We demonstrate deterministic control over the photon-number distribution by interfering two coherent beams within a disordered photonic lattice. By sweeping a relative phase between two equal-amplitude coherent fields with Poissonian statistics that excite adjacent sites in a lattice endowed with disorder-immune chiral symmetry, we measure an output photon-number distribution that changes periodically between super-thermal and sub-thermal photon statistics upon ensemble averaging. Thus, the photon-bunching level is controlled interferometrically at a fixed mean photon-number by gradually activating the excitation symmetry of the chiral-mode pairs with structured coherent illumination and without modifying the disorder level of the random system itself.
We demonstrate deterministic control over the photon-number distribution by interfering two coherent beams within a disordered photonic lattice. By sweeping a relative phase between two equal-amplitude coherent fields with Poissonian statistics that excite adjacent sites in a lattice endowed with disorder-immune chiral symmetry, we measure an output photon-number distribution that changes periodically between super-thermal and sub-thermal photon statistics upon ensemble averaging. Thus, the photon-bunching level is controlled interferometrically at a fixed mean photon-number by gradually activating the excitation symmetry of the chiral-mode pairs with structured coherent illumination and without modifying the disorder level of the random system itself.
Coherent light traversing disordered media usually attains a random field both in amplitude and phase with independent Gaussian statistics and results in thermal light (associated with Bose-Einstein photon-number statistics) upon ensemble averaging. This is expected according to the central limit theorem, which dictates the addition of a large number of independent random variables leads to a normal (Gaussian) distribution. Here, we show that certain network topologies that light travels within preclude the central limit theorem and result in non-Gaussian statistics. We realize such networks in the form of evanescently-coupled waveguide arrays (photonic lattices) and obtain the photon statistics at the output by time gating and averaging over multiple realizations of disordered photonic lattices. The effect of lattice topology, however, only exists when the photonic lattice is endowed with chiral-symmetric eigenmode pairs a disorder-immune symmetry where the eigenmodes appear in pairs with oppositely signed eigenvalues and the coherent input field satisfies certain conditions. We specifically examine one-dimensional arrays of randomly coupled identical waveguides (off-diagonal disorder) arranged on linear and ring topologies. The emerging field exhibits super-thermal statistics (associated with modified Bose-Einstein photon-number statistics) only for ring lattices with even parity and linear lattices (independent of its parity), whereas input coherent fields traversing ring lattices with odd parity attain sub-thermal statistics. By controlling the relative phase of a coherent input field exciting two neighboring lattice sites, we also demonstrate a deterministic tuning of photon-number statistics, namely photon bunching, while maintaining the mean photon number fixed.
While the spontaneous emission from independent emitters provides spatially uncorrelated photons-a typical manifestation of quantum randomness, the interference of the coherent scattering leads to a well-defined intensity pattern-a feature described by linear optics. We here demonstrate experimentally how the interplay between the two mechanisms in large systems of quantum emitters leads to spatial variations of photon correlations. The implementation with trapped ion crystals in free space allows us to observe the anticorrelation between photon rates and variance of the photon number distributions in chains of up to 18 ions. For smaller crystals of four ions, the transition from antibunching to bunching and super-Poissonian statistics of the scattered light is reported. For higher numbers of scatterers, the photon statistics still display a strong deviation from the fully incoherent scattering case. Our results illustrate how the interference of coherent scattering, combined with spontaneous emission, provides a control mechanism for the light statistics.
Controlling the photon statistics of light is paramount for quantum science and technologies. Recently, we demonstrated that transmitting resonant laser light past an ensemble of two-level emitters can result in a stream of single photons or excess photon pairs. This transformation is due to quantum interference between the transmitted laser light and the incoherently scattered photon pairs [Prasad et al., Nat. Photonics 14, 719 (2020)NPAHBY1749-488510.1038/s41566-020-0692-z]. Here, using the dispersion of the atomic medium, we actively control the relative quantum phase between these two components. We thereby realize a tunable two-photon interferometer and observe interference fringes in the normalized photon coincidence rate. When tuning the relative phase, the coincidence rate varies periodically, giving rise to a continuous modification of the photon statistics from antibunching to bunching. Beyond the fundamental insight that there exists a tunable quantum phase between incoherent and coherent light that dictates the photon statistics, our results lend themselves to the development of novel quantum light sources.
In this thesis, we study a system consisting of qubits—two-level quantum systems— interacting in the strong-coupling regime for a finite amount of time with a mode of the electromagnetic field inside a resonator. In particular, we are interested in the dynamics of the photon number and the possibility of controlling its statistics to produce sub-Poissonian light. To describe the system we use quantum trajectory theory where the time evolution of the state vector consists of both a continuous part and jumps, which account for the resonator losses. We also incorporate a variable number of qubits (changing in time) interacting with the mode. A computer simulation was developed and optimised to calculate the quantum trajectories. When one sets the qubits initially to the excited state and fixes the interaction time accordingly, so-called “trapping states” (photon numbers which block the addition of a photon by a single qubit) appear. These states are instrumental in the generation of sub-Poisson fields, for some cases even when the number of interacting qubits follow a Poisson process. We take advantage of the existence of trapping states to implement control protocols based on the measurement of the output field of the resonator, with the objective of lowering the uncertainty in the number of photons. Furthermore, we explore the possibility of creating a source of sub-Poissonian pulses by first using a random interaction time (which bypasses the trapping states) and then utilising the control protocols developed. We are able to achieve fields with a Mandel Q parameter of QM = −0.98 and a photon number of n 68.
Photon anti-bunching, measured via the Hanbury-Brown-Twiss experiment, is one of the key signatures of quantum light and is tied to sub-Poissonian photon number statistics. Recently, it has been reported that photon anti-bunching or conditional sub-Poissonian photon number statistics can be obtained via second-order interference of mutually incoherent weak lasers and heralding based on photon counting [Phys. Rev. A92, 033855 (2015)10.1103/PhysRevA.92.033855; Opt. Express24, 19574 (2016)10.1364/OE.24.019574; https://arxiv.org/abs/1601.08161]. Here, we report theoretical analysis on the limits of manipulating conditional photon statistics via interference of weak lasers. It is shown that conditional photon number statistics can become super-Poissonian in such a scheme. We, however, demonstrate explicitly that it cannot become sub-Poissonian, i.e., photon anti-bunching cannot be obtained in such a scheme. We point out that incorrect results can be obtained if one does not properly account for seemingly negligible higher-order photon number expansions of the coherent state.
We investigate interference of optical fields by examining the probability distribution of photon detection. The usual description of interference patterns in terms of superposition of classical mean fields with definite phases is elucidated in quantum fashion. Especially, for interference of two independent mixtures of number states with Poissonian or sub-Poissonian statistics, despite lack of intrinsic phases, it is found that the joint probability has a distinct peak manifold in the multi-dimensional space of the detector outcomes, which is along the trajectory of the mean-field values as the relative phase varies on the unit circle. Then, an interference pattern should mostly appear in each shot of measurement as a point in the peak manifold with a randomly chosen relative phase. On the other hand, for super-Poissonian sources the mean-field description is likely invalidated with rather broad probability distributions.