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
2023-02-02 (Thursday)
room 1.02, Pasteura 5 at 11:15 
Mohammad Mehboudi (University of Geneva)Nonclassical Gaussian quantum systems can improve thermometry precision
ONSITE ONLY (change of room!)
We study the problem of estimating the temperature of Gaussian systems with Gaussian and photo-detection-like measurements. For Gaussian measurements, we frame a method to identify the optimal measurement numerically and derive the analytical solutions in some relevant cases. For a class of single-mode states that includes thermal ones, the optimal Gaussian measurement is either Heterodyne or Homodyne, depending on the temperature regime. This contrasts with the general setting, in which a projective measurement in the eigenbasis of the Hamiltonian is optimal regardless of temperature. In the general multi-mode case, and unlike the general unrestricted scenario where joint measurements are not helpful for thermometry (nor for any parameter estimation task), it is open whether joint Gaussian measurements provide an advantage over local ones. We conjecture that they are not useful for thermal systems, supported by partial analytical and numerical evidence. We further show that Gaussian measurements become optimal in the limit of large temperatures, while photo-detection-like measurements do it for when the temperature tends to zero. Lastly, we address the operational significance of nonclassicality in nonequilibrium temperature estimation of bosonic baths with Gaussian dynamics and probing with Gaussian states.
Refs:
1. Quantum 6, 743 (2022)
2. arXiv:2207.10742
We study the problem of estimating the temperature of Gaussian systems with Gaussian and photo-detection-like measurements. For Gaussian measurements, we frame a method to identify the optimal measurement numerically and derive the analytical solutions in some relevant cases. For a class of single-mode states that includes thermal ones, the optimal Gaussian measurement is either Heterodyne or Homodyne, depending on the temperature regime. This contrasts with the general setting, in which a projective measurement in the eigenbasis of the Hamiltonian is optimal regardless of temperature. In the general multi-mode case, and unlike the general unrestricted scenario where joint measurements are not helpful for thermometry (nor for any parameter estimation task), it is open whether joint Gaussian measurements provide an advantage over local ones. We conjecture that they are not useful for thermal systems, supported by partial analytical and numerical evidence. We further show that Gaussian measurements become optimal in the limit of large temperatures, while photo-detection-like measurements do it for when the temperature tends to zero. Lastly, we address the operational significance of nonclassicality in nonequilibrium temperature estimation of bosonic baths with Gaussian dynamics and probing with Gaussian states.
Refs:
1. Quantum 6, 743 (2022)
2. arXiv:2207.10742
2023-01-26 (Thursday)
room 1.03, Pasteura 5 at 11:15
Kacper Dębski (IFT UW)Indefinite temporal order without gravity
ONSITE ONLY
According to the general theory of relativity, time can flow at different rates depending on the configuration of massive objects, affecting the temporal order of events. Combined with quantum theory, this gravitational effect can result in events with an indefinite temporal order when a massive object is prepared in a suitable quantum state. This was argued to lead to a theory-independent test of the non-classical order of events through the violation of Bell-type inequalities for temporal order. The theory-independence of this protocol is problematic: one of the auxiliary assumptions in the above approach turns out to be essential, while it is explicitly theory-dependent. To illustrate this problem we can construct a complete scenario where accelerating particles interacting with optical cavities result in a violation of temporal Bell inequalities. Due to the Equivalence Principle, this scenario is fully analogous to the gravitational case, yet avoids the criticism raised in the context of gravitational effects arising from quantum states of massive bodies, that we do not have a full framework describing such scenarios.
According to the general theory of relativity, time can flow at different rates depending on the configuration of massive objects, affecting the temporal order of events. Combined with quantum theory, this gravitational effect can result in events with an indefinite temporal order when a massive object is prepared in a suitable quantum state. This was argued to lead to a theory-independent test of the non-classical order of events through the violation of Bell-type inequalities for temporal order. The theory-independence of this protocol is problematic: one of the auxiliary assumptions in the above approach turns out to be essential, while it is explicitly theory-dependent. To illustrate this problem we can construct a complete scenario where accelerating particles interacting with optical cavities result in a violation of temporal Bell inequalities. Due to the Equivalence Principle, this scenario is fully analogous to the gravitational case, yet avoids the criticism raised in the context of gravitational effects arising from quantum states of massive bodies, that we do not have a full framework describing such scenarios.
