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
2024-04-04 (Thursday)
room 1.02, Pasteura 5 at 11:15 
Srijon Ghosh (IFT UW, Harish-Chandra Research Institute)Quantum sensing beyond quantum criticality
2024-03-21 (Thursday)
join us at 11:15
Marcin Kalinowski (Harvard University)Neutral atom quantum processors and the error correction frontier [ONLINE]
ONLINE SEMINAR: Meeting ID: 928 9413 0767, Passcode: R6Vx6E
We will present an approach to large-scale quantum information processing based on reconfigurable arrays of neutral atoms trapped and transported in optical tweezers. This digital quantum processor features hundreds of individually addressable physical qubits, high two-qubit gate fidelities, arbitrary connectivity with parallel control, and mid-circuit readout. In particular, we use this platform to explore quantum algorithms with encoded logical qubits and quantum error correction techniques. Using this logical processor with various types of error-correcting codes, we demonstrate that we can improve logical two-qubit gates by increasing code size, outperform physical qubit fidelities, create logical GHZ states, and perform computationally complex scrambling circuits using 48 logical qubits and hundreds of logical gates. These results herald the advent of early error-corrected quantum computation, enabling new applications and inspiring a shift in addressing the challenges that lay ahead.
We will present an approach to large-scale quantum information processing based on reconfigurable arrays of neutral atoms trapped and transported in optical tweezers. This digital quantum processor features hundreds of individually addressable physical qubits, high two-qubit gate fidelities, arbitrary connectivity with parallel control, and mid-circuit readout. In particular, we use this platform to explore quantum algorithms with encoded logical qubits and quantum error correction techniques. Using this logical processor with various types of error-correcting codes, we demonstrate that we can improve logical two-qubit gates by increasing code size, outperform physical qubit fidelities, create logical GHZ states, and perform computationally complex scrambling circuits using 48 logical qubits and hundreds of logical gates. These results herald the advent of early error-corrected quantum computation, enabling new applications and inspiring a shift in addressing the challenges that lay ahead.
2024-03-14 (Thursday)
room 1.02, Pasteura 5 at 11:15
Owidiusz Makuta (CFT PAN)Network-based definition of genuine multipartite nonlocality
In a bipartite scenario, nonlocality can be thought of as an indicator of "true bipartiteness" of a correlation, i.e., that the full description of the system cannot be reproduced by studying each party individually. Extending this idea to the multipartite case has led to the introduction of Genuine Multipartite Nonlocality (GMNL); a GMNL correlation is nonlocal with respect to any bipartition of the parties. However, it turns out that in some cases, this definition can be cheated, e.g., two bipartite correlations can pass a test for GMNL in a tripartite scenario. This led to the introduction of a modified GMNL definition, called LOSR-GMNL, based on a network with Local Operations and Shared Randomness. In this talk, I will introduce the concept of LOSR-GMNL, show why it is resilient to the types of "attacks" to which the old definition is vulnerable, and discuss the inflation technique - a useful proof method for a LOSR network scenario. Lastly, I will present the results of our work concerning LOSR-GMNL, in which we show that all caterpillar graph states, as well as some other classes of states, can produce LOSR-GMNL correlations.
2024-03-07 (Thursday)
room 1.02, Pasteura 5 at 11:15
Andrzej Dragan (IFT UW)How to quantize a tachyon?
2024-02-29 (Thursday)
room 1.02, Pasteura 5 at 11:15
Dmitri B. Horoshko (Univ. Lille, CNRS)Time-resolved intensity autocorrelation function of twin beams generated in optical parametric downconversion
Second-order autocorrelation function of the optical field, g(2)(τ), where τ is a time delay, is a powerful tool for determining the physical properties of light beams and their sources. In particular, the value of this function at τ = 0 reveals the statistics of the photons: g(2)(0) = 2 for a Gaussian statistics, g(2)(0) = 1 for a Poissonian one, and the value g(2)(0) < 1 indicates the phenomenon of photon antibunching, which is a signature of field nonclassicality. In my report, I discuss a possibility of measuring the time-resolved second-order autocorrelation function of one of two beams generated in type-II parametric downconversion by means of temporal magnification of this beam, bringing its correlation time from the picosecond to the nanosecond scale, resolvable by modern photodetectors. I show that such a measurement enables one to infer directly the degree of global coherence of that beam, which is linked by a simple relation to the number of Schmidt modes characterizing the entanglement between the two generated beams. I illustrate the proposed method by the photon pairs generated in a periodically poled KTP crystal with a symmetric group velocity matching for various durations of the pump pulse, resulting in different numbers of modes.
2024-01-25 (Thursday)
room 1.03, Pasteura 5 at 11:15
Jing Yang (Royal Institute of Technology and Stockholm University)Precision Limits in Many-body Quantum Sensing [ONLINE!]
ONLINE SEMINAR: Meeting ID: 928 9413 0767, Passcode: R6Vx6E
Many-body interactions can introduce entanglement between particles and hence are valuable resources for quantum information processing. In quantum metrology, the precision can be further boosted by adding many-body interactions. I will discuss an optimal control theory for controlling many-body interaction with restricted operations, in the context of quantum sensing. We show that in a spin chain model the Heisenberg scaling can be still achieved even though the control operations are restricted, given an initial GHZ-like state can be prepared. When the GHZ state cannot be efficiently prepared in experiments, one may consider many-body sensing with separable initial states. We find that using separable initial states cannot beat the shot noise limit in locally interacting systems, unless long-range non-local interactions are utilized. These findings identify two important ingredients in many-body sensing: initial entanglement and long-range interactions. I will conclude briefly commenting the local optimal measurements that can be performed to extract the many-body precision limits.
