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## 2021:

**Name:**Prof. Xiao Yuan**Dates for visit (onsite):**11am, 23rd June 2021 (UTC+8)**Room 725, IFFS building (shahe campus)****Unit:**Center on Frontiers of Computing Studies, Peking University**Talks in IFFS:**- Quantum algorithms for near-term quantum hardware
**Abstract:**Show more

Quantum computer could solve classically intractable problems. While since realizing a universal quantum computer is challenging with current technology, a more practical question before having a fully-fledged quantum computer is what we can do with current and near-term quantum hardware. Focusing on the noisy-intermediate-scaled-quantum regime, we introduce variational quantum algorithms for solving static and dynamic problems of many-body physics. Considering the limitations on the qubits number and circuit depth, we introduce the hybrid tensor network and perturbative quantum simulation methods that allow us to solve practical large problems. With the rapid development of quantum hardware, variational quantum algorithms may finally enable genuine quantum advantage demonstration in the noisy-intermediate-scaled quantum era.Hide

- Quantum algorithms for near-term quantum hardware

**Name:**Prof. Qing-Hu Chen**Dates for visit (online):**2pm, 17th June 2021 (UTC+8)**Zoom link****Unit:**Zhejiang University**Talks in IFFS:**- Quantum phase transition in the Rabi-Stark model at finite frequency ratios
**Abstract:**Show more

The continuous quantum phase transition (QPT) has been observed in the quantum Rabi model in an extreme condition that the ratio of the qubit and cavity frequencies Δ/ω approaches infinity [1]. Interestingly, we find that the Rabi-Stark model can undergo the continuous QPT at finite ratios of the qubit and cavity frequencies Δ/ω, if the stark constant U is equal to twice the cavity frequency [2]. The critical exponents in the Rabi-Stark model are different from those in the quantum Rabi model and the Dicke model. In terms of the bosonic Hilbert space truncation number, universal critical behavior for both the Rabi-Stark model and the quantum Rabi model can be established, and the obtained critical correlation length exponents are different, suggesting they belong to different universality classes. A first-order QPT indicated by level crossing of the ground state and the first excited state can be induced by the positive nonlinear Stark-like coupling constant U [3], which is however absent in any previous isotropic quantum Rabi models. In the anisotropic Rabi-Stark model, we find that the critical gap exponent at the continuous QPTs is independent of the anisotropy as long as the counter-rotating wave coupling is present, but essentially changed if the counter-rotating wave coupling disappears completely [4]. It is suggested that the gapless Goldstone mode excitations could appear above a critical coupling in the Rabi-Stark model under the rotating-wave approximation.

References:

[1] M.-J. Hwang, R. Puebla, and M. B. Plenio, Phys. Rev. Lett. 115, 180404 (2015).

[2] X. Y. Chen, Y. F. Xie, and Qing-Hu Chen, Phys. Rev. A 102, 063721 (2020).

[3] Y. F. Xie, L. W. Duan, and Qing-Hu Chen, J. Phys. A: Math. Theor. 52, 245304 (2019).Hide

- Quantum phase transition in the Rabi-Stark model at finite frequency ratios

**Name:**Prof. Abdollah Langari**Dates for visit (online):**2pm, 3rd June 2021 (UTC+8)**Zoom link****Unit:**Sharif University of Technology, Iran**Talks in IFFS:**- Many-body localization
**Abstract:**Show more

In this talk I will review the basic notions of many-body localization, which is a dynamical phase of systems. Most of recent investigations evince the existence of many-body localization (MBL) in a closed quantum system through the presence of two key ingredients: quenched disorder in the Hamiltonian and localization of all single-particle states. Here, I will discuss the emergence of MBL within two new mechanisms, which do not need the basic requirements of the conventional MBL. In the first approach [1], we consider the Kitaev toric code on the ladder geometry, where anyon statistics leads to an emergent effect of disorder that localizes one sort of anynons. In the second mechanism [2], we introduce a clean cluster spin chain coupled to fully interacting spinless fermions, forming an unconstrained Z2 lattice gauge theory (LGT), which possesses dynamical proximity effect controlled by the entanglement structure of the initial state. We expand the machinery of interaction-driven localization to the realm of LGTs such that for any starting product state, the matter field exhibits emergent statistical bubble localization, which is driven solely by the cluster interaction, having no topologically trivial noninteracting counterpart, and thus is of a pure dynamical many-body effect.

References:

[1] H. Yarloo, A. Langari and A. Vaezi, Phys. Rev. B 97, 054304 (2018), “Anyonic self-induced disorder in a stabilizer code: quasi-many body localization in a translational invariant model”.

[2] H. Yarloo, M. Mohseni-Rajaee, A. Langari, Phys. Rev. B 99, 054403 (2019), “Emergent statistical bubble localization in a Z2 lattice gauge theory”.Hide

- Many-body localization

**Name:**Prof. Longwen Zhou**Dates for visit (online):**2pm, 27th May 2021 (UTC+8)**Zoom link****Unit:**Ocean University of China**Talks in IFFS:**- Non-Hermitian Floquet topological matter
**Abstract:**Show more

Topological states of matter have been investigated intensively in the past decades. Recently, the frontier of topological quantum matter has been extended to nonequilibrium domain, leading to the discoveries of Floquet topological matter, non-Hermitian topological matter, and dynamical quantum phase transitions. In this talk, we introduce a series of topological phases that are unique to periodically driven non-Hermitian systems, including non-Hermitian Floquet topological insulators and superconductors. We will discuss the topological characterization, edge states and bulk-edge correspondence of the non-Hermitian Floquet topological phases, and discuss dynamical observables that can be used to probe the topological invariants of non-Hermitian Floquet matter in experiments.Hide

- Non-Hermitian Floquet topological matter

**Name:**Dr. Thi Ha Kyaw**Dates for visit (online):**10am, 20th May 2021 (UTC+8)**Zoom link****Unit:**University of Toronto**Talks in IFFS:**- Squeezing noisy intermediate-scale quantum (NISQ) lemons
**Abstract:**Show more

As we have come into the second wave of the quantum revolution, the available quantum hardware is often noisy and with no explicit error correction scheme. Quantum computing tasks using such devices are known as NISQ algorithms [1]. Here, in this talk, I will tell you our recent effort in resource-efficient bosonic quantum simulation [2] given a limited number of qubits and gates, in particular, in the area of quantum simulations of quantum processors [3, 4], and adaptive ansatz construction [5]. Lastly, I will end the talk by briefly outlining our recent work on unsupervised classical machine learning quantum trajectories [6], and how one can make use of our proposal to help build better NISQ devices.

