Improved decoy-state quantum key distribution with uncharacterized heralded single-photon sources

Encoding system plays a significant role in quantum key distribution(QKD).However,the security and performance of QKD systems can be compromised by encoding misalignment due to the inevitable defects in realistic devices.To alleviate the influence of misalignments,a method exploiting statistics from mismatched basis is proposed to enable uncharacterized sources to generate secure keys in QKD.In this work,we propose a scheme on four-intensity decoy-state quantum key distribution with uncharacterized heralded single-photon sources.It only requires the source states are prepared in a two-dimensional Hilbert space,and can thus reduce the complexity of practical realizations.Moreover,we carry out corresponding numerical simulations and demonstrate that our present four-intensity decoy-state scheme can achieve a much higher key rate compared than a three-intensity decoy-state method,and meantime it can obtain a longer transmission distance compared than the one using weak coherent sources.

Dynamic modulated single-photon routing

The dynamic control of single-photon scattering in a pair of one-dimensional waveguides mediated by a time-modulated atom-cavity system is investigated.Two cases,where the waveguides are coupled symmetrically or asymmetrically to the atom-cavity system,are discussed in detail.The results show that such time-modulated atom-cavity configuration can behave as a dynamical tunable directional single-photon router.The photons with different frequencies can dynamically be routed from the incident waveguide into any ports of the other with a 100%probability via adjusting the modulated amplitude or phases of the time-modulated atom-cavity coupling strengths,associate with the help of the asymmetrical waveguide-cavity couplings.Furthermore,the influence of dissipation on the routing capability is investigated.It is shown that the present single-photon router is robust against the dissipative process of the system,especially the atomic dissipation.These results are expected to be applicable in quantum information processing and design quantum devices with dynamical modulation.

Model and data of optically controlled tunable capacitor in silicon single-photon avalanche diode

This paper reports the photocapacitance effect of silicon-based single-photon avalanche diodes(SPADs),and the frequency scattering phenomenon of capacitance.The test results of the small-signal capacitance-voltage method show that light can cause the capacitance of a SPAD device to increase under low-frequency conditions,and the photocapacitance exhibits frequency-dependent characteristics.Since the devices are fabricated based on the standard bipolar-CMOS-DMOS process,this study attributes the above results to the interfacial traps formed by Si-SiO_(2),and the illumination can effectively reduce the interfacial trap lifetime,leading to changes in the junction capacitance inside the SPAD.Accordingly,an equivalent circuit model considering the photocapacitance effect is also proposed in this paper.Accurate analysis of the capacitance characteristics of SPAD has important scientific significance and application value for studying the energy level distribution of device interface defect states and improving the interface quality.

A New Method to Investigate InGaAsP Single-Photon Avalanche Diodes Using a Digital Sampling Oscilloscope

A near-infrared single-photon detection system is established by using pigtailed InGaAs/InP avalanche photodiodes. With a 50GHz digital sampling oscilloscope, the function and process of gated-mode （Geiger-mode） single-photon detection are intuitionally demonstrated for the first time. The performance of the detector as a gated-mode single-photon counter at wavelengths of 1310 and 1550nm is investigated. At the operation temperature of 203K,a quantum efficiency of 52% with a dark count probability per gate of 2.4 × 10 ^-3 ,and a gate pulse repetition rate of 50kHz are obtained at 1550nm. The corresponding parameters are 43%, 8.5 × 10^-3 , and 200kHz at 238K.

