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Secure communication networks enabled by commercial quantum key distribution (QKD) are already available. However, their widespread deployment will require great efforts towards reducing the currently prohibitive cost of QKD systems. Here, we propose a compact and cost-effective alternative to the asymmetric Mach-Zehnder interferometer commonly used to implement phase encoding in the Bennett-Brassard 1984 (BB84) QKD protocol. Our solution consists of an all-fiber, in-line, highly birefringent interferometer (HBI). The HBI shows improved tolerance to length mismatches and a simpler assembly, making it particularly desirable for the fabrication of multi-user systems where several interferometers must have matched delays and where cost and space considerations can be most critical, such as quantum access networks. As a proof-of-principle, we demonstrate point-to-point QKD operation with HBIs over 15.5 km drop fiber and an 8-port passive optical network splitter. We achieve a secure key generation rate of 299.4 ± 16.4 kbit/s with a quantum bit error rate of 2.89 ± 0.31% for a continuous 25 h operation period.

Measurements performed on entangled photon pairs shared between two parties can allow unique quantum cryptographic keys to be formed, creating secure links between users. An advantage of using such entangled photon links is that they can be adapted to propagate entanglement to end users of quantum networks with only untrusted nodes. However, demonstrations of quantum key distribution with entangled photons have so far relied on sources optically excited with lasers. Here, we realize a quantum cryptography system based on an electrically driven entangled-light-emitting diode. Measurement bases are passively chosen and we show formation of an error-free quantum key. Our measurements also simultaneously reveal Bell's parameter for the detected light, which exceeds the threshold for quantum entanglement.

We present an efficient and robust quantum random number generator based upon high-rate room temperature photon number detection. We employ an electric field-modulated silicon avalanche photodiode, a type of device particularly suited to high-rate photon number detection with excellent photon number resolution to detect, without an applied dead-time, up to 4 photons from the optical pulses emitted by a laser. By both measuring and modeling the response of the detector to the incident photons, we are able to determine the illumination conditions that achieve an optimal bit rate that we show is robust against variation in the photon flux. We extract random bits from the detected photon numbers with an efficiency of 99% corresponding to 1.97 bits per detected photon number yielding a bit rate of 143 Mbit/s, and verify that the extracted bits pass stringent statistical tests for randomness. Our scheme is highly scalable and has the potential of multi-Gbit/s bit rates.

Optical properties of single submonolayer InAs structures grown on GaAs (001) matrix are systematically investigated by means of photoluminescence and time‐resolved photoluminescence. It is shown that the formation of InAs dots with 1 ML height leads to localization of excitons under certain submonolayer InAs coverages, which play a key role in the highly improved luminescence efficiency of the submonolayer InAs/GaAs structures.

We demonstrate quantum key distribution (QKD) with bidirectional 10 Gb/s classical data channels in a single fiber using dense wavelength division multiplexing. Record secure key rates of 2.38 Mbps and fiber distances up to 70 km are achieved. Data channels are simultaneously monitored for error-free operation. The robustness of QKD is further demonstrated with a secure key rate of 445 kbps over 25 km, obtained in the presence of data lasers launching conventional 0 dBm power. We discuss the fundamental limit for the QKD performance in the multiplexing environment., 10 pages, 4 figures