2023-01-19 (Thursday)
room 1.03, Pasteura 5 at 11:15
Thomas Theurer (University of Calgary)Resource theory of quantum thermodynamics: State convertibility from qubit cooling and heating
Thermodynamics plays an important role both in the foundations of physics and in technological applications. An operational perspective adopted in recent years is to formulate it as a quantum resource theory. I will begin with a quick introduction to the general framework of quantum resource theories, in particular motivating it and explaining why the convertibility of resourceful states is at its core. I will then specialize to the resource theory of quantum thermodynamics and present recent results that I found in collaboration with Elia Zanoni, Carlo Maria Scandolo, and Gilad Gour: We solved the question how in the quantum limit, thermal non-equilibrium can be used to heat and cool other quantum systems that are initially at thermal equilibrium. We then showed that the convertibility between quasi-classical resources (resources that do not exhibit coherence between different energy eigenstates) is fully characterized by their ability to cool and heat qubits, i.e., by two of the most fundamental thermodynamical tasks on the simplest quantum systems. We therefore characterized the core problem of the resource theory of thermodynamics with operationally relevant tasks.
2023-01-12 (Thursday)
room 1.03, Pasteura 5 at 11:15
Giacomo Sorelli (Fraunhofer Institute for Optronics, System Technology and Image Exploitation - IOSB)Modes and states of light in Gaussian quantum metrology
ONSITE ONLY!
Quantum optical metrology aims to identify ultimate sensitivity bounds for the estimation of parameters encoded into quantum states of the electromagnetic field. In many practical applications, including imaging, microscopy, and remote sensing, the parameter of interest is not only encoded in the quantum state of the field, but also in its spatio-temporal distribution, i.e. in its mode structure. In this mode-encoded parameter estimation setting, we derive an analytical expression for the quantum Fisher information valid for arbitrary multimode Gaussian fields. To illustrate the power of our approach, we apply it to the transverse localisation of a Gaussian beam, and to the temporal separation between two pulses. In both cases, we will show which are the relevant modes where to deploy quantum resources, i.e. squeezing, to achieve a sensitivity enhancement.
Quantum optical metrology aims to identify ultimate sensitivity bounds for the estimation of parameters encoded into quantum states of the electromagnetic field. In many practical applications, including imaging, microscopy, and remote sensing, the parameter of interest is not only encoded in the quantum state of the field, but also in its spatio-temporal distribution, i.e. in its mode structure. In this mode-encoded parameter estimation setting, we derive an analytical expression for the quantum Fisher information valid for arbitrary multimode Gaussian fields. To illustrate the power of our approach, we apply it to the transverse localisation of a Gaussian beam, and to the temporal separation between two pulses. In both cases, we will show which are the relevant modes where to deploy quantum resources, i.e. squeezing, to achieve a sensitivity enhancement.
2023-01-11 (Wednesday)
room 1.03, Pasteura 5 at 12:15
Nathan Walk (Freie Universität Berlin)Quantum photo-thermodynamics on a programmable photonic quantum processor: Emergence of local equilibration from unitary evolution [SPECIAL SEMINAR]
One of the core questions of quantum physics is how to reconcile the unitary evolution of quantum states, which is information-preserving and time-reversible, with evolution following the second law of thermodynamics, which, in general, is neither. The resolution to this paradox is to recognize that global unitary evolution of a multi-partite quantum state causes the state of local subsystems to evolve towards maximum-entropy states [1-3]. In this work, we experimentally demonstrate this effect in linear quantum optics by simultaneously showing the convergence of local quantum states to a generalized Gibbs ensemble constituting a maximum-entropy state under precisely controlled conditions, while using a new, efficient certification method to demonstrate that the state retains global purity [4]. Our quantum states are manipulated by a programmable integrated photonic quantum processor, which simulates arbitrary non-interacting Hamiltonians, demonstrating the universality of this phenomenon. Our results show the potential of photonic devices for quantum simulations involving non-Gaussian states.
[1] Popescu, S., Short, A., & Winter, A. (2006). Entanglement and the foundations of statistical mechanics. Nature Physics, 2(11), 754–758.
[2] Cramer, M., Flesch, A., McCulloch, I. P., Schollwöck, U., & Eisert, J. (2008). Exploring Local Quantum Many-Body Relaxation by Atoms in Optical Superlattices, 101(6), 063001. http://doi.org/10.1103/PhysRevLett.101.063001
[3] Linden, N., Popescu, S., Short, A. J., & Winter, A. (2009). Quantum mechanical evolution towards thermal equilibrium. Physical Review E, 79(6), 061103. http://doi.org/10.1103/PhysRevE.79.061103
[4] Somhorst, F. H. B., van der Meer, R., Anguita, M. C., Schadow, R., Snijders, H. J., de Goede, M., et al. (2021). Quantum photo-thermodynamics on a programmable photonic quantum processor. arXiv:2201.00049
[1] Popescu, S., Short, A., & Winter, A. (2006). Entanglement and the foundations of statistical mechanics. Nature Physics, 2(11), 754–758.
