Temporal Multiplexing Research Articles

Active temporal multiplexing of indistinguishable heralded single photons.

It is a fundamental challenge in quantum optics to deterministically generate indistinguishable single photons through non-deterministic nonlinear optical processes, due to the intrinsic coupling of single- and multi-photon-generation probabilities in these processes. Actively multiplexing photons generated in many temporal modes can decouple these probabilities, but key issues are to minimize resource requirements to allow scalability, and to ensure indistinguishability of the generated photons. Here we demonstrate the multiplexing of photons from four temporal modes solely using fibre-integrated optics and off-the-shelf electronic components. We show a 100% enhancement to the single-photon output probability without introducing additional multi-photon noise. Photon indistinguishability is confirmed by a fourfold Hong–Ou–Mandel quantum interference with a 91±16% visibility after subtracting multi-photon noise due to high pump power. Our demonstration paves the way for scalable multiplexing of many non-deterministic photon sources to a single near-deterministic source, which will be of benefit to future quantum photonic technologies.

Active temporal and spatial multiplexing of photons

The maturation of many photonic technologies from individual components to next-generation system-level circuits will require exceptional active control of complex states of light. A prime example is in quantum photonic technology: while single-photon processes are often probabilistic, it has been shown in theory that rapid and adaptive feedforward operations are sufficient to enable scalability. Here, we use simple “off-the-shelf” optical components to demonstrate active multiplexing—adaptive rerouting to single modes—of eight single-photon “bins” from a heralded source. Unlike other possible implementations, which can be costly in terms of resources or temporal delays, our new configuration exploits the benefits of both time and space degrees of freedom, enabling a significant increase in the single-photon emission probability. This approach is likely to be employed in future near-deterministic photon multiplexers with expected improvements in integrated quantum photonic technology.

Evolutionary Clustering for Mining and Tracking Dynamic Multilayer Networks

This article proposes a framework for community discovery in temporal multiplex networks by extending the evolutionary clustering approach to encompass both time and multiple dimensions. In this extended framework, the problem of finding community structures for time‐evolving networks with multiple types of ties is reformulated by adding the concept of dimensional smoothness, relative to a single timestamp, to that of temporal smoothness, at the base of evolutionary clustering. At each timestamp, the method tries to maximize the quality of the clustering obtained for the current multidimensional network and to minimize the differences with respect to that obtained at the previous timestamp. Moreover, the evolution of a community between two consecutive timestamps is maintained by exploiting the Hungarian approach, which determines the best cluster correspondence between two consecutive timestamps. Experiments on synthetic and real‐world networks show the capability of the approach in discovering and tracking group organization of actors constituting the network.

Multiplexed on-demand storage of polarization qubits in a crystal

A long-lived and multimode quantum memory is a key component needed for the development of quantum communication. Here we present temporally multiplexed storage of five photonic polarization qubits encoded onto weak coherent states in a rare-earth-ion doped crystal. Using spin refocusing techniques we can preserve the qubits for more than half a millisecond. The temporal multiplexing allows us to increase the effective rate of the experiment by a factor of five, which emphasizes the importance of multimode storage for quantum communication. The fidelity upon retrieval is higher than the maximum classical fidelity achievable with qubits encoded onto single photons and we show that the memory fidelity is mainly limited by the memory signal-to-noise ratio. These results show the viability and versatility of long-lived, multimode quantum memories based on rare-earth-ion doped crystals.

Elliptic solitons in optical fiber media

We examine the evolution of a time-varying perturbation signal pumped into a mono-mode fiber in the anomalous dispersion regime. We analytically establish that the perturbation evolves into a conservative pattern of periodic pulses which structures and profiles share close similarity with the so-called soliton-crystal states recently observed in fiber media [see e.g. A. Haboucha et al., Phys. Rev. A\textbf{78}, 043806 (2008); D. Y. Tang et al., Phys. Rev. Lett. \textbf{101}, 153904 (2008); F. Amrani et al., Opt. Express \textbf{19}, 13134 (2011)]. We derive mathematically and generate numerically a crystal of solitons using time division multiplexing of identical pulses. We suggest that at very fast pumping rates, the pulse signals overlap and create an unstable signal that is modulated by the fiber nonlinearity to become a periodic lattice of pulse solitons which can be described by elliptic functions. We carry out a linear stability analysis of the soliton-crystal structure and establish that the correlation of centers of mass of interacting pulses broadens their internal-mode spectrum, some modes of which are mutually degenerate. While it has long been known that high-intensity periodic pulse trains in optical fibers are generated from the phenomenon of modulational instability of continuous waves, the present study provides evidence that they can also be generated via temporal multiplexing of an infinitely large number of equal-intensity single pulses to give rise to stable elliptic solitons.

