Joint Seminar on Quantum Information and Technologies
2012/2013 | 2013/2014 | 2014/2015 | 2015/2016 | 2016/2017 | 2017/2018 | 2018/2019 | 2019/2020 | 2020/2021 | 2021/2022 | 2022/2023 | 2023/2024 | 2024/2025 | Seminar YouTube channel
until 2023/2024 Quantum Information Seminar
2019-09-13 (Friday)
room 1.02, Pasteura 5 at 11:15 
Przemysław Bienias (Joint Quantum Institute, NIST, University of Maryland)Self-bound clusters made out of light
Recently, the combination of slow light polaritons with the stronginteractions between Rydberg atoms has emerged as a promising systemfor inducing a strong interaction between photons. Potentialapplications range from the implementation of phase gate for photonsto single-photon sources, as well as the generation of stronglycorrelated states of photons. I present our theoretical andexperimental studies of strongly interacting photons. I show thatthese interacting photons can be used to prepare exotic andnon-classical states of light. In the regime of attractiveinteractions, we identify multiple two-polariton bound states,calculate their dispersion, and study the resulting scatteringresonances. For three-polaritons, we study the influence of thethree-body force on the bound states. Finally, we propose a method ofengineering novel molecular-like interactions between polaritonsleading to self-bound clusters made out of light.
2019-09-05 (Thursday)
room 1.03, Pasteura 5 at 11:15
Jadwiga Wilkens (Freie Universitat Berlin)Blind Tomography
For the last 20 years, the research on quantum information processing is experiencing a rapid growthand holds great promise for revolutionary new technology. In the development of these quantumtechnologies efficient and flexibel methods for extracting information about a quantum state frommeasurements are required. One important task is to fully determine a quantum state from the measured data with only mild structure assumptions on the state. This is the problem of quantum statetomography.Using a signal processing paradigm called compressed sensing, quantum tomography schemes for lowrank states were developed that are resource-optimal. But to date compressed sensing schemes forquantum state tomography lack robustness against imperfection of the measurement devices. For thisreason, experimental setups performing these schemes need to have measurements devices that arecalibrated to a high precision. In this work we develop the framework of blind calibration tomography which allows for incomplete knowledge of the measurement device during the tomography of aquantum state. It simultaneously determines both the device calibration and the quantum state withminimal resources and efficient classical post-processing.Building on recent techniques from the field of compressed sensing, we derive algorithmic strategiesfor blind calibration tomography and provide analytical performance guarantees. We further demonstrate their performance in numerical simulations.
2019-08-08 (Thursday)
room 1.03, Pasteura 5 at 11:15
Alex Retzker (Hebrew University of Jerusalem)Quantum spectroscopy of single spins assisted by a classical clock
Quantum spectroscopy with single two-level systems has considerably improved our ability to detect weak signals. Recently it was realized that for classical signals, precision and resolution of quantum spectroscopy is limited mainly by coherence of the signal and the stability of the clock used to measure time. The coherence time of the quantum probe, which can be significantly shorter, is not a major limiting factor in resolution measurements. In this talk I will address a similar question for spectroscopy of quantum signals, for example, a quantum sensor that is used to detect a single nuclear spin. I will present an analysis of a novel correlation spectroscopy technique with performance that is limited by the coherence time of the target spins and the stability of the clock.
2019-07-23 (Tuesday)
room 1.03, Pasteura 5 at 16:15
Vadim Makarov (Russian Quantum Center, Moscow)Can quantum physics break cryptography's curse? [ROOM CHANGED: CeNT building Aula 00.143]
The history of cryptography is a history of failures. Stronger ciphersreplaced broken ones, to be in turn broken again. Quantum cryptographyoffers a hope to end this replacement cycle, for its security premises on the laws of physics and not on limitations of human ingenuity andcomputing power. I will tell about ongoing deployment of quantumcryptography networks using fiber-optic and satellite links, and effortsto certify their implementation as secure. [Prof. Vadim Makarov heads the Quantum hacking lab http://www.vad1.com/lab/at the Russian Quantum Center, Moscow, where he tests commercial quantumcryptography systems for implementation loopholes.]
2019-07-18 (Thursday)
room 1.01, Pasteura 5 at 11:15
Marcio Mendes Taddei (Federal University of Rio de Janeiro)Error-run-time trade-off in the adiabatic approximation beyond scaling relations
The use of the adiabatic approximation in practical applications, as in adiabatic quantum computation, demands an assessment of the errors made in finite-time evolutions. Aiming at such scenarios, we derive bounds relating error and evolution time in the adiabatic approximation that go beyond typical scaling relations. Using the Adiabatic Perturbation Theory, we obtain leading-order expressions valid for long evolution time T, while explicitly determining the shortest time T and the largest error ε for which they are valid. Restricting our considerations to this validity regime, we can make clear and precise statements about the evolution time necessary to reach a given error and vice-versa. As an example of practical importance, we apply these results to the adiabatic version of Grover's quantum search algorithm and obtain highly accurate trade-off relations between run time and error for several evolution schedules. Their examination indicates that different strategies are required to optimize for either shorter time or minimal error. (arXiv:1907.03769).