[2] Cramer, M., Flesch, A., McCulloch, I. P., Schollwöck, U., & Eisert, J. (2008). Exploring Local Quantum Many-Body Relaxation by Atoms in Optical Superlattices, 101(6), 063001. http://doi.org/10.1103/PhysRevLett.101.063001
[3] Linden, N., Popescu, S., Short, A. J., & Winter, A. (2009). Quantum mechanical evolution towards thermal equilibrium. Physical Review E, 79(6), 061103. http://doi.org/10.1103/PhysRevE.79.061103
[4] Somhorst, F. H. B., van der Meer, R., Anguita, M. C., Schadow, R., Snijders, H. J., de Goede, M., et al. (2021). Quantum photo-thermodynamics on a programmable photonic quantum processor. arXiv:2201.00049
2022-12-15 (Thursday)
room 1.03, Pasteura 5 at 11:15
Aleksander Kubica (AWS Center for Quantum Computing)Single-shot quantum error correction
ONSITE ONLY!
Fault-tolerant protocols and quantum error correction (QEC) are essential to building reliable quantum computers from imperfect erroneous components. Optimizing the resource and time overheads needed to implement QEC is one of the most pressing challenges. In this talk, I will introduce a new topological QEC code, the three-dimensional subsystem toric code (3D STC). I will explain how the 3D STC allows for single-shot QEC, i.e., reliable QEC with no time overhead even in the presence of measurement errors. Due to its high QEC threshold together with local parity checks of small weight, the 3D STC is particularly appealing for realizing fault-tolerant quantum computing.Based on A. Kubica, M. Vasmer, Nat. Commun. 13, 6272 (2022).
Fault-tolerant protocols and quantum error correction (QEC) are essential to building reliable quantum computers from imperfect erroneous components. Optimizing the resource and time overheads needed to implement QEC is one of the most pressing challenges. In this talk, I will introduce a new topological QEC code, the three-dimensional subsystem toric code (3D STC). I will explain how the 3D STC allows for single-shot QEC, i.e., reliable QEC with no time overhead even in the presence of measurement errors. Due to its high QEC threshold together with local parity checks of small weight, the 3D STC is particularly appealing for realizing fault-tolerant quantum computing.Based on A. Kubica, M. Vasmer, Nat. Commun. 13, 6272 (2022).
2022-12-08 (Thursday)
room 1.03, Pasteura 5 at 11:15
Alexander Streltsov (QOT CENT UW)The power of noisy quantum states and the advantage of resource dilution
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).
Entanglement distillation allows to convert noisy quantum states into singlets, which can in turn be used for various quantum technological tasks, such as quantum teleportation and quantum key distribution. Entanglement dilution is the inverse process: singlets are converted into quantum states with less entanglement. While the usefulness of distillation is apparent, practical applications of entanglement dilution are less obvious. Here, we show that entanglement dilution can increase the resilience of shared quantum states to local noise. The increased resilience is observed even if diluting singlets into states with arbitrarily little entanglement. We extend our analysis to other quantum resource theories, such as quantum coherence, quantum thermodynamics, and purity. For these resource theories, we demonstrate that diluting pure quantum states into noisy ones can be advantageous for protecting the system from noise. Our results demonstrate the usefulness of quantum resource dilution, and provide a rare example for an advantage of noisy quantum states over pure states in quantum information processing.
Entanglement distillation allows to convert noisy quantum states into singlets, which can in turn be used for various quantum technological tasks, such as quantum teleportation and quantum key distribution. Entanglement dilution is the inverse process: singlets are converted into quantum states with less entanglement. While the usefulness of distillation is apparent, practical applications of entanglement dilution are less obvious. Here, we show that entanglement dilution can increase the resilience of shared quantum states to local noise. The increased resilience is observed even if diluting singlets into states with arbitrarily little entanglement. We extend our analysis to other quantum resource theories, such as quantum coherence, quantum thermodynamics, and purity. For these resource theories, we demonstrate that diluting pure quantum states into noisy ones can be advantageous for protecting the system from noise. Our results demonstrate the usefulness of quantum resource dilution, and provide a rare example for an advantage of noisy quantum states over pure states in quantum information processing.
2022-12-01 (Thursday)
room 1.03, Pasteura 5 at 11:15
Adam Sawicki (CFT PAN)Random approximate t-designs
The talk will be based on a joint work with Piotr Dulian: arXiv:2210.07872and arXiv:2202.05371
ONSITE ONLY!
ONSITE ONLY!
2022-11-24 (Thursday)
room 1.03, Pasteura 5 at 11:15
Karol Łlukanowski (QOT CENT UW)Upper bounds on key rates in DIQKD based on convex-combination attacks
ONSITE ONLY!
2022-11-17 (Thursday)