Stable and wavelength-tunable multiwavelength laser for high-rate measurement device independent quantum key distribution.

Measurement device independent quantum key distribution (MDI QKD) has attracted growing attention for its immunity to attacks at the measurement unit, but its unique structure limits the secret key rate. Utilizing the wavelength division multiplexing (WDM) technique and reducing error rates are effective strategies for enhancing the secret key rate. Reducing error rates often requires active feedback control of wavelengths using precise external references. However, for a multiwavelength laser, employing multiple references to stabilize each wavelength output places stringent demands on these references and significantly increases system complexity. Here, we demonstrate a stable, wavelength-tunable multiwavelength laser with an output wavelength ranging from 1270 to 1610 nm. Through precise temperature control and stable drive current, we passively lock the laser wavelength, achieving remarkable wavelength stability. This significantly reduce the error rate, leading to an almost doubling of the secret key rate compared to previous experiments. Furthermore, the exceptional wavelength stability offered by our multiwavelength laser, combined with the WDM technique, has further boosted the secret key rate of MDI QKD. With a wide wavelength tuning range of 5.1 nm, our multiwavelength laser facilitates flexible operation across multiple dense wavelength division multiplexing channels. Coupled with high wavelength stability and multiple wavelength outputs simultaneously, this laser offers a promising solution for a high-rate MDI QKD system.

Determination of the total antioxidant capacity of the Chinese tea based on a novel "peroxidase/zirconium phosphonate"composite electrochemical sensor.

In this work, a novel zirconium phosphonate (ZrPR1R2) was prepared by decorating both the aminoethoxy- group (R1) and the carboxypropyl- group (R2) on the zirconium phosphate layers in order to manipulate further the immobilization of the peroxidase (POD), and an antioxidant biosensor with higher sensitivity was constructed by dropping the POD/ZrPR1R2 composite onto the glassy carbon electrode surface. The activity of the POD/ZrPR1R2 composite was detected by Uv-vis spectra. The direct electrochemical behavior, the electrocatalytic response to dissolved oxygen and hydrogen peroxide, as well as the ability to detect total antioxidant capacity in tea sample were investigated by the methods of cyclic voltammetry. The results indicated that the immobilization of POD in ZrPR1R2 nanosheets matrix enhanced the enzymatic activity, and achieved the fast and direct electron transfer between POD and glassy carbon electrode. Moreover, the POD/ZrPR1R2 composite modified electrode show the electrocatalytic response to hydrogen peroxide in the linear range of 8.8×10-8 to 8.8×10-7 mol L-1, with the detection limit of 3.3×10-8 mol L-1. Attributing to the sensitive response to dissolved oxygen, the total antioxidant capacity can be detected directly in the real tea water by this POD/ZrPR1R2 composite modified electrode.

Performance Investigation of Asphaltene Adsorption at a Carbonate Rock Surface: Spectroscopy Experiment and Molecular Simulation.

Investigation of asphaltene adsorption at rock surfaces plays an important role in enhanced oil recovery (EOR) for the petroleum industry. In this work, the interaction performances of asphaltene adsorption at carbonate dolomite and calcite surfaces are investigated based on experimental and simulation insights. On the one hand, macroscopic interaction performances were investigated by spectroscopy experiments to obtain the Langmuir thermodynamic model and pseudo-second-order (PSO) kinetic model. The results indicated monolayer molecular asphaltene adsorption for both dolomite and calcite, while they showed 'slow adsorption-slow desorption' for dolomite but 'fast adsorption-fast desorption' for calcite. Meanwhile, dolomite showed a higher adsorption capacity with qm(dol 1) = 5.35 mg/g > qm(cal 1) = 1.28 mg/g and a stronger adsorption spontaneity with ΔGm(dol 1)θ = -7.76 kJ/mol < ΔGm(cal 1)θ = -4.76 kJ/mol. On the other hand, microscopic interaction performances were investigated for three asphaltene molecules by molecular dynamics simulation (MDS) with ∼8 Å distance-placing and 500 ps time-running. According to the results, dolomite showed higher system stability than calcite with a lower final energy of ΔEdol-cal = -58 kJ/mol, and archipelago asphaltene showed higher adsorption stability with the smallest equilibrium energy of Earch(dol) = -147 kJ/mol for albite and Earch(cal) = -89 kJ/mol for calcite. The model of molecular orientation and force dominance was proposed as the interaction mechanism for asphaltene adsorption, which "lie sideways" at low concentrations but "stands upright" at high concentrations. This work allows the performance investigation and mechanism illustration of asphaltene adsorption at rock surfaces, which can help gain a fundamental understanding of the EOR during reservoir exploitation.

