Mordechai Segev

We propose and demonstrate numerically a measurement scheme for complete reconstruction of the 2D quantum wave function of a Bose-Einstein condensate, amplitude and phase, from a time-of-flight measurement. We identify a fundamental ambiguity present in the measurement of phase structures of high-symmetry excitations (eg, vortices) and show how to overcome it by allowing for different expansion durations in different directions. We demonstrate this approach with the reconstruction of matter-wave vortices and arrays of vortices.

In this opinion article, we briefly outline some historical highlights and the most recent developments in the novel and exciting field of photonic time-crystals and present the challenges, disruptive opportunities and potential impact on both the fundamental science of light and on photonic technologies.

Coherent high-quality laser sources are important for a variety of photonic devices and applications, yet they are especially challenging for high-power semiconductor lasers, because high-power and single-mode lasing present conflicting requirements; high power drives nonlinear processes causing multimode lasing and instabilities. Here, we present a distributed feedback (DFB) microring laser coupled to a uniform ring, which displays single-mode lasing up to 10 times the threshold value. This is the first microring DFB laser in gallium nitride (Ga N) and the first monolithic coupled microring Ga N lasers. Our work paves the way for future designs of high-quality high-power coupled lasers for applications, such as topological lasers, high-power monolithic frequency combs, optical clocks, gyros, sensing, and quantum systems.

We study light propagation of electromagnetic waves in a medium with a homogeneous refractive index varying quasi-periodically in time. We study two types of time-varying quasi-crystals: Andrey Aubrey and Fibonacci sequence.

We present an experimental study of optical time-refraction caused by time-interfaces as short as a single optical cycle. Specifically, we study the propagation of a probe pulse through a sample undergoing a large refractive index change induced by an intense modulator pulse. In these systems, increasing the refractive index abruptly leads to time-refraction where the spectrum of all the waves propagating in the medium is red-shifted, and subsequently blue-shifted when the refractive index relaxes back to its original value. We observe these phenomena in the single-cycle regime. Moreover, by shortening the temporal width of the modulator to ∼5–6 fs, we observe that the rise time of the red-shift associated with time-refraction is proportionally shorter. The experiments are carried out in transparent conducting oxides acting as epsilon-near-zero materials. These observations raise multiple questions on the …

We observe non-classical correlations between single photons outcoupled from surface plasmon modes carrying a superposition of angular momenta. The spatial distribution of the correlation indicates entanglement of surface plasmon pairs in the mode basis.

When an electromagnetic (EM) wave is propagating in a medium whose properties are varied abruptly in time, the wave experiences refractions and reflections known as "time-refractions" and "time-reflections", both manifesting spectral translation as a consequence of the abrupt change of the medium and the conservation of momentum. However, while the time-refracted wave continues to propagate with the same wave-vector, the time-reflected wave is propagating backward with a conjugate phase, despite the lack of any spatial interface. Importantly, while time-refraction is always significant, observing time-reflection poses a major challenge - because it requires a large change in the medium occurring within a single cycle. For that reason, time-reflection of EM waves was observed only recently. Here, we present the observation of microwave pulses at the highest frequency ever observed (0.59 GHz), and the experimental evidence of the phase-conjugation nature of time-reflected waves. Our experiments are carried out in a periodically-loaded microstrip line with optically-controlled picosecond-switchable photodiodes. Our system paves the way to the experimental realization of Photonic Time-Crystals at GHz frequencies.

Photonic Time-Crystals (PTCs) are materials in which the refractive index varies periodically and abruptly in time. This medium exhibits unusual properties such as momentum bands separated by gaps within which waves can be amplified exponentially, extracting energy from the modulation. This article provides a brief review on the concepts underlying PTCs, formulates the vision and discusses the challenges.

We propose superluminal solitons residing in the momentum gap (k gap) of nonlinear photonic time crystals. These gap solitons are structured as plane waves in space while being periodically self-reconstructing wave packets in time. The solitons emerge from modes with infinite group velocity causing superluminal evolution, which is the opposite of the stationary nature of the analogous Bragg gap soliton residing at the edge of an energy gap (or a spatial gap) with zero group velocity. We explore the faster-than-light pulsed propagation of these k-gap solitons in view of Einstein’s causality by introducing a truncated input seed as a precursor of a signal velocity forerunner, and find that the superluminal propagation of k-gap solitons does not break causality.