References:

[1] KB, ACL, THK, et. al., Noisy intermediate-scale quantum (NISQ) algorithms, arXiv:2101.08448 (2021)

[2] NPDS, TM, THK, et. al., Resource-efficient digital quantum simulation of d-level systems for photonic, vibrational, and spin-s Hamiltonians, npjQI (2020)

[3] THK, et. al., Quantum computer-aided design: digital quantum simulation of quantum processors, arXiv:2006.03070 (2020)

[4] JK, MK, THK, et. al., Quantum Computer-Aided design of Quantum Optics Hardware, QST (2021)

[5] ZJZ, THK, et. al., Mutual information-assisted adaptive variational quantum eigensolver, QST (2021)

[6] MC, DFS, THK, AAG, Unsupervised machine learning quantum dynamics, (in preparation)Hide

- Squeezing noisy intermediate-scale quantum (NISQ) lemons

**Name:**Dr. Carlos Navarrete-Benlloch**Dates for visit (online):**2pm, 14th May 2021 (UTC+8)**Zoom link****Unit:**Shanghai Jiaotong University**Talks in IFFS:**- The age of Many-Body Quantum Optics: time-crystalline order in Van der Pol lattices
**Abstract:**Show more

Many-body physics and quantum optics have been traditionally working on opposite sides of the physics spectrum. Quantum optics deals with the precise quantum control of atomic, optical, and solid state systems with a small number of degrees of freedom. On the other hand, many-body physics is typically associated with the statistical behavior of condensed-matter systems containing an infinite number of constituents. However, we are now entering a new era where our level of control of quantum-optical systems is so precise, that we can apply it to the design of devices that implement many-body models capable of reaching phases traditionally inaccessible to condensed-matter systems. In this talk, I will show how the addition of particle-non-conserving processes (pumping and dissipation) to bosonic many-body models allows reaching elusive phases such as supersolids or time crystals. In particular, I will focus on time-crystalline order, which we find in a model consisting of coupled quantum Van der Pol oscillators. Combining approximate analytical techniques with exact phase-space stochastic numerical simulations, we prove the existence of spontaneous self-sustained oscillations that are robust against quantum fluctuations.Hide

- The age of Many-Body Quantum Optics: time-crystalline order in Van der Pol lattices

**Name:**Dr. Marco Genoni**Dates for visit (online):**3pm, 6th May 2021 (UTC+8)**Zoom link****Unit:**University of Milan**Talks in IFFS:**- Continuous monitoring for super-classical quantum magnetometry
**Abstract:**Show more

In this talk I will address the estimation of a magnetic field acting on an ensemble of N atoms initially prepared ina coherent spin state and whose collective spin is continuously monitored. I will first show that, thanks to the spin-squeezing generated by the measurement during the evolution, one can achieve a Heisenberg-limited precision, i.e. scaling as 1/N^2 with the total number of atoms.

I will then consider the noisy scenario, assuming that independent dephasing acts on each atom. This kind of noise is known to be highly detrimental for standard magnetometry protocols employing initial entangled states, leading to a standard quantum limited precision, i.e. scaling as 1/N. I will discuss the results of our numerical simulation of the dynamics of an ensemble with up to N=150 atoms initially prepared in a (classical) spin coherent state and subjected to continuous monitoring. I will show that, also in the presence of noise, spin squeezing can be dynamically generated by the measurement, and that the estimation precision obtainable from the continuous photocurrent scales superclassically with respect to the number of atoms. I will provide evidence that such superclassical scaling holds for different values of dephasing and monitoring efficiency. Finally I will also briefly discuss the difference between our protocol and standard estimation schemes, where the state preparation time is neglected.Hide

- Continuous monitoring for super-classical quantum magnetometry

**Name:**Dr. Adolfo del Campo**Dates for visit (online):**3pm, 29th April 2021 (UTC+8)**Zoom link****Unit:**University of Luxembourg**Talks in IFFS:**- Probing the Universality of Topological Defect Formation in a Quantum Annealer
**Abstract:**Show more

The number of topological defects created in a system driven through a quantum phase transition exhibits a power-law scaling with the driving time. This universal scaling law is the key prediction of the Kibble-Zurek mechanism (KZM), and testing it using a hardware-based quantum simulator is a coveted goal of quantum information science. Here we provide such a test using quantum annealing. Specifically, we report on extensive experimental tests of topological defect formation via the one-dimensional transverse-field Ising model on two different D-Wave quantum annealing devices. We find that the quantum simulator results can indeed be explained by the KZM for open-system quantum dynamics with phase-flip errors, with certain quantitative deviations from the theory likely caused by factors such as random control errors and transient effects. In addition, we probe physics beyond the KZM by identifying signatures of universality in the distribution and cumulants of the number of kinks and their decay, and again find agreement with the quantum simulator results. This implies that the theoretical predictions of the generalized KZM theory, which assumes isolation from the environment, applies beyond its original scope to an open system. We support this result by extensive numerical computations. To check whether an alternative, classical interpretation of these results is possible, we used the spin-vector Monte Carlo model, a candidate classical description of the D-Wave device. We find that the degree of agreement with the experimental data from the D-Wave annealing devices is better for the KZM, a quantum theory, than for the classical spin-vector Monte Carlo model, thus favoring a quantum description of the device. Our work provides an experimental test of quantum critical dynamics in an open quantum system, and paves the way to new directions in quantum simulation experiments.Hide

