Eleni Diamanti

Highly efficient single-photon detection at communication wavelengths by use of upconversion in reverse-proton-exchanged periodically poled LiNbO 3 waveguides

Conventional single-photon detectors at communication wavelengths suffer from low quantum efficiencies and large dark counts. We present a single-photon detection system, operating at communication wavelengths, based on guided-wave frequency upconversion in a nonlinear crystal with an overall system detection efficiency (upconversion + detection) exceeding 46% at 1.56 microm. This system consists of a fiber-pigtailed reverse-proton-exchanged periodically poled LiNbO3 waveguide device in conjunction with a silicon-based single-photon counting module.

Quantum key distribution over 25 km with an all-fiber continuous-variable system

We report on the implementation of a reverse-reconciliated coherent-state continuous-variable quantum key distribution system, with which we generated secret keys at a rate of more than 2 kb/s over 25 km of optical fiber. Time multiplexing is used to transmit both the signal and phase reference in the same optical fiber. Our system includes all experimental aspects required for a field implementation of a quantum key distribution setup. Real-time reverse reconciliation is achieved by using fast and efficient low-density parity check error correcting codes.

Differential phase shift quantum key distribution experiment over 105 km fibre

We report a quantum key distribution experiment based on the differential phase shift keying (DPSK) protocol with a Poissonian photon source, in which secure keys were generated over >100 km fibre for the first time. We analysed the security of the DPSK protocol and showed that it is robust against strong attacks by Eve, including a photon number splitting attack. To implement this protocol, we developed a new detector for the 1.5 μm band based on frequency up-conversion in a periodically poled lithium niobate waveguide followed by an Si avalanche photodiode. The use of detectors increased the sifted key generation rate up to >1 Mbit s−1 over 30 km fibre, which is two orders of magnitude larger than the previous record.

Quantum teleportation with a quantum dot single photon source

We report the experimental demonstration of a quantum teleportation protocol with a semiconductor single photon source. Two qubits, a target and an ancilla, each defined by a single photon occupying two optical modes (dual-rail qubit), were generated independently by the single photon source. Upon measurement of two modes from different qubits and postselection, the state of the two remaining modes was found to reproduce the state of the target qubit. In particular, the coherence between the target qubit modes was transferred to the output modes to a large extent. The observed fidelity is 80%, in agreement with the residual distinguishability between consecutive photons from the source. An improved version of this teleportation scheme using more ancillas is the building block of the recent Knill, Laflamme, and Milburn proposal for efficient linear optics quantum computation.

High-efficiency photon-number detection for quantum information processing

The visible light photon counter (VLPC) features high quantum efficiency (QE) and low pulse height dispersion. These properties make it ideal for efficient photon-number state detection. The ability to perform efficient photon-number state detection is important in many quantum information processing applications, including recent proposals for performing quantum computation with linear optical elements. In this paper, we investigate the unique capabilities of the VLPC. The efficiency of the detector and cryogenic system is measured at 543 nm wavelengths to be 85%. A picosecond pulsed laser is then used to excite the detector with pulses having average photon numbers ranging from 3-5. The output of the VLPC is used to discriminate photon numbers in a pulse. The error probability for number state discrimination is an increasing function of the number of photons, due to buildup of multiplication noise. This puts an ultimate limit on the ability of the VLPC to do number state detection. For many applications, it is sufficient to discriminate between 1 and more than one detected photon. The VLPC can do this with 99% accuracy.

Preventing Calibration Attacks on the Local Oscillator in Continuous-Variable Quantum Key Distribution

Establishing an information-theoretic secret key between two parties using a quantum key distribution (QKD) system is only possible when an accurate characterization of the quantum channel and proper device calibration routines are combined. Indeed, security loopholes due to inappropriate calibration routines have been shown for discrete-variable QKD. Here, we propose and provide experimental evidence of an attack targeting the local oscillator calibration routine of a continuous-variable QKD system. The attack consists in manipulating the classical local oscillator pulses during the QKD run in order to modify the clock pulses used at the detection stage. This allows the eavesdropper to bias the shot noise estimation usually performed using a calibrated relationship. This loophole can be used to perform successfully an intercept-resend attack. We characterize the loophole and suggest possible countermeasures.

100 km differential phase shift quantum key distribution experiment with low jitter up-conversion detectors

We present a quantum key distribution experiment in which keys that were secure against all individual eavesdropping attacks allowed by quantum mechanics were distributed over 100 km of optical fiber. We implemented the differential phase shift quantum key distribution protocol and used low timing jitter 1.55 microm single-photon detectors based on frequency up-conversion in periodically poled lithium niobate waveguides and silicon avalanche photodiodes. Based on the security analysis of the protocol against general individual attacks, we generated secure keys at a practical rate of 166 bit/s over 100 km of fiber. The use of the low jitter detectors also increased the sifted key generation rate to 2 Mbit/s over 10 km of fiber.

Distributing Secret Keys with Quantum Continuous Variables: Principle, Security and Implementations

The ability to distribute secret keys between two parties with information-theoretic security, that is regardless of the capacities of a malevolent eavesdropper, is one of the most celebrated results in the field of quantum information processing and communication. Indeed, quantum key distribution illustrates the power of encoding information on the quantum properties of light and has far-reaching implications in high-security applications. Today, quantum key distribution systems operate in real-world conditions and are commercially available. As with most quantum information protocols, quantum key distribution was first designed for qubits, the individual quanta of information. However, the use of quantum continuous variables for this task presents important advantages with respect to qubit-based protocols, in particular from a practical point of view, since it allows for simple implementations that require only standard telecommunication technology. In this review article, we describe the principle of continuous-variable quantum key distribution, focusing in particular on protocols based on coherent states. We discuss the security of these protocols and report on the state-of-the-art in experimental implementations, including the issue of side-channel attacks. We conclude with promising perspectives in this research field.

Field test of classical symmetric encryption with continuous variables quantum key distribution

We report on the design and performance of a point-to-point classical symmetric encryption link with fast key renewal provided by a Continuous Variable Quantum Key Distribution (CVQKD) system. Our system was operational and able to encrypt point-to-point communications during more than six months, from the end of July 2010 until the beginning of February 2011. This field test was the first demonstration of the reliability of a CVQKD system over a long period of time in a server room environment. This strengthens the potential of CVQKD for information technology security infrastructure deployments.

Analysis of Imperfections in Practical Continuous-Variable Quantum Key Distribution

As quantum key distribution becomes a mature technology, it appears clearly that some assumptions made in the security proofs cannot be justified in practical implementations. This might open the door to possible side-channel attacks. We examine several discrepancies between theoretical models and experimental setups in the case of continuous-variable quantum key distribution. We study in particular the impact of an imperfect modulation on the security of Gaussian protocols and show that approximating the theoretical Gaussian modulation with a discrete one is sufficient in practice. We also address the issue of properly calibrating the detection setup, and in particular the value of the shot noise. Finally, we consider the influence of phase noise in the preparation stage of the protocol and argue that taking this noise into account can improve the secret key rate because this source of noise is not controlled by the eavesdropper.