Quantum key distribution (QKD) has been proven theoretically secure at the protocol level. However, the security may be compromised through deviation from theoretical models in device implementation or operation. For each deviation, thorough understanding must be achieved in order for subsequent construction of robust countermeasures. In Ref. 1, we have studied the effectiveness of bright illumination attacks, a group of attacks targeting gated InGaAs avalanche photodiodes (APDs). We found that gated APDs are naturally resilient against continuous-wave (CW) attacks through the gain modulation effect. The finding is contrary to the claim by Lydersen et al. that “the loophole is likely to be present in most QKD systems using APDs to detect single photons.” The detector loophole reported by Lydersen et al. was in fact due to inappropriate settings in the discrimination level of single photon APDs. Furthermore, we discussed the respective effectiveness for temporally tailored bright illumination and after-gate attacks, as summarized in Table I. Against bright illumination attacks, “monitoring the photocurrent” was proposed as a counter-measure, which was based on the vast difference in the measured APD currents as compared with normal operation, see Fig. 1. In the preceding comment, we note that Lydersen et al. do not dispute that bright light attacks would be ineffective if the detector parameters were set correctly, which was our main finding. Instead, they challenge the robustness of the counter-measure we proposed. To serve this challenge, they have designed a new attack that uses faint optical pulses ( 120 photons/pulse) only. The attack exploits the superlinear count dependence achieved by restricting the avalanche duration. In the absence of bright illumination, Lydersen et al. claim, unfortunately without experimental elaboration, that the attack “would not be detectable” by monitoring the current and “the afterpulsing is negligible.” It should be pointed out that Lydersen et al. reported a high quantum bit error ratio (QBER) of >12% during the attack, which would be easily detected by the legitimate users of the system, even for a 100 kHz clock rate which significantly favours the attacker. Although the illumination during the attack is weak, Lydersen et al. have overlooked the fact that the APD gain is still large, and thus, the attack generates a sizable photocurrent that can easily be detected by the users. We prove this with a simple experiment. A gated InGaAs APD is subjected to faint illumination by a 50 ps pulsed laser. The APD is gated with 2 MHz pulses of 4 V amplitude and 3.5 ns duration, a gating condition identical to previously used in an actual QKD system. Its single photon detection efficiency is measured to be 15% with a dark count probability of 3.5 10 5 per gate. In the attack, the pulse delay is optimized to have a minimum QBER that the attack would have caused with a flux of 120 photons. Under this optimized delay, the count probability and photocurrent are measured as a function of attacking flux, as shown in Fig. 1. Superlinearity regime used in the faint after-gate attack is identified as occurring at fluxes between 70–300 photons/pulse. Within this flux range, the APD current is macroscopic and actually easily detectable. At around 9 lA, this current is more than 40 times stronger than would be under normal QKD operation for this APD, see Fig. 1. Macroscopic current causes afterpulsing. In Fig. 1, for an attacking flux less than 60 photons/pulse, the count probability shows a slightly sub-linear dependence, closely resembling that of the photocurrent. This sub-linear behavior can be explained only by the dominance of afterpulsing, and this assignment has been verified by the gated afterpulse measurement technique. As can be extrapolated from the linear dependence, the afterpulsing is not negligible in the super-linear regime. Therefore, contrary to the intuitive claims by Lydersen et al., the faint after-gate attack not only causes significant current, but also produces non-negligible afterpulses. In addition to the resultant QBER, monitoring the APD current is an effective counter-measure against this attack, although it was initially proposed for bright illumination attacks.

We report here a complete experimental realization of one-way decoy-pulse quantum key distribution, demonstrating an unconditionally secure key rate of 5.51 kbps for a 25.3 km fibre length. This is two orders of magnitudes higher than the value that can be obtained with a non-decoy system. We introduce also a simple test for detecting the photon number splitting attack and highlight that it is essential for the security of the technique to fully characterize the source and detectors used.

An electrically driven ∼1.3μm single-photon source is demonstrated. The source contains InAs quantum dots within a planar cavity light-emitting diode. Electroluminescence (EL) spectra show clear emission lines and from time resolved EL we estimate a primary decay time of ∼1ns. Time-varying Stark shifts are studied and proposed for truncating the emission in jitter-sensitive applications (optimization for 2ns detector gate width demonstrated) and for relaxing excitation pulse-length requirements. A correlation measurement demonstrates suppression of multiphoton emission to below 28% of the Poissonian level before correction for detector dark counts, suggesting g(2)(0)∼0.19 for the source itself.

The authors report here a complete experimental realization of one-way decoy pulse quantum key distribution, demonstrating an unconditionally secure key rate of 5.51kbits∕s for a 25.3km fiber length. This is two orders of magnitudes higher than the value that can be obtained with a nondecoy system. They also introduce a simple test for detecting the photon number splitting attack and highlight that it is essential for the security of the technique to fully characterize the source and detectors used.

We demonstrate an on-demand single-photon source that is compatible with standard telecom optical fiber. Through careful control of the critical strains of InAs∕GaAs self-assembled quantum dots, we produce a microcavity sample with a low density of large dots emitting into the fiber-optic transmission band at 1.3μm. The second-order correlation function of the source reveals a strong suppression in the rate of multiphoton pulses at both 5K and above 30K. The source may be useful for fiber-optic-based single-photon applications, such as quantum metrology, quantum communications, and distributed quantum computing.

Photoluminescence and time‐resolved photoluminescence were used to study the heterointerface configuration in GaAs/AlGaAs quantum wells grown by molecular‐beam epitaxy with growth interruption. Photoluminescence spectra of the growth‐interrupted sample are characterized by multiplet structures, with energy separation corresponding to a 0.8 monolayer difference in well width, rather than 1 monolayer as expected from the ‘‘atomically smooth island’’ picture. By analyzing the thermal transfer process of the photogenerated carriers and luminescence decay process, we further exploit the exciton localization at the interface microroughness superimposed on the extended growth islands. The lateral size of the microroughness in our sample was estimated to be 5 nm, less than the exciton diameter of 15 nm. Our results strongly support the bimodal roughness model proposed by Warwick et al. [Appl. Phys. Lett. 56, 2666 (1990)].