2019-07-11 (Thursday)
room 1.01, Pasteura 5 at 11:15
Suchetana Goswami (S. N. Bose National Centre for Basic Sciences, Kolkata, India)Universal detection of entanglement in two-qubit states using only two copies
We revisit the problem of detection of entanglement of an unknown two-qubit state using minimal resources. Using weak values and just two copies of an arbitrary two-qubit state, we present a protocol where a postselection measurement in the computational basis provides enough information to identify if the state is entangled or not. Our protocol enables complete state identification with a single-setting postselection measurement on two copies of the state. It follows that by restricting to pure states, the global interaction required for determining the weak values can be realized by local operations. We further show that our protocol is robust against errorsarising from inappropriate global interactions applied during weak valuedetermination. Reference: Phys. Rev. A 99, 012327 (2019).
2019-06-13 (Thursday)
room 1.03, Pasteura 5 at 11:15
Alex Davis (Laboratoire Kastler-Brossel, UPMC-Sorbonne Universities, Paris)Multimode quantum optics in the continuous variable regime
Quantum states of light featuring high-dimensional entanglement are of growing interest as a platform for quantum information processing. Furthermore, complex optical states with non-Gaussian phase space distributions have been shown to be a vital resource for optical quantum simulation and computation. Multimode squeezed vacuum, a useful resource for preparing such states, can be generated experimentally using parametric downconversion of a broadband optical frequency comb in a synchronously-pumped optical parametric oscillator (SPOPO). I will expand on how we can achieve a high degree of control over the mode structure of the quantum light at the SPOPO output by shaping the pulse train used to pump the SPOPO, as well as the techniques we have developed to characterise these states, including spectrally multipixel homodyne detection. I will also describe how conditional single-photon subtraction enables us to use multimode squeezed vacuum as a resource for preparing non-Gaussian states.
2019-06-12 (Wednesday)
room 1.03, Pasteura 5 at 15:15
Boulat Bash (Department of Electrical and Computer Engineering, University of Arizona)Fundamental Limits of Low Probability of Covert Communications: Core Results and Future Directions [SPECIAL EXTRA SEMINAR NOTE DIFFERENT TIME]
Hiding transmitted signals is of paramount importance in many communication settings. While traditional security (e.g., encryption) prevents unauthorized access to message content, detection of the mere presence of a message by the adversary can have significant negative impact. This necessitates the use of covert communication, which not only protects the information contained in a transmission from unauthorized decoding, but also prevents the detection of a transmission in the first place. In this talk, I will present the fundamental classical and quantum limits of covert communication, and overview future research directions.
2019-06-06 (Thursday)
room 0.06, Pasteura 5 at 11:15
Andreas Poppe (Huawei Technologies Duesseldorf GmbH)Development of a continuous-variable QKD system and its first deployment in the field [UWAGA ZMIANA SALI!]
“No SPADs in Spain” as the subtitle in http://optics.org/news/9/6/26 will remain the most appropriate way to describe our work in 4 words ever. I will first outline the motivation to abandon widespread single photon detectors (as SPADs) but change to homodyne detection and work on continuous-variable (CV) technologies. I will further show the transition from the first optical test setups to fully functional CV-QKD systems we shipped to Madrid in 2018 to deploy these fully functional devices in the telecom network of Telefonica. This of cause was only the first step to establish quantum technologies in real-world scenarios as to secure critical telecom infrastructures in future.
2019-05-30 (Thursday)
room 1.03, Pasteura 5 at 11:15
Filip Rozpędek (QuTech, Delft University of Technology)Near-term quantum repeater experiments with NV centers: overcoming the limitations of direct transmission
Quantum channels enable the implementation of communication tasks inaccessible to their classical counterparts. However, in the absence of quantum repeaters the rate at which these tasks can be performed is dictated by the losses in the quantum channel. In practice, channel losses have limited the reach of quantum protocols to short distances. Quantum repeaters have the potential to significantly increase the rates and reach beyond the limits of direct transmission. In the first part of this talk I will present our proposal of four quantum repeater schemes and assess their ability to generate secret key when implemented on a setup using NV centers in diamond with near-term experimental parameters. We find that one of these schemes surpasses the capacity - the highest secret-key rate achievable with direct transmission - establishing it as a prime candidate for the experimental realization of a scalable quantum repeater. In the second part of this talk I will focus on another building block of quantum repeater networks – entanglement distillation. Specifically, I will present our framework for optimizing practical entanglement distillation, which we used to prove optimality of various experimentally relevant distillation schemes. Finally, I will describe our on-going efforts to integrate our models of scalable proof of principle repeaters with practical entanglement distillation and classical control to simulate a pan-European quantum Internet.