The peculiar properties of quantum mechanics allow two remote parties to communicate a private, secret key, which is protected from eavesdropping by the laws of physics(1-4). So-called quantum key distribution (QKD) implementations always rely on detectors to measure the relevant quantum property of single photons(5). Here we demonstrate experimentally that the detectors in two commercially available QKD systems can be fully remote-controlled using specially tailored bright illumination. This makes it possible to tracelessly acquire the full secret key; we propose an eavesdropping apparatus built from off-the-shelf components. The loophole is likely to be present in most QKD systems using avalanche photodiodes to detect single photons. We believe that our findings are crucial for strengthening the security of practical QKD, by identifying and patching technological deficiencies

Coherent coupling between single quantum objects is at the heart of modern quantum physics. When coupling is strong enough to prevail over decoherence, it can be used for the engineering of correlated quantum states. Especially for solid-

Random numbers are a valuable component in diverse applications that range from simulations(1) over gambling to cryptography(2,3). The quest for true randomness in these applications has engendered a large variety of different proposals for producing random numbers based on the foundational unpredictability of quantum mechanics(4-11). However, most approaches do not consider that a potential adversary could have knowledge about the generated numbers, so the numbers are not verifiably random and unique(12-15). Here we present a simple experimental setup based on homodyne measurements that uses the purity of a continuous-variable quantum vacuum state to generate unique random numbers. We use the intrinsic randomness in measuring the quadratures of a mode in the lowest energy vacuum state, which cannot be correlated to any other state. The simplicity of our source, combined with its verifiably unique randomness, are important attributes for achieving high-reliability, high-speed and low-cost quantum random number generators

The quantum state of a single photon stands amongst the most fundamental and intriguing manifestations of quantum physics [1]. At the same time single photons and pairs of single photons are important building blocks in the fields of linear optical based quantum computation [2] and quantum repeater infrastructure [3] . These fields possess enormous potential [4] and much scientific and technological progress has been made in developing individual components, like quantum memories and photon sources using various physical implementations [5][6][7][8][9][10][11]. However, further progress suffers from the lack of compatibility between these different components. Ultimately, one aims for a versatile source of single photons and photon pairs in order to overcome this hurdle of incompatibility. Such a photon source should allow for tuning of the spectral properties (wide wavelength range and narrow bandwidth) to address different implementations while retaining high efficiency. In addition, it should be able to bridge different wavelength regimes to make implementations compatible. Here we introduce and experimentally demonstrate such a versatile single photon and photon pair source based on the physics of whispering gallery resonators. A diskshaped, monolithic and intrinsically stable resonator is made of lithium niobate and supports a cavity-assisted triply-resonant spontaneous parametric down-conversion process. Measurements show that photon pairs are efficiently generated in two highly tunable resonator modes. We verify wavelength tuning over 100 nm between both modes with a controllable bandwidth between 7.2 and 13 MHz. Heralding of single photons yields anti-bunching with g (2) (0) < 0.2. This compact source provides unprecedented possibilities to couple to different physical quantum systems and makes it ideal for the implementation of quantum repeaters and optical quantum information processing.It is known that in a nonlinear medium a photon can spontaneously decay into a pair of photons, usually called signal and idler. This process, referred to as spontaneous parametric down-conversion (SPDC), preserves the energy and momentum of the parent photon. The resulting pair of photons posses the ability to bridge different wavelength ranges. At the same time detecting one photon of this pair unambiguously heralds the presence of a single photon. In principle, the process of SPDC has a very high bandwidth. By assisting it with a high quality factor (high-Q) resonator, the desired narrow bandwidth of a few MHz for the individual photons can be ensured [12]. A thorough description of this resonator-assisted SPDC leads to a two-mode EPR entangled state [13] and has successfully been used to generate heralded single photons [14]. Recently, resonator-assisted SPDC has led to a substantial progress towards an efficient narrow-band source [15]. However, the wavelength and bandwidth tunability remained a major challenge.We overcome this problem by using an optical whispering gallery mode resonator (WGMR). These resonators a...

Single defect centers in diamond have been generated via nitrogen implantation. The defects have been investigated by single defect center fluorescence microscopy. Optical and EPR spectra unambiguously show that the produced defect is the nitrogen-vacancy colour center.An analysis of the nitrogen flux together with a determination of the number of nitrogenvacancy centers yields that on average two 2 MeV nitrogen atoms need to be implanted per defect center. However, a much lower flux has been chosen such that at some implantation spots no NV center has been generated. We estimate that on average two nitrogen atoms are implanted per spot. To determine the number of NV centers per spot, the photon statistics of the spot emission is evaluated. For this, the normalized second-order autocorrelation function

Characterizing the physical channel and calibrating the cryptosystem hardware are prerequisites for establishing a quantum channel for quantum key distribution (QKD). Moreover, an inappropriately implemented calibration routine can open a fatal security loophole. We propose and experimentally demonstrate a method to induce a large temporal detector efficiency mismatch in a commercial QKD system by deceiving a channel length calibration routine. We then devise an optimal and realistic strategy using faked states to break the security of the cryptosystem. A fix for this loophole is also suggested.

Entanglement of Gaussian states and the applicability to quantum key distribution over fading channels T h e o p e n -a c c e s s j o u r n a l f o r p h y s i c s New Journal of PhysicsEntanglement of Gaussian states and the applicability to quantum key distribution over fading channels Abstract. Entanglement properties of Gaussian states of light as well as the security of continuous variable quantum key distribution with Gaussian states in free-space fading channels are studied. These qualities are shown to be sensitive to the statistical properties of the transmittance distribution in the cases when entanglement is strong or when channel excess noise is present. Fading, i.e. transmission fluctuations, caused by beam wandering due to atmospheric turbulence, is a frequent challenge in free-space communication.We introduce a method of fading discrimination and subsequent post-selection of the corresponding sub-states and show that it can improve the entanglement 6 Authors to whom any correspondence should be addressed.Content from this work may be used under the terms of the Creative Commons Attribution-NonCommercialShareAlike 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. New Journal of Physics 14 (2012) 0930481367-2630/12/093048+20$33.00 © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft 2 resource and restore the security of the key distribution over a realistic fading link. Furthermore, the optimal post-selection strategy in combination with an optimized entangled resource is shown to drastically increase the protocol's robustness to excess noise, which is confirmed for experimentally measured fading channel characteristics. The stability of the result against finite data ensemble size and imperfect channel estimation is also addressed. Contents

We present a method to control the detection events in quantum key distribution systems that use gated single-photon detectors. We employ bright pulses as faked states, timed to arrive at the avalanche photodiodes outside the activation time. The attack can remain unnoticed, since the faked states do not increase the error rate per se. This allows for an intercept-resend attack, where an eavesdropper transfers her detection events to the legitimate receiver without causing any errors. As a side effect, afterpulses, originating from accumulated charge carriers in the detectors, increase the error rate. We have experimentally tested detectors of the system id3110 (Clavis2) from ID Quantique. We identify the parameter regime in which the attack is feasible despite the side effect. Furthermore, we outline how simple modifications in the implementation can make the device immune to this attack.