Abstracts
Abstract for posters can be found below.
Oral presentations
Click on the title to see an abstract. Oral presentations sessions are on Wednesday 2:00 - 2:45 PM and Friday 11:00 AM - 12:30 PM. Abstracts are in the order they will be presented.
| 1. | Timothy Heightman | Invited talk on behalf of Prof. Antonio Acin |
| 2. | Alexandre Orthey | Quantum switch instabilities with an open control |
| 3. | Marek Tylutki | Dynamics of one-dimensional ultradilute quantum liquids |
| 4. | Adam Bednorz | Null witnessing of quantum resources |
| 5. | Anubhav Chaturvedi | Robust self-testing of Bell inequalities tilted for maximal loophole-free nonlocality |
| 6. | Grzegorz Rajchel-Mieldzioć | Entanglement classification and non-k-separability certification via Greenberger-Horne-Zeilinger-class fidelity |
| 7. | Kaushik Joarder | Towards entanglement-based QKD in a hybrid channel, for the daylight and uplink satellite scenario |
| 8. | Paweł Gora | Solving Vehicle Routing Problems using Quantum Computing |
Invited talk on behalf of Prof. Antonio Acin
Timothy Heightman (ICFO)
Understanding and characterising quantum many-body dynamics remains a significant challenge due to both the exponential complexity required to represent quantum many-body Hamiltonians, and the need to accurately track states in time under the action of such Hamiltonians. This inherent complexity limits our ability to characterise quantum many-body systems, highlighting the need for innovative approaches to unlock their full potential. To address this challenge, we propose a novel method to solve the Hamiltonian Learning (HL) problem-inferring quantum dynamics from many-body state trajectories-using Neural Differential Equations combined with an Ansatz Hamiltonian. Our method is reliably convergent, experimentally friendly, and interpretable, making it a stable solution for HL on a set of Hamiltonians previously unlearnable in the literature. In addition to this, we propose a new quantitative benchmark based on power laws, which can objectively compare the reliability and generalisation capabilities of any two HL algorithms. Finally, we benchmark our method against state-of-the-art HL algorithms with a 1D spin-1/2 chain proof of concept.
Quantum switch instabilities with an open control
Alexandre Orthey (IPPT PAN)
The superposition of causal order shows promise in various quantum technologies. However, the fragility of quantum systems arising from environmental interactions, leading to dissipative behavior and irreversibility, demands a deeper understanding of the possible instabilities in the coherent control of causal orders. In this work, we employ a collisional model to investigate the impact of an open control system on the generation of interference between two causal orders. We present the environmental instabilities for the switch of two arbitrary quantum operations and examine the influence of environmental temperature on each potential outcome of control post-selection. Additionally, we explore how environmental instabilities affect protocol performance, including switching between mutually unbiased measurement observables and refrigeration powered by causal order superposition, providing insights into broader implications.
Dynamics of one-dimensional ultradilute quantum liquids
Marek Tylutki (Warsaw University of Technology)
The progress in cooling and control of ultracold quantum gases and their mixtures has allowed for engineering new states of matter in laboratories. For example, such system could be an ultradilute quantum liquid in a binary mixture of Bose-Einstein condensates, where a low compressibility favours constant bulk density and allows for formation of finite-size quantum droplets. We study one-dimensional ultradilute quantum liquids in the context of their collective excitations and Josephson dynamics. For instance, the breathing mode is the only excitation that remains trapped in a one-dimensional quantum droplet and its frequency is uniquely determined by the particle number. The Josephson oscillations are characterized by dynamics that differs from that of a single-component Bose-Einstein condensate and is characterized by non-linear localization revivals.
Null witnessing of quantum resources
Adam Bednorz (University of Warsaw)
We have developed a precise method of certification of the Hilbert space of single and bipartite quantum states. We constructed a witness, a special determinant function of probabilities of results of a specified measurement protocol, which is zero up to finite statistics error if the system is of a limited dimension. The proposed protocol involves prepare and measure scenario in the case of single systems, in which the two stages are independent or measure and measure scenario for a bipartite system, assuming the independence of the two parties. The application to IBM Quantum network revealed inconsistencies in the simple qubit picture of a qubit network, indicating a contribution of extra space beyond standard technical imperfections. The method demonstrates the usefulness of null witnessing in the diagnostics of quantum computers. We also show that even simple tests of no-signaling, which also count as null witnesses, fail in particular cases of IBM Quantum.