Enhancing the heralded single-photon rate from a silicon nanowire by time and wavelength division multiplexing pump pulses.

Heralded single photons produced on a silicon chip represent an integrated photon source solution for scalable photonic quantum technologies. The key limitation of such sources is their non-deterministic nature introduced by the stochastic spontaneous four-wave mixing (SFWM) process. Active spatial and temporal multiplexing can improve this by enhancing the single-photon rate without degrading the quantum signal-to-noise ratio. Here, taking advantage of the broad bandwidth of SFWM in a silicon nanowire, we experimentally demonstrate heralded single-photon generation from a silicon nanowire pumped by time and wavelength division multiplexed pulses. We show a 90±5% enhancement on the heralded photon rate at the cost of only 14±2% reduction to the signal-to-noise ratio, close to the performance found using only time division multiplexed pulses. As single-photon events are distributed to multiple wavelength channels, this new scheme overcomes the saturation limit of avalanche single-photon detectors and will improve the ultimate performance of such photon sources.

Model-based low bit-rate video coding for resource-deficient wireless visual communication

In this paper, an effective low bit-rate video coding scheme is developed to realize state-of-the-art video coding efficiency with lower encoder complexity, while supporting standard compliance and error resilience. Such an architecture is particularly attractive for application scenarios involving resource-deficient wireless video communications. At the encoder, in order to increase resilience to channel error, multiple descriptions of a video sequence are generated in the spatio-temporal domain by temporal multiplexing and spatial adaptive downsampling. The resulting side descriptions are interleaved with each other in temporal domain, while still with conventional square sample grids in spatial domain. As such, each side description can be compressed without any change to existing video coding standards. At the decoder, each side description is first decompressed, and then reconstructed to the original resolution with the help of the other side description. In this procedure, the decoder recovers the original video sequence in a constrained least squares regression process, in which 2D or 3D piecewise autoregressive model is adaptively chosen according to different predictive modes. In this way, the spatial and temporal correlation is sufficiently explored to achieve superior quality. Experimental results demonstrate that the proposed video coding scheme outperforms H.264/AVC and other state-of-the-art methods in rate–distortion performance at low bit-rates and achieves superior visual quality at medium bit rates as well, while with lower encoding computational complexity.

Optimization of a heralded single-photon source with spatial and temporal multiplexing

The properties of a heralded single-photon source with temporal and spatial multiplexing are studied with the aim to maximize its efficiency for a given value of the second-order zero-time autocorrelation function. We show that the variable time delay, which is used for temporal multiplexing, can be optimized so that the mean number of photon passes through the switches and the total number of switches are respectively reduced to ∼ log2N and ∼ (1/2)log2N, where N is the temporal multiplexing degree. The total efficiency of such an optimized source is calculated for typical switching losses and the autocorrelation function is calculated in the presence of the detector dark-count noise.

Homeostatic plasticity for single node delay-coupled reservoir computing.

Supplementing a differential equation with delays results in an infinite-dimensional dynamical system. This property provides the basis for a reservoir computing architecture, where the recurrent neural network is replaced by a single nonlinear node, delay-coupled to itself. Instead of the spatial topology of a network, subunits in the delay-coupled reservoir are multiplexed in time along one delay span of the system. The computational power of the reservoir is contingent on this temporal multiplexing. Here, we learn optimal temporal multiplexing by means of a biologically inspired homeostatic plasticity mechanism. Plasticity acts locally and changes the distances between the subunits along the delay, depending on how responsive these subunits are to the input. After analytically deriving the learning mechanism, we illustrate its role in improving the reservoir's computational power. To this end, we investigate, first, the increase of the reservoir's memory capacity. Second, we predict a NARMA-10 time series, showing that plasticity reduces the normalized root-mean-square error by more than 20%. Third, we discuss plasticity's influence on the reservoir's input-information capacity, the coupling strength between subunits, and the distribution of the readout coefficients.

