Quantum key distribution (QKD) promises unconditional security in data communication and is currently being deployed in commercial applications. Nonetheless, before QKD can be widely adopted, it faces a number of important challenges such as secret key rate, distance, size, cost and practical security. Here, we survey those key challenges and the approaches that are currently being taken to address them. For thousands of years, human beings have been using codes to keep secrets. With the rise of the Internet and recent trends to the Internet of Things, our sensitive personal financial and health data as well as commercial and national secrets are routinely being transmitted through the Internet. In this context, communication security is of utmost importance. In conventional symmetric cryptographic algorithms, communication security relies solely on the secrecy of an encryption key. If two users, Alice and Bob, share a long random string of secret bits-the key-then they can achieve unconditional security by encrypting their message using the standard one-time-pad encryption scheme. The central question then is: how do Alice and Bob share a secure key in the first place? This is called the key distribution problem. Unfortunately, all classical methods to distribute a secure key are fundamentally insecure because in classical physics there is nothing preventing an eavesdropper, Eve, from copying the key during its transit from Alice to Bob. On the other hand, standard asymmetric or public-key cryptography solves the key distribution problem by relying on computational assumptions such as the hardness of factoring. Therefore, such schemes do not provide information-theoretic security because they are vulnerable to future advances in hardware and algorithms, including the construction of a large-scale quantum computer.

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In the quantum version of a Trojan-horse attack, photons are injected into the optical modules of a quantum key distribution system in an attempt to read information direct from the encoding devices. To stop the Trojan photons, the use of passive optical components has been suggested. However, to date, there is no quantitative bound that specifies such components in relation to the security of the system. Here, we turn the Trojan-horse attack into an information leakage problem. This allows us quantify the system security and relate it to the specification of the optical elements. The analysis is supported by the experimental characterization, within the operation regime, of reflectivity and transmission of the optical components most relevant to security.

We have investigated the temperature dependence of photoluminescence ͑PL͒ properties of a number of self-organized InAs/GaAs heterostructures with InAs layer thickness ranging from 0.5 to 3 ML. The temperature dependence of InAs exciton emission and linewidth was found to display a significant difference when the InAs layer thickness is smaller or larger than the critical thickness around 1.7 ML. The fast redshift of PL energy and an anomalous decrease of linewidth with increasing temperature were observed and attributed to the efficient relaxation process of carriers in multilayer samples, resulting from the spread and penetration of the carrier wave functions in coupled InAs quantum dots. The measured thermal activation energies of different samples demonstrated that the InAs wetting layer may act as a barrier for the thermionic emission of carriers in high-quality InAs multilayers, while in InAs monolayers and submonolayers the carriers are required to overcome the GaAs barrier to escape thermally from the localized states. ͓S0163-1829͑96͒06440-5͔Spontaneous formation of three-dimensional ͑3D͒ islands during Stranski-Krastanov-like growth of highly strained InAs layers on GaAs substrates by molecular-beam epitaxy ͑MBE͒ has been proposed as a promising way for fabricating high-quality InAs quantum dots ͑QD's͒ in GaAs.1-6 When the thickness of an InAs layer is beyond a critical thickness of around 1.7 ML, the structure is usually composed of an InAs wetting layer and conelike InAs islands deposited on it. The density, size distribution, uniformity, and coverage of such InAs islands were found to be growth condition dependent, and have been investigated by TEM and atomic force microscopy ͑AFM͒ in recent years. [7][8][9] The optical studies 10-13 revealed its excellent radiative recombination, which usually gives a broadband with a reported full width at half maximum ͑FWHM͒ in the range of 50-130 meV. Recent work by Lubyshev et al. 13 presented the unusual temperature dependence of exciton energy in InAs multilayer structures. An anomalous decrease of the FWHM was detected and explained in terms of the tunneling process between InAs dots.In this paper, we studied the exciton relaxation and thermal activation in InAs multilayer structures via the analysis of cw photoluminescence ͑PL͒ data under different temperatures. It is found that the temperature dependence of the exciton energy and linewidth is significantly different from that obtained in InAs monolayers and submonolayers. The unusual temperature behavior in InAs multilayers is associated with the relaxation effect of carriers, resulting from the spread and penetration of the wave functions of carriers in coupled InAs QDs. In the study of the thermal activation process, we found that potential barriers for InAs excitons to escape thermally from the localized states are different for different structures. For high-quality InAs multilayers, the barrier could be the wetting layer, while in the case of monolayers or submonolayers the carriers are required to ove...