- Probing the Universality of Topological Defect Formation in a Quantum Annealer

**Name:**Dr. Angelo Carollo**Dates for visit (online):**3pm, 8th April 2021 (UTC+8)**Zoom link****Unit:**University of Palermo, Italy**Talks in IFFS:**- Multi-parameter quantum critical metrology: a compatibility index
**Abstract:**Show more

I will introduce a measure of compatibility in quantum multi-parameter estimation problems. One can show that the ratio between the mean Uhlmann Curvature and the Fisher Information provides a figure of merit which estimates the amount of (in)compatibility arising from the quantum nature of the underlying physical system. This ratio accounts for the discrepancy between the attainable precision in the simultaneous estimation of multiple parameters and the precision predicted by the Cram\’er-Rao bound. We apply this measure to quantitatively assess the quantum character of phase transition phenomena in peculiar quantum critical models. We consider a paradigmatic class of lattice fermion systems, which shows equilibrium quantum phase transition and dissipative non-equilibrium steady-state phase transitions.Hide

- Multi-parameter quantum critical metrology: a compatibility index

**Name:**Dr. Guillermo Romero**Dates for visit (online):**10am, 2nd April 2021 (UTC+8)**Zoom link****Unit:**Universidad de Santiago de Chile, Chile**Talks in IFFS:**- Dimerized dynamics in one-dimensional lattice systems
**Abstract:**Show more

The range of atomic interactions in many-body systems depends on the interparticle distance. However, in state-of-the-art quantum simulators, short-range interactions make correlations spread over the system via delocalized excitations. Even though long-range correlations may dominate the collective behavior in a lattice, a dynamical dimerized phase emerges in the strong interaction limit of lattice systems in the non-equilibrium regime. The latter can be quantitatively characterized by the emergence of the dimer’s frequency splitting as one increases the lattice size. We use this characteristic of the many-body dynamics to define the dynamical dimerization phase (DDP). We report the latter on a family of one-dimensional lattices that include interacting bosons, polaritons, and spin-1 systems, as we quench the system from an initial state deep in the strong interaction regime of each many-body Hamiltonian. The DDP is demonstrated by numerically computing time-dependent density correlations and their corresponding discrete Fourier transform. For the particular case of interacting bosons (polaritons), the DDP is an emergent phenomenon that results from strong correlations of nearest-neighbor correlations. Recognizing the emergence of the dimer’s frequency splitting in many-body lattice systems allows us to activate strong short-range correlations while minimizing the spreading of correlations. Our findings could be tested in state-of-the-art quantum simulators such as cold atoms and superconducting circuits.Hide

- Dimerized dynamics in one-dimensional lattice systems

**Name:**Dr. Sofia Qvarfort**Dates for visit (online):**4pm, 25th March 2021 (UTC+8)**Zoom link****Unit:**University College London, UK**Talks in IFFS:**- Solving nonlinear optomechanical dynamics with a Lie-algebra method
**Abstract:**Show more

Hamiltonians that contain products of more than two operators often give rise to nonlinear equations of motions. These equations and the time-evolution of the Hamiltonian are often notoriously difficult to solve, but the nonlinear dynamics allow for the generation of non-Gaussian states and is a key component for many quantum-information processing schemes.

One example of such a nonlinear Hamiltonian is the radiation-pressure Hamiltonian that arises from light interacting with a mechanical element. Here, the photon number operator couples to the centre-of-mass position of the mechanical mode. The dynamics was solved for a constant optomechanical coupling back in 1997 [1,2], however, certain experimental implementations also allow for time-modulated couplings, for which the solution becomes more involved.

In this talk, I will provide an introduction to a Lie algebraic method [3] that allowed us to solve the dynamics for a time-dependent nonlinear optomechanical coupling. I will also quickly highlight some results that were derived from these solutions, which range from sensing to the inclusion of optical decoherence in the dynamics.

**Reference:**

[1] Bose, S., K. Jacobs, and P. L. Knight. “Preparation of nonclassical states in cavities with a moving mirror.” Physical Review A 56.5 (1997): 4175.

[2] Mancini, S., V. I. Man’ko, and P. Tombesi. “Ponderomotive control of quantum macroscopic coherence.” Physical Review A 55.4 (1997): 3042.

[3] Wei, James, and Edward Norman. “Lie algebraic solution of linear differential equations.” Journal of Mathematical Physics 4.4 (1963): 575-581.Hide

- Solving nonlinear optomechanical dynamics with a Lie-algebra method

**Name:**Prof. Jamir Morino**Dates for visit (online):**4pm, 18th March 2021 (UTC+8)**Zoom link****Unit:**University of Mainz, Germany**Talks in IFFS:**- Correlation engineering via non-local dissipation
**Abstract:**Show more

Controlling the spread of correlations in quantum many-body systems is a key challenge at the heart of quantum science and technology. Correlations are usually destroyed by dissipation arising from coupling between a system and its environment. Here, we show that dissipation can instead be used to engineer a wide variety of spatio-temporal correlation profiles in an easily tunable manner. We describe how dissipation with any translationally-invariant spatial profile can be realized in cold atoms trapped in an optical cavity. A uniform external field and the choice of spatial profile can be used to design when and how dissipation creates or destroys correlations. We demonstrate this control by preferentially generating entanglement at a desired wavevector. We thus establish non-local dissipation as a new route towards engineering the far-from-equilibrium dynamics of quantum information, with potential applications in quantum metrology, state preparation, and transport.Hide

- Correlation engineering via non-local dissipation

**Name:**Dr. Kishor Bhrati**Dates for visit (online):**3pm, 11th March 2021 (UTC+8)**Zoom link****Unit:**Center for Quantum Technologies, Singapore**Talks in IFFS:**- A blueprint for practical quantum advantage
**Abstract:**Show more

We are in the regime of noisy intermediate-scale quantum (NISQ) devices. The “quantum supremacy” experiments by the Google team and Jian-Wei Pan’s group have demonstrated such devices’ power for computationally contrived examples. So far, a practical quantum advantage is missing. Our best hope has been variational quantum algorithms (VQA) in the search for quantum advantage. These algorithms employ a classical-quantum feedback loop to update the parameters of a parametric quantum circuit. However, these algorithms have many limitations related to trainability (the barren plateau problem) and expressibility. There is no systematic way to construct a problem-aware Ansatz which can be efficiently implemented on the existing hardware.