Insights into Zwitterionic Surfactant Interactions at the Oil-Water Interface by Interferometry Experiments and MDS Calculations.

In this work, the interaction performance of zwitterionic surfactant [dodecyl dimethyl sulfopropyl betaine (DSB-12) and hexadecyl dimethyl sulfopropyl betaine (DSB-16)] at the n-octadecane oil surface is investigated from experimental and simulation insights. For a macroscopic experiment, interfacial interferometry technology was developed for real-time monitor interaction performances and to obtain the quantitative interfacial thickness and mass results. The Langmuir model was characterized by thermodynamic analysis, deducing the aggregation spontaneity of DSB-16 > DSB-12 with ΔGagg(DSB-16) = -5.94 kJ mol-1 < ΔGagg(DSB-12) = 24.08 kJ mol-1. A three-step dynamic model (adsorption, arrangement, and aggregation) was characterized by kinetic analysis, indicating arrangement process as slow-limiting step with k2(arr) < k1(ads), k3(agg). For microscopic simulation, and molecular dynamic (MD) method was utilized to theoretically investigate interaction performances and obtain the interfacial configuration and energy results. The interaction stability and interaction strength were indicated to be DSB-16 > DSB-12 with differences of final energy ΔEfin = 48-88 kcal mol-1. The interaction mechanism was explained by proposing the model of "response enhancement" and "deposition activity" for DSB-16 interactions, and "response decrease" and "elution activity" for DSB-12 interactions. The different performances can be attributed to the different interaction forms and forces of surfactants. This work provided a platform for performance and mechanism investigation between the surfactant molecule and oil surface, which is of great significance in reservoir exploitation and enhanced oil recovery (EOR).

Experimental Demonstration of Fully Passive Quantum Key Distribution.

The passive approach to quantum key distribution (QKD) consists of removing all active modulation from the users' devices, a highly desirable countermeasure to get rid of modulator side channels. Nevertheless, active modulation has not been completely removed in QKD systems so far, due to both theoretical and practical limitations. In this Letter, we present a fully passive time-bin encoding QKD system and report on the successful implementation of a modulator-free QKD link. According to the latest theoretical analysis, our prototype is capable of delivering competitive secret key rates in the finite key regime.

Transmittance-invariant phase modulator for chip-based quantum key distribution.

In chip-based quantum key distribution (QKD) systems, the non-ideal quantum state preparation due to the imperfect electro-optic phase modulators (EOPM) decreases the secret key rate and introduces potential vulnerabilities. We propose and implement an on-chip transmittance-invariant phase modulator (TIPM) to solve this problem. Simulated and experimental results show that TIPM can eliminate the correlation between phase, intensity, and polarization of quantum states caused by phase-dependent loss. The design can tolerate a significant fabrication mismatch and is universal to multi-material platforms. Furthermore, TIPM increases the modulation depth achievable by EOPMs in standard process design kit (PDK). The proposal of TIPM can improve the practical security and performance of the chip-based QKD systems.

Afterpulse effect in measurement-device-independent quantum key distribution.

There is no doubt that measurement-device-independent quantum key distribution (MDI-QKD) is a crucial protocol that is immune to all possible detector side channel attacks. In the preparation phase, a simulation model is usually employed to get a set of optimized parameters, which is utilized for getting a higher secure key rate in reality. With the implementation of high-speed QKD, the afterpulse effect which is an intrinsic characteristic of the single-photon avalanche photodiode is no longer ignorable, this will lead to a great deviation compared with the existing analytical model. Here we develop an afterpulse-compatible MDI-QKD model to get the optimized parameters. Our results indicate that by using our afterpulse-compatible model, we can get a much higher key rate than the prior afterpulse-omitted model. It is significant to take the afterpulse effect into consideration because of the improvement of the system working frequency.