Multi-User Autostereoscopic 2D/3D Switchable Flat-Panel Display

An autostereoscopic display system which can allow multiple users to see 3D perception of the images without loss of resolution is introduced in this paper. It is a kind of time-sequential 3D systems incorporating eye tracking and directional optical design to realize free viewing. The whole system consists of five main parts: multi-user eye tracker, direct-view LCD, steerable LED array, MCU controller, and a suit of directional optics. It employs the theory of temporal multiplexing by the use of a LCD available to run at 120 Hz and utilizes a multi-user eye tracker to get the viewers' eye positions so as to control the steerable backlight to produce multiple pairs of exit pupils to the viewers' eyes. This paper describes the detailed structure of the system, the operation principle and the results obtained are also described. This approach differs from many existing ones in that it is switchable between 2D and 3D mode both with full resolution and is much thinner compared to other bulky systems with a width less than 20 cm.

Motion artifacts on 240-Hz OLED stereoscopic 3D displays

Temporal multiplexing is a popular approach for presenting different images to the two eyes in stereoscopic 3D (S3D) displays. We examined the visibility of flicker and motion artifacts—judder, motion blur, and edge banding—on a 240-Hz temporally multiplexed S3D OLED display. Traditionally, a frame rate of 120 Hz (60 Hz per eye) is used to avoid visible flicker, but there is evidence that higher frame rates provide visible benefits. In a series of psychophysical experiments, we measured the visibility of artifacts on the OLED display using temporal multiplexing and those of a 60-Hz S3D LCD using spatial multiplexing. We determined the relative contributions of the frame rate of the content, update rate of the display, duty cycle, and number of flashes. We found that short duty cycles and low flash numbers reduce the visibility of motion artifacts, while long duty cycles and high flash numbers reduce flicker visibility.

Cascaded displays

We demonstrate that layered spatial light modulators (SLMs), subject to fixed lateral displacements and refreshed at staggered intervals, can synthesize images with greater spatiotemporal resolution than that afforded by any single SLM used in their construction. Dubbed cascaded displays , such architectures enable superresolution flat panel displays (e.g., using thin stacks of liquid crystal displays (LCDs)) and digital projectors (e.g., relaying the image of one SLM onto another). We introduce a comprehensive optimization framework, leveraging non-negative matrix and tensor factorization, that decomposes target images and videos into multi-layered, time-multiplexed attenuation patterns---offering a flexible trade-off between apparent image brightness, spatial resolution, and refresh rate. Through this analysis, we develop a real-time dual-layer factorization method that quadruples spatial resolution and doubles refresh rate. Compared to prior superresolution displays, cascaded displays place fewer restrictions on the hardware, offering thin designs without moving parts or the necessity of temporal multiplexing. Furthermore, cascaded displays are the first use of multi-layer displays to increase apparent temporal resolution. We validate these concepts using two custom-built prototypes: a dual-layer LCD and a dual-modulation liquid crystal on silicon (LCoS) projector, with the former emphasizing head-mounted display (HMD) applications.

240 Hz OLED technology properties that can enable improved image quality

AbstractLarge screen Organic Light Emitting Diode (OLED) technology is finally emerging as a technology ready for consumer use. OLED has a number of areas in which it can in principle be better than Liquid Crystal Displays (LCDs); two ways in particular are that OLED technology can achieve black levels that are much darker than LCDs, and the OLED pixels can switch extremely rapidly. These differences can permit OLED displays to outperform LCD under a number of conventional scenarios, but it can also enable some dramatically improved stereoscopic 3D (s3D) displays, including full‐resolution temporal multiplexing with more than two views. In this paper, we review a set of experiments evaluating the strengths and weaknesses of OLED technology in general with a particular emphasis on understanding how the strengths of OLED technology can be leveraged to maximize the performance of an s3D display system.

Extended depth of field using a liquid crystal annular spatial light modulator.

A detailed investigation is presented on the tunable extended depth of field (EDOF) method, proposed recently by Klapp etal. [Opt. Lett.39, 1414 (2014)]. This method is based on temporal multiplexing of phase masks, using an annular liquid crystal spatial light modulator possessing a small number of rings. Examples of 3D simulations used to determine the phase profiles in the pupil plane are presented, as well as more detailed experimental results. Both the experimental and numerical results include comprehensive analysis of contrast dependence on both the spatial spectrum of the object and the amount of defocus. In addition, for the first time, we present the EDOF order inversion in the experimental and simulated data. The results show a profound performance of the proposed system and method.