Robust self-testing of Bell inequalities tilted for maximal
loophole-free nonlocality
Anubhav Chaturvedi (Gdańsk University of Technology)
The degree of experimentally attainable nonlocality, as gauged by the amount of loophole-free violation of Bell inequalities, remains severely limited due to inefficient detectors. We address an experimentally motivated question: Which quantum strategies attain the maximal loophole-free nonlocality in the presence of inefficient detectors? For any Bell inequality and any specification of detection efficiencies, the optimal strategies are those that maximally violate a tilted version of the Bell inequality in ideal conditions. In the simplest scenario, we demonstrate that the quantum strategies that maximally violate the tilted versions of Clauser-Horne-Shimony-Holt inequality are unique up to local isometries. However, self-testing via the standard sum of squares decomposition method turns out to be analytically intractable since even high levels of the Navascués-Pironio-Acín hierarchy are insufficient to saturate the maximum quantum violation of these inequalities. Instead, we utilize a novel Jordan's lemma-based proof technique to obtain robust analytical self-testing statements for the entire family of tilted-Bell inequalities. These results allow us to unveil intriguing aspects of the effect of inefficient detectors and the complexity of characterizing the set of quantum correlations, in the simplest Bell scenario.
Entanglement classification and non-k-separability
certification via Greenberger-Horne-Zeilinger-class fidelity
Grzegorz Rajchel-Mieldzioć
Many-body quantum systems can be characterised using the notions of k-separability and entanglement depth. A quantum state is k-separable if it can be expressed as a mixture of k entangled subsystems, and its entanglement depth is given by the size of the largest entangled subsystem. In this paper we propose a multipartite entanglement measure that satisfies the following criteria: (i) it can be used with both pure and mixed states; (ii) it is encoded in a single element of the density matrix, so it does not require knowledge of the full spectrum of the density matrix; (iii) it can be applied to large systems; and (iv) it can be experimentally verified. The proposed method allows the certification of non-k-separability of a given quantum state. We show that the proposed method successfully classifies three-qubit systems into known stochastic local operations and classical communication (SLOCC) classes, namely bipartite, W-, and GHZ-type entanglement. Furthermore, we characterise the non-k-separability in known nine SLOCC classes of four-qubit states, absolutely maximally entangled states for five and six qubits and for arbitrary size qubit Dicke states.
Towards entanglement-based QKD in a hybrid channel, for the
daylight
and uplink satellite scenario
Kaushik Joarder (Nicolaus Copernicus University, Toruń)
We present a proof-of-principle demonstration of entanglement-based quantum key distribution (QKD) in a satellite uplink channel during a noisy daylight background. This configuration is challenging due to the low signal-to-noise ratio (SNR). Hence, to ensure secure QKD, it is essential to enhance the SNR by filtering out noise, which makes SPDC-based entangled-photon sources less attractive due to their lower brightness and broader spectral width. To address this, we first demonstrate an SPDC-based ultra-bright source (~8 Mcps/mW) of polarization-entangled photon pairs characterized by a narrow temporal jitter and spectral bandwidth of 0.35 ns and 0.54 nm FWHM respectively. Using this source, we measure the secret key rate and QBER for a hybrid QKD channel, with Alice connected via free space and Bob connected via a fiber channel. Our simulated uplink channel attenuation is varied by attenuating the signal photon counts (from 20 dB to 50 dB). The background noise level at the satellite is simulated using a light source emitting unpolarized photons at a rate of up to a few MHz, with the same central wavelength as the signal photons. we also propose a novel theoretical model of polarization visibility that aligns well with our experimental data and is consistent with prevailing models in the field. Unlike other methods, this model is suitable for both continuous wave (CW) and pulse-pumped sources, providing a more intuitive and comparative understanding of both processes. Our model considers the density matrix of a non-ideal entangled photon source instead of an idealized Hamiltonian, as conventionally considered. Using this model, we demonstrate that entanglement-based QKD in uplink-daylight conditions is feasible only up to 400 km in a lower LEO orbit, using single-mode fiber (SMF) coupling and state-of-the-art technology.
