Minimum-leakage continuous-variable quantum key distribution suppresses Eve's Holevo information by engineering the signal ensemble at the state-preparation stage. Existing symmetric minimum-leakage protocols achieve this goal by heralding: Alice interferes two squeezed ensembles, measures one output mode, and sends the other to Bob. Here we propose a deterministic two-mode protocol that removes the Alice-side heralding step. Alice combines two oppositely squeezed Gaussian ensembles on a balanced beam splitter and transmits both output modes, which form phase-conjugated twin beams. We show that this protocol is related to the heralding protocol through a common entanglement-based source but corresponds to a different prepare-and-measure decomposition. In the very-large-squeezing limit, the two protocols give the same secret key rate per transmitted optical mode. For finite squeezing, however, the phase-conjugated twin-beam protocol requires approximately 3 dB less squeezing to achieve the same key rate. We further analyze correlated two-mode Gaussian attacks in which Eve injects ancillary modes with optimized inter-mode correlations. We find that the correlated attacks are slightly more efficient than independent attacks, but the advantage remains limited under the minimum-leakage condition. These results show that phase-conjugated twin beams provide a deterministic and experimentally appealing route to symmetric minimum-leakage CV-QKD.

The mid-infrared (MIR) spectral region is crucial for various applications due to its unique properties, but traditional spectrometers are often bulky. Miniaturized spectrometers face a trade-off between spectral and spatial resolution. Here, a novel approach to MIR spectroscopy is numerically demonstrated by employing an electrically controlled phase-change metasurface. This method fully exploits the high optical contrast and the quasi-continuous phase change characteristics of chalcogenide phase change materials, enabling the construction of a set of spectral responses that provide broad spectral coverage with low correlation, utilizing a single metasurface pixel. With this innovative strategy, a broadband and high-resolution spectral reconstruction is numerically demonstrated with a full width at half maximum (FWHM) resolution of 20 nm and a dual-peak resolution of 160 nm within a 2400 nm bandwidth. Furthermore, the potential of the spectral detection scheme is underscored by the successful numerical reconstruction of the absorption peaks of methane and carbon dioxide, highlighting its capability for gas analysis and molecular identification. The integration of the spectral detection method into the field of spectral imaging is anticipated to have significant implications, suggesting substantial improvements in chemical process monitoring, and rapid diagnostic techniques in combustion environments.

Quantum emitters driven by resonant two-photon excitation are a leading source for deterministically generated entangled photon pairs, essential for scalable photonic quantum technologies. However, conventional resonant schemes are highly sensitive to laser power fluctuations and pose additional experimental challenges for emitters with small biexciton binding energies. Here, we demonstrate how biexciton preparation schemes with significantly improved robustness and reduced laser filtering requirements can be identified using a novel design principle beyond resonant and adiabatic driving: ultrafast all-optical Floquet engineering. This is achieved by employing two strongly and symmetrically detuned dichromatic pulses, whose superposition generates a stroboscopic Hamiltonian that enables direct coupling between ground and biexciton states. Moreover, a pulse delay serves as a tuning knob, introducing an effective magnetic field that concentrates the Bloch sphere trajectory at the biexciton state for a wide range of parameters, making biexciton preparation particularly robust. Experimentally, we achieve a biexciton occupation exceeding 96% and preserve photon-pair entanglement with a fidelity of 93.4%. Our scheme highlights the great impact of Floquet-engineered multicolour excitation protocols for on-demand quantum light sources.

The performances and cost of the phase-sensitive optical time-domain reflectometry (Φ-OTDR) systems are heavily influenced by the lasers used. Traditionally, Φ-OTDR systems rely on highly coherent ultra-narrow linewidth lasers (NLL). This paper proposes a Φ-OTDR system that utilizes self-mixing interferometry to mitigate the impact of laser phase noise and a triple-frequency scheme to achieve fading-free detection over 40 km. The proposed scheme employs an inexpensive fixed wavelength distributed feedback semiconductor laser (DFB-SL) with a 93 kHz linewidth as the light source and successfully mitigates the noise floor by 8 to 22 dB within a range of 35 km compared to the performance of conventional systems. Leveraging the high-power output of the DFB-SL, the proposed scheme eliminates the need for an online erbium-doped optical fiber amplifier (EDFA) and achieves fading-free detection over 10 km. The results in this study offer a practical solution to address the bottleneck issue of laser phase noise in Φ-OTDR systems and contribute to the development of cost-effective systems and on-chip integration, eliminating the requirement for NLL and online amplifiers.

We demonstrated over field-deployed fibers that CVQKD using 10 MHz linewidth LO injected by a 10 kHz seed laser with simplified DSP and no active phase locking performs better than LLO using two 10 kHz lasers.

