Optical Neural Network Architecture for Deep Learning with Temporal Synthetic Dimension

The physical concept of synthetic dimensions has recently been introduced into optics.The fundamental physics and applications are not yet fully understood,and this report explores an approach to optical neural networks using synthetic dimension in time domain,by theoretically proposing to utilize a single resonator network,where the arrival times of optical pulses are interconnected to construct a temporal synthetic dimension.The set of pulses in each roundtrip therefore provides the sites in each layer in the optical neural network,and can be linearly transformed with splitters and delay lines,including the phase modulators,when pulses circulate inside the network.Such linear transformation can be arbitrarily controlled by applied modulation phases,which serve as the building block of the neural network together with a nonlinear component for pulses.We validate the functionality of the proposed optical neural network for the deep learning purpose with examples handwritten digit recognition and optical pulse train distribution classification problems.This proof of principle computational work explores the new concept of developing a photonics-based machine learning in a single ring network using synthetic dimensions,which allows flexibility and easiness of reconfiguration with complex functionality in achieving desired optical tasks.

Extreme single-excitation subradiance from two-band Bloch oscillations in atomic arrays

Atomic arrays provide an important quantum optical platform with photon-mediated dipole–dipole interactions that can be engineered to realize key applications in quantum information processing.A major obstacle for such applications is the fast decay of the excited states.By controlling two-band Bloch oscillations of single excitation in an atomic array under an external magnetic field,here we show that exotic subradiance can be realized and maintained with orders of magnitude longer than the spontaneous decay time in atomic arrays with the finite size.The key finding is to show a way for preventing the wavepacket of excited states scattering into the dissipative zone inside the free space light cone,which therefore leads to the excitation staying at a subradiant state for an extremely long decay time.We show that such operation can be achieved by introducing a spatially linear potential from the external magnetic field in the atomic arrays and then manipulating interconnected two-band Bloch oscillations along opposite directions.Our results also point out the possibility of controllable switching between superradiant and subradiant states,which leads to potential applications in quantum storage.

Ultracompact topological photonic switch based on valley-vortex-enhanced high-efficiency phases shift

Topologically protected edge states based on valley photonic crystals(VPCs)have been widely studied,from theoretical verifcation to technical applications.However,research on integrated tuneable topological devices is still lacking.Here,we study the phase-shifting theory of topological edge modes based on a VPC structure.Benefiting from the phase vortex formed by the VPC structure,the optical path of the topological edge mode in the propagation direction is approximately two-fold that of the conventional optical mode in a strip waveguide.In experiments,we show a 1.57-fold improvement inπ-phase tuning efficiency.By leveraging the highefficiency phase-shifting properties and the sharp-turn features of the topological waveguide,we demonstrate an ultracompact 1×2 thermo-optic topological switch(TOTS)operating at telecommunication wavelengths.A switching power of 18.2 mW is needed with an ultracompact device footprint of 25.66×28.3μm in the wavelength range of 1530-1582 nm.To the best of our knowledge,this topological photonic switch is the smallest switch of any dielectric or semiconductor 1×2/2×2 broadband optical switches,including thermo-optic and electro-optic switches.In addition,a high-speed transmission experiment employing the proposed TOTS is carried out to demonstrate the robust transmission of high-speed data.Our work reveals the phase shifting mechanism of valley edge modes,which may enable diverse topological functional devices in many fields,such as optical communications,nanophotonics,and quantum information processing.

Photon retention in coherently excited nitrogen ions

Quantum coherence in quantum optics is an essential part of optical information processing and light manipulation.Alkali metal vapors,despite the numerous shortcomings,are traditionally used in quantum optics as a working medium due to convenient near-infrared excitation,strong dipole transitions and long-lived coherence.Here,we proposed and experimentally demonstrated photon retention and subsequent re-emittance with the quantum coherence in a system of coherently excited molecular nitrogen ions(N_(2)^(+))which are produced using a strong 800 nm femtosecond laser pulse.Such photon retention,facilitated by quantum coherence,keeps releasing directly-unmeasurable coherent photons for tens of picoseconds,but is able to be read out by a time-delayed femtosecond pulse centered at 1580 nm via two-photon resonant absorption,resulting in a strong radiation at 329.3 nm.We reveal a pivotal role of the excited-state population to transmit such extremely weak re-emitted photons in this system.This new finding unveils the nature of the coherent quantum control in N_(2)^(+)for the potential platform for optical information storage in the remote atmosphere,and facilitates further exploration of fundamental interactions in the quantum optical platform with strong-field ionized molecules.