In this talk, I will discuss a new generation of algorithms alternative to VQA. Our algorithms do not require any classical-quantum feedback loop and can be executed exceptionally faster than their VQA counterparts. These algorithms render a systematic approach to construct ansatz, circumvent the barren plateau problem, and do not require any complicated controlled multi-qubit unitaries. Our algorithms can be implemented immediately on the current-term hardware and thus provide the hope for quantum advantage in the NISQ era.Hide

- A blueprint for practical quantum advantage

**Name:**Prof. Huangjun Zhu**Dates for visit (online):**2pm, 14th January 2021 (UTC+8)**Tencent meeting link****Unit:**Fudan University**Talks in IFFS:**- Efficient verification of quantum states and quantum gates
**Abstract:**Show more

Quantum states and quantum gates are basic ingredients in quantum information processing. Efficient verification of quantum states and quantum gates based on local operations is a key to the development of quantum technologies, but is a daunting task as the system size increases. Here we present optimal or efficient protocols for verifying a number of important quantum states, including bipartite pure states, GHZ states, graph states, hypergraph states, and Dicke states. Furthermore, we present a simple and general framework for verifying unitary transformations that can be applied to both individual quantum gates and gate sets, including quantum circuits. This framework enables efficient verification of all bipartite unitaries, Clifford unitaries, generalized controlled-Z gates, generalized CNOT gates, and CSWAP gate.Hide

- Efficient verification of quantum states and quantum gates

## 2020:

**Name:**Prof. Xiong-Jun Liu**Dates for visit (online):**3pm, 31st December 2020 (UTC+8)**Zoom link****Unit:**Peking University**Talks in IFFS:**- Universal far-from-equilibrium quantum dynamics in topological systems
**Abstract:**Show more

A most important theme in condensed matter physics is how to characterize the fundamental states of quantum matter. The celebrated paradigms include Landau symmetry-breaking and topological quantum phases, whose characterizations have been broadly developed based on the equilibrium theories over the past half century. In comparison, the non-equilibrium quantum dynamics are far less understood. In this talk, I will present pedagogically a generic theory to characterize the far-from-equilibrium quantum dynamics induced in topological quantum systems, by showing a universal correspondence between equilibrium topological phases and non-equilibrium quantum dynamics, which in turn provides the non-equilibrium characterization of equilibrium topological phases. This generic theory is built on the so-called dynamical bulk-surface correspondence, which connects dD bulk topology of an equilibrium phase to topological pattern of quench dynamics emerging in the (d-1)D momentum subspace, dubbed band-inversion surfaces (BISs), rendering a momentum-space counterpart of the well-known bulk-boundary correspondence for equilibrium topological phases in the real space. This dynamical bulk-surface correspondence can be extended to broad topological systems, including the correlated phases, Floquet phases, and topological states realized in synthetic dimensions. The rich nontrivial and universal far-from-equilibrium topological dynamics can be predicted. These results also provide conceptually new schemes to detect equilibrium topological phases by non-equilibrium quantum dynamics, which exhibit clear advantages in the detection and show the capabilities beyond equilibrium schemes. Finally, the future interesting issues will be commented.Hide

- Universal far-from-equilibrium quantum dynamics in topological systems

**Name:**Prof. Animesh Datta**Dates for visit (online):**5pm, 10th December 2020 (UTC+8)**Zoom link****Unit:**University of Warwick, UK**Talks in IFFS:**- Quantum-enhanced sensing of single and multiple parameters
**Abstract:**Show more

Quantum-enhanced sensing is one of the several potential applications of quantum technologies. I will discuss the quantum information theoretic aspects of estimating single and multiple parameters, with an emphasis on recent advances and applications. The latter will include applications in imaging and 3D magnetometry.Hide

- Quantum-enhanced sensing of single and multiple parameters

**Name:**Prof. Luyan Sun**Dates for visit (online):**2pm, 3rd December 2020 (UTC+8)**Zoom link****Unit:**Tsinghua University**Talks in IFFS:**- Quantum error correction and error-transparent gate based on a binomial bosonic code
**Abstract:**Show more

Quantum error correction (QEC) is necessary for a practical quantum computer because of the inevitable coupling of quantum systems with the uncontrolled environment. A measurement-based QEC requires rapid extraction of error syndromes without disturbing the stored information and fast real-time feedback control for error corrections. Encoding quantum information on photonic states in a microwave cavity for QEC has attracted a lot of interests because of its hardware efficiency [1]. This scheme benefits from the infinite dimensional Hilbert space of a harmonic oscillator for redundant information encoding and only one error syndrome that needs to be monitored. In this talk, I will describe our experimental realizations of repetitive QEC and full control of a binomial bosonic code in a circuit quantum electrodynamics architecture [2], as well as error transparent gates based on the binomial code [3].**Reference:**

[1] W. Cai, Y. Ma, W. Wang, C.-L. Zou and L. Sun, arXiv:2010.08699 (2020).

[2] L. Hu, Y. Ma, W. Cai, X. Mu, Y. Xu, W. Wang, Y. Wu, H. Wang, Y. P. Song, C.-L. Zou, S. M. Girvin, L-M. Duan and L. Sun, Nature Physics 15, 503 (2019).