Many-body interactions can introduce entanglement between particles and hence are valuable resources for quantum information processing. In quantum metrology, the precision can be further boosted by adding many-body interactions. I will discuss an optimal control theory for controlling many-body interaction with restricted operations, in the context of quantum sensing. We show that in a spin chain model the Heisenberg scaling can be still achieved even though the control operations are restricted, given an initial GHZ-like state can be prepared. When the GHZ state cannot be efficiently prepared in experiments, one may consider many-body sensing with separable initial states. We find that using separable initial states cannot beat the shot noise limit in locally interacting systems, unless long-range non-local interactions are utilized. These findings identify two important ingredients in many-body sensing: initial entanglement and long-range interactions. I will conclude briefly commenting the local optimal measurements that can be performed to extract the many-body precision limits.
2024-01-18 (Thursday)
room 1.03, Pasteura 5 at 11:15
Emilia Witkowska (IFPAN)Spin squeezing and Bell correlations with ultra-cold atoms in one-dimensional optical lattice
I will discuss a method to obtain scalable spin squeezing in isotropic Heisenberg spin chains that can be obtained using either bosonic or fermionic ultra-cold atoms in optical lattices. In this scheme, spin-flip coupling induces effective interactions among individual spins allowing the production of scalable spin squeezing. I will discuss the influence of boundary conditions on squeezing dynamics. In addition, I will show how to quantify the level of many-body Bell correlations generated in the systems. The schemes can be directly implemented experimentally with state-of-the-art techniques.
2024-01-11 (Thursday)
room 1.03, Pasteura 5 at 11:15
Marcin Koźbiał (QOT CENT UW)Spin noise spectroscopy of an alignment-based atomic magnetometer
Optically pumped magnetometers (OPMs) are revolutionising the task of magnetic-field sensing due to their extremely high sensitivity combined with technological improvements in miniaturisation which have led to compact and portable devices. OPMs can be based on spin-oriented or spin-aligned atomic ensembles which are spin-polarized through optical pumping with circular or linear polarized light, respectively. Characterisation of OPMs and the dynamical properties of their noise is important for applications in real-time sensing tasks. In our work, we experimentally perform spin noise spectroscopy of an alignment-based magnetometer. Moreover, we propose a stochastic model that predicts the noise power spectra exhibited by the device when, apart from the strong magnetic field responsible for the Larmor precession of the spin, white noise is applied in the perpendicular direction aligned with the pumping-probing beam. By varying the strength of the noise applied as well as the linear-polarisation angle of incoming light, we verify the model to accurately predict the heights of the Larmor-induced spectral peaks and their corresponding line-widths. Our work paves the way for alignment-based magnetometers to become operational in real-time sensing tasks.
2023-12-21 (Thursday)
room 1.03, Pasteura 5 at 11:15
Alicja Dutkiewicz (Google Quantum AI)The advantage of quantum control in many-body Hamiltonian learning
2023-12-14 (Thursday)
room 0.06, Pasteura 5 at 11:15
Kostas Mouloudakis (ICFO Barcelona)Probing spin fluctuations in dense alkali-metal media
Spin-based devices, like optically pumped magnetometers (OPMs), constitute a great example of quantum sensing by offering a testbed for both applied and fundamental investigations. OPMs utilizing the spin-degrees of freedom of high-density alkali-metal vapors are currently at the forefront of applied magnetometry, yet involving intriguing quantum effects at the atom-light interface.
In this talk, we report on the spontaneous spin fluctuations of the collective spin of such atomic ensembles, that ultimately limit the performance of OPMs. We will give a brief introduction to spin-noise spectroscopy, the field studying these fluctuations, and we will discuss recent advances both in single- and dual-species atomic ensembles. In particular, we will discuss spin noise in the spin-exchange-relaxation-free (SERF) regime, a parameter-regime where some of the most sensitive magnetometer operate. We do that by considering a mean-field spin evolution augmented by stochastic Ornstein-Uhlenbeck dynamics. We will derive the full two-time spin covariance matrix and the observable noise spectra in Faraday rotation experiments and we will compare the model against precision measurements in a 87Rb, as well as in a mixture of 87Rb-133Cs vapors.
References
[1] Phys. Rev. A 106, 023112 (2022)
[2] Phys. Rev. A 108, 052822 (2023)
[3] arXiv: 2307. 16869v1 (2023)
In this talk, we report on the spontaneous spin fluctuations of the collective spin of such atomic ensembles, that ultimately limit the performance of OPMs. We will give a brief introduction to spin-noise spectroscopy, the field studying these fluctuations, and we will discuss recent advances both in single- and dual-species atomic ensembles. In particular, we will discuss spin noise in the spin-exchange-relaxation-free (SERF) regime, a parameter-regime where some of the most sensitive magnetometer operate. We do that by considering a mean-field spin evolution augmented by stochastic Ornstein-Uhlenbeck dynamics. We will derive the full two-time spin covariance matrix and the observable noise spectra in Faraday rotation experiments and we will compare the model against precision measurements in a 87Rb, as well as in a mixture of 87Rb-133Cs vapors.
References
[1] Phys. Rev. A 106, 023112 (2022)
[2] Phys. Rev. A 108, 052822 (2023)
[3] arXiv: 2307. 16869v1 (2023)