Imperfection-insensitivity quantum random number generator with untrusted daily illumination.

Quantum random number generators (QRNGs) promise secure randomness generation based on the foundational unpredictability of quantum mechanics. However, the unavoidable gaps between theoretical models and practical devices could lead to security invalidation. Recently, a source-independent quantum random number generator (SI-QRNG) has been proposed to solve the issue of uncharacteristic sources. However, in most current analyses of SI-QRNG protocols, the security proofs with imperfect measurements are individual for different factors and very sensitive to small deviations from theoretical models. Here, we establish a unified model for imperfect measurements in the SI-QRNG and provide a tight rate bound based on the uncertainty relation for smooth entropies. Then the performance with large device imperfections is evaluated and the randomness rate in our model can approach a similar order of magnitude of the rate upper bound in common discrete variable QRNGs. In addition, by utilizing the daily illumination and measurement devices with large imperfections, we experimentally demonstrate our scheme at the rate of the order of magnitude of Mbps.

Integration in the C-band between quantum key distribution and the classical channel of 25 dBm launch power over multicore fiber media.

The quantum-classical coexistence can be implemented based on wavelength division multiplexing (WDM), but due to Raman noise, the wavelength spacing between quantum and classical signals and launch power from classical channels are restricted. Space division multiplexing (SDM) can now be availably achieved by multicore fiber (MCF) to reduce Raman noise, thereby loosening the restriction for coexistence in the same band and obtaining a high communication capacity. In this paper, we realize the quantum-classical coexistence over a 7-core MCF. Based on the SDM, the highest launch power of 25 dBm is achieved which has been extended nearly 19 times in previous work. Moreover, both the quantum and classical channels are allocated in the C-band and the minimum wavelength spacing between them is only 1.6 nm. The coexistence system eliminates the need for adding a narrowband filter.

Transverse Mode-Encoded Quantum Gate on a Silicon Photonic Chip.

As an important degree of freedom (d.o.f.) in photonic integrated circuits, the orthogonal transverse mode provides a promising and flexible way to increase communication capability, for both classical and quantum information processing. To construct large-scale on-chip multimode multi-d.o.f.s quantum systems, a transverse mode-encoded controlled-NOT (CNOT) gate is necessary. Here, with the help of our new transverse mode-dependent directional coupler and attenuator, we demonstrate the first multimode implementation of a 2-qubit quantum gate. The ability of the gate is demonstrated by entangling two separated transverse mode qubits with an average fidelity of 0.89±0.02 and the achievement of 10 standard deviations of violations in the quantum nonlocality verification. In addition, a fidelity of 0.82±0.01 is obtained from quantum process tomography used to completely characterize the CNOT gate. Our work paves the way for universal transverse mode-encoded quantum operations and large-scale multimode multi-d.o.f.s quantum systems.

Quantum key distribution integrating with ultra-high-power classical optical communications based on ultra-low-loss fiber.

The demand for the integration of quantum key distribution (QKD) and classical optical communication in the same optical fiber medium greatly increases as fiber resources and the flexibility of practical applications are taken into consideration. To satisfy the needs of the mass deployment of ultra-high power required for classical optical networks integrating QKD, we implement the discrete variable quantum key distribution (DV-QKD) under up to 25 dBm launch power from classical channels over 75 km on an ultra-low-loss (ULL) fiber by combining a finite-key security analysis method with the noise model of classical signals. To the best of our knowledge, this is the highest power launched by classical signals on the coexistence of DV-QKD and classical communication. The results exhibit the feasibility and tolerance of our QKD system for use in ultra-high-power classical communications.

Compact quantum random number generation using a linear optocoupler.

