Transmission of photonic polarization states through 55-m water: towards air-to-sea quantum communication

Quantum communication has been rapidly developed due to its unconditional security and successfully implemented through optical fibers and free-space air in experiments. To build a complete quantum communication network involving satellites in space and submersibles in ocean, the underwater quantum channel has been investigated in both theory and experiment. However, the question of whether the polarization encoded qubit can survive through a long-distance and high-loss underwater channel, which is considered as the restricted area for satellite-borne radio waves, still remains. Here, we experimentally demonstrate the transmission of blue-green photonic polarization states through 55-m-long water. We prepare six universal quantum states at the single photon level and observe their faithful transmission in a large marine test platform. We obtain complete information of the channel by quantum process tomography. The distance demonstrated in this work reaches a region allowing potential real applications, representing a step further towards air-to-sea quantum communication.

Erratum to‘Topologically protected polarization quantum entanglement on a photonic chip’[Chip 1,100003(2022)]

Given his role as Executive Editor of this journal,Xian-Min Jin had no involvement in the peer-review of the article titled‘Topologically pro-tected polarization quantum entanglement on a photonic chip’(Chip 1,100003),and had no access to information regarding its peer-review.Full responsibility for the editorial process for this article was delegated to Xibo Feng(fred.feng@chipress.org)at Chip Editorial Office.

Topologically protecting squeezed light on a photonic chip

Squeezed light is a critical resource in quantum sensing and information processing. Due to the inherently weak optical nonlinearity and limited interaction volume, considerable pump power is typically needed to obtain efficient interactions to generate squeezed light in bulk crystals. Integrated photonics offers an elegant way to increase the nonlinearity by confining light strictly inside the waveguide. For the construction of large-scale quantum systems performing many-photon operations, it is essential to integrate various functional modules on a chip. However, fabrication imperfections and transmission cross talk may add unwanted diffraction and coupling to other photonic elements, reducing the quality of squeezing. Here, by introducing the topological phase, we experimentally demonstrate the topologically protected nonlinear process of four-wave mixing, enabling the generation of squeezed light on a silica chip. We measure the cross-correlations at different evolution distances for various topological sites and verify the nonclassical features with high fidelity. The squeezing parameters are measured to certify the protection of cavity-free, strongly squeezed states. The demonstration of topological protection for squeezed light on a chip brings new opportunities for quantum integrated photonics,opening novel approaches for the design of advanced multi-photon circuits.

Topologically Protected Polarization Quantum Entanglement on a Photonic Chip

Quantum entanglement,as the strictly non-classical phenomenon,is the kernel of quantum computing and quantum simulation,and has been widely applied ranging from fundamental tests of quantum physics to quantum information processing.Meanwhile,the topolog-ical phase is found inherently capable of protecting physical fields from unavoidable fabrication-induced disorder,which inspires the po-tential application of topological protection to quantum states.Here,we present the experimental demonstration of topologically protected quantum entangled states on a photonic chip.The process tomogra-phy shows that quantum entanglement can be well preserved by the topological states even when the chip material introduces disorder and relative polarization rotation in phase space.Our work links the fields of materials,topological science and quantum physics,opening the door to wide applications of topological enhancement in quantum regime.

Experimental quantum simulation of dynamic localization on curved photonic lattices

Dynamic localization,which originates from the phenomena of particle evolution suppression under an externally applied AC electric field,has been simulated by suppressed light evolution in periodically curved photonic arrays.However,experimental studies on their quantitative dynamic transport properties and application for quantum information processing are rare.Here we fabricate one-dimensional and hexagonal two-dimensional arrays both with sinusoidal curvatures.We successfully observe the suppressed single-photon evolution patterns,and for the first time,to the best of our knowledge,measure the variances to study their transport properties.For onedimensional arrays,the measured variances match both the analytical electric-field calculation and the quantum walk Hamiltonian engineering approach.For hexagonal arrays as anisotropic effective couplings in four directions are mutually dependent,the analytical approach suffers,whereas quantum walk conveniently incorporates all anisotropic coupling coefficients in the Hamiltonian and solves its exponential as a whole,yielding consistent variances with our experimental results.Furthermore,we implement a nearly complete localization to show that it can preserve both the initial injection and the wave packet after some evolution,acting as a memory of a flexible time scale in integrated photonics.We demonstrate a useful quantum simulation of dynamic localization for studying their anisotropic transport properties and a promising application of dynamic localization as a building block for quantum information processing in integrated photonics.

Experimental Test of Tracking the King Problem

In quantum theory, the retrodiction problem is not as clear as its classical counterpart because of the uncertainty principle ofquantum mechanics. In classical physics, the measurement outcomes of the present state can be used directly for predicting thefuture events and inferring the past events which is known as retrodiction. However, as a probabilistic theory, quantummechanical retrodiction is a nontrivial problem that has been investigated for a long time, of which the Mean King Problem isone of the most extensively studied issues. Here, we present the first experimental test of a variant of the Mean King Problem,which has a more stringent regulation and is termed “Tracking the King.” We demonstrate that Alice, by harnessing the sharedentanglement and controlled-not gate, can successfully retrodict the choice of King’s measurement without knowing anymeasurement outcome. Our results also provide a counterintuitive quantum communication to deliver information hidden inthe choice of measurement.

Quantum advantage with membosonsampling

Quantum computer,harnessing quantum superposition to boost a parallel computational power,promises to outperform its classical counterparts and offer an exponentially increased scaling.The term“quantum advantage”was proposed to mark the key point when peo-ple can solve a classically intractable problem by artificially control-ling a quantum system in an unprecedented scale,even without er-ror correction or known practical applications.Boson sampling,a problem about quantum evolutions of multi-photons on multimode photonic networks,as well as its variants,has been considered as a promising candidate to reach this milestone.However,the current photonic platforms suffer from the scaling problems,both in pho-ton numbers and circuit modes.Here,we propose a new variant of the problem,membosonsampling,exploiting the scaling of the prob-lem can be in principle extended to a large scale.We experimentally verify the scheme on a self-looped photonic chip inspired by mem-ristor,and obtain multi-photon registrations up to 56-fold in 750,000 modes with a Hilbert space up to 10254.The results exhibit an inte-grated and cost-efficient shortcut stepping into the“quantum advan-tage”regimeina photonic systemfarbeyondpreviousscenarios,and provide a scalable and controllable platform for quantum information processing.