We introduce topological edge states in photonic space-time crystals. We show that photonic space-time crystals support propagating edge states and find a unique edge state that grows exponentially in energy whilst following the spatio-temporal edge.

A laser source is presented a plurality of unit cells of a selected number of partially physically coupled lasing units arranged within a plane and configured to form a topological structure, wherein each of the lasing units is configured to emit radiation component substantially perpendicular to said plane, said plurality of the unit cells comprising at least a first sub-array of the unit cells located in a first region interfacing with a second region of a different type than said first region, thereby defining an arrangement of optically coupled lasing units along an interface region between the first and second adjacent regions, forming at least one topological state along a topological path within said interface region.

Recent advances in ultrafast, large-modulation photonic materials have opened the door to many new areas of research. One specific example is the exciting prospect of photonic time crystals. In this perspective, we outline the most recent material advances that are promising candidates for photonic time crystals. We discuss their merit in terms of modulation speed and depth. We also investigate the challenges yet to be faced and provide our estimation on possible roads to success.

We present an experimental study of time-refraction with propagating time-interfaces. We observe the special spectral signature of time-reflections generated by these interfaces, paving the way for their experimental demonstration in the optical regime.

We present algorithmic reconstruction of the phase imprinted on the wavefunction of 2D Bose gas from experimental in-situ and time-of-flight measurements of atom density, without interferometry.

Quantum communication is based on the generation of quantum states and exploitation of quantum resources for communication protocols. Currently, photons are considered as the optimal carrier of information, because they enable long-distance transition with resilience to decoherence, and they are relatively easy to create and detect. Entanglement is a fundamental resource for quantum communication and information processing, and it is of particular importance for quantum repeaters [1]. Hyperentanglement [2], a state where parties are entangled with two or more degrees of freedom (DoFs), provides an important additional resource because it increases data rates and enhances error resilience. However, in photonics, the channel capacity, i.e. the ultimate throughput, is fundamentally limited when dealing with linear elements. We propose a technique for achieving higher transmission rates for quantum communication by using hyperentangled states, based on multiplexing multiple DoFs on a single photon, transmitting the photon, and eventually demultiplexing the DoFs to different photons at the destination, using a Bell state measurement. Following our scheme, one can generate two entangled qubit pairs by sending only a single photon. The proposed transmission scheme lays the groundwork for novel quantum communication protocols with higher transmission rate and refined control over scalable quantum technologies.

We study interaction between two k-gap solitons in Photonic-Time-Crystals. We show that they always annihilate each other, regardless of initial power, phase, and direction. The residuals of interaction strongly affect proceeding solitons as cascaded collision.

We study how thermodynamic behavior in classical nonlinear optical multimode systems presents itself in quantum many-body settings, and find multimode entanglement arising in the thermalization process.

Propagating topological edge states have been extensively studied in two-dimensional photonic topological insulators (TIs)[1]. Apart from other photonic TIs in higher dimensions, 3D TIs have recently been implemented with microwaves [2] and acoustic waves [3], the latter inducing topology through a dislocation. Here we report the first experimental demonstration of a three-dimensional topological insulator at optical frequencies and observe the propagation dynamics of its unidirectional edge states in two spatial and one modal dimensions [4]. In our system, the introduction of a screw dislocation into a weak 3DTI serves to establish a closed 3-dimensional path that encircles the 3D structure around the outer parameter and loops back along the axis of the dislocation.

We study the time-reflection and time-refraction of waves caused by a spatial interface with a medium undergoing a sudden temporal change in permittivity. We show that monochromatic waves are transformed into a pulse by the permittivity change, and that time-reflection is enhanced at the vicinity of the critical angle for total internal reflection. In this regime, we find that the evanescent field is transformed into a propagating pulse by the sudden change in permittivity. These effects display enhancement of the time-reflection and high sensitivity near the critical angle, paving the way to experiments on time-reflection and photonic time-crystals at optical frequencies.

We propose a method for chirality selection of edge modes in topological laser arrays by applying two-stage turn-on. Symmetry breaking is based on the nonlinearity of gain saturation which is always present in lasers.