Solving Vehicle Routing Problems using Quantum Computing
Paweł Gora (Quantum AI Foundation)
In this talk, I will present some approaches to solving the Vehicle Routing Problem and its variants using quantum computing. First, I will formally define the considered optimization problems and the corresponding QUBO formulations. Then, I will explain how these problems can be tackled using quantum annealing, QAOA, and VQE, and present the results of the experiments. Finally, I will explain how these approaches have been improved using graph coarsening and how they can be further improved using other classical methods.
Posters
Click on the title to see an abstract. Poster session is on Wednesday 3:30 - 5:30 PM.
Hilbert curve vs Hilbert space: exploiting fractal 2D covering to
increase tensor network efficiency
Ashkan Abedi (Scuola Normale Superiore di Pisa)
We present a novel mapping for studying 2D many-body quantum systems by solving an effective, one-dimensional long-range model in place of the original two-dimensional short-range one. In particular, we address the problem of choosing an efficient mapping from the 2D lattice to a 1D chain that optimally preserves the locality of interactions within the TN structure. By using Matrix Product States (MPS) and Tree Tensor Network (TTN) algorithms, we compute the ground state of the 2D quantum Ising model in transverse field with lattice size up to 64x64, comparing the results obtained from different mappings based on two space-filling curves, the snake curve and the Hilbert curve. We show that the locality-preserving properties of the Hilbert curve leads to a clear improvement of numerical precision, especially for large sizes, and turns out to provide the best performances for the simulation of 2D lattice systems via 1D TN structures.
Quantum observables over time for information recovery
Gabriele Bressanini (Imperial College London)
We introduce the concept of quantum observables over time (QOOT), an operator that jointly describes two observables at two different time points, as a generalization of the quantum state over time formalism. We provide a full characterization of the conditions under which a QOOT can be properly defined. We then use this object to establish a systematic way to construct time reversal for a generic quantum channel that preserves a given reference observable. Finally, we compare our approach ito existing error mitigation models, demonstrating that in the examples provided our method achieves optimal performance.
OAM-Based Quantum-OCT and Noise Source Analysis
Crislane de Brito (Nicolaus Copernicus University, Toruń)
We take into account the most relevant effects that can cause noise in the signal in a quantum optical coherence tomography (QOCT) setup, such as optical losses occurring in the QOCT interferometer and the quantum detection efficiency of the single photon avalanche diode (SPAD) camera. Optical loss is a term that comprises a very wide spectrum of physical processes, including reflection, scattering on defects of the coating and absorption of light. As for the SPAD detector, the statistic of photons detected may differ from the initial state due to 1- non-perfect photon detection efficiency (loss), 2- false detections caused by thermal excitations (dark counts), and 3- by secondary photons detected by a neighboring element (optical crosstalk). Considering beams carrying orbital angular momentum (OAM) in the context of the spontaneous parametric down-conversion (SPDC) process, calculations were performed aimed at predicting the OAM mode distribution at the output of the SPDC crystal with no spatial-spectral coupling. Not properly mitigated, the coupling between these degrees of freedom leads to reduced-quality signals in the practical applications of SPDC, such as quantum imaging or sensing. Finally, we assume that an object with two interfaces is placed in one arm of a Hong-Ou-Mandel interferometer. One of the OAM entangled photons travels along the reference arm, while the other passes through the object. The sample in the object arm might alter the propagation time and OAM of the photon traveling along that arm. Thus, we investigate the spectral and OAM interference effects produced.
Quantum switch instabilities with an open control
Pedro Dieguez (ICTQT, Gdańsk)
The superposition of causal order shows promise in various quantum technologies. However, the fragility of quantum systems arising from environmental interactions, leading to dissipative behavior and irreversibility, demands a deeper understanding of the possible instabilities in the coherent control of causal orders. In this work, we employ a collisional model to investigate the impact of an open control system on the generation of interference between two causal orders. We present the environmental instabilities for the switch of two arbitrary quantum operations and examine the influence of environmental temperature on each potential outcome of control post-selection. Additionally, we explore how environmental instabilities affect protocol performance, including switching between mutually unbiased measurement observables and refrigeration powered by causal order superposition, providing insights into broader implications
Quantum unital Otto Heat Engines: universal bounds
Abdelkader El Makouri (Mohammed V University in Rabat)
In this poster presentation, I'm going to focus on and present my results on a special type of quantum heat engine called quantum unital Otto heat engines. An example of this engine is when we replace the hot heat bath of the Otto cycle with quantum projective measurement. I consider a working medium to be an arbitrary qubit. This engine has the property that when there is no monitoring of the working medium, coherence can contribute to heat and work. However, when the system is monitored, in this case, coherence in the energy basis is killed. When the quantum coherence is not erased, I call the engine the undephased engine. On the other hand, when this is not the case I call the engine dephased engine.