To date, various quantum random number schemes have been demonstrated. However, the cost, size, and final random bit generation rate usually limits their wide application on-shelf. To overcome these limitations, we propose and demonstrate a compact, simple, and low-cost quantum random number generation based on a linear optocoupler. Its integrated structure consists mainly of a light emitting diode and a photodetector. Random bits are generated by directly measuring the intensity noise of the output light, which originates from the random recombination between holes of the p region and electrons of the n region in a light emitting diode. Moreover, our system is robust against fluctuation of the operating environment, and can be extended to a parallel structure, which will be of great significance for the practical and commercial application of quantum random number generation. After post-processing by the SHA-256 algorithm, a random number generation rate of 43 Mbps is obtained. Finally, the final random bit sequences have low autocorrelation coefficients with a standard deviation of 3.16×10-4 and pass the NIST-Statistical Test Suite test.

Coexistence of quantum key distribution and optical transport network based on standard single-mode fiber at high launch power.

There is an increasing demand for multiplexing of quantum key distribution with optical communications in single fiber in consideration of high costs and practical applications in the metropolitan optical network. Here, we realize the integration of quantum key distribution and an optical transport network of 80 Gbps classical data at 15 dBm launch power over 50 km of the widely used standard (G.652 Recommendation of the International Telecom Union Telecom Standardization Sector) telecom fiber. A secure key rate of 11 Kbps over 20 km is obtained. By tolerating a high classical optical power up to 18 dBm of 160 Gbps classical data on single-mode fiber, our result shows the potential and tolerance of quantum key distribution being used in future large capacity transmission systems, such as metropolitan area networks and data centers. The quantum key distribution system is stable, practical, and insensitive to the polarization disturbance of channels by using a phase coding system based on a Faraday-Michelson interferometer. We also discuss the fundamental limit for quantum key distribution performance in the multiplexing environment.

Supercompact Photonic Quantum Logic Gate on a Silicon Chip.

To build universal quantum computers, an essential step is to realize the so-called controlled-NOT (CNOT) gate. Quantum photonic integrated circuits are well recognized as an attractive technology offering great promise for achieving large-scale quantum information processing, due to the potential for high fidelity, high efficiency, and compact footprints. Here, we demonstrate a supercompact integrated quantum CNOT gate on silicon by using the concept of symmetry breaking of a six-channel waveguide superlattice. The present path-encoded quantum CNOT gate is implemented with a footprint of 4.8×4.45 µm^{2} (∼3λ×3λ) as well as a high-process fidelity of ∼0.925 and a low excess loss of <0.2 dB. The footprint is shrunk significantly by ∼10 000 times compared to those previous results based on dielectric waveguides. This offers the possibility of realizing practical large-scale quantum information processes and paving the way to the applications across fundamental science and quantum technologies.

Quantum random number generation based on spontaneous Raman scattering in standard single-mode fiber.

We investigate quantum random number generation based on backward spontaneous Raman scattering in standard single-mode fiber, where the randomness of photon wavelength superposition and arrival time is simultaneously utilized. The experiment uses four avalanche photodiodes working in gated Geiger mode to detect backward Raman scattering photons from four different wavelength channels and a time-to-digital converter placed behind the detectors to record their arrival time. Both information of the wavelength and arrival time interval of photons from different channels are applied to generate random bits. Due to the independence of these two entropy sources, the random number resource of the present system is fully utilized. Five-bit raw data can be obtained for every effective click, which contains 2.87-bit min-entropy. To obtain the optimal generation rate of random bits, appropriate pump power and fiber length are adopted. The post-processing method by the SHA-256 hashing algorithm is used to remove the bias of the raw data, after which the final random bit sequences pass the NIST statistical test.

Extracting information from single qubits among multiple observers with optimal weak measurements.

In the context of quantum information, major efforts have been made to maximize the mutual information by measuring single copies of signal states. In general, one execution of optimal projective measurement extracts all the accessible mutual information. However, in some scenarios, weak measurements are preferred because of kinds of specific requirements, e.g., to distribute secret keys to multi-observers. In this study, we propose a method to construct optimal weak measurements for multi-party quantum communications. Utilizing the method in [Physical Review Letters 120, 160501 (2018)] to classify the mutual information, the theoretical study shows that by successively performing this optimal weak measurement, all accessible information can be obtained by multiple observers. This conclusion is experimentally verified by a cascaded measurement apparatus that can perform six successive weak measurements on heralded single photons. The experimental results clearly indicate that almost all accessible mutual information is extracted by this sequence of optimal weak measurements; meanwhile, none of the information is destroyed or residual. Thus, this optimal weak measurement is an efficient and reliable tool for performing quantum communication tasks. The consistence between the experimental and theoretical results verifies that the classifying method in [Phys. Rev. Lett.120, 160501 (2018)] can be applied to characterize realistic quantum measurements.

