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 | Seminar YouTube channel
until 2023/2024 Quantum Information Seminar
2022-03-17 (Thursday)
room 1.02, Pasteura 5 at 11:15 
Gabriel Pereira Alves (IFT UW)Any pair of incompatible rank-one projective measurements is optimal for some Bell inequality
Bell non-locality represents one of the most striking departures of quantum mechanics from classical physics. It shows that correlations between space-like separated systems allowed by quantum mechanics are stronger than those present in any classical theory. In a recent work [Sci. Adv. 7, eabc3847 (2021)], a family of Bell functionals tailored to mutually unbiased bases (MUBs) is proposed. For these functionals, the maximal quantum violation is achieved if the two measurements performed by one of the parties are constructed out of MUBs of a fixed dimension. In this work, we generalize this construction to an arbitrary incompatible pair of rank-one projective measurements. By constructing a new family of Bell functionals, we show that for any such pair there exists a Bell inequality that is maximally violated by this pair. Moreover, when investigating the robustness of these violations to noise, we demonstrate that the realization which is most robust to noise is not generated by MUBs.
The Seminar will take a HYBRID form. It will take place in room 1.02 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)
The Seminar will take a HYBRID form. It will take place in room 1.02 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)
2022-03-10 (Thursday)
room 1.02, Pasteura 5 at 11:15
Stanisław Sołtan (IFT UW)Weak measurements and the quantum Zeno effect
A quantum decaying system can reveal its nonclassical behavior by being noninvasively measured.Correlations of weak measurements in the noninvasive limit violate the classical bound for a universalclass of systems. The violation is related to incompatibility between exponential decay and unitaryevolution, and as such are closely related to the quantum Zeno effect. The phenomenon can be experimentallyobserved by a continuous weak measurement
The Seminar will take a HYBRID form. It will take place in room 1.03 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)
The Seminar will take a HYBRID form. It will take place in room 1.03 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)
2022-03-03 (Thursday)
room 1.02, Pasteura 5 at 11:15
Saronath Halder (QOT CENT UW)Unextendible entangled bases and more nonlocality with less entanglement
We consider a general version of the phenomenon of more nonlocality with less entanglement, within the framework of the conclusive quantum state discrimination problem under local quantum operations and classical communication. We show that although the phenomenon was obtained before for two qutrits, it can also be observed for two qubits. We establish that the phenomenon is intrinsically connected to the concept of unextendible entangled bases, in the two-qubit case. In the process, we demonstrate a hierarchy of nonlocality among sets of two-qubit orthogonal pure states, where the nonlocality is in the sense of a difference between global and local abilities of quantum state discrimination. We present a complete characterization of two-qubit pure orthogonal state sets of cardinality three with respect to their nonlocality in terms of conclusive local distinguishability, the status for other cardinalities being already known.
The Seminar will take a HYBRID form. It will take place in room 1.03 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)
The Seminar will take a HYBRID form. It will take place in room 1.03 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)
2022-01-27 (Thursday)
room 1.03, Pasteura 5 at 11:15
Michal Jachura (CENT UW)Intensity modulation/direct detection optical key distribution
The Seminar will take a HYBRID form. It will take place in room 1.03 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)
While the ambition of QKD technology is to make the cryptographic key distribution secure even against the most sophisticated physical attacks permitted by quantum theory, it falls short of delivering high key rates when the distance between Alice and Bob is large enough that one has to use either satellite communication or a series of quantum repeaters. Hence, it is interesting to consider alternative key distribution protocols which are still secure against restricted class of attacks such as passive eavesdropping where an unauthorized third party collects passively a fraction of the signal that escapes detection by the legitimate recipient. Here we introduce and analyze theoretically the performance of the optical key distribution (OKD) relying on intensity modulation/direct detection of the optical carrier, where the security is ensured by the presence of the shot noise that inevitably accompanies eavesdropper’s attempt to detect the collected signal. We show that the key rate depends on a ratio that compares legitimate recipient’s and eavesdropper’s capabilities to detect the signal, including noise contributed by their respective detectors. Finally we present a realistic numerical example to estimate an attainable key length distributed during a single low-earth-orbit satellite pass which significantly exceeds key lengths attained in satellite QKD missions realized until now.
While the ambition of QKD technology is to make the cryptographic key distribution secure even against the most sophisticated physical attacks permitted by quantum theory, it falls short of delivering high key rates when the distance between Alice and Bob is large enough that one has to use either satellite communication or a series of quantum repeaters. Hence, it is interesting to consider alternative key distribution protocols which are still secure against restricted class of attacks such as passive eavesdropping where an unauthorized third party collects passively a fraction of the signal that escapes detection by the legitimate recipient. Here we introduce and analyze theoretically the performance of the optical key distribution (OKD) relying on intensity modulation/direct detection of the optical carrier, where the security is ensured by the presence of the shot noise that inevitably accompanies eavesdropper’s attempt to detect the collected signal. We show that the key rate depends on a ratio that compares legitimate recipient’s and eavesdropper’s capabilities to detect the signal, including noise contributed by their respective detectors. Finally we present a realistic numerical example to estimate an attainable key length distributed during a single low-earth-orbit satellite pass which significantly exceeds key lengths attained in satellite QKD missions realized until now.