Diffractive Optical Neural Networks with Arbitrary Spatial
Coherence
Matthew Filipovich (University of Oxford)
Diffractive optical neural networks (DONNs) have emerged as a promising optical hardware platform for energy-efficient and ultra-fast signal processing. However, previous experimental demonstrations of DONNs have only been performed using coherent light, which is not present in the natural world. Here, we study the role of spatial optical coherence in DONN operation. We propose a numerical approach to efficiently simulate DONNs under input illumination with arbitrary spatial coherence and discuss the corresponding computational complexity using coherent, partially coherent, and incoherent light. We also investigate the expressive power of DONNs and examine how coherence affects their performance. We show that under fully incoherent illumination, the DONN performance cannot surpass that of a linear model. As a demonstration, we train and evaluate simulated DONNs on the MNIST dataset using light with varying spatial coherence.
Experimental study of two color quantum heralded imaging
Nur Duwi Fat Fitri (KAIST, Daejeon)
Heralded imaging systems based on parametric down-conversion use one beam to illuminate the object which is detected by a single-pixel detector. An image is recovered from the signal beam with a spatially resolved detector, which never interacted with the object. In this study, we report a camera based on a heralding imaging system where the correlated photons were at non-degenerate wavelength. Infrared photons of signal at 1550 nm wavelength illuminate the object and detected by InGaAs single photon detector. We record the image data from the coincidently detected photon at wavelength 810 nm using an electron multiplying charged coupled device (EMCCD) camera. By applying the photon counting technique to our image reconstruction, we obtain a better signal-to-noise ratio from correlated photons than direct imaging.
Selective organic polariton condensation in individual states
of a
1D topological lattice
Ioannis Georgakilas (IBM Research Zurich)
We use exciton-polariton condensates trapped in potential landscapes as a room-temperature platform for analogue simulations. In this work, we develop and investigate a 1D polariton lattice with alternating coupling strengths, a so-called Su-Schrieffer-Heeger chain, that exhibits topological effects. Our results below polariton condensation threshold reveal the non-trivial topological nature of the studied lattice structure. When exciting above threshold, while tuning our cavity, we show selective condensation in different modes of the lattice. These results demonstrate the tunability and robustness of our platform, making it an ideal candidate for exploring a variety of polariton lattices at ambient conditions.
Bell Correlations of a Spin Chain System near Critical Point
Danish Ali Hamza (University of Warsaw)
We provide a detailed analysis of the character and strength of many-body Bell correlations in interacting multi-qubit systems with particle exchange symmetry. Such a configuration can be described by an effective Schrödinger-like equation, which allows precise analytical predictions. We show that in the vicinity of the quantum critical point, these correlations quickly become so strong that only a fraction of the qubits remain uncorrelated. We also identify the threshold temperature above which thermal fluctuations destroy Bell correlations.
Effects of Higher Josephson Harmonics in tunnel junctions on
superconducting qubit designs
Abbas Hirkani (SISSA, Trieste)
Long relaxation and dephasing times for qubits are crucial for large-scale fault-tolerant quantum computers. In our efforts to prolong the lifetimes of quantum superpositions, one must either run error correction codes or designs qubits with inherent noise protection. Targeting the specific parameter regime is crucial for achieving precise control and characterization of qubits. With the presence of Josephson harmonics in Al-AlOx-Al junctions detected and its effect on transmon artificial atoms already explored, in this presentation we examine how the presence of these higher-harmonics affects the various qubit designs like the 0-Pi qubit, cos2phi, and fluxonium qubit. We study how the presence a of few percent higher harmonics changes the useful qubit parameter regimes which are crucial for accurately targeting the implementation of desired Hamiltonians on superconducting hardware.
