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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Experimental realization of a resource-saving polarization-independent orbital-angular-momentum-preserving tunable beam splitter.

The tunable beam splitter (TBS) is a fundamental component used in optical experiments. A TBS can preserve the orbital angular momentum (OAM) states; in addition, the polarization states of photons are valuable for some particular experiments, such as high-dimensional quantum information processing. We use polarization beam splitters and half-wave plates to realize such a TBS under a compact structure, which can reduce the number of elements that require comparing with existing works. The experiments verify that the TBS has good performances in tunability, polarization, and OAM state preservation. A Sagnac interferometer is implemented with the proposed TBS to evaluate its practical usability, and the mean visibilities greater than 99.30% under varying polarization states demonstrate its potential for optical information processing.

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.

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.

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

Quantum digital signature (QDS) can guarantee message integrity and non-repudiation with information-theoretical security, and it has attracted more attention recently. Since proposed by Andersson et al. [Phys. Rev. A93, 032325 (2016)PLRAAN1050-294710.1103/PhysRevA.93.032325], a quantum digital signature protocol using an insecure channel has been realized with several different quantum key distribution (QKD) systems. Here we report an experimental QDS based on a BB84 QKD system. An asymmetric Faraday-Sagnac-Michelson interferometer structure has been designed in our system, which is intrinsically stable against channel disturbance. The innovatory structure supports the system to work at high speed and, in practice, the repetition rate is in gigahertz. A 0.044 bit/s signature rate has been attained with a 25 dB channel loss composed of a 25 km installed fiber with additional optical attenuation in a 10-10 security level. Thus, our QDS device is stable and highly efficient. This Letter provides a further step for the practical application of QDS.

Practical gigahertz quantum key distribution robust against channel disturbance.

Quantum key distribution (QKD) provides an attractive solution for secure communication. However, channel disturbance severely limits its application when a QKD system is transferred from the laboratory to the field. Here a high-speed Faraday-Sagnac-Michelson QKD system is proposed that can automatically compensate for the channel polarization disturbance, which largely avoids the intermittency limitations of environment mutation. Over a 50 km fiber channel with 30 Hz polarization scrambling, the practicality of this phase-coding QKD system was characterized with an interference fringe visibility of 99.35% over 24 h and a stable secure key rate of 306 k bits/s over seven days without active polarization alignment.

Achieving Heisenberg-Scaling Precision with Projective Measurement on Single Photons.

It has been suggested that both quantum superpositions and nonlinear interactions are important resources for quantum metrology. However, to date the different roles that these two resources play in the precision enhancement are not well understood. Here, we experimentally demonstrate a Heisenberg-scaling metrology to measure the parameter governing the nonlinear coupling between two different optical modes. The intense mode with n (more than 10^{6} in our work) photons manifests its effect through the nonlinear interaction strength which is proportional to its average photon number. The superposition state of the weak mode, which contains only a single photon, is responsible for both the linear Hamiltonian and the scaling of the measurement precision. By properly preparing the initial state of single photon and making projective photon-counting measurements, the extracted classical Fisher information (FI) can saturate the quantum FI embedded in the combined state after coupling, which is ∼n^{2} and leads to a practical precision ≃1.2/n. Free from the utilization of entanglement, our work paves a way to realize Heisenberg-scaling precision when only a linear Hamiltonian is involved.

Polarization variations in installed fibers and their influence on quantum key distribution systems.

Polarization variations in the installed fibers are complex and volatile, and would severely affect the performances of polarization-sensitive quantum key distribution (QKD) systems. Based on the recorded data about polarization variations of different installed fibers, we establish an analytical methodology to quantitatively evaluate the influence of polarization variations on polarization-sensitive QKD systems. Using the increased quantum bit error rate induced by polarization variations as a key criteria, we propose two parameters - polarization drift time and required tracking speed - to characterize polarization variations. For field buried and aerial fibers with different length, we quantitatively evaluate the influence of polarization variations, and also provide requirements and suggestions for polarization basis alignment modules of QKD systems deployed in different kind of fibers.

Field and long-term demonstration of a wide area quantum key distribution network.

A wide area quantum key distribution (QKD) network deployed on communication infrastructures provided by China Mobile Ltd. is demonstrated. Three cities and two metropolitan area QKD networks were linked up to form the Hefei-Chaohu-Wuhu wide area QKD network with over 150 kilometers coverage area, in which Hefei metropolitan area QKD network was a typical full-mesh core network to offer all-to-all interconnections, and Wuhu metropolitan area QKD network was a representative quantum access network with point-to-multipoint configuration. The whole wide area QKD network ran for more than 5000 hours, from 21 December 2011 to 19 July 2012, and part of the network stopped until last December. To adapt to the complex and volatile field environment, the Faraday-Michelson QKD system with several stability measures was adopted when we designed QKD devices. Through standardized design of QKD devices, resolution of symmetry problem of QKD devices, and seamless switching in dynamic QKD network, we realized the effective integration between point-to-point QKD techniques and networking schemes.