Polarization scrambling models (PSM) are useful in various areas of optics research. The conventional rotatable waveplate-based PSM is easy to implement physically yet exhibits limitations in model characterization. This paper develops two novel PSMs based on cascaded extrinsic and intrinsic elementary rotations of the state of polarization (SOP). The new models exhibit rich features in terms of deterministic polarization scrambling speed and tunable distribution of SOP variation rate. The development of new models also provides us with novel theoretical tools for characterizing various PSMs. We study the new models with detailed mathematical derivations and analyses, verified by extensive numerical simulations. The developed models could be useful for theoretical study and building simulation platforms for polarization testing.

Quantum emitters driven by resonant two-photon excitation are a leading source for deterministically generated entangled photon pairs, essential for scalable photonic quantum technologies. However, conventional excitation schemes pose various constraints, limiting their scalability and practicability. Here, we demonstrate how biexciton preparation schemes with significantly improved robustness and reduced experimental demands can be identified using a novel design principle: ultrafast single-shot Floquet driving. This is achieved by employing two strongly and symmetrically detuned dichromatic pulses, whose superposition generates a stroboscopic Hamiltonian that enables direct coupling between ground and biexciton states. Moreover, a pulse delay serves as a tuning knob, introducing an effective magnetic field making biexciton preparation particularly robust. Experimentally, we achieve a biexciton occupation exceeding 96

The type of system employing continuous variables, such as light field quadrature for signal modulation, is an important branch for quantum key distribution. Achieving a high key rate using quantum key distribution technology is beneficial for realizing more frequent key updates or enabling one-time pad encryption for high-speed communication tasks. However, the deficiencies of typical large-bandwidth balanced photodetectors prevent the continuous variable quantum key distribution from matching the speed of state-of-the-art classical optical communications. Previous attempts to improve the key rate have focused on designing and manufacturing high-speed balanced photodetectors. It is well known that phase-sensitive amplifiers also improve the performance of balanced photodetectors and thus the performance of key distribution, but the concept remains purely theoretical. Here, for the first time, we experimentally demonstrate a high-speed continuous-variable quantum key distribution operating with a 10 GHz balanced photodetector enhanced by phase-sensitive amplification and detection. A conjugated multi-mode continuous variable protocol is used to accommodate the phase-sensitive operations. The optical amplifier offers a 10 dB increase in the photodetector’s clearance (quantum noise over electrical noise) and improves the detection efficiency from 72% to 96%, thus enabling an overall 248.9 Mb/s key distribution rate through a 16.7 km field-deployed optical fiber.

Analytical solutions for phase retardation in polarization control systems based on cascaded waveplates are developed. Combined with phase interpolation, arbitrary polarization path planning on the Poincare sphere is demonstrated.

Space division multiplexing(SDM)can achieve higher communication transmission capacity by exploiting more spatial channels in a single optical fiber.For weakly coupled few-mode fiber,different mode groups(MGs)are highly isolated from each other,so the SDM system can be simplified by utilizing MG multiplexing and intensity modulation direct detection.A key issue to be addressed here is MG demultiplexing,which requires processing all the modes within a single MG in contrast to MG multiplexing.Benefiting from the great light manipulation freedom of the diffractive optical network(DON),we achieve efficient separation of the MGs and receive them with the multimode fiber(MMF)array.To fully exploit the mode field freedom of the MMF,a non-deterministic mode conversion strategy is proposed here to optimize the DON,which enables high-efficiency demultiplexing with a much smaller number of phase plates.As a validation,we design a 6-MG demultiplexer consisting of only five phase plates;each MG is constituted by several orbital angular momentum modes.The designed average loss and crosstalk at the wavelength of 1550 nm are 0.5 dB and-25 dB,respectively.In the experiment,the loss after coupling to the MMF ranged from 4.1 to 4.9 dB,with an average of 4.5 dB.The inter-MG crosstalk is better than-12 dB,with an average of-18 dB.These results well support the proposed scheme and will provide a practical solution to the MG demultiplexing problem in a short-distance SDM system.

The performance of local local oscillator-based continuous-variable quantum key distribution (LLO-CVQKD) systems is critically limited by phase recovery accuracy. To address the phase mismatch between independently operated lasers at the transmitter and receiver, this work proposes a multi-pilot scheme integrated with advanced digital filtering for optimized phase estimation. Through systematic comparisons of the rectangular filter, Savitzky-Golay filter, and Wiener filter, experimental results confirm that Wiener filtering has superior noise suppression capabilities based on multiple frequency pilot-tones. Implemented under the Gaussian-modulated coherent state (GMCS) protocol, our findings reveal that dual-pilot, four-pilot, and eight-pilot configurations all enhance phase recovery precision compared to single-pilot approaches, with the dual-pilot scheme achieving optimal balance between system complexity and performance. In a 20 km fiber link with a 1 GHz system repetition rate and a 2.83 SNU modulation variance, the dual-pilot Wiener-filtered configuration achieves an asymptotic secret key rate of almost 80 Mbps, establishing an innovative approach for practical LLO-CVQKD phase recovery.

We successfully recovered the low-frequency vibration signals from 1 to 500 Hz based on fiber interferometry constructed by seven-core fiber while during communication. We report field trial results over installed fiber cable to detect real-world vibrations.

