Observation of optical precursors at the biphoton level

Abstract

We describe the observation of a sharp leading edge spike in a biphoton wave packet that is produced using slow light and measured by two photon correlation. Using the stationary phase approximation we characterize this spike as a Sommerfeld-Brillouin precursor resulting from the interference of low and high frequency spectral components. c 2008 Optical Society of America OCIS codes: 270.0270, 070.7345.Optical precursors were first described by Sommerfeld and Brillouin in 1907, and are of importance in electromagnetic theory because they ensure that the front edge of a wave packet will always travel at the velocity of light in vacuum. It was previously believed that optical precursors existed for only a few optical cycles and contributed little to the amplitude of the field [1]. However, recent theoretical [2] and experimental [3] work has shown that in a narrow-resonance atomic medium the precursor magnitude can be of the same order as the primary field and its length may be much longer than a few cycles. Precursors have been extensively studied in the gamma ray [4], microwave [5], and optical regimes [3,[6][7][8][9].In this Letter, we extend the classical concept of precursors to describe the behavior of biphoton wavepackets that are measured by correlation using single photon counting modules (SPCM). The measured wave packets are generated by using electromagnetically induced transparency (EIT) and slow light [10,11] so that the correlation time of the main body of the wave packet can be varied over the range of 50-900 ns [12]. A requirement for the sharp front edge spike to be apparent is that the optical depth (OD) be sufficiently large that the pulse length is determined by the group delay. We remark that the observation of the sharp leading edge spike, see Fig. 2(b), was not expected, and its nature was first suggested to us by Daniel J. Gauthier.A schematic of biphoton generation is shown in Fig. 1. In the presence of counter-propagating cw pump (ω p ) and coupling (ω c ) lasers, phase-matched, paired Stokes (ω s ) and anti-Stokes (ω as ) photons are spontaneously generated and propagate in opposite directions. We use a two-dimensional (2D)85 Rb magneto-optical trap (MOT) with a longitudinal length L = 1.7 cm and an aspect ratio of 25. The Stokes (σ − ) and anti-Stokes (σ + ) photons are coupled into opposing single mode fibers, and detected by two SPCMs after passage through two narrow-band optical filters (F1 and F2). With the dephasing rate of the |1 → |3 transition denoted by γ 13 = 2π × 3 × 10 6 sec −1 , the pump laser has a Rabi frequency of Ω p = 1.16γ 13 , is σ − circularly polarized, and is blue detuned from the |1 → |4 transition by ∆ p = 48.67γ 13 . The coupling laser (Ω c ) is σ + circularly polarized, and is on resonance with the |2 → |3 transition. Further experimental details are described in [12].We first note that the Glauber correlation function, or equivalently the coincidence rate versus the relative time delay τ for an ideal parametric down converter with a frequency independent gro...

Optical Precursor of a Single Photon

“…Therefore, detection of single-photon precursors may shine light in the understanding of quantum information transmission. Recent progress in generating narrow-band nonclassical photon pairs [19][20][21] using EIT led to the observation of optical precursors in two-photon quantum correlation [22,23]. However, the observed biphoton precursors are produced during a nonlinear optical process and are not separable from the entire two-photon wave packet.…”

“…Figure 2(a) displays the two-photon correlation without passing the anti-Stokes photons through the EOM and MOT2. The spikeslike oscillatory structure at the leading edge results from the interference between the precursor and main waveform [22]. Figure 2(b) shows two-photon coincidence counts when the MOT2 is present with OD ¼ 14 and c2 ¼ 3:5 13 .…”

Optical Precursors with Electromagnetically Induced Transparency in Cold Atoms

“…As a result, the precursor signal cannot be separated clearly from the main pulse or otherwise the main field is absorbed. Recently, motivated by the observation of optical precursors at the biphoton level [15], the advantage of using electromagnetically induced transparency (EIT) and slow light [16,17] was pointed out [6,7]. In this case, in a single experiment, both the precursor and the main pulse can be observed, and with proper parameters, they can be well separated in time with comparable amplitudes.…”

Narrowband Biphotons: Generation, Manipulation, and Applications

“…9(b) is the first observed Sommerfeld-Brillouin precursor at the biphoton level [52]. In the stationary-phase approximation, starting from Eq.…”