Free-space optical (FSO) communications are becoming promising schemes for high-capacity wireless links due to their plentiful characteristics originated from higher carrier frequency. These characteristics also yield a greater security advantage over radio frequency counterparts: the physical ability of a wiretapper is reasonably restricted due to the high directionality of communication beam and the line-of-sight configuration of the link. Secret key agreement over FSO links (FSO-SKA) employs this security advantage as well as the post-processing over an authenticated public channel to establish an information-theoretic secure key which cannot be broken even with unbounded computer resources. In the previous works, the authors demonstrated the full-field implementations of FSO-SKA with a 7.8-km FSO link testbed including a probing station to estimate the possible wiretapping risks from the sidelobe of the communication beam. In the demonstration, however, there is still room to improve the secret key rate by exploiting the optical fading which contains additional information about random states of the FSO links. We here propose a novel protocol for FSO-SKA employing such channel state information. In the protocol, the legitimate receiver decides whether to discard the received symbols or not according to the received optical power at the time. Based on the experimental data from the FSO link testbed, we demonstrate that the proposed protocol improves the secret key rate compared with our previous result. To our best knowledge, this is the first demonstration that exploits the effect of atmospheric turbulences to improve the security performance of communication systems. We anticipate that this idea will be applicable on the broader areas of FSO communications and opens a way toward practical wireless network spanned by FSO links.
Research and development of a novel method for a secure free-space optical communication system has been done in NICT since 2018, and demonstration experiments between an aircraft and a transportable optical ground station are planned in near future. In order to establish a stable and highly accurate optical communication link, the system must have a fine pointing mechanism in both the aircraft and the ground station. A compact and light-weight tracking system is required to be mounted on the aircraft, and there will be needed to have an adjustment function of the beam divergence control to allow stable communication under various altitude and atmospheric conditions. The transportable optical ground station should maintain vibration resistance when moving, and it must be easily deployed on each site where is the appropriate optical ground station site with respect to atmospheric turbulence condition.
The transmission and reception of polarized quantum-limited signals from space is of capital interest for a variety of fundamental-physics experiments and quantum-communication protocols. Specifically, Quantum Key Distribution (QKD) deals with the problem of distributing unconditionally-secure cryptographic keys between two parties. Enabling this technology from space is a critical step for developing a truly-secure global communication network. The National Institute of Information and Communications Technology (NICT, Japan) performed the first successful measurement on the ground of a quantum-limited signal from a satellite in experiments carried out on early August in 2016. The SOTA (Small Optical TrAnsponder) lasercom terminal onboard the LEO satellite SOCRATES (Space Optical Communications Research Advanced Technology Satellite) was utilized for this purpose. Two non-orthogonally polarized signals in the ~800-nm band and modulated at 10 MHz were transmitted by SOTA and received in the single-photon regime by using a 1-m Cassegrain telescope on a ground station located in an urban area of Tokyo (Japan). In these experiments, after compensating the Doppler effect induced by the fast motion of the satellite, a QKD-enabling QBER (Quantum Bit Error Rate) below 5% was measured with estimated key rates in the order of several Kbit/s, proving the feasibility of quantum communications in a real scenario from space for the first time.
Recently, the free space optical (FSO) communications have been widely studied as an alternative for large capacity communications and its possible implementation in satellite and terrestrial laser links. In satellite communications, clouds can strongly attenuate the laser signal that would lead to high bit-error rates or temporal unavailability of the link. To overcome the cloud coverage effects, often site diversity technique is implemented. When using multiple ground stations though, simplified optical system is required to allow the usage of more flexible approaches. In terrestrial laser communications, several methods for optical system simplification by using a multimode fiber (MMF) have been proposed.
The security of electronic communications is a topic that has gained noteworthy public interest in recent years. As a result, there is an increasing public recognition of the existence and importance of mathematically based approaches to digital security. Many of these implement digital signatures to ensure that a malicious party has not tampered with the message in transit, that a legitimate receiver can validate the identity of the signer and that messages are transferable. <p> </p>The security of most digital signature schemes relies on the assumed computational difficulty of solving certain mathematical problems. However, reports in the media have shown that certain implementations of such signature schemes are vulnerable to algorithmic breakthroughs and emerging quantum processing technologies. Indeed, even without quantum processors, the possibility remains that classical algorithmic breakthroughs will render these schemes insecure.<p> </p> There is ongoing research into information-theoretically secure signature schemes, where the security is guaranteed against an attacker with arbitrary computational resources. One such approach is quantum digital signatures. Quantum signature schemes can be made information-theoretically secure based on the laws of quantum mechanics while comparable classical protocols require additional resources such as anonymous broadcast and/or a trusted authority. <p> </p>Previously, most early demonstrations of quantum digital signatures required dedicated single-purpose hardware and operated over restricted ranges in a laboratory environment. Here, for the first time, we present a demonstration of quantum digital signatures conducted over several kilometers of installed optical fiber. The system reported here operates at a higher signature generation rate than previous fiber systems.