Rational Design and Intramolecular Cyclization of Hotspot Peptide Segments at YAP-TEAD4 Complex Interface.

BACKGROUND: The Yes-Associated Protein (YAP) is a central regulator of Hippo pathway involved in carcinogenesis, which functions through interaction with TEA Domain (TEAD) transcription factors. Pharmacological disruption of YAP-TEAD4 complexes has been recognized as a potential therapeutic strategy against diverse cancers by suppressing the oncogenic activity of YAP. OBJECTIVE: We systematically examine the crystal structure of YAP complex with TEAD4 and rationally identify two hotspot segments at the complex interface; they could be exploited as self-inhibitory peptides to target the complex interaction. METHODS: Two peptides, termed PS-1 and PS-2 are split from the interfacial context of YAP protein. Dynamics simulations, energetics analyses and fluorescence polarizations are employed to characterize the intrinsic disorder as well as binding energy/affinity of the two YAP peptides to TEAD4 protein. RESULT: The native conformation of PS-2 peptide is a cyclic loop, which is supposed to be constrained by adding a disulfide bond across the spatially vicinal residue pair Arg87-Phe96 or Met86- Phe95 at the peptide's two ends, consequently resulting in two intramolecular cyclized counterparts of linear PS-2 peptide, namely PS-2(cyc87,96) and PS-2(cyc86,95). The linear PS-2 peptide is determined as a weak binder of TEAD4 (Kd = 190 µM), while the two cyclic PS-2(cyc87,96) and PS-2(cyc86,95) peptides are measured to have moderate or high affinity towards TEAD4 (Kd = 21 and 45 µM, respectively). CONCLUSION: PS-1 and PS-2 peptides are highly flexible and cannot maintain in native active conformation when splitting from the interfacial context, and thus would incur a considerable entropy penalty upon rebinding to the interface. Cyclization does not influence the direct interaction between PS-2 peptide and TEAD4 protein, but can largely reduce the intrinsic disorder of PS-2 peptide in free state and considerably minimize indirect entropy effect upon the peptide binding.

Experimental realization of a reference-frame-independent decoy BB84 quantum key distribution based on Sagnac interferometer.

Quantum key distribution (QKD) can generate secure key bits between remote users employing the features of quantum physics. However, a shared reference frame is necessary for QKD systems in most scenarios. A reference-frame-independent (RFI) scheme can tolerate the reference frame drifting between legitimate remote users, which is significant in the operation of relative moving terminals such as satellites and aircraft. We design and experimentally demonstrate an RFI-BB84-QKD system by joint encoding with the polarization and orbital angular momentum states of the photons. We use self-compensating fiber Sagnac interferometers to perform high-speed polarization modulation, and q-plates to passively manipulate the rotation-invariant photon states, which makes the system feasible for high-speed operation using off-the-shelf components.

Temporal ghost imaging for quantum device evaluation.

Ghost imaging (GI) can reconstruct the image of an object by measuring the correlation function of two beams, none of which carries the structure information of the object independently. This powerful technology makes it possible to obtain high-quality imaging of the object even in the presence of noise. Here, we introduce the GI method into quantum device evaluation in the time domain. We realized a proof-of-principle experiment to evaluate the temporal detection efficiency of a gated-mode single-photon avalanche detector (SPAD). The experimental results show that high-quality evaluation of temporal characteristics of the SPAD can be realized by the method of temporal GI (TGI). Our work indicates that the TGI method is an effective tool to monitor the temporal characteristics of quantum devices in real time and will bring a new perspective to the security evaluation of quantum communication.

Practical quantum digital signature with a gigahertz BB84 quantum key distribution system: erratum.

In this erratum the formulas (6) and (8) of Opt. Lett.44, 139 (2019) OPLEDP0146-959210.1364/OL.44.000139 have been updated.