Lana Sheridan

Efficient excitation of a two-level atom by a single photon in a propagating mode

State mapping between atoms and photons, and photon-photon interactions play an important role in scalable quantum information processing. We consider the interaction of a two-level atom with a quantized propagating pulse in free space and study the probability P{sub e}(t) of finding the atom in the excited state at any time t. This probability is expected to depend on (i) the quantum state of the pulse field and (ii) the overlap between the pulse and the dipole pattern of the atomic spontaneous emission. We show that the second effect is captured by a single parameter {Lambda}(set-membership sign)[0,8{pi}/3], obtained by weighting the dipole pattern with the numerical aperture. Then, P{sub e}(t) can be obtained by solving time-dependent Heisenberg-Langevin equations. We provide detailed solutions for both single-photon Fock state and coherent states and for various temporal shapes of the pulses.

More randomness from the same data

Correlations that cannot be reproduced with local variables certify the generation of private randomness. Usually, the violation of a Bell inequality is used to quantify the amount of randomness produced. Here, we show how private randomness generated during a Bell test can be directly quantified from the observed correlations, without the need to process these data into an inequality. The frequency with which the different measurement settings are used during the Bell test can also be taken into account. This improved analysis turns out to be very relevant for Bell tests performed with a finite collection efficiency. In particular, applying our technique to the data of a recent experiment (Christensen et al 2013 Phys. Rev. Lett. 111 130406), we show that about twice as much randomness as previously reported can be potentially extracted from this setup.

Finite-key security against coherent attacks in quantum key distribution

Christandl et al (2009 Phys. Rev. Lett. 102 020504) provide, in particular, the possibility of studying unconditional security in the finite-key regime for all discrete-variable protocols. We spell out this bound from their general formalism. Then, we apply it in the analysis of a recently proposed protocol (Laing et al 2010 Phys. Rev. A 82 012304). This protocol is meaningful when the alignment of Alices and Bobs reference frames is not monitored and may vary with time. In this scenario, the notion of asymptotic key rate has hardly any operational meaning, because if one waits too long a time, the average correlations are smeared out and no security can be inferred. Therefore, finite-key analysis is necessary for finding the maximal achievable secret key rate and the corresponding optimal number of signals.

Bell tests with min-entropy sources

Device independent protocols rely on the violation of Bell inequalities to certify properties of the resources available. The violation of the inequalities is meaningless without a few well-known assumptions. One of these is measurement independence, the property that the source of the states measured in an inequality is uncorrelated from the measurements selected. Since this assumption cannot be confirmed, we consider the consequences of relaxing it and find that the definition chosen is critically important to the observed behavior. Considering a definition that is a bound on the min-entropy of the measurement settings, we find lower bounds on the min-entropy of the source used to choose the inputs required to deduce any quantum or nonlocal behavior from a Bell inequality violation. These bounds are significantly more restrictive than the ones obtained by endowing the measurement-input source with the further structure of a Santha-Vazirani source. We also outline a procedure for finding tight bounds and study the set of probabilities that can result from relaxing measurement dependence.

TOMOGRAPHIC QUANTUM CRYPTOGRAPHY PROTOCOLS ARE REFERENCE FRAME INDEPENDENT

We consider the class of reference frame independent protocols in d dimensions for quantum key distribution, in which Alice and Bob have one natural basis that is aligned and the rest of their measurement bases are unaligned. We relate existing approaches to tomographically complete protocols. We comment on two different approaches to finite key bounds in this setting, one direct and one using the entropic uncertainty relation and suggest that the existing finite key bounds can still be improved.

Quantum Bell Inequalities from Macroscopic Locality

We propose a method to generate analytical quantum Bell inequalities based on the principle of macroscopic locality. By imposing locality over binary processings of virtual macroscopic intensities, we establish a correspondence between Bell inequalities and quantum Bell inequalities in bipartite scenarios with dichotomic observables. We discuss how to improve the latter approximation and how to extend our ideas to scenarios with more than two outcomes per setting.

Insider-Proof Encryption with Applications for Quantum Key Distribution

We introduce insider-proof private channels which are private channels that additionally allow for security even when the key is correlated with the message. This prevents an insider, who has access to secret keys and the capability of choosing messages to be sent on the channel, from signalling to someone who can read the ciphertexts. We give a construction for approximately insider-proof private channels using 2-universal hash functions.