We present the latest results on two kinds of photon detectors: single photon detectors (SPDs) and photon number resolving detector (PNRD). We developed high speed and low noise SPDs using superconducting nano-wire (abbreviated by SNSPD) and semiconductor (InGaAs) avalanche photodiode (APD). The SNSPD system has totally four channels all of which have the detection eciency higher than 16% at 100Hz dark count rate. The InGaAs APD system also has four channels and the best performance is represented by the after-pulse probability of 0.61%, the dark count probability of 0.71×10<sup>-6</sup> (~1kHz), and the detection eciency of 10.9%. Both systems were applied to wavelength division multiplexing quantum key distribution (WDM-QKD) operated at 1.2GHz repetition rate in a eld environment. The PNRD is made of superconducting transition edge sensor. It was applied to the implementation of quantum receiver which could beat the homodyne limit of the bit error rate of binary coherent states. We discuss future perspective of quantum communications with those photon detection technologies, including multi-user QKD networks and low-power high capacity communications.
Single-photon sources and detectors are key enabling technologies for photonics in quantum information science and
technology (QIST). QIST applications place high-level demands on the performance of sources and detectors; it is
therefore essential that their properties can be characterized accurately. Superconducting nanowire single-photon
detectors (SNSPDs) have spectral sensitivity from visible to beyond 2 μm in wavelength, picosecond timing resolution
(Jitter <100 ps FWHM) and the capacity to operate ungated with low dark counts (<1 kHz). This facilitates data
acquisition at high rates with an excellent signal-to-noise ratio.
We report on the construction and characterization of a two-channel SNSPD system. The detectors are mounted in a
closed-cycle refrigerator, which eliminates reliance on liquid cryogens. Our specification was to deliver a system with
1% efficiency in both channels at a wavelength of 1310 nm with 1 kHz dark count rate. A full width at half maximum
timing jitter of less than 90 ps is achieved in both channels. The system will be used to detect individual photons
generated by quantum-optical sources at telecom wavelengths. Examples include single-photon sources based on
quantum dots (emitting at 1310 nm). The SNSPD system's spectral sensitivity and timing resolution make it suited to
characterization of such sources, and to wider QIST applications.
A free-space quantum key distribution system is being developed by the National Institute of Information and
Communications Technology (NICT) in Koganei, Japan. Quantum cryptography is a new technique for transmitting
information where the security is guaranteed by the laws of physics. In such systems, a single photon is used for the
quantum information. However, since the transmission distance in optical fibers is limited by the absorption of photons
by the fiber, the maximum demonstrated range has been limited to about 100 km. Free-space quantum cryptography
between an optical ground station and a satellite is a possible solution to extend the distance for a quantum network
beyond the limits of optical fibers. At NICT, a laser communication demonstration between the NICT optical ground
station and a low earth orbit satellite was successfully conducted in 2006. The use of free-space quantum key
distribution for such space communication links is considered an important future application. This paper presents
conceptual designs for the onboard transceivers for satellite quantum cryptography
Quantum cryptography is a new technique for transmitting quantum information. The information is securely transmitted due to the laws of physics. In such systems, the vehicle that transfers quantum information is a single photon. The problem with using photons is that the transmission distance is limited by the absorption of the photons by the optical fiber along which they pass. The maximum demonstrated range so far is approximately 100 km. Using free-space
quantum cryptography between a ground station and a satellite is a possible way of sending quantum information farther than is possible with optical fibers. This is because there is no birefringence effect in the atmosphere. However, there is a complication in that the directions of the polarization basis between the transmitter and the receiver must coincide with each other. This polarization changes because the mobile terminals for free-space transmission continuously change their attitudes. If the transmission protocol is based on polarization, it is necessary to compensate for the change in attitude between the mobile terminals. We are developing a scheme to track the polarization basis between the transceivers. The preliminary result is presented.
A measurement system is described for acquiring the gain distributions of avalanche photodiodes (APDs) in a range of
low average gain. The system is based on an ultralow-noise capacitive transimpedance amplifier to readout the charges
generated in an APD. The low noise level of the readout circuit about 7 electrons at the sampling rate of 200 Hz enables
us to characterize the gain distributions. The gain distribution of a commercial silicon (Si) APD measured at gain of 3.29
using this system is presented.
We study various non-Gaussian states generated by photon subtrastion from a squeezed light source. The source is a cw beam generated by optical parametric oscillator. The photon subtraction
is made by tapping a small fraction of the squeezed light source and by guiding it into two Si-APDs, which enable the subtraction of one to two photons. Trigger photon clicks specify a certain temporally
localized mode in the remaining squeezed beam. By filtering the remaining squeezed beam through an appropriate mode function, one can generate a variety of non-Gaussian states. This includes single and two photon states, the NOON state (N = 2), Schrödinger kitten states of both odd and even parities, and their arbitrarily desired superposition.
