
Wang, T., Ghobadi, R., Raeisi, S., & Simon, C. (2013). Precision requirements for observing macroscopic quantum effects. Phys. Rev. A, 88(6), 5 pp.
Abstract: It has recently been conjectured that detecting quantum effects such as superposition or entanglement for macroscopic systems always requires high measurement precision. Analyzing an apparent counterexample involving macroscopic coherent states and Kerr or higherorder nonlinearities, we find that while measurements with coarse outcomes can be sufficient, the phase control precision of the necessary nonlinear operations has to increase with the size of the system. This suggests a refined conjecture that either the outcome precision or the control precision of the measurements has to increase with system size.


Liu, Q., LamasLinares, A., Kurtsiefer, C., Skaar, J., Makarov, V., & Gerhardt, I. (2014). A universal setup for active control of a singlephoton detector. Rev. Sci. Instrum., 85(1), 9 pp.
Abstract: The influence of bright light on a singlephoton detector has been described in a number of recent publications. The impact on quantum key distribution (QKD) is important, and several hacking experiments have been tailored to fully control singlephoton detectors. Special attention has been given to avoid introducing further errors into a QKD system. We describe the design and technical details of an apparatus which allows to attack a quantumcryptographic connection. This device is capable of controlling freespace and fiberbased systems and of minimizing unwanted clicks in the system. With different control diagrams, we are able to achieve a different level of control. The control was initially targeted to the systems using BB84 protocol, with polarization encoding and basis switching using beamsplitters, but could be extended to other types of systems. We further outline how to characterize the quality of active control of singlephoton detectors. (C) 2014 AIP Publishing LLC.


Fisher, K. A. G., Broadbent, A., Shalm, L. K., Yan, Z., Lavoie, J., Prevedel, R., et al. (2014). Quantum computing on encrypted data. Nat. Commun., 5, 7 pp.
Abstract: The ability to perform computations on encrypted data is a powerful tool for protecting privacy. Recently, protocols to achieve this on classical computing systems have been found. Here, we present an efficient solution to the quantum analogue of this problem that enables arbitrary quantum computations to be carried out on encrypted quantum data. We prove that an untrusted server can implement a universal set of quantum gates on encrypted quantum bits (qubits) without learning any information about the inputs, while the client, knowing the decryption key, can easily decrypt the results of the computation. We experimentally demonstrate, using single photons and linear optics, the encryption and decryption scheme on a set of gates sufficient for arbitrary quantum computations. As our protocol requires few extra resources compared with other schemes it can be easily incorporated into the design of future quantum servers. These results will play a key role in enabling the development of secure distributed quantum systems.


Berry, D. W., Cleve, R., & Gharibian, S. (2014). GATEEFFICIENT DISCRETE SIMULATIONS OF CONTINUOUSTIME QUANTUM QUERY ALGORITHMS. Quantum Inform. Comput., 14(12), 1–30.
Abstract: We show how to efficiently simulate continuoustime quantum query algorithms that run in time T in a manner that preserves the query complexity (within a polylogarithmic factor) while also incurring a small overhead cost in the total number of gates between queries. By small overhead, we mean T within a factor that is polylogarithmic in terms of T and a cost measure that reflects the cost of computing the driving Hamiltonian. This permits any continuoustime quantum algorithm based on an efficiently computable driving Hamiltonian to be converted into a gateefficient algorithm with similar running time.
Keywords: Quantum computation; quantum query complexity


JochymO'Connor, T., & Laflamme, R. (2014). Using Concatenated Quantum Codes for Universal FaultTolerant Quantum Gates. Phys. Rev. Lett., 112(1), 5 pp.
Abstract: We propose a method for universal faulttolerant quantum computation using concatenated quantum error correcting codes. The concatenation scheme exploits the transversal properties of two different codes, combining them to provide a means to protect against lowweight arbitrary errors. We give the required properties of the error correcting codes to ensure universal fault tolerance and discuss a particular example using the 7qubit Steane and 15qubit ReedMuller codes. Namely, other than computational basis state preparation as required by the DiVincenzo criteria, our scheme requires no special ancillary state preparation to achieve universality, as opposed to schemes such as magic state distillation. We believe that optimizing the codes used in such a scheme could provide a useful alternative to state distillation schemes that exhibit high overhead costs.