Dopamine transporter distribution in patients with Parkinson disease of different stages detected using single-photon emission computed tomography brain imaging

BACKGROUND： Literatures have reported that the density changes of dopamine transporter is negatively correlated with the severity degree and grading of disease condition of Parkinson disease （PD）. However, the distribution ofdopamine transporter in each nucleus of corpora striatum at each period is still unclear. OBJECTIVE： To observe the radioactive uptake distribution of dopamine transporter in bilateral corpora striata of patients with different stages of PD using single photon emission computed tomography （SPECT）, and make a comparison with healthy controls. DESIGN： Case-control analysis. SETTING： Department of Imageology, Second Hospital Affiliated to Guangzhou University of Chinese Medicine. PARTICIPANTS： Thirty patients with PD admitted to Second Hospital Affiliated to Guangzhou University of Traditional Chinese Medicine between January and December 2005 were recruited. The involved patients, 19 male and 11 female, were aged from 36 to 80 years and with disease course of 2.5 months to 10 years. They all met the clinical diagnosis criteria of Britain Parkinson＇s disease Association Think Tank; Following Hoehn-Yahr grading： grade Ⅰ ： unilateral morbidity; grade Ⅱ： bilateral morbidity, but without balance disorder; grade Ⅲ： bilateral morbidity, accompanied with early posture balance disorder; grade Ⅳ： severe morbidity, needs more help; grade V ： without help, only in bed or wheelchair. There were 11 patients with mild PD （grade Ⅰ - Ⅱ ）, 9 patients with moderate PD （grade Ⅲ） and 10 patients with advanced PD （grade Ⅳ - Ⅴ ）. Meanwhile, 6 healthy persons were selected as normal controls. Informed consents were obtained from all the subjects. METHODS： Twenty-four hours after withdrawal of PD drugs, 30 patients with PD and 6 healthy controls took kalium perchloricum 400 mg orally. After lying down for 30 minutes, all the subjects were intravenously injected with 740 MBq 99Tc m-TRODAT-1 （Jiangsu Institute of Atomic Medicine, Batch No. 20040310） at elbow part. Following injection, image was collected using scanner for single photon emission computed tomography （ADAC Company, USA）. Matrix was 64 × 64, each detecting head revolved 180 ° , 1 frame/60 s. Sixty-four frames were collected with double detecting heads, 50 K/frame. Faultages with clearest image of corpora striatum were selected. Regions of interest （ROI） of caudate nucleus, anterior and posterior putamen and thalamic region in bilateral corpora striata were radioactively counted, and mean value of radioactive counting of ROI was used as the mean value ofpixel in each region of bilateral corpora striata. MAIN OUTCOME MEASURES： Comparison of radioactive uptake in each region of brain between healthy persons and patients with PD. RESULTS： Thirty patients with PD and six healthy persons who received body examination participated in the final result. Comparison of radioactive uptake in each region between healthy persons and patients with PD： ①In the healthy persons, high-density radioactive uptake was found in bilateral corpora striata; Structures of caudate nucleus, anterior and posterior putamen, and thalamus were clear with eudipleural radioactive distribution, and the background of peripheral brain tissue was very low. ②Radioactive intakes in opposite anterior and posterior putamen of patients with mild PD were significantly inferior to those in homolateral ones（70.45±3.35, 87.64±2.65, t =15.82, P 〈 0.05）. Structures of bilateral caudate nucleus and thalamus were clear with eudipleural radioactive distribution （P 〉 0.05）. ③Radioactive intakes in anterior and posterior putamen and thalamus of patients with moderate PD were significantly reduced as compared with healthy persons. There were significant differences in mean radioactive counting of ROI between patients with PD and healthy persons （t =5.20, P 〈 0.05： t =3.95, P 〈 0.05）; The structure of opposite caudate nucleus was not very clear, radioactive distribution of opposite caudate nucleus was significantly reduced as compared with homolateral one （81.11±4.25, 104.56±3.64, t = 14.65, P 〈 0.05）. ④As for patients with advanced PD, the structure of bilateral corpora striata was not clear, radioactive intake was significantly reduced and peripheral background was heightened, even higher than the distribution of the whole corpora striatum. CONCLUSION： SPECT DAT imaging of brain can show the distribution of radioactive uptake in each region of bilateral corpora striata of patients with different stages of PD, which is helpful to diagnose and evaluate the severity of PD.

