Joint Seminar on Quantum Information and Technologies
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until 2023/2024 Quantum Information Seminar | YouTube channel
2022-11-10 (Thursday)
room 1.03, Pasteura 5 at 11:15 
Eliška Greplová (Delft University of Technology)Quantum Matter and the Multiverse of Engineered Topology
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 field of condensed matter physics is currently being transformed by a series of exciting theoretical discoveries of intriguing properties of quantum materials and remarkable experimental progress that allows us to test the novel theories contemporaneously. In this talk, I will illustrate this condensed matter renaissance with two examples that also connect condensed matter to the two emerging fields of artificial intelligence and quantum computing. First, I will discuss a top-down example that considers solving existing models withthe aid of artificial intelligence tools. In a second, bottom-up approach, I will discuss on-chip engineering of topological features using scalable quantum computing building blocs.
The field of condensed matter physics is currently being transformed by a series of exciting theoretical discoveries of intriguing properties of quantum materials and remarkable experimental progress that allows us to test the novel theories contemporaneously. In this talk, I will illustrate this condensed matter renaissance with two examples that also connect condensed matter to the two emerging fields of artificial intelligence and quantum computing. First, I will discuss a top-down example that considers solving existing models withthe aid of artificial intelligence tools. In a second, bottom-up approach, I will discuss on-chip engineering of topological features using scalable quantum computing building blocs.
2022-10-27 (Thursday)
room 1.03, Pasteura 5 at 11:15
Wojciech Górecki (IFT UW)Quantum metrology of noisy spreading channels
ONSITE ONLY
We provide the optimal measurement strategy for a class of noisy channels that reduce to the identity channel for a specific value of a parameter (spreading channels). We provide an example that is physically relevant: the estimation of the absolute value of the displacement in the presence of phase randomizing noise. Surprisingly, this noise does not affect the effectiveness of the optimal measurement. We show that, for small displacement, a squeezed vacuum probe field is optimal among strategies with same average energy. A squeezer followed by photodetection is the optimal detection strategy that attains the quantum Fisher information, whereas the customarily used homodyne detection becomes useless in the limit of small displacements, due to the same effect that gives Rayleigh's curse in optical superresolution. There is a quantum advantage: a squeezed or a Fock state with N average photons allow to asymptotically estimate the parameter with a sqrt(N) better precision than classical states with same energy.
We provide the optimal measurement strategy for a class of noisy channels that reduce to the identity channel for a specific value of a parameter (spreading channels). We provide an example that is physically relevant: the estimation of the absolute value of the displacement in the presence of phase randomizing noise. Surprisingly, this noise does not affect the effectiveness of the optimal measurement. We show that, for small displacement, a squeezed vacuum probe field is optimal among strategies with same average energy. A squeezer followed by photodetection is the optimal detection strategy that attains the quantum Fisher information, whereas the customarily used homodyne detection becomes useless in the limit of small displacements, due to the same effect that gives Rayleigh's curse in optical superresolution. There is a quantum advantage: a squeezed or a Fock state with N average photons allow to asymptotically estimate the parameter with a sqrt(N) better precision than classical states with same energy.
2022-10-20 (Thursday)
room 1.03, Pasteura 5 at 11:15
Dan McNulty (CFT PAN)Estimating Quantum Hamiltonians via Joint Measurements of Noisy Non-Commuting Observables
SEMINAR ONSITE ONLY!!!
Estimation of expectation values of incompatible observables is an essential practical task in quantum computing, especially for approximating energies of chemical and other many-body quantum systems. In this work we introduce a method for this purpose based on performing a single joint measurement that can be implemented locally and whose marginals yield noisy (unsharp) versions of the target set of non-commuting Pauli observables. We derive bounds on the number of experimental repetitions required to estimate energies up to a certain precision. We compare this strategy to the classical shadow formalism and show that our method yields the same performance as the locally biased classical shadow protocol. We also highlight some general connections between the two approaches by showing that classical shadows can be used to construct joint measurements and vice versa. Finally, we adapt the joint measurement strategy to minimise the sample complexity when the implementation of measurements is assumed noisy. This can provide significant efficiency improvements compared to known generalisations of classical shadows to noisy scenarios.
Estimation of expectation values of incompatible observables is an essential practical task in quantum computing, especially for approximating energies of chemical and other many-body quantum systems. In this work we introduce a method for this purpose based on performing a single joint measurement that can be implemented locally and whose marginals yield noisy (unsharp) versions of the target set of non-commuting Pauli observables. We derive bounds on the number of experimental repetitions required to estimate energies up to a certain precision. We compare this strategy to the classical shadow formalism and show that our method yields the same performance as the locally biased classical shadow protocol. We also highlight some general connections between the two approaches by showing that classical shadows can be used to construct joint measurements and vice versa. Finally, we adapt the joint measurement strategy to minimise the sample complexity when the implementation of measurements is assumed noisy. This can provide significant efficiency improvements compared to known generalisations of classical shadows to noisy scenarios.
