility of the HOM dip is ascribed to the pure single photon in the Fock state that was generated from a dense-cold-atom cloud pumping by a short pulse. The visibility is always the same regardless of the time response of the single-photon detectors. This result experimentally shows that the single photons retrieved are in a separable temporal state with their idler photons.

Coherence time limit of entangled paired photons generated in a cold atom cloud

Through electromagnetically-induced transparency (EIT) assisted spontaneous four-wave mixing, we produce the entangled paired photons with a coherence time of 2.34 μs from a cold atom cloud. The EIT dephasing rate is the ultimate limit.

Searching for Precursors: From Microwave to Primary Optical Experiments

In this chapter, we review some early experimental works on precursor observation. The first experimental observation of Sommerfeld-Brillouin precursor is reported in coaxial transmission line, where SB precursors exist in microwave. Later, precursors are studied and measured experimentally in sound-wave domain, with superfluid 3He-B prepared closer to “resonant regime”. The first optical precursor was reported in infrared light propagated through GaAs single-crystal layer. We review and discuss these experimental works within the theoretical framework in Chap. 2.

Theory of Optical Precursors

In this chapter, we discuss the theoretical framework of optical precursors based on the incident electromagnetic waves interacting with dielectric media. The simplest way to interpret the light-matter interaction is the medium optical response to the incident light characterized by dielectric constant \( \varepsilon (\omega ) \) as a function of incident light frequency \( \omega \). To build a theoretical model of optical precursors, first we derive macroscopic dielectric constant \( \varepsilon (\omega ) \) starting from the microscopic dipole moment \( p(\omega ) \). Based on Maxwell’s equation and transfer function, the transmitted step-modulated electromagnetic field through dielectric media will be derived in general form of inverse Fourier transform of transmitted spectrum. The general expression of transmitted field can be solved numerically or analytically depending on the specific parameter regimes, such as Brillouin regime or resonant regime. The discussion extends from single Lorentz medium to electromagnetically-induced transparency medium, where the main signal transmits without loss.

Observation of Optical Precursors in Cold Atoms

Cold atomic source was first introduced into the community of precursor in 2006, when Jeong (Phys. Rev. Lett. 96:143901, 2006) reported the direct observation in a cloud of potassium cold atoms. Later, Wei et al. reported observation in a 2-dimensional magneto-optical trap with optical depth as high as 50, with the assistance of EIT effect. With the advent of cold atom traps and tunable diode lasers, we now have a single physical system with parameters that can be widely tuned to cover both physical regimes. Also, in the chapter, we discuss and interpret the slow and fast light phenomena while comparing with the precursor propagation in the same system. In the last section, we review the stacked precursors measured with a multiple-step function.

Optical precursors in slow and fast light media

We observe optical precursors generated from slow and fast light cold atomic media. Using constructive interference between sequenced precursors, we produce optical transient pulses with peak powers of about 9 times the input power.