Round-Robin Differential Phase Shift with Heralded Single-Photon Source

Round-robin differential phase shift （RRDPS） is a novel quantum key distribution protocol which can bound information leakage without monitoring signal disturbance. In this work, to decrease the effect of the vacuum component in a weak coherent pulses source, we employ a practical decoy-state scheme with heralded singlephoton source for the RRDPS protocol and analyze the performance of this method. In this scheme, only two decoy states are needed and the yields of single-photon state and multi-photon states, as well as the bit error rates of each photon states, can be estimated. The final key rate of this scheme is bounded and simulated over transmission distance. The results show that the two-decoy-state method with heralded single-photon source performs better than the two-decoy-state method with weak coherent pulses.

A High-Efficiency Broadband Superconducting Nanowire Single-Photon Detector with a Composite Optical Structure

Superconducting nanowire single-photon detectors （SNSPDs） with a composite optical structure composed of phase-grating and optical cavity structures are designed to enhance both the system detection efficiency and the response bandwidth. Numerical simulation by the finite-difference time-domain method shows that the photon absorption capacity of SNSPDs with a composite optical structure can be enhanced significantly by adjusting the parameters of the phase-grating and optical cavity structures at multiple frequency bands. The absorption capacity of the superconducting nanowires reaches 70%, 72%, 60.73%, 61.7%, 41.2%, and 46.5% at wavelengths of 684, 850, 732, 924, 1256, and 1426nm, respectively. The use of a composite optical structure reduces the total filling factor of superconducting nanowires to only 0.25, decreases the kinetic inductance of SNSPDs, and improves the count rates.

Time-Bin Phase-Encoding Measurement-Device-Independent Quantum Key Distribution with Four Single-Photon Detectors

Measurement-device-independent quantum key distribution （MDI-QKD） eliminates all loopholes on detection. 3 loss in the final key for the Previous experiments of time-bin phase-encoding MDI-QKD allow a factor of incapability of identifying two successive detection events by a single photon detector. Here we propose a new scheme to realize the time-bin phase-encoding MDI-QKD. The polarization states are used to generate the time bins and the phase-encoding states. The factor of loss in the final key is eliminated by using four single photon detectors at the measurement site. We show the feasibility of our scheme with a proof-of-principle experimental demonstration. The phase reference frame is rotated extremely slowly with only passive stabilization measures. The quantum bit error rate can reach 0.8% in the Z-basis and 26.2% in the X-basis.

Controlling a sine wave gating single-photon detector by exploiting its filtering loophole

GHz single-photon detector （SPD） is a crucial part in the practical high speed quantum key distribution （QKD） system. However, any imperfections in a practical QKD system may be exploited by an eavesdropper （Eve） to collect information about the key without being discovered. The sine wave gating SPD （SG-SPD） based on InGaAs/InP avalanche photodiode, one kind of practical high speed SPD, may also contain loopholes. In this paper, we study the principle and characteristic of the SG-SPD and find out the filtering loophole of the SG-SPD for the first time. What is more, the proof-of-principle experiment shows that Eve could blind and control Bob＇s SG-SPD by exploiting this loophole. We believe that giving enough attention to this loophole can improve the practical security of the existing QKD system. GHz single-photon detector （SPD） is a crucial part in the practical high speed quantum key distribution （QKD） system. However, any imperfections in a practical QKD system may be exploited by an eavesdropper （Eve） to collect information about the key without being discovered. The sine wave gating SPD （SG-SPD） based on InGaAs/InP avalanche photodiode, one kind of practical high speed SPD, may also contain loopholes. In this paper, we study the principle and characteristic of the SG-SPD and find out the filtering loophole of the SG-SPD for the first time. What is more, the proof-of-principle experiment shows that Eve could blind and control Bob＇s SG-SPD by exploiting this loophole. We believe that giving enough attention to this loophole can improve the practical security of the existing QKD system.