[3] Y. Ma, Y. Xu, X. Mu, W. Cai, L. Hu, W. Wang, X. Pan, H. Wang, Y. P. Song, C.-L. Zou, L. Sun, Nature Physics 16, 827 (2020).Hide

- Quantum error correction and error-transparent gate based on a binomial bosonic code

**Name:**Prof. Victor M. Bastidas**Dates for visit (online):**2pm, 26th November 2020 (UTC+8)**Zoom link****Unit:**NTT Basic Research Laboratories, Japan**Talks in IFFS:**- Crystallization of time: Quantum simulation of complex networks and beyond
**Abstract:**Show more

Recently time crystals have attracted major interest from the physics community as they are exotic phases of matter characterized by a robust dynamic response that breaks discrete time-translation symmetry [1–4]. The development of new insights into how such new phase of matter arise as well as determining what applications they may be useful for is much in demand. The use of graphs to analyze large sets of data has proven to be extremely useful to observe dynamical behaviors giving rise to the subfield of complex networks. In this talk I will discuss how to bridge these two areas by establishing a novel strategy to identify, characterize and analyze time crystals in terms of graphs [5]. Our approach completely changes the way we look at time-periodic systems and related phenomena [6–9]. It is under this perspective that we envisage the use of such unique physical systems as platforms where to simulate complex quantum networks. The simulation of such networks has wide applicability in diverse communities. Importantly, our proposal could be implemented in current noisy intermediate-scale quantum (NISQ) devices, ranging from trapped ions to superconducting qubit chips [10].**Reference:**

[1] F. Wilczek, Phys. Rev. Lett. 109, 160401 (2012).

[2] V. Khemani, A. Lazarides, R. Moessner, S. L. Sondhi, Phys. Rev. Lett. 116, 250401 (2016).

[3] N. Y. Yao, A. C. Potter, I.-D. Potirniche, A. Vishwanath, Phys. Rev. Lett. 118, 030401 (2017).

[4] K. Sacha, J. Zakrzewski, Rep. Prog. Phys. 81, 016401 (2018).

[5] M. P. Estarellas, T. Osada, V. M. Bastidas, B. Renoust, K. Sanaka, W. J. Munro, Kae Nemoto, Science Advances 6, eaay8892 (2020).

[6] A. Lazarides, A. Das, and R. Moessner, Phys. Rev. Lett. 115, 030402 (2015).

[7] S. Restrepo, J. Cerrillo, V. M. Bastidas, D. G. Angelakis, and T. Brandes, Phys. Rev. Lett. 117, 250401 (2016).

[8] A. Eckardt, Rev. Mod. Phys. 89, 011004 (2017).

[9] V. M. Bastidas, B. Renoust, Kae Nemoto, W. J. Munro, Phys. Rev. B 98, 224307 (2018).

[10] P. Roushan, C. Neill, J. Tangpanitanon, V. M. Bastidas, A. Megrant, R. Barends, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, A. Fowler, B. Foxen, M. Giustina, E. Jeffrey, J. Kelly, E. Lucero, J. Mutus,1 M. Neeley, C. Quintana, D. Sank, A. Vainsencher, J. Wenner, T. White, H. Neven, D. G. Angelakis, J. Martinis, Science 358, 1175 (2017).Hide

- Crystallization of time: Quantum simulation of complex networks and beyond

**Name:**Prof. Dongling Deng**Dates for visit (online):**2pm, 12th November 2020 (UTC+8)**Zoom link****Unit:**Tsinghua University**Talks in IFFS:**- Quantum Artificial Intelligence — The Present and Future
**Abstract:**Show more

Quantum artificial intelligence (Quantum AI) is an emergent interdisciplinary field that explores the interplay between artificial intelligence and quantum physics. On the one hand, judiciously designed quantum algorithms may exhibit exponential advantages in solving certain AI problems; on the other hand, ideas and techniques from AI can also be exploited to tackle challenging problems in the quantum domain. In this talk, I will first make a brief introduction to this field and review some recent progresses. I will talk about several concrete examples to illustrate how AI and quantum physics can promote studies in both fields. At the end of the talk, I will pose ten fundamental challenges facing quantum AI that, if overcome, would give a significant boost to this fledgling field full of uncertainties and opportunities.Hide

- Quantum Artificial Intelligence — The Present and Future

**Name:**Prof. Haidong Yuan**Dates for visit (onsite):**11am, 6th November 2020 (UTC+8)**Room 816, IFFS building (shahe campus)****Unit:**Chinese University of Hong Kong**Talks in IFFS:**- Ultimate precision limit for quantum magnetometry
**Abstract:**Show more

Measurement and estimation of the magnetic field is essential for many practical applications, where the main quest is to find out the highest achievable precision with given resources and design schemes to attain it. In this talk I will present the ultimate precision that can be achieved for quantum magnetometry with the optimal entangled probe states. I will talk about how to further improve the precision with optimal quantum controls.Hide

- Ultimate precision limit for quantum magnetometry

**Name:**Prof. Yue Ban**Dates for visit (online):**4pm, 5th November 2020 (UTC+8)**Zoom link****Unit:**The University of the Basque Country UPV/EHU, Bilbao, Spain**Talks in IFFS:**- Speeding up quantum perceptron via shortcuts to adiabaticity
**Abstract:**Show more

The quantum perceptron is a fundamental building block for quantum machine learning. This is a multidisciplinary field that incorporates abilities of quantum computing, such as state superposition and entanglement, to classical machine learning schemes. Motivated by the techniques of shortcuts to adiabaticity, we propose a speed-up quantum perceptron where a control field on the perceptron is inversely engineered leading to a rapid nonlinear response with a sigmoid activation function. This results in faster overall perceptron performance compared to quasi-adiabatic protocols, as well as in enhanced robustness against imperfections in the controls.Hide

- Speeding up quantum perceptron via shortcuts to adiabaticity

**Name:**Prof. Oscar Dahlsten**Dates for visit (online):**2pm, 29th October 2020 (UTC+8)**Zoom link****Unit:**SUSTech (Southern University of Science and Technology), China**Talks in IFFS:**- Quantum generalisations of feedforward neural nets, designs and applications
**Abstract:**Show more

Classical feedforward neural nets are tunable logical gates. Training the neural nets means tuning these gates. These gates can be generalised to parametrised quantum unitaries. Training the network classically amounts to tuning these parameters. This can be used for quantum generalisations of classical neural nets like auto-encoders, as well as for discovering quantum algorithms for classical tasks. One can also train the network in a superposition of parameters, in a Grover-like training procedure which has a speed-up relative to classical brute force global optimisation.Hide

- Quantum generalisations of feedforward neural nets, designs and applications

**Name:**Dr. Daniel Ebler**Dates for visit (online):**2pm, 23rd October 2020 (UTC+8)**Zoom link****Unit:**SUSTech (Southern University of Science and Technology), China**Talks in IFFS:**- Annealing as a paradigm for solving hard combinatorial problems
**Abstract:**Show more

Combinatorial problems are ubiquitous for a large variety of scientific disciplines, encompassing portfolio optimization in finance, molecular dynamics in biology and drug design in chemistry. However, solving such problems is extremely challenging. Indeed, even elementary instances have been shown to be NP-complete, such that the fastest supercomputers today get pushed beyond their limits when addressing real-life problems.

