Expanded quantum cryptographic entangling probe

A generalized quantum circuit and design are given for an optimal entangling probe to be used in attacking the BB84 protocol of quantum key distribution and yielding maximum information to the probe. Probe photon polarization states become optimally entangled with the BB84 signal states on their way between the legitimate transmitter and receiver. The present design generalizes an earlier one by Brandt [J. Mod. Optics 52 2177 (2005)] to include a complete range of error rates that can be induced by the probe.     

Simulations of continuum coherent states and its use in quantum cryptographic systems

Abstract

The continuum states formalism is suitable for field quantization in optical fibre; however, they are harder to use than discrete states. On the other hand, a Hermitian phase operator can be defined only in a finite dimensional space. We approximated a coherent continuum state by a finite tensor product of coherent states, each one defined in a finite dimensional space. Using this, in the correct limit, we were able to obtain some statistical properties of the photon number and phase of the continuum coherent states from the probability density functions of the individual, finite dimensional, coherent states. Then, we performed a simulation of the BB84 protocol, using the continuum coherent states, in a fibre interferometer commonly used in quantum cryptography. We observed the fluctuations of the mean photon number in the pulses that arrive at Bob, which occurs in the practical system, introduced by the statistical property of the simulation.     

Defense frontier analysis of quantum cryptographic systems

When a quantum cryptographic system operates in the presence of background noise, security of the key can be recovered by a procedure called key distillation. A key-distillation scheme effective against so-called individual (bitwise-independent) eavesdropping attacks involves sacrifice of some of the data through privacy amplification. We derive the amount of data sacrifice sufficient to defend against individual eavesdropping attacks in both BB84 and B92 protocols and show in what sense the communication becomes secure as a result. We also compare the secrecy capacity of various quantum cryptosystems, taking into account data sacrifice during key distillation, and conclude that the BB84 protocol may offer better performance characteristics than the B92.     

Phase auto-compensating high-dimensional quantum cryptography in elliptical-core few-mode fibres

High-dimensional quantum cryptography through optical fibres with several spatial modes requires an efficient quantum key distribution (QKD). However, optical modes acquire different phases and lags due to modal dispersion and random fluctuations, and a modal crosstalk appears under propagation. At present, special optical fibres for spatial multiplexing are being proposed in order to reduce notably the modal crosstalk, however, arbitrary relative phases and lags between modes are always present, which prevents getting an efficient phase encoding QKD. In this work, we take advantage of elliptical-core few-mode optical fibres presenting a very low modal crosstalk and propose an exact phase auto-compensating method by making photons travel several times the path between Alice and Bob (rounds) and by using appropriate modal inversions in each round trip. In order to make clear the proposed phase auto-compensating method, we study in detail a four-dimensional BB84 QKD case with single photon states excited in both polarization and spatial LP modes.     

An Efficient Quantum Key Distribution Protocol with Dense Coding on Single Photons

Combined with the dense coding mechanism and the bias-BB84 protocol, an efficient quantum key distribution protocol with dense coding on single photons (QDKD-SP) is proposed. Compared with the BB84 or bias-BB84 protocols based on single photons, our QDKD-SP protocol has a higher capacity without increasing the difficulty of its experiment implementation as each correlated photon can carry two bits of useful information. Compared with the quantum dense key distribution (QDKD) protocol based on entangled states, our protocol is more feasible as the preparation and the measurement of a single-photon quantum state is not difficult with current technology. In addition, our QDKD-SP protocol is theoretically proved to be secure against the intercept-resend attack.     

Tailored bright illumination attack on distributed-phase-reference protocols

Detector control attacks on quantum key distribution systems exploit the linear mode of avalanche photodiode in single photon detectors. So far, the protocols under consideration have been the BB84 protocol and its derivatives. Here we present how bright tailored illumination exploiting the linear mode of detectors can be used to eavesdrop on distributed-phase-reference protocols, such as differential-phase-shift and coherent-one-way.     

Probing Defect‐Induced Midgap States in MoS2 Through Graphene–MoS2 Heterostructures

Crystalline defects in MoS₂ may induce midgap states, resulting in low carrier mobility. These midgap states are usually difficult to probe by conventional transport measurement. The quantum capacitance of single‐layer graphene is sensitive to defect‐induced states near the Dirac point, at which the density of states is extremely low. It is reported that the hexagonal‐boron nitride/graphene/MoS₂ sandwich structure facilitates the exploration of the properties of those midgap states in MoS₂. Comparative results of the quantum capacitance of pristine graphene indicate the presence of several midgap states with distinct features. Some of these states donate electrons while some states lead to localization of electrons. It is believed that these midgap states originate from intrinsic point defects such as sulfur vacancies, which have a significant impact on the property of the MoS₂/graphene interface. They are responsible for the contact problems of metal/MoS­₂ interfaces.     