Quantum Authentication with Quantum Dots
Lennart Jehle (University of Vienna)
A future quantum internet will combine a cornucopia of interconnected quantum systems, and much interest has focused on securing the exchange of information between dissimilar nodes. Near-deterministic and highly pure single-photon sources rise to the challenge and semiconductor quantum dots can additionally alleviate the need of phase randomisation when excited incoherently. While impressive results have been achieved in quantum key distribution many applications widely used in classical communication require more advanced cryptographic primitives. Message or identity authentication is particularly important as it is prerequisite for most communication protocols including QKD. We demonstrate the transmission of more than 109 single-photons generated by a quantum dot emitting in the telecom C-band over 641m of deployed fibre. The BB84-like states are polarisation encoded and we report an average QBER of 0.49% while maintaining an end-to-end efficiency of 2.9%. I will then describe how to use the transmitted data to perform an informationally-secure authentication based on a NP-hard problem.
High-robust topological states in Q1D systems via orbital-induced
flux
Valerii Kachin (University of Warsaw)
In recent years, the study of topologically non-trivial structures in one-dimensional models has been dominated by the Su-Schrieffer-Heeger model due to its simplicity in design and ability to support edge states with robustness to disorder in couplings, protected by chiral and inversion symmetries. Here, we present a novel study on a zigzag quasi-one-dimensional model, which supports topologically protected edge states without relying on conventional symmetries. Our model utilizes next-neighbor couplings and unit cell threaded halfway by a flux of π to mediate edge states and is simultaneously resilient to dissipation, couplings, and on-site energy disorders. To understand the topological properties of this model, we introduce a novel way to demonstrate the bulk-boundary correspondence of the edge states and construct a topological invariant that returns quantized values. We propose a practical implementation of this model using photonic waveguide arrays with orbital-induced synthetic flux, demonstrating good agreement between coupled mode theory and full-wave simulations. Our numerical analysis of coupled mode dynamics reveals the robust nature of edge states and engineered defect states in the presence of strong disorder. Our study sheds light on the possibility of constructing topological phases in new ways, even in the absence of conventional symmetries, and opens up new avenues for research in topological photonics.
Electro-optic spectral shift of single-photons from a quantum dot
source
Sanjay Kapoor (University of Warsaw)
Efficient control of the spectro-temporal properties of single photons is crucial for interfacing different quantum information processing systems in hybrid networks. There is a growing interest in using electro-optic phase modulators to manipulate quantum light. In this work, we show how a phase-locked electro-optic phase modulator can shift the frequency of single photons emitted by a quantum dot while maintaining the indistinguishability of the photon source. This advancement has numerous applications, such as interfering photons from two different quantum dots, tuning quantum dots to quantum memories, and frequency-bin encoding for quantum key distribution. It represents a significant step towards shaping arbitrary phase-only waveforms of single photons from a quantum dot.
Enabling two-photon interference with time lensing
Jan Krzyżanowski (University of Warsaw)
Two-photon interference is essential for many quantum communication and sensing devices. This phenomenon is quite hard to observe, because it requires the indistinguishability of the interfering photons. The photons from various quantum devices may have mismatched spectrotemporal profiles, disturbing the two-photon interference. An interface is needed to obtain the indistinguishability of the photons and enable quantum information exchange between the mismatched devices. In this work we present a proof-of-concept experiment in which we perform a two-photon interference of photons with mismatched spectral bandwidths. We correct the spectrum of one photon to match it to the other one by applying a spectral compression to one of the photons using an electro-optic time lens. We can get about 60% interference visibility by using the time lens starting from almost no visibility.
Random Dynamical Systems Approach to Study
(non-IID)
Repeated Interaction Dynamics, Disordered Quantum Trajectories
Lubashan Pathirana Karunarathna (University of Copenhagen)
A discretization of the dynamics of an open quantum system is the repeated interaction or collision model, where one assumes that the system interacts with IID copies of the environment or an IID collection of smaller units (ancillas). However, using methods in random dynamical systems (RDS) one may study repeated interaction dynamics for the non-IID case. In the context of repeated-interactions one assumes that the interactions are modeled by a random stationary sequence of quantum operations-valued maps, defined on the measure-preserving system. We study the asymptotic behaviour of iterated compositions of such sequences using RDS tools and obtain limiting results for both the discrete-parameter dynamics. As applications, a class of random matrix products states (MPS) obtained by random CP-maps are introduced and certain probabilistic correlation inequalities are presented. Furthermore we generalize the asymptotic purification (or the none-existence of dark subspaces) of quantum trajectories in this random regim.