Lately, a fundamentally different approach towards solving combinatorial problems has arisen attention. It has been shown that all of Karp’s 21 NP-complete computational problems can be formulated as finding round states of spin-glass systems. This shifted the focus shifted towards special-purpose machines – made to address solely the specific problems of interest – rather than using conventional classical universal computers. The fundamental idea is to let a system evolve naturally – in accordance to the laws of physics – instead of constructing algorithms to search for the optimal solution. Spin-glass systems are microscopic, and, hence, it deems crucial to utilize quantum systems for the efficient system evolution.

This talk provides an overview of one of the promising approaches to find the ground states of quantum spin-glass models: quantum annealing. Basic concepts, comparisons to classical approaches, as well as limitations of quantum annealing and future directions of research will be discussed.Hide

- Annealing as a paradigm for solving hard combinatorial problems

**Name:**Prof. Zhaohui Wei**Dates for visit (online):**2pm, 22nd October 2020 (UTC+8)**Zoom link****Unit:**Tsinghua University**Talks in IFFS:**- Quantifying unknown entanglement in a device-independent manner
**Abstract:**Show more

Certifying unknown quantum entanglement experimentally is a fundamental problem in quantum information. However, due to the imperfection of quantum operations, this is a very challenging task, not to mention quantifying unknown (multipartite) entanglement experimentally. Fortunately, it turns out these two tasks can be fulfilled reliably and efficiently using device-independent approaches, where the certification and quantification are achieved based on the observed nonlocality only and one does not have to care about the precision and internal workings of involved quantum devices. In this talk, we will introduce several recent progresses on this problem.Hide

- Quantifying unknown entanglement in a device-independent manner

**Name:**Prf. Lorenzo Maccone**Dates for visit (online):**3pm, 15th October 2020 (UTC+8)**Zoom link****Unit:**University of Pavia, Italy**Talks in IFFS:**- Squeezing Metrology: a unified framework
**Abstract:**Show more

I will briefly introduce quantum metrology and then focus on quantum metrology for squeezing. Usually the theory uses the resolution gains obtainable thanks to the entanglement among N probes. Typically, a quadratic gain in resolution is achievable, going from the 1/\sqrt(N) of the central limit theorem to the 1/N of the Heisenberg bound. We focus on quantum squeezing and provide a unified framework for metrology with squeezing, showing that, similarly, one can generally attain a quadratic gain when comparing the resolution achievable by a squeezed probe to the best N-probe classical strategy achievable with the same energy. Namely, here we give a quantification of the Heisenberg squeezing bound for arbitrary estimation strategies that employ squeezing. Our theory recovers known results (e.g. in quantum optics and spin squeezing), but it uses the general theory of squeezing and holds for arbitrary quantum systems. The talk is based on the paper:

L. Maccone, A. Riccardi, Squeezing Metrology: a unified framework, Quantum 4, 292 (2020).Hide

- Squeezing Metrology: a unified framework

**Name:**Prof. Tony J. G. Apollaro**Dates for visit (online):**3pm, 9th October 2020 (UTC+8)**Zoom link****Unit:**Department of Physics, University of Malta**Talks in IFFS:**- Many qubit quantum state transfer and routing via resonant tunnelling in spin chains
**Abstract:**Show more

Quantum-State Transfer (QST) is an important requisite in many Quantum Information Processing (QIP) protocols [1]. For short-distance communication, interacting spin-1/2 chains as a data bus fulfilling faithful QST between a sender and a receiver qubit have been widely investigated and different protocols have been proposed. A step forward would be to allow for the use of a single data bus by many senders and receivers for QIP protocols. In this talk a protocol for high-fidelity QST from one sender to a selected receiver out of many possible ones is proposed by exploiting a weak-coupling mechanics inducing resonant tunnelling of the excitation between the sender and the selected receiver location. The same protocol is also explored in order to achieve QST of more than a single qubit [2], generation of multiple pairs of Bell states [3], and transfer of multipartite entanglement [4].**Reference:**

[1] S. Bose (2007) Quantum communication through spin chain dynamics: an introductory overview, Contemporary Physics, 48:1, 13-30, http://dx.doi.org/10.1080/00107510701342313

[2] W.J. Chetcuti, C. Sanavio, S. Lorenzo and T.J.G. Apollaro, Perturbative many-body transfer, 2020 New J. Phys. 22 033030 https://doi.org/10.1088/1367-2630/ab7a33

[3] T.J.G. Apollaro, G.M.A. Almeida, S. Lorenzo, A. Ferraro, and S. Paganelli, Spin chains for two-qubit teleportation, Phys. Rev. A 100, 052308 (2019) https://doi.org/10.1103/PhysRevA.100.052308

[4] T.J.G. Apollaro, C. Sanavio, W.J. Chetcuti, S. Lorenzo, Multipartite entanglement transfer in spin chains, Physics Letters A 384, 126306 2020 https://doi.org/10.1016/j.physleta.2020.126306onyHide

- Many qubit quantum state transfer and routing via resonant tunnelling in spin chains

**Name:**Prof. Ali Rezakhani**Dates for visit (online):**2pm, 24th September 2020 (UTC+8)**Zoom link****Unit:**Sharif University of Technology, Iran**Talks in IFFS:**- Dynamics of Open Quantum Systems from a Correlation Perspective
**Abstract:**Show more