Passive Decoy-State Reference-Frame-Independent Quantum Key Distribution with Heralded Single-Photon Source

Reference-frame-independent（RFI）quantum key distribution（QKD）is a protocol which can share unconditional secret keys between two remote users without the alignment of slowly varying reference frames.We propose a passive decoy-state RFI-QKD protocol with heralded single-photon source（HSPS）and present its security analysis.Compared with RFI QKD using a weak coherent pulse source（WCPS）,numerical simulations show that the passive decoy-state RFI QKD with HSPS performs better not only in secret key rate but also in secure transmission distance.Moreover,our protocol is robust against the relative motion of the reference frames as well as RFI QKD with the WCPS.In addition,we also exploit Hoeffding＇s inequality to investigate the finite-key effect on the security of the protocol.

A Bright Single-Photon Source from Nitrogen-Vacancy Centers in Diamond Nanowires

Single-photon flux is one of the crucial properties of nitrogen vacancy （NV） centers in diamond for its application in quantum information techniques. Here we fabricate diamond conical nanowires to enhance the single-photon count rate. Through the interaction between tightly confined optical mode in nanowires and NV centers, the single-photon lifetime is much shortened and the collection efficiency is enhanced. As a result, the detected single-photon rate can be at 564 kcps, and the total detection coefficient can be 0.8%, which is much higher than that in bulk diamond. Such a nanowire single-photon device with high photon flux can be applied to improve the fidelity of quantum computation and the precision of quantum sensors.

Indocyanine green plasma clearance rate and 99mTc-galactosyl human serum albumin single-photon emission computed tomography evaluated preoperative remnant liver

BACKGROUND Preoperative evaluation of future remnant liver reserves is important for safe hepatectomy.If the remnant is small,preoperative portal vein embolization(PVE)is useful.Liver volume analysis has been the primary method of preoperative evaluation,although functional examination may be more accurate.We have used the functional evaluation liver using the indocyanine green plasma clearance rate(KICG)and 99mTc-galactosyl human serum albumin single-photon emission computed tomography(99mTc-GSA SPECT)for safe hepatectomy.AIM To analyze the safety of our institution’s system for evaluating the remnant liver reserve.METHODS We retrospectively reviewed the records of 23 patients who underwent preoperative PVE.Two types of remnant liver KICG were defined as follows:Anatomical volume remnant KICG(a-rem-KICG),determined as the remnant liver anatomical volume rate×KICG;and functional volume remnant KICG(frem-KICG),determined as the remnant liver functional volume rate based on 99mTc-GSA SPECT×KICG.If either of the remnant liver KICGs were>0.05,a hepatectomy was performed.Perioperative factors were analyzed.We defined the marginal group as patients with a-rem-KICG of<0.05 and a f-rem-KICG of>0.05 and compared the postoperative outcomes between the marginal and not marginal(both a-rem-KICG and f-rem-KICG>0.05)groups.RESULTS All 23 patients underwent planned hepatectomies.Right hepatectomy,right trisectionectomy and left trisectionectomy were in 16,6 and 1 cases,respectively.The mean of blood loss and operative time were 576 mL and 474 min,respectively.The increased amount of frem-KICG was significantly larger than that of a-rem-KICG after PVE(0.034 vs 0.012,P=0.0273).The not marginal and marginal groups had 17(73.9%)and 6(26.1%)patients,respectively.The complications of Clavian-Dindo classification grade II or higher and post-hepatectomy liver failure were observed in six(26.1%)and one(grade A,4.3%)patient,respectively.The 90-d mortality was zero.The marginal group had no significant difference in postoperative outcomes(prothrombin time/international normalised ratio,total bilirubin,complication,post-hepatectomy liver failure,hospital stay,90-d,and mortality)compared with the not-marginal group.CONCLUSION Functional evaluation of the remnant liver enabled safe hepatectomy and may extend the indication for hepatectomy after PVE treatment.