Toward a demonstration of a fully photonic
device-independent QKD
Ewan Mer (Imperial College)
The rise of quantum computers threatens communication protocols relying on RSA encryption. While post-quantum algorithms offer alternatives, they lack the inherent security of RSA. Quantum key distribution (QKD) protocols leverage the fundamental property of entanglement in quantum mechanics to ensure intrinsic security against eavesdropping. However, ensuring the security of the protocol remains challenging due to the need for continuous monitoring of various devices: the photon source, the communication channel and the detectors. Device-independent QKD (DI-QKD) removes the need for trusted devices. Security in DI-QKD hinges on violating a Bell inequality after local measurements. While DI-QKD has already been demonstrated with atomic systems, a fully photonic protocol has yet to be implemented. The poster will focus on a proof-of-concept of DI-QKD protocol that utilizes weak-field homodyne detection (WFHD) by Alice and Bob with an untrusted party, called Charlie, in a twin-field QKD fashion. Alice and Bob have their own spontaneous-parametric down-conversion (SPDC) photon source, one mode is measured with WFHD using transition-edge sensors (TES) with high-detection efficiency, the other mode is sent to Charlie to perform entanglement swapping between Alice and Bob. The challenges are to ensure high-purity photon source, low-loss and phase stability during the duration of the protocol.
Improving the secure key rate in a photon counting based DIQKD
protocol
Morteza Moradi (University of Warsaw)
Recently, there has been a growing focus on quantum communication due to its inherent security advantages. However, several key challenges remain, including the distribution of entangled states over long distances, closing Bell test loopholes, and enhancing the secure key rate. In this work, we propose an innovative device-independent QKD protocol based on the transmission of near-maximally entangled multiphoton states over long distances. The protocol leverages available resources such as two mode squeezed vacuum (TMSV) states and photon-number-resolving detectors. We also implement two numerical methods—min entropy and relative entropy—into the protocol. These methods are integrated with the NPA hierarchy to lower bound the eavesdropper's conditional entropy and optimize the secure key rate. Additionally, we employ two pre-processing techniques—noisy pre-processing and post-selection—to further optimize the final secure key rate.
A central limit theorem for partially distinguishable bosons
Marco Robbio (Université libre de Bruxelles)
The quantum central limit theorem derived by Cushen and Hudson provides the foundations for understanding how subsystems of large bosonic systems evolving unitarily do reach equilibrium. It finds important applications in the context of quantum interferometry, for example, with photons. A practical feature of current photonic experiments, however, is that photons carry their own internal degrees of freedom pertaining to, e.g., the polarization or spatiotemporal mode they occupy, which makes them partially distinguishable. The ensuing deviation from ideal indistinguishability is well known to have observable consequences, for example in relation with boson bunching, but an understanding of its role in bosonic equilibration phenomena is still missing. Here, we generalize the Cushen-Hudson quantum central limit theorem to encompass scenarios with partial distinguishability, implying an asymptotic convergence of the subsystem's reduced state towards a multimode Gaussian state defined over the internal degrees of freedom. While these asymptotic internal states may not be directly accessible, we show that particle number distributions carry important signatures of distinguishability, which may be used to diagnose experimental imperfections in large boson sampling experiments.
Certification of multi-qubit quantum systems with temporal
inequality
Sazim Sk (CFT PAN Warsaw)
Self-testing of quantum devices based on observed measurement statistics is a method to certify quantum systems using minimal resources. In Ref. [PRA 101, 032106 (2020)], a scheme based on observing measurement statistics that demonstrate Kochen-Specker contextuality has been shown to certify two-qubit entangled states and measurements without the requirement of spatial separation between the subsystems. However, this scheme assumes a set of compatibility conditions on the measurements which are crucial to demonstrating Kochen-Specker contextuality. In this work, we propose a self-testing protocol to certify the multi-qubit states and measurements without the assumption of the compatibility conditions, and at the same time without requiring the spatial separation between the subsystems. Our protocol is based on the observation of sequential correlations leading to the maximal violation of a temporal inequality derived from non-contextuality inequality. Moreover, our protocol is robust to small experimental errors or noise.