Isolating a quantum system from ambient environment is practically impossible. Interaction with the environment makes obtaining an exact master equation for the dynamics of an open system hard. Most of the existing techniques for deriving a dynamical master equation for an open system are approximative and based on weak-coupling expansions. I give a brief review of such techniques and in particular how the celebrated Lindblad equation is obtained under the weak-coupling, Markovian assumption. I then introduce a novel correlation picture which enables us to derive an exact Lindblad-like master equation, with system-environment correlations included. I also introduce a weak-correlation expansion that allows to perform systematic perturbative approximations. This expansion provides approximate master equations which can feature advantages over existing weak-coupling techniques in the sense that they are more accurate or as accurate as weak-coupling master equations.Hide

- Dynamics of Open Quantum Systems from a Correlation Perspective

**Name:**Prof. Luis Correa**Dates for visit (online):**3pm, 17th September 2020 (UTC+8)**Zoom link****Unit:**University of Exeter, UK**Talks in IFFS:**- Getting “quantum” about thermometry
**Abstract:**Show more

Recent experimental advances have made record-breaking cooling possible in ultracold gases. And yet, state-of-the-art thermometric techniques fall short of the challenge of measuring their temperatures with high accuracy in the subnanokelvin range. The limitations are not just technical—there are stringent fundamental bounds on the precision of low-temperature thermometry. To approach them as closely as possible, it becomes necessary to revise every step in the estimation process: from the design of the probe, to the choice of temperature estimator, and the physical modelling of probe and sample, which dictates how the measured data must be processed into a temperature estimate. The emerging field of quantum thermometry aims at answering these questions, by blending quantum parameter estimation with the theory of open quantum systems. In this talk I will give an overview of the field and focus on applications to impurity thermometry in ultracold atomic gases.Hide

- Getting “quantum” about thermometry

**Name:**Dr. Rubem Mondaini**Dates for visit (online):**2pm, 10th September 2020 (UTC+8)**Zoom link****Unit:**Beijing Computational Science Research Center, China**Talks in IFFS:**- Many-body localization: When thermalization fails and how to experimentally observe it
**Abstract:**Show more

The observation of many-body localization is a paradigmatic example of the amount of time an idea takes to get mature enough, and the numerical and experimental methods to sufficiently develop, in order to settle its existence. After the original study of Philip Anderson in 1958, demonstrating localization of non-interacting quantum particles in disordered settings, a natural question is on the resulting effects of the inter-particle interactions on this phenomenon. Only after 50 years, substantial theoretical progress was made in solving this puzzle and, in 2016 the first experimental observation of this phenomenon was realized. The advent of platforms involving ultracold atoms trapped by optical lattices allowed the inspection of an inherently dynamical quantum phase transition, that goes beyond the standard ground-state classification of the quantum matter, and its associated low-lying excitations. Instead, it is described by a high-energy phase transition, inherently manifested via the unitary dynamics of an isolated quantum system, wherein by tuning the strength of disorder, one is able to halt the onset of ergodic behavior and thermalization. In this talk, after introducing the general conditions where it occurs, and review the experiments tackling it so far, I will show numerical and experimental results using quantum circuits of superconducting qubits that shed light on yet another highly debated aspect: the possible existence of many-body mobility edges.**Reference:**arXiv preprint, arXiv:1912.02818, “Observation of energy resolved many-body localization”, Qiujiang Guo, Chen Cheng, Zheng-Hang Sun, Zixuan Song, Hekang Li, Zhen Wang, Wenhui Ren, Hang Dong, Dongning Zheng, Yu-Ran Zhang, Rubem Mondaini, Heng Fan, H. Wang. — Nature Physics (in press)Hide

- Many-body localization: When thermalization fails and how to experimentally observe it

**Name:**Dr. Amir Mohammadi**Dates for visit (online):**3pm, 3rd September 2020 (UTC+8)**Zoom link****Unit:**Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Germany**Talks in IFFS:**- Towards simulating of chemical reactions at the quantum limits
**Abstract:**Show more

The rates of chemical reactions are predicted to be increased rapidly at very low temperatures where the quantum mechanical treatments start dominating the classical processes. In such an interaction regime, one can get precise control over the chemical processes by finding a full understanding from the dynamics of involved parameters acting on the reactions, including but not limited to initial quantum states of the reactants and applied external fields. The system under investigation can be even more complicated if reactants are molecules with microscopic degrees of freedom. In this work, we show how such a system can be simulated via a hybrid atom-ion system in which the dynamics and evolution of a single molecular ion inside a bath of ultracold neutral Rb atoms are investigated at very low collision energies of a few mK.Hide

- Towards simulating of chemical reactions at the quantum limits

**Name:**Dr. Kishor Bharti**Dates for visit (online):**2pm, 27th August 2020 (UTC+8)**Zoom link****Unit:**Center for Quantum Technologies, Singapore**Talks in IFFS:**- Near-term quantum algorithms for linear systems of equations
**Abstract:**Show more

Solving linear systems of equations is essential for many problems in science and technology, including problems in machine learning. Existing quantum algorithms have demonstrated the potential for large speedups, but the required quantum resources are not immediately available on near-term quantum devices. In this work, we study near-term quantum algorithms for linear systems of equations of the form Ax = b. We investigate the use of variational algorithms and analyze their optimization landscapes. There exist types of linear systems for which variational algorithms designed to avoid barren plateaus, such as properly initialized imaginary time evolution and adiabatic-inspired optimization, suffer from a different plateau problem. To circumvent this issue, we design near-term algorithms based on a core idea: the classical combination of variational quantum states (CQS). We exhibit several provable guarantees for these algorithms, supported by the representation of the linear system on a so-called Ansatz tree. The CQS approach and the Ansatz tree also admit the systematic application of heuristic approaches, including a gradient-based search. We have conducted numerical experiments solving linear systems as large as 2 300 × 2 300 by considering cases where we can simulate the quantum algorithm efficiently on a classical computer. These experiments demonstrate the algorithms’ ability to scale to system sizes within reach in near-term quantum devices of about 100-300 qubits.Hide