2022-10-13 (Thursday)
room 1.03, Pasteura 5 at 11:15
Kamil Korzekwa (UJ Kraków)Finite-size effects in quantum thermodynamics
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 necessity to go beyond classical thermodynamics is usually motivated by the fact that at the nanoscale quantum effects, like coherence and entanglement, start playing an important role. However, in the quantum regime one also deals with systems composed of a finite number $n$ of particles, whereas the theory of thermodynamics is traditionally constrained to the study of macroscopic systems with $n\to\infty$, whose energy fluctuations are negligible compared to their average energy. In this talk I will address this problem and describe recent developments allowing one to go beyond the thermodynamic limit and rigorously investigate thermodynamic transformations of finite-size systems. I will explain why such transformations are generally irreversible and consume free energy, and how this affects the performance of thermodynamic protocols [1]. A new version of the famous fluctuation-dissipation theorem will also be presented, linking the minimal amount of free energy dissipated in the process to the amount of free energy fluctuations present in the system’s initial state [2]. Moreover, I will discuss a novel resource resonance phenomenon, which allows one to significantly reduce dissipation for transformations between states whose fluctuations are properly tuned [3,4]. Finally, I will also explain how quantum coherence may bring states closer to resonance effectively decreasing the dissipation of free energy [5].
[1] Quantum 2, 108 (2018).
[2] Phys. Rev. E 105, 054127 (2022).
[3] Phys. Rev. A 99, 032332 (2019).
[4] Phys. Rev. Lett. 122, 110403 (2019).
[5] In preparation (arXiv 2022).
The necessity to go beyond classical thermodynamics is usually motivated by the fact that at the nanoscale quantum effects, like coherence and entanglement, start playing an important role. However, in the quantum regime one also deals with systems composed of a finite number $n$ of particles, whereas the theory of thermodynamics is traditionally constrained to the study of macroscopic systems with $n\to\infty$, whose energy fluctuations are negligible compared to their average energy. In this talk I will address this problem and describe recent developments allowing one to go beyond the thermodynamic limit and rigorously investigate thermodynamic transformations of finite-size systems. I will explain why such transformations are generally irreversible and consume free energy, and how this affects the performance of thermodynamic protocols [1]. A new version of the famous fluctuation-dissipation theorem will also be presented, linking the minimal amount of free energy dissipated in the process to the amount of free energy fluctuations present in the system’s initial state [2]. Moreover, I will discuss a novel resource resonance phenomenon, which allows one to significantly reduce dissipation for transformations between states whose fluctuations are properly tuned [3,4]. Finally, I will also explain how quantum coherence may bring states closer to resonance effectively decreasing the dissipation of free energy [5].
[1] Quantum 2, 108 (2018).
[2] Phys. Rev. E 105, 054127 (2022).
[3] Phys. Rev. A 99, 032332 (2019).
[4] Phys. Rev. Lett. 122, 110403 (2019).
[5] In preparation (arXiv 2022).
2022-10-06 (Thursday)
room 1.03, Pasteura 5 at 11:15
Sagnik Chakraborty (UMK, Toruń)Strongly coupled quantum Otto cycle with single qubit bath
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).
We discuss a model of a closed quantum evolution of two-qubits where the joint Hamiltonian is so chosen that one of the qubits acts as a bath and thermalize the other qubit which is acting as the system. The corresponding exact master equation for the system is derived. Interestingly, for a specific choice of parameters the master equation takes the Gorini-Kossakowski-LindbladSudarshan (GKLS) form with constant coefficients, representing pumping and damping of a single qubit system. Based on this model we construct an Otto cycle connected to a single qubit bath and study its thermodynamic properties. Our analysis goes beyond the conventional weak coupling scenario and illustrates the effects of finite bath including non-Markovianity. We find closed form expressions for efficiency (coefficient of performance), power (cooling power) for heat engine regime (refrigerator regime) for different modifications of the joint Hamiltonian.
Ref: arXiv:2206.14751
We discuss a model of a closed quantum evolution of two-qubits where the joint Hamiltonian is so chosen that one of the qubits acts as a bath and thermalize the other qubit which is acting as the system. The corresponding exact master equation for the system is derived. Interestingly, for a specific choice of parameters the master equation takes the Gorini-Kossakowski-LindbladSudarshan (GKLS) form with constant coefficients, representing pumping and damping of a single qubit system. Based on this model we construct an Otto cycle connected to a single qubit bath and study its thermodynamic properties. Our analysis goes beyond the conventional weak coupling scenario and illustrates the effects of finite bath including non-Markovianity. We find closed form expressions for efficiency (coefficient of performance), power (cooling power) for heat engine regime (refrigerator regime) for different modifications of the joint Hamiltonian.
Ref: arXiv:2206.14751