The effect of annealing and photoactivation on the optical transitions of band-band and surface trap states of colloidal quantum dots in PMMA

For optoelectronic applications, colloidal CdSe quantum dots (QDs) have been integrated into solid devices by using optically transparent polymer matrices that embedded the colloidal QDs. We systematically studied the effect of annealing and photoactivation on the band-band (BB) and surface trap state (STS) transitions of colloidal CdSe QDs embedded in polymethylmethacrylate (QDs-PMMA). The QDs-PMMA composites demonstrate enhancement of the STS emissions while their annealing leads to an intensity quench of both BB and STS emissions. The annealing process also causes the red shift of the BB emission. By contrast, photoactivation of QDs-PMMA composites results in the remarkable recovery of luminescence intensity accompanied by blue shift of the emissions. Furthermore, it is found in the photoactivation process that the STS emission can be saturated earlier than the BB emission, which renders it possible to tune the light color of the emissions. The combination of annealing and photoactivation could undoubtedly provide an effective way to precisely tune the colors of light-emitting devices that use colloidal CdSe QDs.     

Frequency shift attack on ‘plug-and-play’ quantum key distribution systems

Many imperfections in a practical quantum key distribution (QKD) system have been exploited by an eavesdropper (Eve) to attack the system. However, most of these attacks will introduce perturbations to the system while collecting information about the key. For example, the phase-remapping attack [Phys. Rev. A2007,75, 032314], in which Eve performs time shift on the signal pulse from the constant acting range of the phase modulation voltage to its rising edge to introduce an imperfection, results in an quantum bit error rate (QBER) of 14.6%, which is too high and will be discovered by careful users. In this paper, a frequency shift (FS) attack on ‘plug-and-play’ QKD systems with phase-coding BB84 protocol is proposed, in which Eve introduces an imperfection by the same method as she used in the phase-remapping attack. The most novel advantage of our FS attack is that Eve can get full information without introducing detectable QBER, which is more deceptive than the phase-remapping attack.     

Influence of electron irradiation on the electronic transport mechanisms during the conductive AFM imaging of InAs/GaAs quantum dots capped with a thin GaAs layer

We have used conductive atomic force microscopy (C-AFM) to study the electronic transport mechanisms through InAs quantum dots (QDs) grown by molecular beam epitaxy on an n-type GaAs(001) substrate and covered with a 5?nm thick GaAs cap layer. The study is performed with a conductive atomic force microscope working inside a scanning electron microscope. Electric images can be obtained only if the sample is preliminarily irradiated with an electron probe current sufficiently high to generate strong electron beam induced current. In these conditions holes are trapped in QDs and surface states, so allowing the release of the Fermi level pinning and thus conduction through the sample. The electronic transport mechanism depends on the type of AFM probe used; it is explained for a metal (Co/Cr) coated probe and p-doped diamond coated probe with the aid of energy band diagrams. The writing (charge trapping) and erasing (untrapping) phenomena is conditioned by the magnitude of the electron probe current. A strong memory effect is evidenced for the sample studied.     

Conduction intersubband transitions at normal incidence in Si(1-x)Ge(x) quantum well devices

We show theoretically that it is possible to design SiGe-based quantum well structures in which conduction intersubband transitions are induced by normal incidence infrared radiation. A sp(3)d(5)s(*) tight binding model has been adopted to evaluate the electronic states and optical transitions between lowest conduction confined states of a superlattice composed of one pure Ge quantum well separated by SiGe alloys, grown along the [001] direction. We find that significant optical coupling between confined states in the Ge wells is achieved at normal incidence radiation by the off-diagonal elements of the mass tensor. The minimum energy Ge conduction valleys are, in fact, tilted with respect to the [001] growth axis. For comparison we show that no such coupling can be realized for the conduction states confined in a similar structure composed by Si quantum wells because the ellipsoids of the lowest conduction valleys are oriented along the growth direction.     

Electron eigenstates in quantum dots revealed by temperature derivative capacitance spectroscopy

A novel method is presented for detecting confined energy states in quantum dots embedded in a junction space charge region, where the reverse bias is used to discharge the initially occupied energy levels. By determining the temperature derivative of junction capacitance as a function of bias voltage and temperature, spectra are obtained with peaks revealing the existence of electron states in InGaAs/GaAs quantum dots structure grown by molecular beam epitaxy. The method is compared with admittance based techniques and theory used earlier for interpretation of experimental data.     