- Near-term quantum algorithms for linear systems of equations

**Name:**Dr. Tommaso Macrì**Dates for visit (online):**5pm, 20th August 2020 (UTC+8)**Zoom link****Unit:**Universidade Federal do Rio Grande do Norte, Natal, Brazil**Talks in IFFS:**- Superfluidity and quasicrystals with nonlocal interactions
**Abstract:**Show more

In recent years, propelled by the progress in the field of quantum simulations with ultracold atoms, there has been an increasing interest of the condensed matter community in what is generally called quasicrystal lattices, long-range ordered but non-periodic structures. Besides retaining intrinsic relevant questions that range from the stability of tiled structures at zero temperature to their relation to fractal lattices, quasicrystals have also shown to support quantum phases of matter such as superconductors and Bose-Einstein condensates. Nonetheless, in spite of important works that address the emergence of quasicrystalline order in classical systems, a deeper understanding of the role of quantum fluctuations in these structures still lacks. Here we present our proposal to realize quasi-crystalline states in ultra cold setups with non-local interactions.Hide

- Superfluidity and quasicrystals with nonlocal interactions

**Name:**Dr. Nana Liu**Dates for visit (online):**13th August 2020**Unit:**Shanghai Jiao Tong University**Talks in IFFS:**- Adversarial quantum learning
**Abstract:**Show more

In the classical world, there is a powerful interplay between security and machine learning deployed in a network, like on the modern internet. What happens when the learning algorithms and the network itself can be quantum? What are the new problems that can arise and can quantum resources offer advantages to their classical counterparts?

We explore these questions in a new area called adversarial quantum learning, that combines the area of adversarial machine learning, which investigates security questions in machine learning, and quantum information.

For the first part of the talk, I’ll introduce adversarial machine learning and some exciting potential prospects for contributions from quantum information and computation. For the second part of the talk, I’ll present two recent works on adversarial quantum learning. Here we are able to quantify the vulnerability of quantum algorithms for classification against adversaries and learn how to leverage quantum noise to improve its robustness against attacks.Hide

- Adversarial quantum learning

**Name:**Dr. Xingyao Wu**Dates for visit (online):**10th August 2020**Unit:**HiQ team in Huawei**Talks in IFFS:**- An introduction to Huawei’s HiQ simulator and quantum research
**Abstract:**Show more

HiQ is Huawei’s high-performance cloud platform for large-scale quantum circuit simulations based on HUAWEI CLOUD’s powerful computing and storage infrastructure. It plays an important role in quantum hardware design and verification, quantum algorithm research and quantum software design. The world largest VQE simulation has been carried out on HiQ (in terms of number of orbitals). In this talk, I’ll introduce the basic functionalities of HiQ and its application scenarios. I’ll also briefly introduce our recent research on quantum chemistry VQE and quantum optimization.Hide

- An introduction to Huawei’s HiQ simulator and quantum research

**Name:**Dr. Tao Wang**Dates for visit (online):**30th July 2020**Unit:**Department of Physics, and Center of Quantum Materials and Devices, Chongqing University (重庆大学)**Talks in IFFS:**- Floquet engineering on Sr-87 optical lattice clock platform
**Abstract:**Show more

Floquet engineering is a powerful tool to customize quantum states and Hamiltonians via time-periodic fields. On the other hand, the optical lattice clock (OLC) is among the most accurate precision measurement devices. Considering the long life time of the clock state, the OLC becomes an ideal candidate for Floquet engineering the internal (atomic) energy distribution. In this talk, I will introduce the theoretical and experimental research on this project. Our work aims to design of a new generation of devices for sensing, metrology and simulating exotic spin-orbit Hamiltonian.Hide

- Floquet engineering on Sr-87 optical lattice clock platform

## 2019:

**Name:**Xuefeng Zhang (张学锋)**Dates for visit:**27th June 2019 – 28th June 2019**Unit:**Chongqing University (重庆大学)**Talks in IFFS:**- Frustrated optical lattice: from topological excitations to deconfined phase transition
- Machine learning of quantum phase transitions

**Name:**Chiranjib Mukhopadhyay**Dates for visit:**11st July 2019 – 15th July 2019**Unit:**Harish-Chandra Research Institute**Talks in IFFS:**- Thermometric schemes using superposition of temporal order and weak measurement

**Name:**Matteo Paris**Dates for visit:**13rd July 2019 – 20th July 2019**Unit:**Harish-Chandra Research Institute**Talks in IFFS:**- Universal Quantum Magnetometry with Spin States at Equilibrium
- Quantum sensing and metrology: the quantum Cramer-Rao bound and beyond

**Name:**Haidong Yuan (袁海东)**Dates for visit:**22nd July 2019 – 25th July 2019**Unit:**The Chinese University of Hong Kong (香港中文大学)**Talks in IFFS:**- Ultimate precision limit for quantum parameter estimation

**Name:**John Charles Mcintosh Gray**Dates for visit:**22nd Sep 2019 – 29th Sep 2019**Unit:**Imperial College London**Talks in IFFS:**- Tensor Network Tomography
- Tensor Network Basics
- Tensor Network Algorithms

**Name:**Mikio Nakahara + Shingo Kukita**Dates for visit:**24th Nov 2019 – 27th Nov 2019**Unit:**Shanghai University (上海大学)**Talks in IFFS:**- Spin-selective electron transfer in a quantum dot array
- An upper bound on the number of compatible parameters in simultaneous quantum estimation

**Name:**Marc-Antoine Lemonde**Dates for visit:**26th Nov 2019 – 1st Dec 2019**Unit:**National University of Singapore**Talks in IFFS:**- Phonon Networks with Silicon-Vacancy Centers in Diamond Waveguides

**Name:**Yuji Hirono**Dates for visit:**16th Dec 2019 – 21st Dec 2019**Unit:**Asia Pacific Center for Theoretical Physics (South Korea)**Talks in IFFS:**- Topological order, higher-form symmetry, and dense quark matter