Hepatic clearance measured with ^(99m)Tc-GSA single-photon emission computed tomography to estimate liver fibrosis

AIM: To evaluate the clinical utility of hepatic clearance(HC) measured with technetium-99m-diethylenetriaminepenta-acetic acid-galactosyl human serum albumin(99mTc-GSA) single-photon emission computed tomography(SPECT) to estimate the degree of liver fibrosis.METHODS:Seventy-eight consecutive patients who underwent initial hepatectomy due to hepatocellular carcinoma were enrolled in this study.Indocyanine green clearance(ICG R15),quantitative indices estimated by 99mTc-GSA[the receptor index(LHL15 and HH15)and HC via SPECT analysis],and conventional liver function tests were performed before hepatectomy.Correlations among the quantitative indices for liver functional reserve,conventional liver function tests,andthe degree of liver fibrosis were evaluated.RESULTS:The degree of liver fibrosis was correlated with ICG R15,HH15,LHL15,and HC.HC showed the best correlation with conventional liver function tests.According to multivariate analysis,HC and LHL15 were significant independent predictors of severe fibrosis.HC was the most valuable index for predicting severe fibrosis.CONCLUSION:HC measured with 99mTc-GSA SPECT is a reliable index for assessing liver fibrosis before hepatectomy.

Cerebral functional imaging of Waiguan (SJ 5) acupoint specificity using single-photon emission computed tomography

The action of needling in acupoint therapy has to first be regulated and integrated by the brain, and then it affects the target organ and manifests its therapeutic effects, which is dependent on the specificity of the acupoints. The authors put forward the hypothesis of the ＂acupoint-related brain＂. Single-photon emission computed tomography was used to explore the activation of brain regions following true needling in true acupoint Waiguan （SJ 5）, sham needling in true acupoint Waiguan, true needling in a sham point, and sham needling in a sham point. The relative specificity of Waiguan in normal persons was analyzed by observing changes in regional cerebral blood flow. Compared with the sham needling in true acupoint group and sham needling in the sham point group, acupuncture at Waiguan can activate brain regions controlling movement. Compared with true needling in the sham point group, acupuncture at Waiguan can also activate brain regions controlling movement. The results suggest that the specificity of needling at an acupoint is related to certain activated cerebral functional regions, which are associated with the clinical application of the acupoint.

Cavity optomechanics： Manipulating photons and phonons towards the single-photon strong coupling

Cavity optomechanical systems provide powerful platforms to manipulate photons and phonons, open potential ap- plications for modern optical communications and precise measurements. With the refrigeration and ground-state cooling technologies, studies of cavity optomechanics are making significant progress towards the quantum regime including non- classical state preparation, quantum state tomography, quantum information processing, and future quantum internet. With further research, it is found that abundant physical phenomena and important applications in both classical and quan- tum regimes appeal as they have a strong optomechanical nonlinearity, which essentially depends on the single-photon optomechanical coupling strength. Thus, engineering the optomechanical interactions and improving the single-photon optomechanical coupling strength become very important subjects. In this article, we first review several mechanisms, theoretically proposed for enhancing optomechanical coupling. Then, we review the experimental progresses on enhancing optomechanical coupling by optimizing its structure and fabrication process. Finally, we review how to use novel structures and materials to enhance the optomechanical coupling strength. The manipulations of the photons and phonons at the level of strong optomechanical coupling are also summarized.

Noiseless linear amplification for the single-photon entanglement of arbitrary polarization–time-bin qudit

Single-photon entanglement(SPE) is an important source in quantum communication. In this paper, we put forward a single-photon-assisted noiseless linear amplification protocol to protect the SPE of an arbitrary polarization–time-bin qudit from the photon transmission loss caused by the practical channel noise. After the amplification, the fidelity of the SPE can be effectively increased. Meanwhile, the encoded polarization–time-bin features of the qudit can be well preserved. The protocol can be realized under the current experimental conditions. Moreover, the amplification protocol can be extended to resist complete photon loss and partial photon loss during the photon transmission. After the amplification, we can not only increase the fidelity of the target state, but also solve the decoherence problem simultaneously. Based on the above features, our amplification protocol may be useful in future quantum communication.

