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 | YouTube channel
until 2023/2024 Quantum Information Seminar | YouTube channel
2023-12-07 (Thursday)
room 1.03, Pasteura 5 at 11:15 
Marek Kopciuch (Uniwersytet Jagieloński)Optimal tomography of collective quantum-states in room temperature gases
One of the most important challenges of quantum information science is to create, manipulate, and reconstructa desired quantum state. Conventional media for realization of such experiments include single microscopicalobjects such as ions, photons, superconducting circuits, etc. However, such choice comes with a number ofexperimental challenges. This seminar introduces an alternative approach centered around collective quantumstates formed within a macroscopic cloud comprising approximately 109 atoms of room-temperature alkali-metalvapors.
In this presentation I will show a method for conducting quantum-state tomography in this medium througha few simple measurements of the Faraday effect. I will also demonstrate how careful tuning of one parameter(in our case laser detuning) can significantly enhance tomographic robustness. Furthermore, I will describe howtomography may aid in verifying and exploring quantum metrological concepts, such as ’quantum protractorstates’.
In this presentation I will show a method for conducting quantum-state tomography in this medium througha few simple measurements of the Faraday effect. I will also demonstrate how careful tuning of one parameter(in our case laser detuning) can significantly enhance tomographic robustness. Furthermore, I will describe howtomography may aid in verifying and exploring quantum metrological concepts, such as ’quantum protractorstates’.
2023-11-30 (Thursday)
room 1.03, Pasteura 5 at 11:15
Stanisław Kurdziałek (IFT UW)Using adaptiveness (and causal superpositions) against noise in quantum metrology
2023-11-23 (Thursday)
join us at 11:15
Johannes Meyer (Freie Universität Berlin)[ONLINE] Quantum metrology in the finite-sample regime
ONLINE SEMINAR: Meeting ID: 928 9413 0767, Passcode: R6Vx6E
In quantum metrology, one of the major applications of quantum technologies, the ultimate precision of estimating an unknown parameter is often stated in terms of the Cramér-Rao bound. Yet, the latter is no longer guaranteed to carry an operational meaning in the regime where few measurement samples are obtained. We instead propose to quantify the quality of a metrology protocol by the probability of obtaining an estimate with a given accuracy. This approach, which we refer to as probably approximately correct (PAC) metrology, ensures operational significance in the finite-sample regime. The accuracy guarantees hold for any value of the unknown parameter, unlike the Cramér-Rao bound which assumes it is approximately known. We establish a strong connection to multi-hypothesis testing with quantum states, which allows us to derive an analogue of the Cramér-Rao bound which contains explicit corrections relevant to the finite-sample regime. We further study the asymptotic behavior of the success probability of the estimation procedure for many copies of the state and apply our framework to the example task of phase estimation with an ensemble of spin-1/2 particles. Overall, our operational approach allows the study of quantum metrology in the finite-sample regime and opens up a plethora of new avenues for research at the interface of quantum information theory and quantum metrology.
TL;DR: In this talk, I will motivate why the Cramér-Rao bound might not always be the tool of choice to quantify the ultimate precision attainable in a quantum metrology task and give a (hopefully) intuitive introduction of how we propose to instead quantify it in a way that is valid in the single- and few-shot settings. We will together unearth a strong connection to quantum multi-hypothesis testing and conclude that there are many exiting and fundamental open questions in single-shot metrology!
In quantum metrology, one of the major applications of quantum technologies, the ultimate precision of estimating an unknown parameter is often stated in terms of the Cramér-Rao bound. Yet, the latter is no longer guaranteed to carry an operational meaning in the regime where few measurement samples are obtained. We instead propose to quantify the quality of a metrology protocol by the probability of obtaining an estimate with a given accuracy. This approach, which we refer to as probably approximately correct (PAC) metrology, ensures operational significance in the finite-sample regime. The accuracy guarantees hold for any value of the unknown parameter, unlike the Cramér-Rao bound which assumes it is approximately known. We establish a strong connection to multi-hypothesis testing with quantum states, which allows us to derive an analogue of the Cramér-Rao bound which contains explicit corrections relevant to the finite-sample regime. We further study the asymptotic behavior of the success probability of the estimation procedure for many copies of the state and apply our framework to the example task of phase estimation with an ensemble of spin-1/2 particles. Overall, our operational approach allows the study of quantum metrology in the finite-sample regime and opens up a plethora of new avenues for research at the interface of quantum information theory and quantum metrology.
TL;DR: In this talk, I will motivate why the Cramér-Rao bound might not always be the tool of choice to quantify the ultimate precision attainable in a quantum metrology task and give a (hopefully) intuitive introduction of how we propose to instead quantify it in a way that is valid in the single- and few-shot settings. We will together unearth a strong connection to quantum multi-hypothesis testing and conclude that there are many exiting and fundamental open questions in single-shot metrology!