The population and coherent dynamics of excitons in InAs quantum dots were investigated using transient pump-probe and four-wave mixing spectroscopies in the telecommunications wavelength range. The sample
was fabricated on an InP(311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to stack over a hundred QD layers, which resulted in a significant enhancement of nonlinear signals. By controlling the polarization directions of incident pulses, we precisely estimated the radiative and non-radiative lifetimes, the transition dipole moment, and the dephasing time while taking into account their anisotropic properties. The measured radiative lifetime and dephasing time shows large anisotropies with respect to the crystal axes, which results from the anisotropic nature of the transition dipole moment. The
anisotropy is larger than that for InAs quantum dots on a GaAs(100) substrate, which seems to reflect a lack of symmetry on an (311)B substrate. A quantitative comparison of these anisotropies demonstrates that nonradiative population relaxation and pure dephasing are quite small in our QDs.
We investigate the dephasing of excitons in InAs self-assembled
quantum dots by using a transient four-wave-mixing technique. A used sample is specially designed to compensate the strain. We observe long-lived coherence of excitons at 5 K which corresponds to the dephasing time longer than a nanosecond, where the photon energy of
the excitation pulse is 0.874~eV. We find that a pure dephasing due to exciton-phonon interactions dominates in the exciton dephasing
rather than the population decay and the exciton-exciton interaction
in the weak excitation region, by analyzing the population lifetime and the polarization-dependent dephasing time.
Optical coherence time and nonlinear optical response of excitons localized to quantum dots in a GaAs/AlGaAs single quantum well are investigated by the three-pulse photon echo technique. At low temperature, the optical coherence time is estimated to be 1.8 ns from the decay curve of the photon echo signal. The decay profile for short time delay shows the strong dependence on the input pulse area of the third pulse. Rabi oscillations are also observed. The oscillations exhibit nonlinear behavior, which is affected by the pre-excited polarization. Such behavior can be qualitatively explained by the local field effect due to the exciton-exciton interaction between the excitons.
We investigate the possibility of implementing a given measurement using linear optics and continuous measurement. In particular, we revisit the so-called Dolinar receiver, a quasi-physical model attaining the minimum error discrimination of binary coherent states, to give an alternative derivation for the optimal discrimination scheme. Our approach is rather simple and can be applied to various kinds of measurements such as projection measurements in the regime of photonic-qubit signals.
The photon subtraction operation is an effective method to enhance an entanglement of the two-mode squeezed vacuum state. We show that it can be well approximated to the two-mode squeezed state with an enhanced squeezing although it is basically a mixed and non-Gaussian state. Such property can be directly observed by the current technology. We also study the continuous variable quantum dense coding with the photon subtracted two-mode squeezed state. It is shown that, in particular squeezing region, its error probability performance can be greatly improved from the original two-mode squeezed state. Since quantum dense coding is the protocol to transmit classical information, our analysis provides an alternative operational meaning of the entanglement involved in the on-off photon subtracted state, compared to the previous results, such as teleportation or the nonlocality, that could help to quantify the entanglement of mixed and non-Gaussian states.
A GaAs junction field-effect transistor (JFET) is a promising candidate for low noise at low frequency, and low-power cryogenic electronics to read out high-impedance photo-detectors. We report on the spectral noise characteristics, gate leak current, and gate capacitance of a SONY n-type GaAs JFET at a cryogenic temperature of 4.2 K. In our papers reporting performances of FETs, the noise and leak current have been measured separately. However, noise should be measured in the case of the gate terminal surrounded by high impedance devices, so-called in high impedance circumstance. In such high impedance circumstance, the dielectric polarization noise occurs, which is generated in devices and material around the gate terminal. Materials with dielectric loss generate dielectric polarization noise, which cannot be measured in low impedance circumstance because the electric charges compensate the dipoles. Therefore, to estimate the performance of the FETs for cryogenic readout electronics, noise measurement should be done in high impedance circumstance. In our previous work on a GaAs FET, low noise at low-frequency, i.e. ~500 nV/Hz<sup>1/2</sup> at 1 Hz was achieved by the thermal cure (TC) in low-impedance circumstance. By using the thermal curing technique, we have obtained a low noise level of ~500 nV/Hz<sup>1/2</sup> at 1Hz. Moreover, we have demonstrated faint light detection at 4.2 K using an InGaAs pin photodiode and a charge integration amplifier composed of the GaAs JFET. We have succeeded in detecting such ultra low power influx as a few photons per second with a quantum efficiency of ~80% and tolerance of 0.5 photons. The leak current of this detection system is ~500 electrons/hour.