SNSPD Array with High Photon Number Resolution Efficiency for
Quantum
Communication
Lorenzo Stasi (ID Quantique / University of Geneva)
In quantum communication and information, single photon detectors with the photon-number-resolving (PNR) functionality are key to many protocols, such as quantum key distribution, the implementation of quantum repeaters using heralded single-photon sources and the realization of a photonic quantum computer. Superconducting nanowire single photon detector (SNSPD) arrays are one of the most promising platforms for realizing such detectors with high photon number resolution efficiency while maintaining a high detection rate. Here, we develop an SNSPD array consisting of 28 pixels in a parallel architecture (P-SNSPD), requiring only one coaxial cable output for the whole array. The 28-pixel P-SNSPD presented here can reach ~90% system detection efficiency (SDE) at the single photon, 75% at the 2-photon, and 60% at the 3-photon level at 1550nm. We'll present a detailed analysis of the PNR capabilities of these detectors in real-case applications. These detectors can also count at a high rate achieving a maximum count rate of 250 Mcps. Using only 4 of them, we could achieve a count rate of 1 Gcps at 45% of absolute efficiency. In addition, these detector have the unique ability to maintain <60 ps timing jitter at 100 Mcps, making them extremely suitable to achieve high secret key rate for QKD.
Integrated Ultra-fast All-optical Transistors
Pietro Tassan (IBM Research Zurich)
The clock frequency of electronic circuits has been stagnant at a few gigahertz for almost two decades because of the breakdown of Dennard scaling, which suggests that by miniaturizing transistors, they can run faster while consuming the same power. Optical-based computing offers a potential solution to this challenge. However, the lack of materials with sufficiently strong nonlinear interactions necessary for achieving all-optical switches led to large device sizes and high optical energy requirements that hindered scalable architectures. Recently, microcavities utilizing polymers as photoactive material in the regime of strong light-matter interaction have enabled the development of all-optical transistors capable of operating at room temperature with switching times below one picosecond. Nonetheless, the realization of complex circuits was restricted due to limitations posed by non-integrated vertical cavity geometry. Here, by leveraging silicon-on-insulator technology, we realize fully integrated metamaterial-based high-index contrast grating (HCG) microcavities filled with an organic polymer (MeLPPP) as photoactive material. This platform, capable of hosting a strong-light matter interaction regime system, shows integrated on-chip exciton-polariton condensation at ambient conditions. Furthermore, by exploiting the outcoupling resonance from one (control) cavity as input for the next (transistor) cavity, through seeded polariton condensation, we demonstrate ultrafast all-optical transistor action with switching time in the order of 1ps. Finally, the coupling of two resonators proves the cascadability of this technology which paves the way for the implementation of more complex logic circuits.
Quantum addition implementation
Andrii Tereshchenko (GIC of NAS of Ukraine)
The presentation considers the operation of addition as a bitwise arithmetic operation in the quantum model of computation. The implementation of the full adder is considered. The implementation of the full adder, given by Feynman, based on Toffoli gates and the implementation of the full adder based on Fredkin and Peres gates are considered. The optimization method of the implementation of the quantum full adder, given by Feynman, using the IBM Composer simulator is shown. For the sequential model of computation, the methods of building the adders of higher orders based on half-adders and full adders are shown. Addition is the most challenging operation to implement efficiently because of the need for carries. For the parallel model of calculations, the method of multi-bit addition with "carry lookahead method" (CLA) is demonstrated, which has a logarithmic complexity in terms of the number of steps performed by one parallel processor. The addition method with "carry signs prediction" based on Toffoli gates is considered. For the quantum computing model, the Toffoli gate is briefly considered as one of the methods of transferring the methods of implementing the addition operation from classical computational models (sequential and parallel) to the quantum computational model. The method of multi-bit addition based on a constant number of qubits is considered from a theoretical point of view. Using the example of adding two- and three-bit numbers, the implementation of the addition operation based on the quantum Fourier transform using the IBM composer simulator is shown. A program of adding two N-bit numbers based on the quantum Fourier transform is provided, using the Qiskit library to build a quantum circuit on 2N qubits and on N qubits. The quantum circuit includes few optimization steps.