Ahmadzadegan, A., MartinMartinez, E., & Mann, R. B. (2014). Cavities in curved spacetimes: The response of particle detectors. Phys. Rev. D, 89(2), 8 pp.
Abstract: We introduce a method to compute a particle detector transition probability in spacetime regions of general curved spacetimes provided that the curvature is not above a maximum threshold. In particular we use this method to compare the response of two detectors, one in a spherically symmetric gravitational field and the other one in Rindler spacetime to compare the Unruh and Hawking effects: we study the vacuum response of a detector freely falling through a stationary cavity in a Schwarzschild background as compared to the response of an equivalently accelerated detector traveling through an inertial cavity in the absence of curvature. We find that as we set the cavity at increasingly further radii from the black hole, the thermal radiation measured by the detector approaches the quantity recorded by the detector in Rindler background showing in which way and at what scales the equivalence principle is recovered in the HawkingUnruh effect. i.e. when the Hawking effect in a Schwarzschild background becomes equivalent to the Unruh effect in Rindler spacetime.


Coles, P. J., & Piani, M. (2014). Complementary sequential measurements generate entanglement. Phys. Rev. A, 89(1), 5 pp.
Abstract: We present a paradigm for capturing the complementarity of two observables. It is based on the entanglement created by the interaction between the system observed and the two measurement devices used to measure the observables sequentially. Our main result is a lower bound on this entanglement and resembles wellknown entropic uncertainty relations. Besides its fundamental interest, this result directly bounds the effectiveness of sequential bipartite operationscorresponding to the measurement interactionsfor entanglement generation. We further discuss the intimate connection of our result with two primitives of information processing, namely, decoupling and coherent teleportation.


Gittsovich, O., Beaudry, N. J., Narasimhachar, V., Alvarez, R. R., Moroder, T., & Lutkenhaus, N. (2014). Squashing model for detectors and applications to quantumkeydistribution protocols. Phys. Rev. A, 89(1), 25 pp.
Abstract: We develop a framework that allows a description of measurements in Hilbert spaces that is smaller than their natural representation. This description, which we call a “squashing model,” consists of a squashing map that maps the input states of the measurement from the original Hilbert space to the smaller one, followed by a targeted prescribed measurement on the smaller Hilbert space. This framework has applications in quantum key distribution, but also in other cryptographic tasks, as it greatly simplifies the theoretical analysis under adversarial conditions.


Deng, C. Q., Otto, M., & Lupascu, A. (2014). Characterization of lowtemperature microwave loss of thin aluminum oxide formed by plasma oxidation. Appl. Phys. Lett., 104(4), 3 pp.
Abstract: We report on the characterization of microwave loss of thin aluminum oxide films at low temperatures using superconducting lumped resonators. The oxide films are fabricated using plasma oxidation of aluminum and have a thickness of 5 nm. We measure the dielectric loss versus microwave power for resonators with frequencies in the GHz range at temperatures from 54 to 303 mK. The power and temperature dependence of the loss are consistent with the tunneling twolevel system theory. These results are relevant to understanding decoherence in superconducting quantum devices. The obtained oxide films are thin and robust, making them suitable for capacitors in compact microwave resonators. (C) 2014 AIP Publishing LLC.


Papic, Z., & Abanin, D. A. (2014). Topological Phases in the Zeroth Landau Level of Bilayer Graphene. Phys. Rev. Lett., 112(4), 5 pp.
Abstract: We analyze the phase diagram of the zeroth Landau level of bilayer graphene, taking into account the realistic effects of screening of the Coulomb interaction and strong mixing between two degenerate sublevels. We identify robust quantum Hall states at filling factors nu = 1, 4/3, 5/3, 8/5, 1/2 and discuss the nature of their ground states, collective excitations, and relation to the more familiar states in GaAs using a tractable model. In particular, we present evidence that the nu = 1/2 state is nonAbelian and described by either the MooreRead wave function or its particlehole conjugate, while ruling out other candidates such as the 331 state.