2023-11-16 (Thursday)
room 1.03, Pasteura 5 at 11:15
Moein Naseri (QOT CENT UW)Scalable noisy quantum circuits for biased noise qubits
2023-11-09 (Thursday)
room 1.03, Pasteura 5 at 11:15
Guillem Muller-Rigat (ICFO, Barcelona)Certifying metrologically useful entanglement via semidefinite programming
Quantum systems may be investigated from the perspective of the resource they represent in quantum metrology applications. This resource is quantified by the so-called quantum Fisher information (QFI). In this work, we introduce a mathematical technique to quantify the minimal QFI in a given metrology scenario, compatible with some given measured mean values. We show that some popular experiments on spin ensembles allow one to prepare very useful states for metrology, beyond what was previously envisioned.
Reference: Quantum 7, 1152 (2023)
Reference: Quantum 7, 1152 (2023)
2023-10-26 (Thursday)
join us at 11:15
Jessica Bavaresco (University of Geneva)[ONLINE] Unitary channel discrimination beyond group structures: Advantages of sequential and indefinite-causal-order strategies
ZOOM ID: 928 9413 0767 Passcode: R6Vx6E
We will discuss the formalism of higher-order operations and see how it can be used to efficiently find the optimal strategies for a channel discrimination task. Such strategies may be subjected to different causal constraints, such as being parallel (non-adaptive) or sequential (adaptive). Furthermore, we will see how indefinite causal order naturally arises when considering tasks of channel discrimination. Focusing on the discrimination of unitary channels, we show that sequential strategies may outperform parallel ones, and that, in turn, general strategies with indefinite causal order may outperform sequential ones. This hierarchy of discrimination is only possible when at least one of two things are true: either the prior distribution of the unitary channels is not uniform, or the unitaries being discriminated do not form a group. When the hypothesis that the prior is uniform and the unitaries form a group holds, we show that parallel strategies are optimal even when compared to indefinite-causal-order strategies. Finally, we present an ultimate upper bound for the probability of successfully discriminating a set of unitary channels and study the application of strategies based on the quantum switch for this task.
We will discuss the formalism of higher-order operations and see how it can be used to efficiently find the optimal strategies for a channel discrimination task. Such strategies may be subjected to different causal constraints, such as being parallel (non-adaptive) or sequential (adaptive). Furthermore, we will see how indefinite causal order naturally arises when considering tasks of channel discrimination. Focusing on the discrimination of unitary channels, we show that sequential strategies may outperform parallel ones, and that, in turn, general strategies with indefinite causal order may outperform sequential ones. This hierarchy of discrimination is only possible when at least one of two things are true: either the prior distribution of the unitary channels is not uniform, or the unitaries being discriminated do not form a group. When the hypothesis that the prior is uniform and the unitaries form a group holds, we show that parallel strategies are optimal even when compared to indefinite-causal-order strategies. Finally, we present an ultimate upper bound for the probability of successfully discriminating a set of unitary channels and study the application of strategies based on the quantum switch for this task.
2023-10-19 (Thursday)
room 1.03, Pasteura 5 at 11:15
Ray Ganardi (QOT CENT UW)Catalytic and asymptotic equivalence for quantum entanglement
2023-10-12 (Thursday)
room 1.03, Pasteura 5 at 11:15
Brian J. Smith (University of Oregon)Challenges in Fluorescence-Detected Entangled Two-Photon-Absorption Experiments: Exploring the Low-to-High-Gain Squeezing Regimes
The rate of two-photon absorption of time-frequency-entangled photon pairs has been the subject of much study for its potential to enable quantum-enhanced molecular spectroscopy and imaging. We closely replicated recent experiments that reportedly observed such enhancement and have found that in the low-photon-flux regime the signal is below detection threshold. Using an optical parametric down-conversion photon-pair source that can be varied from the low-gain spontaneous regime to the high-gain squeezing regime, we observe two-photon absorption with a molecular sample in solution for the high-gain regime, but not for the low-gain regime where time-frequency correlations provide an advantage over uncorrelated laser light of the same flux. The observed rates are consistent with theoretical predictions and indicate that time-frequency photon entanglement does not yet provide a practical means to enhance spectroscopy or imaging with current techniques.
2023-10-05 (Thursday)
room 1.03, Pasteura 5 at 11:15
Karol Gietka (Universitat Innsbruck)Combining critical and quantum metrology
Critical metrology relies on the precise preparation of a system's ground state near the quantum phase transition point. This facilitates the creation of quantum correlations, known for their ability to enhance the quantum Fisher information and thus improve measurement precision within the confines of the Cramér-Rao bound. Essentially, critical metrology involves encoding information about the unknown parameter within the system's ground state. Conversely, in conventional metrology methods like Ramsey interferometry, the eigenstates of the system remain unchanged, and information about the unknown parameter is stored within the phase accumulated during free evolution. I will introduce an approach that combines these two methodologies into a unified protocol applicable to both closed and driven-dissipative systems. To achieve this, I will concentrate on the squeezing Hamiltonian, which characterizes the thermodynamic limit of Dicke and Lipkin-Meshkov-Glick Hamiltonians.