We developed a novel data-aided timing error detector for one sample per symbol Nyquist optical communication systems and experimentally demonstrated its robustness to multiple channel impairments including CD and PMD. (C) 2024 The Author(s)

Chromatic dispersion estimation (CDE) and timing error detection (TED) are essential components of digital signal processing for coherent optical communication systems. Despite the extensive research conducted in coherent single-mode fiber transmission systems, traditional algorithms are less efficient or ineffective when operating in few-mode fiber transmission. We develop new cyclostationary-based methods for CDE and TED, suitable for mode-division multiplexing systems employing weakly coupled orbital angular momentum fiber. The novel methods feature strong resistance to polarization mode dispersion and mode coupling. Simulations and experimental evaluations confirm the superior performance of the proposed methods.

AbstractThe construction of a large-scale quantum internet requires quantum repeaters containing multiple entangled photon sources with identical wavelengths. Semiconductor quantum dots can generate entangled photon pairs deterministically with high fidelity. However, realizing wavelength-matched quantum-dot entangled photon sources faces two difficulties: the non-uniformity of emission wavelength and exciton fine-structure splitting induced fidelity reduction. Typically, these two factors are not independently tunable, making it challenging to achieve simultaneous improvement. In this work, we demonstrate wavelength-tunable entangled photon sources based on droplet-etched GaAs quantum dots through the combined use of AC and quantum-confined Stark effects. The emission wavelength can be tuned by ~1 meV while preserving an entanglement fidelity f exceeding 0.955(1) in the entire tuning range. Based on this hybrid tuning scheme, we finally demonstrate multiple wavelength-matched entangled photon sources with f > 0.919(3), paving the way towards robust and scalable on-demand entangled photon sources for quantum internet and integrated quantum optical circuits.

The optical multipath interference (MPI) effect is detrimental to the widely deployed intensity modulation direct detection (IMDD) systems. Here, we develop a balanced coding scheme-minimum running digital sum (MRDS), applied at the transmitter, shaping the signal's spectrum to suppress the low-frequency carrier-carrier beat noise. Thus, it helps mitigate MPI with a much smaller power penalty. Compared to other reported methods, it requires only simple operations with low computational complexity at the expense of small overhead. Simulation and experimental results confirm the coding scheme can effectively improve the bit error rate (BER) performance of a high baudrate four-level pulse amplitude modulation (PAM4) IMDD system under various signal-to-interference ratio (SIR), received optical power (ROP), and laser linewidth. Assisted with the cascade of feed-forward equalization (FFE), post filtering (PF), and maximum likelihood sequence estimation (MLSE), the MRDS coding performs the best among all the compared MPI mitigation methods in an experimental 30 Gbaud PAM4-IMDD system.

We propose a novel chip-scale integrated self-interference micro-disk resonator-based spectrometer that realizes a high resolution of 0.02 nm across a bandwidth of 1500–1600 nm using 401 sampling channels by regularization optimization.

Inspired by the reverse thrust generated by fuel injection, micromachines that are self-propelled by bubble ejection are developed, such as microrods, microtubes, and microspheres. However, controlling bubble ejection sites to build micromachines with programmable actuation and further enabling mechanical transmission remain challenging. Here, bubble-propelled mechanical microsystems are constructed by proposing a multimaterial femtosecond laser processing method, consisting of direct laser writing and selective laser metal reduction. The polymer frame of the microsystems is first printed, followed by the deposition of catalytic platinum into the desired local site of the microsystems by laser reduction. With this method, a variety of designable microrotors with selective bubble ejection sites are realized, which enable excellent mechanical transmission systems composed of single and multiple mechanical components, including a coupler, a crank slider, and a crank rocker system. We believe the presented bubble-propelled mechanical microsystems could be extended to applications in microrobotics, microfluidics, and microsensors.

We propose a novel joint SNR and mode coupling estimation method for weakly coupled mode-division multiplexing systems. This method offers potential vibration detection in integrated sensing and communication scenarios. It is based on an extended cyclic matrix and is robust against random polarization effects.

Microbot collectives can cooperate to accomplish complex tasks that are difficult for a single individual. However, various force-induced microbot collectives maintained by weak magnetic, light, and electric fields still face challenges such as unstable connections, the need for a continuous external stimuli source, and imprecise individual control. Here, we construct magnetic and light-driven ant microbot collectives capable of reconfiguring multiple assembled architectures with robustness. This methodology utilizes a flexible two-photon polymerization strategy to fabricate microbots consisting of magnetic photoresist, hydrogel, and metal nanoparticles. Under the cooperation of magnetic and light fields, the microbots can reversibly and selectively assemble (e.g., 90° assembly and 180° assembly) into various morphologies. Moreover, we demonstrate the ability of assembled microbots to cross a one-body-length gap and their adaptive capability to move through a constriction and transport microcargo. Our strategy will broaden the abilities of clustered microbots, including gap traversal, micro-object manipulation, and drug delivery.