2022-01-20 (Thursday)
room 1.03, Pasteura 5 at 11:15
Maciej Ogrodnik (Wydział Fizyki UW)Overview of quantum key distribution efforts
The Seminar will take a HYBRID form. It will take place in room 1.03 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)
2022-01-13 (Thursday)
room 1.03, Pasteura 5 at 11:15
Jan Chwedeńćzuk (IFT UW)Many-body nonlocality as a resource for quantum-enhanced metrology
The Seminar will take a HYBRID form. It will take place in room 1.03 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)
In this talk I will argue that the strongest quantum correlation, manifested by a many-body nonlocality, is a resource for ultra-precise metrology. I will show that the sensitivity of a quantum sensor can be expressed in terms of many-body correlation functions (such that witness the nonlocality) of all orders. I will illustrate this general result with some prominent examples, such as a collection of spins forming an Ising chain or a gas of ultra-cold bosons in any two-mode configuration.
In this talk I will argue that the strongest quantum correlation, manifested by a many-body nonlocality, is a resource for ultra-precise metrology. I will show that the sensitivity of a quantum sensor can be expressed in terms of many-body correlation functions (such that witness the nonlocality) of all orders. I will illustrate this general result with some prominent examples, such as a collection of spins forming an Ising chain or a gas of ultra-cold bosons in any two-mode configuration.
2021-12-16 (Thursday)
room 1.03, Pasteura 5 at 11:15
Giovanni Scala (IFT UW)Entanglement witnesses: overview of the technique and a new construction
The Seminar will take a HYBRID form. It will take place in room 1.03 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)
In Entanglement theory, it is still open the question if a given quantum state is separable or entangled. To tackle this problem many separability criteria have been derived in the last decades. In this talk, we discuss the mathematical methods that have been developed to detect entanglement, as well as the state of the art of the problem, and we introduce our promising multipartite separability criteria based on the correlation tensor.Specifically, via the correlation tensor we construct the related entanglement witnesses and show how they unifies known criteria as the realignment criterion, the De Vincente’s criterion, the SIC-POVM criterion and the others. Interestingly, our criteria is linear in the density operator, and thus, we find an unexplored classes of entanglement witnesses.
In Entanglement theory, it is still open the question if a given quantum state is separable or entangled. To tackle this problem many separability criteria have been derived in the last decades. In this talk, we discuss the mathematical methods that have been developed to detect entanglement, as well as the state of the art of the problem, and we introduce our promising multipartite separability criteria based on the correlation tensor.Specifically, via the correlation tensor we construct the related entanglement witnesses and show how they unifies known criteria as the realignment criterion, the De Vincente’s criterion, the SIC-POVM criterion and the others. Interestingly, our criteria is linear in the density operator, and thus, we find an unexplored classes of entanglement witnesses.
2021-12-09 (Thursday)
room 1.03, Pasteura 5 at 11:15
Paweł Machnikowski (Politechnika Wrocławska)Resonance fluorescence of a quantum system with modulated transition energy
The Seminar will take a HYBRID form. It will take place in room 1.03 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)
I will discuss the spectra of resonance fluorescence for a two-level system in which the transition energy varies in time in a deterministic or random way. A particular system that we studied both theoretically and experimentally is a semiconductor quantum dot interacting with the strain field of a surface acoustic wave. I will show how a modulation with one or two acoustic tones allows one to control the frequency bin to which the photons are scattered in the weak excitation regime. Next, I will discuss acoustic sidebands of the Mollow triplet in the strong excitation regime and show how the double dressing of quantum states leads to resonances and central line suppression under strong excitation. Finally, I will briefly report results on random modulation by a telegraph noise.
I will discuss the spectra of resonance fluorescence for a two-level system in which the transition energy varies in time in a deterministic or random way. A particular system that we studied both theoretically and experimentally is a semiconductor quantum dot interacting with the strain field of a surface acoustic wave. I will show how a modulation with one or two acoustic tones allows one to control the frequency bin to which the photons are scattered in the weak excitation regime. Next, I will discuss acoustic sidebands of the Mollow triplet in the strong excitation regime and show how the double dressing of quantum states leads to resonances and central line suppression under strong excitation. Finally, I will briefly report results on random modulation by a telegraph noise.
2021-12-02 (Thursday)
room 1.03, Pasteura 5 at 11:15
Nicolas Gigena (IFT UW)Computable and operationally meaningful multipartite entanglement measures
The Seminar will take a HYBRID form. It will take place in room 1.03 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)
Multipartite entanglement is a key resource for quantum communication, quantum computing and quantum cryptography. Therefore, efficiently quantifying and characterizing multipartite entanglement is of paramount importance. In this work we introduce a family of multipartite entanglement measures called "concentratable entanglements" that includes, as special cases, previously defined entanglement measures. We prove that all quantities in the family are non-increasing under Local Operations and Classical Communication (LOCC) and provide an operational meaning for them in terms of probabilistic concentration of entanglement into Bell pairs. Finally, we show that these quantities can be estimated on a quantum computer by implementing a parallelized SWAP test, opening a research direction for measuring multipartite entanglement measures on quantum devices.
Multipartite entanglement is a key resource for quantum communication, quantum computing and quantum cryptography. Therefore, efficiently quantifying and characterizing multipartite entanglement is of paramount importance. In this work we introduce a family of multipartite entanglement measures called "concentratable entanglements" that includes, as special cases, previously defined entanglement measures. We prove that all quantities in the family are non-increasing under Local Operations and Classical Communication (LOCC) and provide an operational meaning for them in terms of probabilistic concentration of entanglement into Bell pairs. Finally, we show that these quantities can be estimated on a quantum computer by implementing a parallelized SWAP test, opening a research direction for measuring multipartite entanglement measures on quantum devices.
2021-11-25 (Thursday)
room 1.03, Pasteura 5 at 11:15
Wojciech Górecki (IFT UW)Multiple-phase quantum interferometry: beyond Quantum Fisher Information
The Seminar will take a HYBRID form. It will take place in room 1.03 but will be simmultaneously tranmitted via ZOOM under the following link: https://zoom.us/j/92894130767 (Passcode: R6Vx6E)