Direct extraction of topological Zak phase with the synthetic dimension

Measuring topological invariants is an essential task in characterizing topological phases of matter.They are usually obtained from the number of edge states due to the bulk-edge correspondence or from interference since they are integrals of the geometric phases in the energy band.It is commonly believed that the bulk band structures could not be directly used to obtain the topological invariants.Here,we implement the experimental extraction of Zak phase from the bulk band structures of a Su-Schrieffer-Heeger(SSH)model in the synthetic frequency dimension.Such synthetic SSH lattices are constructed in the frequency axis of light,by controlling the coupling strengths between the symmetric and antisymmetric supermodes of two bichromatically driven rings.We measure the transmission spectra and obtain the projection of the time-resolved band structure on lattice sites,where a strong contrast between the non-trivial and trivial topological phases is observed.The topological Zak phase is naturally encoded in the bulk band structures of the synthetic SSH lattices,which can hence be experimentally extracted from the transmission spectra in a fiber-based modulated ring platform using a laser with telecom wavelength.Our method of extracting topological phases from the bulk band structure can be further extended to characterize topological invariants in higher dimensions,while the exhibited trivial and non-trivial transmission spectra from the topological transition may find future applications in optical communications.

Topological holographic quench dynamics in a synthetic frequency dimension

The notion of topological phases extended to dynamical systems stimulates extensive studies,of which the characterization of nonequilibrium topological invariants is a central issue and usually necessitates the information of quantum dynamics in both the time and momentum dimensions.Here,we propose the topological holographic quench dynamics in synthetic dimension,and also show it provides a highly efficient scheme to characterize photonic topological phases.A pseudospin model is constructed with ring resonators in a synthetic lattice formed by frequencies of light,and the quench dynamics is induced by initializing a trivial state,which evolves under a topological Hamiltonian.Our key prediction is that the complete topological information of the Hamiltonian is encoded in quench dynamics solely in the time dimension,and is further mapped to lower-dimensional space,manifesting the holographic features of the dynamics.In particular,two fundamental time scales emerge in the dynamical evolution,with one mimicking the topological band on the momentum dimension and the other characterizing the residue time evolution of the state after the quench.For this,a universal duality between the quench dynamics and the equilibrium topological phase of the spin model is obtained in the time dimension by extracting information from the field evolution dynamics in modulated ring systems in simulations.This work also shows that the photonic synthetic frequency dimension provides an efficient and powerful way to explore the topological nonequilibrium dynamics.

Temporal modulation brings metamaterials into new era

Temporal modulations in photonics bring many exotic optical phenomena in the time dimension while metamaterials provide powerful ways in manipulating light in the spatial domain.The authors envision the connection,Floquet Metamaterials,may deliver novel opportunities in nanophotonics.

Observation of flat-band and band transition in the synthetic space

.Constructions of synthetic lattices in modulated ring resonators attract growing attention to interesting physics beyond the geometric dimensionality,where complicated connectivities between resonant frequency modes are explored in many theoretical proposals.We implement experimental demonstration of generating a stub lattice along the frequency axis of light,in two coupled ring resonators of different lengths,with the longer one dynamically modulated.Such a synthetic photonic structure intrinsically exhibits the physics of flat band.We show that the time-resolved band structure read-out from the drop-port output of the excited ring is the intensity projection of the band structure onto a specific resonant mode in the synthetic momentum space,where gapped flat band,mode localization effect,and flat-to-nonflat band transition are observed in experiments and verified by simulations.This work provides evidence for constructing a synthetic stub lattice using two different rings,which,hence,makes a solid step toward experimentally constructing complicated lattices in multiple rings associated with synthetic frequency dimensions.

Creating locally interacting Hamiltonians in the synthetic frequency dimension for photons

The recent emerging field of synthetic dimension in photonics offers a variety of opportunities for manipulating different internal degrees of freedom of photons such as the spectrum of light.While nonlinear optical effects can be incorporated into these photonic systems with synthetic dimensions,these nonlinear effects typically result in long-range interactions along the frequency axis.Thus,it has been difficult to use the synthetic dimension concept to study a large class of Hamiltonians that involves local interactions.Here we show that a Hamiltonian that is locally interacting along the synthetic dimension can be achieved in a dynamically modulated ring resonator incorporatingχ3nonlinearity,provided that the group velocity dispersion of the waveguide forming the ring is specifically designed.As a demonstration we numerically implement a Bose–Hubbard model and explore photon blockade effect in the synthetic frequency space.Our work opens new possibilities for studying fundamental many-body physics in the synthetic space in photonics,with potential applications in optical quantum communication and quantum computation.

Directional coherent light via intensity-induced sideband emission

We introduce a unique technique for generating directional coherent emissions that could be utilized to create coherent sources in a wide range of frequencies from the extreme ultraviolet(XUV)to the deep infrared.This is accomplished without population inversion by pumping a two-level system with a far-detuned strong optical field that induces the splitting of the two-level system.A nonlinear process of four-wave mixing then occurs across the split system,driving coherent emission at sidebands both redand blue-detuned from the pump frequency,and propagates both forward and backward along the pump beam path.We observed this phenomenon in dense rubidium vapor along both the D_(1) and D_(2) transitions.The sideband emission exhibits a short pulse duration(o1 ns)with threshold-like behavior dependent on both the pump intensity and Rb vapor density.This technique offers a new capability for manipulating the emission frequency simply through intensity-induced atomic modulation that can be scaled to most frequency regimes using various atomic/molecular ensembles and pump energies.