Micropillar Cavity Design for 1.55-μm Quantum-Dot Single-Photon Sources

The 1.55-μm quantum-dot (QD) micropillar cavities are strongly required as single photon sources (SPSs) for silica-fiber-based quantum information processing. Theoretical analysis shows that the adiabatic distributed Bragg reflector (DBR) structure may greatly improve the quality of a micropillar cavity. An InGaAsP/InP micropillar cavity is originally difficult, but it becomes more likely usable with inserted tapered (thickness decreased towards the center) distributed DBRs. Simulation turns out that, incorporating adiabatically tapered DBRs, a Si/SiO2- InP hybrid micropillar cavity, which enables weakly coupling InAs/InP quantum dots (QDs), can even well satisfy strong coupling at a smaller diameter. Certainly, not only the tapered structure, other adiabatic designs, e.g., both DBR layers getting thicker and one thicker one thinner, also improve the quality, reduce the diameter, and degrade the fabrication difficulty of Si/SiO2-InP hybrid micropillar cavities. Furthermore, the problem of the thin epitaxial semiconductor layer can also be greatly resolved by inserting adiabatic InGaAsP/InP DBRs. With tapered DBRs, the InGaAsP/InP-air-aperture micro-pillar cavity serves as an efficient, coherent, and monolithically producible 1.55-μm single-photon source (SPS). The adiabatic design is thus an effective way to obtain prospective candidates for 1.55-μm QD SPSs.

Wavelength dependence of intrinsic detection efficiency of NbN superconducting nanowire single-photon detector

Superconducting nanowire single-photon detectors(SNSPDs) have attracted considerable attention owing to their excellent detection performance;however, the underlying physics of the detection process is still unclear.In this study, we investigate the wavelength dependence of the intrinsic detection efficiency(IDE) for NbN SNSPDs.We fabricate various NbN SNSPDs with linewidths ranging from 30 nm to 140 nm.Then, for each detector, the IDE curves as a function of bias current for different incident photon wavelengths of 510–1700 nm are obtained.From the IDE curves, the relations between photon energy and bias current at a certain IDE are extracted.The results exhibit clear nonlinear energy–current relations for the NbN detectors, indicating that a detection model only considering quasiparticle diffusion is unsuitable for the meander-type NbN-based SNSPDs.Our work provides additional experimental data on SNSPD detection mechanism and may serve as an interesting reference for further investigation.

Dark count in single-photon avalanche diodes:A novel statistical behavioral model

Dark count is one of the inherent noise types in single-photon diodes,which may restrict the performances of detectors based on these diodes.To formulate better designs for peripheral circuits of such diodes,an accurate statistical behavioral model of dark current must be established.Research has shown that there are four main mechanisms that contribute to the dark count in single-photon avalanche diodes.However,in the existing dark count models only three models have been considered,thus leading to inaccuracies in these models.To resolve these shortcomings,the dark current caused by carrier diffusion in the neutral region is deduced by multiplying the carrier detection probability with the carrier particle current at the boundary of the depletion layer.Thus,a comprehensive dark current model is constructed by adding the dark current caused by carrier diffusion to the dark current caused by the other three mechanisms.To the best of our knowledge,this is the first dark count simulation model into which incorporated simultaneously are the thermal generation,trap-assisted tunneling,band-to-band tunneling mechanisms,and carrier diffusion in neutral regions to evaluate dark count behavior.The comparison between the measured data and the simulation results from the models shows that the proposed model is more accurate than other existing models,and the maximum of accuracy increases up to 31.48%when excess bias voltage equals 3.5 V and temperature is 50℃.

Invariant operator theory for the single-photon energy in time-varying media

After the birth of quantum mechanics, the notion in physics that the frequency of light is the only factor that determines the energy of a single photon has played a fundamental role. However, under the assumption that the theory of Lewis-Riesenfeld invariants is applicable in quantum optics, it is shown in the present work that this widely accepted notion is valid only for light described by a time-independent Hamiltonian, i.e., for light in media satisfying the conditions, ε（t） = ε（0）, u（t） = u（0）, and σ（t） = 0 simultaneously. The use of the Lewis Riesenfeld invariant operator method in quantum optics leads to a marvelous result： the energy of a single photon propagating through time-varying linear media exhibits nontrivial time dependence without a change of frequency.