Second-order coherence measurements can unambiguously determine whether a light source is generating coherent radiation, that is, whether it is a laser. In a systematic study reported by Kreinberg et al. , this measurement is used to additionally reveal intriguing physics associated with ultra-small resonators, such as the identification of sub- or super-radiance emission.
Second-order coherence measurements can unambiguously determine whether a light source is generating coherent radiation, that is, whether it is a laser. In a systematic study reported by Kreinberg et al. , this measurement is used to additionally reveal intriguing physics associated with ultra-small resonators, such as the identification of sub- or super-radiance emission.
Properties of light sources based on amplified spontaneous emission (ASE) are similar to the properties of lasers in many regards. However, even though ASE has been widely studied, its photon statistics have not been settled. There are no reliable theoretical estimates or unambiguous experimental data for the second-order coherence function of photons that characterizes the coherence properties of a light source. Our computer simulation clearly establishes that, independently of pump power, the light produced by ASE is similar to that of a thermal source. This result lays bare the fundamental difference between ASE radiation and laser radiation.
Developments in quantum technologies lead to new applications that require radiation sources with specific photon statistics. A widely used Poissonian statistics are easily produced by lasers; however, some applications require super- or sub-Poissonian statistics. Statistical properties of a light source are characterized by the second-order coherence function g^(2)(0). This function distinguishes stimulated radiation of lasers with g^(2)(0)=1 from light of other sources. For example, g^(2)(0)=2 for black-body radiation, and g^(2)(0)=0 for single-photon emission. One of the applications requiring super-Poissonian statistics (g^(2)(0)>1) is ghost imaging with thermal light. Ghost imaging also requires light with a narrow linewidth and high intensity. Currently, rather expensive and inefficient light sources are used for this purpose. In the last year, a superluminescent diode based on amplified spontaneous emission (ASE) has been considered as a new light source for ghost imaging. Even though ASE has been widely studied, its photon statistics has not been settled - there are neither reliable theoretical estimates of the second-order coherence function nor unambiguous experimental data. Our computer simulation clearly establishes that coherence properties of light produced by ASE are similar to that of a thermal source with g^(2)(0)=2 independent of pump power. This result manifests the fundamental difference between ASE and laser radiation.
We examine the coherence properties of stationary light obtained by the superposition of nonstationary independent emissions occurring at random space–time points. The positions of the points are independent and uniformly distributed over the volume of the source. The emission times fluctuate in accordance with a stationary renewal point process. This process admits of super-Poisson, sub-Poisson, or Poisson behavior. The individual emissions are assumed to be in a coherent, chaotic, or n state. The statistical nature of the random-emission space–time points plays an important role in determining the coherence properties and photon statistics of the total field. This is manifested in the normalized second-order correlation function, which turns out to have the usual form for chaotic light with two additional terms. The first of these is determined by the statistical nature of the individual emissions (it is positive for coherent or chaotic, but zero for single-photon, emissions). The second term is governed by the statistics of the primary excitations (it is positive for super-Poisson, zero for Poisson, and negative for sub-Poisson excitations). Both additional terms become small for light with a high degeneracy parameter (many total photons per emission lifetime). The behavior of the light is then asymptotically chaotic. In the opposite limit, when the degeneracy parameter is small (or the emissions are instantaneous), the correlation properties of the primary excitations are directly transferred to the correlation properties of the photons. The first-order spatial-coherence properties of the field turn out to be identical with those of chaotic light (i.e., the van Cittert–Zernike theorem is obeyed), although the second-order properties differ. The photon-counting distribution reflects the character of the correlation function. Thus sub-Poisson primary excitations, together with n-state emissions, can lead to sub-Poisson photon counts under certain conditions. Such nonclassical light may be made arbitrarily intense if interference effects are eliminated by detecting many spatial modes. As an example of our theory, we cite a Franck–Hertz experiment excited by a space-charge-limited electron beam. If the electron excitations are represented as a sub-Poisson renewal point process, and the photon emissions as one-photon states, the light generated should be antibunched and sub-Poisson.
Abstract Accurate detection and identification of space-based faint targets under strong background is a pressing challenge for all‑weather deep space exploration. However, active deep space detection is often hampered by the disturbances of intense stellar stray light and multi-source heterogeneous noise, making it difficult to achieve noise‑blind source separation and accurate identification of weak target signals under extremely low signal-to-noise ratio. To address these challenges, we develop a random dynamic mask modulation scheme to enhance higher-order coherence, unveiling power-law photon number distributions and super-bunching correlations that enable higher-order photon correlation imaging with a single-photon detector array (SPDA). The imaging scheme decouples the target signal from multi-source disturbances including coherent, thermal, and super-bunching light, as well as multiple background noises such as shot noise, environmental stray light, and dark counts. Here, we experimentally demonstrate a super-bunching light with a fiftieth-order photon correlation of 1.45×10 72 at a mean photon number , achieving a 10 42 enhancement in the high-to-second-order correlation ratio. The super-bunching light source enables continuously tunable output intensity from ultra‑weak to extremely bright, overcoming the limitation of conventional sources that produce only ultra‑weak photon numbers. Strikingly, we observe the ratio R n >2 of the multiphoton probability between the signal power-law light and a noise coherent laser reaching 5.29×10 269 at for 281 photons. The contrast in higher-order photon correlation imaging reaches 0.99, increasing by 0.26 over the second-order case. At an imaging exposure time of 5 µs and a mean photon number , we achieve 512×512‑pixel SPDA imaging with a dynamic range of 252.93 dB despite simultaneous photon‑starved signal and intense background. Our noise‑blind scheme establishes ultra‑wide dynamic range, high‑speed, sub-megapixel single‑photon imaging under multi‑source noise and intense optical disturbances, opening new routes for space surveillance and quantum‑enhanced detection.
The fascination of macroscopic quantum coherence formed in a degenerate Bose gas drives the worldwide work on atomic Bose-Einstein condensates. Semiconductor polaritons1 are another promising system to explore the physics of degenerate Bose gases in a stronger interaction regime. It is also a system easily produced and probed through optical methods. In this work we present the first observation of the onset of a quantum coherence formed out of an initially thermal population of polaritons in a microcavity quantum well system. The quantum coherence is measured by a time domain second order coherence g(2)(0) of the light emitted from the polaritons. Second and higher order coherence functions are crucial in quantum optics to distinguish among various pure states of light, such as coherent state or squeezed state, from classical thermal light2. They have been measured indirectly in the spatial domain for atomic BECs3,4. In our system, with supporting evidence from measurements of the input-output, dispersion, and time evolution of the emission, the phase transition was identified by a decrease of g(2)(0) ≈2, the value for a thermal state, toward g(2)(0) ≈1 for a coherent state. This is a direct evidence of the formation of a macroscopic quantum coherence in the lower polariton ground state.