Distribution of genuine time-bin entanglement at telecom
wavelength
Kannan Vijayadharan (University of Padova)
Entanglement is a unique and invaluable resource for quantum information processing because it highlights the non-locality property, and enables various applications like distributed quantum computing, quantum teleportation and quantum key distribution (QKD). While various methods exist for harnessing it, time-bin entanglement is promising for long distance distribution over fiber due to its robustness against polarization mode distortion effects. However, its most common implementation suffers from a from a post-selection loophole (PSL) which invalidates the local-realism tests and prevents its use for device independent protocols. Here we present an experimental scheme to close the PSL by using fast and stable optical switches to deterministically route the photons in the state analyzers and avoid postselection. The implemented scheme working with 1550 nm biphotons allows for distribution of "genuine" time-bin entanglement over existing telecom networks and we show a CHSH violation of S=2.56.
Groups in quantum secure communication
Dorota Wedmann (Warsaw University of Technology)
Quantum algorithms that are able to break cryptographic schemes are relying on solving some group problems, especially hidden subgroup problem in finite abelian groups. On the other hand, combinatorial group theory gives us a way to realize different types of quantum resistant communication: from digital signatures, through multiparty key establishment, to fully homomorphic encryption. Therefore it gives us an opportunity to diversify algorithmic problems we use for communication that is secure against quantum attacks. In this poster I present how solving some group-theoretic problems on quantum computer affects our post-quantum solutions and how we can use groups to construct quantum resistant cryptographic schemes.
Quantum algorithms and fine grained conplexity
Adam Wesołowski (Royal Holloway University of London)
The landscape of quantum algorithms for graph problems from complexity class P, the challenge of closing the gap between upper bounds and conditional or unconditional lower bounds. The techniques for quantum fine-grained reductions and major open problems.
Non-equilibrium thermometry with bosonic sample and probe.
Marek Winczewski (University of Gdańsk)
We characterize the measurement sensitivity, quantified by the Quantum Fisher Information (QFI), of a thermometric probe of quantum harmonic oscillator (QHO) strongly coupled to the sample of interest, a bosonic bath at temperature T. For non-equilibrium protocols, in which the probe is measured before reaching equilibrium with the sample, new behavior of the measurement sensitivity arising due to non-Markovian dynamics is expected, as in the fermionic case [arXiv:2310:14655]. We investigate whether QFI rate is maximized at a finite interrogation time t* as in fermionic case, or the solution t* → 0, known in the Markovian limit, is reproduced [Quantum 6, 869 (2022)].
Leveraging Hybrid Classical-Quantum Methods for Efficient Load
Rebalancing in HPC
Justyna Zawalska (AGH University of Krakow / ACC Cyfronet AGH)
Load imbalance is a challenge for parallel applications in High Performance Computing (HPC). It is caused by processes having different execution times or load values, leading to idle or wait times at synchronization points, where faster processes must wait for the slowest process to catch up. To mitigate this issue, applications can employ load balancing (LB) strategies, which migrate load between processes to even out load. This is often referred to as the Load Rebalancing Problem (LRP). While many approaches to solving the LRP exist, they can only be heuristics and hence further optimization potential exists. In our work, we turn to a novel approach by using hybrid classical-quantum approaches and present two versions of the constrained quadratic model for solving the LRP; the two differ in how they balance the number of qubits required with the types of applied constraints. We compare the quantum-based methods with classical methods using heuristic algorithms. We evaluate our approaches using imbalance ratio and speedup as metrics, as well as the number of migrated tasks to indicate overhead caused by migrations. Our results show that the quantum-based methods outperform the classic methods. For example, we need only 1/4 of the number of migrated tasks in a realistic use case compared with classical methods to balance the load.
Discovery of Novel High-Entropy Materials via Quantum Computing
Houlong Zhuang (Arizona State University)
High-entropy materials (HEMs), which consist of multi-principal elements and a wide range of molar ratios of constituent elements, have emerged as a promising group of materials to tackle pressing energy and environmental challenges. These challenges encompass issues such as climate change and semiconductor chip shortages. This family of materials includes various variants, such as high-entropy catalysts, high-entropy oxides, high-entropy semiconductors, high-entropy superconductors, high-entropy ceramics, and more. A common challenge shared by these frontiers is the selection of elements and their molar ratios from the vast compositional space. In this presentation, I will provide examples of how emerging quantum computers, including quantum simulators and quantum hardware, can address this elemental design problem and contribute to the discovery of novel HEMs. I will also demonstrate how quantum machine learning algorithms exhibit the potential to accelerate the training process. Finally, I will outline several potential directions for HEM research that can benefit from the power of quantum computing.