Wood, C. J., Borneman, T. W., & Cory, D. G. (2014). Cavity Cooling of an Ensemble Spin System. Phys. Rev. Lett., 112(5), 5 pp.
Abstract: We describe how sideband cooling techniques may be applied to large spin ensembles in magnetic resonance. Using the TavisCummings model in the presence of a Rabi drive, we solve a Markovian master equation describing the joint spincavity dynamics to derive cooling rates as a function of ensemble size. Our calculations indicate that the coupled angular momentum subspaces of a spin ensemble containing roughly 1011 electron spins may be polarized in a time many orders of magnitude shorter than the typical thermal relaxation time. The described techniques should permit efficient removal of entropy for spinbased quantum information processors and fast polarization of spin samples. The proposed application of a standard technique in quantum optics to magnetic resonance also serves to reinforce the connection between the two fields, which has recently begun to be explored in further detail due to the development of hybrid designs for manufacturing noiseresilient quantum devices.


Salim, A. J., Eftekharian, A., & Majedi, A. H. (2014). High quantum efficiency and low dark count rate in multilayer superconducting nanowire singlephoton detectors. J. Appl. Phys., 115(5), 4 pp.
Abstract: In this paper, we theoretically show that a multilayer superconducting nanowire singlephoton detector (SNSPD) is capable of approaching characteristics of an ideal SNSPD in terms of the quantum efficiency, dark count, and bandwidth. A multilayer structure improves the performance in two ways. First, the potential barrier for thermally activated vortex crossing, which is the major source of dark counts and the reduction of the critical current in SNSPDs is elevated. In a multilayer SNSPD, a vortex is made of 2Dpancake vortices that form a stack. It will be shown that the stack of pancake vortices effectively experiences a larger potential barrier compared to a vortex in a singlelayer SNSPD. This leads to an increase in the experimental critical current as well as significant decrease in the dark count rate. In consequence, an increase in the quantum efficiency for photons of the same energy or an increase in the sensitivity to photons of lower energy is achieved. Second, a multilayer structure improves the efficiency of singlephoton absorption by increasing the effective optical thickness without compromising the singlephoton sensitivity. (c) 2014 AIP Publishing LLC.


Puzzuoli, D., Granade, C., Haas, H., Criger, B., Magesan, E., & Cory, D. G. (2014). Tractable simulation of error correction with honest approximations to realistic fault models. Phys. Rev. A, 89(2), 18 pp.
Abstract: In previous work, we proposed a method for leveraging efficient classical simulation algorithms to aid in the analysis of largescale faulttolerant circuits implemented on hypothetical quantum information processors. Here, we extend those results by numerically studying the efficacy of this proposal as a tool for understanding the performance of an errorcorrection gadget implemented with fault models derived from physical simulations. Our approach is to approximate the arbitrary error maps that arise from realistic physical models with errors that are amenable to a particular classical simulation algorithm in an “honest” way; that is, such that we do not underestimate the faults introduced by our physical models. In all cases, our approximations provide an “honest representation” of the performance of the circuit composed of the original errors. This numerical evidence supports the use of our method as a way to understand the feasibility of an implementation of quantum information processing given a characterization of the underlying physical processes in experimentally accessible examples.


Garay, L. J., MartinBenito, M., & MartinMartinez, E. (2014). Echo of the quantum bounce. Phys. Rev. D, 89(4), 6 pp.
Abstract: We identify a signature of quantum gravitational effects that survives from the early Universe to the current era: Fluctuations of quantum fields as seen by comoving observers are significantly influenced by the history of the early Universe. In particular, we show how the existence (or not) of a quantum bounce leaves a trace in the background quantum noise that is not damped and would be nonnegligible even nowadays. Furthermore, we estimate an upper bound for the typical energy and length scales where quantum effects are relevant. We discuss how this signature might be observed and therefore used to build falsifiability tests of quantum gravity theories.


Bugge, A. N., Sauge, S., Ghazali, A. M. M., Skaar, J., Lydersen, L., & Makarov, V. (2014). Laser Damage Helps the Eavesdropper in Quantum Cryptography. Phys. Rev. Lett., 112(7), 5 pp.
Abstract: We propose a class of attacks on quantum key distribution (QKD) systems where an eavesdropper actively engineers new loopholes by using damaging laser illumination to permanently change properties of system components. This can turn a perfect QKD system into a completely insecure system. A proofofprinciple experiment performed on an avalanche photodiodebased detector shows that laser damage can be used to create loopholes. After similar to 1 W illumination, the detectors' dark count rate reduces 25 times, permanently improving singlephoton counting performance. After similar to 1.5 W, the detectors switch permanently into the linear photodetection mode and become completely insecure for QKD applications.


