Modern communication in absolute secrecy requires creation of new intrinsically secure quantum communication channels. It is particularly necessary during the first connection between two parties establishing then in assumed unconditional security the secret cryptographic key which is supposed to be used afterwards during normal information exchanging. This new emerging field of quantum information technology is based on a new type of light sources, in which numbers of emitted photons can be carefully controlled. Especially advantageous are sources of single photons emitted at strictly predetermined moments, so called single-photon devices. Then any possible eavesdropper activity will be followed by some unavoidable disturbance which alerts both communication parties to an event. In the present paper, the Purcell effect associated with enhancement of spontaneous emission coupled to a resonator is explained, methods used to produce streams of antibunched photons are given, mechanisms applied to control carrier injection into quantum dots are shown and some possible designs of single-photon devices are presented and described. These devices are based on taking advantage of both the Purcell effect and the atom-like energy spectrum of quantum dots.
Modern communication in absolute secrecy requires creation of new intrinsically secure quantum communication channels. It is particularly necessary during the first connection between two parties establishing then in assumed unconditional security the secret cryptographic key which is supposed to be used afterwards during normal information exchanging. This new emerging field of quantum information technology is based on a new type of light sources, in which numbers of emitted photons can be carefully controlled. Especially advantageous are sources of single photons emitted at strictly predetermined moments, so called single-photon devices. Then any possible eavesdropper activity will be followed by some unavoidable disturbance which alerts both communication parties to an event. In the present paper, the Purcell effect associated with enhancement of spontaneous emission coupled to a resonator is explained, methods used to produce streams of antibunched photons are given, mechanisms applied to control carrier injection into quantum dots are shown and some possible designs of single-photon devices are presented and described. These devices are based on taking advantage of both the Purcell effect and the atom-like energy spectrum of quantum dots.
Abstract In quantum cryptography, fundamental laws of quantum physics are exploited to enhance the security of cryptographic tasks. Quantum key distribution (QKD) is by far the most studied protocol to date, enabling the establishment of a secret key between trusted parties. Many practical use-cases in communication networks, however, involve parties who do not trust each other. The most fundamental quantum cryptographic building block in such a distrustful setting is quantum coin flipping, which has been studied only in few experimental studies to date, all of which used probabilistic quantum light sources facing fundamental limitations. Here, we experimentally implement a quantum strong coin flipping protocol using single-photon states and demonstrate an advantage compared to both classical realizations and implementations using faint laser pulses. We achieve this by employing a state-of-the-art deterministic single-photon source based on the Purcell-enhanced emission of a semiconductor quantum dot in combination with fast polarization-state encoding with sufficiently low quantum bit error ratio. The reduced multi-photon emission of the single-photon source yields a smaller bias of the coin flipping protocol compared to an attenuated laser implementation, both in simulations and in the experiment. By demonstrating a single-photon quantum advantage in a cryptographic primitive beyond QKD, our work represents a major advance towards the implementation of complex cryptographic tasks in a future quantum internet.
Abstract Quantum cryptography harnesses quantum light, in particular single photons, to provide security guarantees that cannot be reached by classical means. For each cryptographic task, the security feature of interest is directly related to the photons’ non-classical properties. Quantum dot-based single-photon sources are remarkable candidates, as they can in principle emit deterministically, with high brightness and low multiphoton contribution. Here, we show that these sources provide additional security benefits, thanks to the tunability of coherence in the emitted photon-number states. We identify the optimal optical pumping scheme for the main quantum-cryptographic primitives, and benchmark their performance with respect to Poisson-distributed sources such as attenuated laser states and down-conversion sources. In particular, we elaborate on the advantage of using phonon-assisted and two-photon excitation rather than resonant excitation for quantum key distribution and other primitives. The presented results will guide future developments in solid-state and quantum information science for photon sources that are tailored to quantum communication tasks.