High-Fidelity Reproduction of Spatiotemporal Visual Signals for Retinal Prosthesis

Natural vision relies on spatiotemporal patterns of electrical activity in the retina. We investigated the feasibility of veridically reproducing such patterns with epiretinal prostheses. Multielectrode recordings and visual and electrical stimulation were performed on populations of identified ganglion cells in isolated peripheral primate retina. Electrical stimulation patterns were designed to reproduce recorded waves of activity elicited by a moving visual stimulus. Electrical responses in populations of ON parasol cells exhibited high spatial and temporal precision, matching or exceeding the precision of visual responses measured in the same cells. Computational readout of electrical and visual responses produced similar estimates of stimulus speed, confirming the fidelity of electrical stimulation for biologically relevant visual signals. These results suggest the possibility of producing rich spatiotemporal patterns of retinal activity with a prosthesis and that temporal multiplexing may aid in reproducing the neural code of the retina.

Optimized Temporal Multiplexing for Reservoir Computing with a Single Delay-Coupled Node

Optimized Temporal Multiplexing for Reservoir Computing with a Single Delay-Coupled Node

Tunable extended depth of field using a liquid crystal annular spatial filter

A tunable extended depth of field (EDOF) imaging is presented using temporal multiplexing and a low-cost eight-ring, annular liquid crystal spatial light modulator. By changing between different phase profiles in the pupil plane of a lens we perform several levels of EDOF. Using these levels as a "database" it is shown by temporal multiplexing how to decompose tunable levels of EDOF.

Tunable lifetime multiplexing using luminescent nanocrystals

Optical multiplexing plays an important role in applications such as optical data storage1, document security2, molecular probes3,4 and bead assays for personalized medicine5. Conventional fluorescent colour coding is limited by spectral overlap and background interference, restricting the number of distinguishable identities. Here, we show that tunable luminescent lifetimes τ in the microsecond region can be exploited to code individual upconversion nanocrystals. In a single colour band, one can generate more than ten nanocrystal populations with distinct lifetimes ranging from 25.6 µs to 662.4 µs and decode their well-separated lifetime identities, which are independent of both colour and intensity. Such ‘τ-dots’ potentially suit multichannel bioimaging, high-throughput cytometry quantification, high-density data storage, as well as security codes to combat counterfeiting. This demonstration extends the optical multiplexing capability by adding the temporal dimension of luminescent signals, opening new opportunities in the life sciences, medicine and data security. Control over the luminescence lifetimes of upconversion nanocrystals allows a new form of temporal multiplexing for imaging and data-storage applications.

Stereoscopic three-dimensional display with a fast-response liquid crystal polarization rotator

A stereoscopic three-dimensional (3D) system using a polarization rotator is proposed. The polarization rotator converts the polarizations of images from different eyes in temporal multiplexing. A novel fast nematic liquid crystal mode is also proposed. The new liquid crystal mode has a fast response time and a high contrast, and is thus a good candidate as a polarization rotator. The proposed 3D system uses only passive polarized glasses, which are more convenient, lightweight, and low-cost than what? In addition, the polarization rotator can be applied to current two-dimensional displays and can become 3D-capable displays.

Edge-guided resolution enhancement in projectors via optical pixel sharing

Digital projection technology has improved significantly in recent years. But, the relationship of cost with respect to available resolution in projectors is still super-linear. In this paper, we present a method that uses projector light modulator panels (e.g. LCD or DMD panels) of resolution n X n to create a perceptually close match to a target higher resolution cn X cn image, where c is a small integer greater than 1. This is achieved by enhancing the resolution using smaller pixels at specific regions of interest like edges. A target high resolution image ( cn X cn ) is first decomposed into (a) a high resolution ( cn X cn ) but sparse edge image , and (b) a complementary lower resolution ( n X n ) non-edge image . These images are then projected in a time sequential manner at a high frame rate to create an edge-enhanced image -- an image where the pixel density is not uniform but changes spatially. In 3D ready projectors with readily available refresh rate of 120Hz, such a temporal multiplexing is imperceptible to the user and the edge-enhanced image is perceptually almost identical to the target high resolution image. To create the higher resolution edge image, we introduce the concept of optical pixel sharing . This reduces the projected pixel size by a factor of 1/ c 2 while increasing the pixel density by c 2 at the edges enabling true higher resolution edges. Due to the sparsity of the edge pixels in an image we are able to choose a sufficiently large subset of these to be displayed at the higher resolution using perceptual parameters. We present a statistical analysis quantifying the expected number of pixels that will be reproduced at the higher resolution and verify it for different types of images.