We introduce the unique class of propagation-invariant surface plasmon polaritons (SPPs) representing pulsed surface wave packets propagating along unpatterned metal-dielectric interfaces and are localized in all dimensions -with potentially subwavelength transverse spatial widths. The characteristic features of such linear diffraction-free, dispersion-free 'plasmonic bullets' stem from tight spatio-temporal correlations incorporated into the SPP spectral support domain, and we thus call them 'space-time' SPPs. We show that the group velocity of space-time SPP wave packets can be readily tuned to subluminal, superluminal, and even negative values by tailoring the spatio-temporal field structure independently of any material properties. We present an analytical framework and numerical simulations for the propagation of space-time SPPs in comparison with traditional pulsed SPPs whose spatial and temporal degrees of freedom are separable, thereby verifying the propagation-invariance of the former.

Given a set of quantum gates and a target unitary operation, the most elementary task of quantum compiling is the identification of a sequence of the gates that approximates the target unitary to a determined precision ε. Solovay-Kitaev theorem provides an elegant solution which is based on the construction of successively tighter 'nets' around the unity comprised of successively longer sequences of gates. The procedure for the construction of the nets, according to this theorem, requires accessibility to the inverse of the gates as well. In this work, we propose a method for constructing nets around unity without this requirement. The algorithmic procedure is applicable to sets of gates which are diffusive enough, in the sense that sequences of moderate length cover the space of unitary matrices in a uniform way. We prove that the number of gates sufficient for reaching a precision ε scales as log(1/ε) log 3/log2 while the pre-compilation time is increased as compared to thatof the Solovay-Kitaev algorithm by the exponential factor 3/2. Approximation up to a given accuracy of an arbitrary unitary transformation by a series of standard transformations is an important ingredient of programming of quantum computers, which was formulated and solved [1,2] in the case where the set of M predetermined standard transformations contains both direct operations and their inverses. The so called Solovay-Kitaev (SK) theorem provides the proof of existence together with the method for constructing the solution. Based on the elements in the proof of the SK theorem, the Dawson-Nielsen (DNSK) algorithm [3] provides the exact steps for identifying a series of length L, which scales with the required accuracy ε as O log(1/ε) 3.97 , and with running time as O log(1/ε) 2.71 . For the special case of qubits, different techniques have been suggested [4,5] improving the running time of this algorithm, while in the general case it has been proved [6,7] that the use of extra ancilla qubits further improves the relations of both the length and the running time, with the accuracy ε.Here we address the question [3] whether it is possible to generalize the results of the SK theorem onto the case where the set of predetermined operations does not contain the inverses. In view of the fast development of quantum technologies, this problem has theoretical but also practical interest since experimentalists sometimes do not have access to inverse operations. For instance, time is the main quantum control (positive) parameter and one may employ it to construct both a gate and its inverse. On the other hand decoherence effect inducing constraints in time might prevent one from doing so in practice.Progress on the possibility of extending the SK theorem has been reported in [8,10] and also in a very recent related work [9]. Our answer is also positive and conditional on a specific property of the given set. We require that sequences of gates of moderate length (composed of 15 − 20 gates) cover the space of unitary matrices in a uniform way. This prop...

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Silicon is the most scalable optoelectronic material but has suffered from its inability to generate directly and efficiently classical or quantum light on-chip. Scaling and integration are the most fundamental challenges facing quantum science and technology. We report an all-silicon quantum light source based on a single atomic emissive center embedded in a silicon-based nanophotonic cavity. We observe a more than 30-fold enhancement of luminescence, a near-unity atom-cavity coupling efficiency, and an 8-fold acceleration of the emission from the all-silicon quantum emissive center. Our work opens immediate avenues for large-scale integrated cavity quantum electrodynamics and quantum light-matter interfaces with applications in quantum communication and networking, sensing, imaging, and computing.

A generic photonic coupler with active and lossy parts, gain saturation and asymmetric characteristics is examined. Saturable activity is shown to be able to enhance the overall stability of the steady states, prevent evolution to undesirable unbounded modes and allow for bistable operation in specific regions of parametric space. Both stability and bistability are studied in the phase space of the system, where the basins of attraction of each state are identified, providing an accurate description of the dependence of the electric fields on the initial conditions. Continuous families of exceptional points are detected via suitable regulation of the coupling and asymmetry features of the configuration. In this way, a complete description of the nonlinear dynamics landscape is provided, which should be crucial for multiple application-driven designs incorporating such a ubiquitous optical component. *