Spontaneous emission of a source, such as an atom, is often thought of as an inherent property of the source, but is a result of the interaction between the radiating source dipole and the vacuum electromagnetic field. Therefore, the radiation emitted from an atom can be altered by suitably modifying the surrounding vacuum field [1] with a cavity. This has been verified by experiments initiated in the 1980s using atoms which are passed through a cavity and show enhanced[2] or inhibited[3] spontaneous emission. This field, called cavity quantum electrodynamics (QED), is currently addressing important issues in semiconductor nanostructures, in particular, high-efficiency light-emitting diodes for communications and single photon sources for quantum cryptography. [4,5] For example, quantum cryptography has emerged as a significant field of study over the last fifteen years, because it offers the promise of private communication whose security is assured by the laws of physics. Most implementations of quantum cryptography so far have used a protocol introduced by Bennet and Brassard, which uses four different states of a quantum system (generally known as BB84) [6 ]. For example, the message can be encoded on the polarization state of single photons, with a random choice between two non-othogonal polarization bases when the photons are sent (by Alice) and recieved (by Bob). Since an eavesdropper (Eve) does not know what bases have been chosen, any measurement she makes will impose a back-action on the states of the transmitted photons, which can be easily detected. Using error correction and privacy amplification, Alice and Bob can distill the transmitted message into a secure key about which Eve knows arbitrarily little.
Quantum cryptography has been proven to be the only practical method for fast distribution of secret keys needed for encryption. It enables absolutely secure information transfer from one party to another and allows for the detection of an eavesdropper without sacrificing security. This project involved designing and building a quantum cryptography device. This device has applications in many fields, including internet security as well as military communications. The data are encoded on single photons, which are sent through free space over a distance of about 30 cm. The electronics, which control the device however, are equally suitable for operation with fibre optic cables. The device was designed to operate in dark conditions within a laboratory. Currently, no research group has demonstrated transmission speeds faster than 8 kb/s. The aim of this project was to achieve bit rates faster than 10 kb/s, while operating under strict economic constraints. Although quantum cryptography was not demonstrated in the laboratory due to problems with the detector, the emission end, which generates single photons of light with one of two different polarisations, was shown to work as it was designed. The current design has limited commercial future due to the dark conditions under which the communication must take place. The project however has demonstrated that quantum cryptography is getting closer to becoming a reality.
The concept of the photon , central to Einstein's explanation of the photoelectric effect, is exactly 100 years old. Yet, while photons have been detected individually for more than 50 years, devices producing individual photons on demand have only appeared in the last few years. New concepts for single-photon sources, or 'photon guns', have originated from recent progress in the optical detection, characterization and manipulation of single quantum objects. Single emitters usually deliver photons one at a time. This so-called antibunching of emitted photons can arise from various mechanisms, but ensures that the probability of obtaining two or more photons at the same time remains negligible. We briefly recall basic concepts in quantum optics and discuss potential applications of single-photon states to optical processing of quantum information: cryptography, computing and communication. A photon gun's properties are significantly improved by coupling it to a resonant cavity mode, either in the Purcell or strong-coupling regimes. We briefly recall early production of single photons with atomic beams, and the operation principles of macroscopic parametric sources, which are used in an overwhelming majority of quantum-optical experiments. We then review the photophysical and spectroscopic properties and compare the advantages and weaknesses of various single nanometre-scale objects used as single-photon sources: atoms or ions in the gas phase and, in condensed matter, organic molecules, defect centres, semiconductor nanocrystals and heterostructures. As new generations of sources are developed, coupling to cavities and nano-fabrication techniques lead to improved characteristics, delivery rates and spectral ranges. Judging from the brisk pace of recent progress, we expect single photons to soon proceed from demonstrations to applications and to bring with them the first practical uses of quantum information.
Quantum cryptography is a method to exchange secret messages with unconditional security over a potentially hostile environment using single photons. Previous implementations of quantum cryptography have relied on highly attenuated laser light to approximate single photo states. Such sources are vulnerable to eavesdropping attacks based on photon splitting. Here we present an experimental demonstration of quantum cryptography using a single photon source based on Indium Arsenide quantum dots. We achieve a communication rate of 25kbits/s. This source allows secure communication over a quantum channel with up to 28dB of channel loss, as opposed to only 23dB for an attenuated laser.