Linear optical setups for active and passive quantum error correction in polarization encoded qubits

In this work, we present active and passive linear optical setups for error correction in quantum communication systems that employ polarization of single-photon and mesoscopic coherent states. The proposed systems are analytically analysed and their applications in quantum communication systems are described. In particular, we show a security analysis of a QKD system employing the active error correction system when an eavesdropper uses the Fuchs–Peres–Brandt attack.     

An entangling-probe attack on Shor’s algorithm for factorization

We investigate how to attack Shor’s quantum algorithm for factorization with an entangling probe. We show that an attacker can steal an exact solution of Shor’s algorithm outside an institute where the quantum computer is installed if he replaces its initialized quantum register with entangled qubits, namely the entangling probe. He can apply arbitrary local operations to his own probe. Moreover, we assume that there is an unauthorized person who helps the attacker to commit a crime inside the institute. He tells garbage data obtained from measurements of the quantum register to the attacker secretly behind a legitimate user’s back. If the attacker succeeds in cracking Shor’s algorithm, the legitimate user obtains a random answer and does not notice the attacker’s illegal acts. We discuss how to detect the attacker. Finally, we estimate a probability that the quantum algorithm inevitably makes an error, of which the attacker can take advantage.     

Efficient quantum secret sharing based on polarization and orbital angular momentum

An efficient quantum secret sharing scheme is proposed. In the proposed scheme, the polarization state and the orbital angular momentum state of the particle can be utilized simultaneously. One state is used to bring the secret information, and the other state is used to check the eavesdropping. So all the particles can be used to transmit the secret, and the utilization efficiency of particles can achieve 100%. Compared to the existing schemes based on BB84 protocol or decoy particles, our scheme can increase the utilization efficiency of particles effectively.     

Quantitative excited state spectroscopy of a single InGaAs quantum dot molecule through multi-million-atom electronic structure calculations

Atomistic electronic structure calculations are performed to study the coherent inter-dot couplings of the electronic states in a single InGaAs quantum dot molecule. The experimentally observed excitonic spectrum by Krenner et al (2005) Phys. Rev. Lett. 94 057402 is quantitatively reproduced, and the correct energy states are identified based on a previously validated atomistic tight binding model. The extended devices are represented explicitly in space with 15-million-atom structures. An excited state spectroscopy technique is applied where the externally applied electric field is swept to probe the ladder of the electronic energy levels (electron or hole) of one quantum dot through anti-crossings with the energy levels of the other quantum dot in a two-quantum-dot molecule. This technique can be used to estimate the spatial electron-hole spacing inside the quantum dot molecule as well as to reverse engineer quantum dot geometry parameters such as the quantum dot separation. Crystal-deformation-induced piezoelectric effects have been discussed in the literature as minor perturbations lifting degeneracies of the electron excited (P and D) states, thus affecting polarization alignment of wavefunction lobes for III-V heterostructures such as single InAs/GaAs quantum dots. In contrast, this work demonstrates the crucial importance of piezoelectricity to resolve the symmetries and energies of the excited states through matching the experimentally measured spectrum in an InGaAs quantum dot molecule under the influence of an electric field. Both linear and quadratic piezoelectric effects are studied for the first time for a quantum dot molecule and demonstrated to be indeed important. The net piezoelectric contribution is found to be critical in determining the correct energy spectrum, which is in contrast to recent studies reporting vanishing net piezoelectric contributions.     

Theoretical and Experimental Studies of Magneto-tunneling in Quantum Dots

We study the magneto-transport in vertical quantum dots boththeoretically and experimentally. We focus our attention onthe non-linear transport regime. We demonstrate that the peakamplitudes in the I–V characteristics show a dramaticallydifferent behaviour as a function of the magnetic field,depending on the value of the angular momentum of the dotstate through which tunnelling occurs. The investigationallows us to probe details of the quantum dot wavefunctionsand to distinguish tunnelling through localised states relatedto impurities or to quantum dots.     

Implementing a non-local xor function with quantum communication

The or-exclusive function (xor) is one of the most important logical functions, and can be used in several protocols and algorithms, as encryption and error correction. This paper discusses several ways to implement the xor function, in a secure way using quantum communication, between users of a network. It is shown how to implement it using bipartite, tripartite and four-partite maximally entangled states, as well using an optical interferometer and strongly attenuated coherent states. Some protocols for secure communication using the xor, as quantum key distribution, are also proposed.