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Quantum Physics (quant-ph)

Tue, 02 May 2023

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1.Low noise quantum frequency conversion of photons from a trapped barium ion to the telecom O-band

Authors:Uday Saha, James D. Siverns, John Hannegan, Qudsia Quraishi, Edo Waks

Abstract: Trapped ions are one of the leading candidates for scalable and long-distance quantum networks because of their long qubit coherence time, high fidelity single- and two-qubit gates, and their ability to generate photons entangled with the qubit state of the ion. One method for creating ion-photon entanglement is to exploit optically transitions from the P_(1/2) to S_(1/2) levels, which naturally emit spin-photon entangled states. But these optical transitions typically lie in the ultra-violet and visible wavelength regimes. These wavelengths exhibit significant fiber-optic propagation loss, thereby limiting the transfer of quantum information to tens of meters. Quantum frequency conversion is essential to convert these photons to telecom wavelengths so that they can propagate over long distances in fiber-based networks, as well as for compatibility with the vast number of telecom-based opto-electronic components. Here, we generate O-band telecom photons via a low noise quantum frequency conversion scheme from photons emitted from the P_(1/2) to S_(1/2) dipole transition of a trapped barium ion. We use a two-stage quantum frequency conversion scheme to achieve a frequency shift of 375.4 THz between the input visible photon and the output telecom photon achieving a conversion efficiency of 11%. We attain a signal-to-background ratio of over 100 for the converted O-band telecom photon with background noise less than 15 counts/sec. These results are an important step toward achieving trapped ion quantum networks over long distances for distributed quantum computing and quantum communication.

2.Steady-state Quantum Thermodynamics with Synthetic Negative Temperatures

Authors:Mohit Lal Bera, Tanmoy Pandit, Kaustav Chatterjee, Varinder Singh, Maciej Lewenstein, Utso Bhattacharya, Manabendra Nath Bera

Abstract: A bath with a negative temperature is a subject of intense debate in recent times. It raises fundamental questions not only on our understanding of negative temperature of a bath in connection with thermodynamics but also on the possibilities of constructing devices using such baths. In this work, we study steady-state quantum thermodynamics involving baths with negative temperatures. A bath with a negative temperature is created synthetically using two baths of positive temperatures and weakly coupling these with a qutrit system. These baths are then coupled to each other via a working system. At steady-state, the laws of thermodynamics are analyzed. We find that whenever the temperatures of these synthetic baths are identical, there is no heat flow, which reaffirms the zeroth law. There is always a spontaneous heat flow for different temperatures. In particular, heat flows from a bath with a negative temperature to a bath with a positive temperature which, in turn, implies that a bath with a negative temperature is `hotter' than a bath with a positive temperature. This warrants an amendment in the Kelvin-Planck statement of the second law, as suggested in earlier studies. In all these processes, the overall entropy production is positive, as required by the Clausius statement of the second law. We construct continuous heat engines operating between positive and negative temperature baths. These engines yield maximum possible heat-to-work conversion efficiency, that is, unity. We also study the thermodynamic nature of heat from a bath with a negative temperature and find that it is thermodynamic work but with negative entropy.

3.Stark tuning of telecom single-photon emitters based on a single Er$^{3+}$

Authors:Jian-Yin Huang, Peng-Jun Liang, Liang Zheng, Pei-Yun Li, You-Zhi Ma, Duan-Chen Liu, Zong-Quan Zhou, Chuan-Feng Li, Guang-Can Guo

Abstract: The implementation of scalable quantum networks requires photons at the telecom band and long-lived spin coherence. The single Er$^{3+}$ in solid-state hosts is an important candidate that fulfills these critical requirements simultaneously. However, to entangle distant Er$^{3+}$ ions through photonic connections, the emission frequency of individual Er$^{3+}$ in solid-state matrix must be the same, which is challenging because the emission frequency of Er$^{3+}$ depends on its local environment. In this study, we propose and experimentally demonstrate the Stark tuning of the emission frequency of a single Er$^{3+}$ in a Y$_2$SiO$_5$ crystal by employing electrodes interfaced with a silicon photonic crystal cavity. We obtain a Stark shift of 182.9 $\pm$ 0.8 MHz which is approximately 27 times of the optical emission linewidth, demonstrating the promising applications in tuning the emission frequency of independent Er$^{3+}$ into the same spectral channels. Our results provide a useful solution for the construction of scalable quantum networks based on single Er$^{3+}$ and a universal tool for tuning the emission of individual rare-earth ions.

4.Short Technical Review of Four Different Quantum Systems: Comparative Analysis of Quantum Correlation, Signal-to-Noise Ratio, and Fidelity

Authors:Ahmad Salmanogli

Abstract: This technical review examines the different methods and approaches used to create microwave modes quantum correlation. Specifically, we consider the electro-opto-mechanical, optoelectronics, 4-coupled qubits, and InP HEMT coupled with two external oscillator methods, and evaluate their effectiveness for quantum applications. As these systems are open quantum systems, they interact with their environment and thermal bath. To ensure an accurate comparison, we analyze all systems using the same gauge. Thus, all systems are shortly introduced, the total Hamiltonian is theoretically derived, and finally, the system dynamics are analogously analyzed using the Lindblad master equation. We then calculate the quantum correlation between cavity modes, signal-to-noise ratio, and fidelity for each system to evaluate their performance. The study result shows that the strength and nature of the calculated quantities vary among the systems. One interesting result is the emergence of mixing behavior in the quantum correlation and signal-to-noise ratio for systems that use different cavities. It also identified a significant similarity between the 4-coupled qubits and InP HEMT coupled with external oscillators methods, where an avoided-level crossing occurs in the quantum correlation. Additionally, the study reveals that the signal-to-noise ratio and classical discord are more consistent than quantum discord.

5.Transformations between arbitrary (quantum) objects and the emergence of indefinite causality

Authors:Simon Milz, Marco Túlio Quintino

Abstract: Many fundamental and key objects in quantum mechanics are linear mappings between particular affine/linear spaces. This structure includes basic quantum elements such as states, measurements, channels, instruments, non-signalling channels and channels with memory, and also higher-order operations such as superchannels, quantum combs, n-time processes, testers, and process matrices which may not respect a definite causal order. Deducing and characterising their structural properties in terms of linear and semidefinite constraints is not only of foundational relevance, but plays an important role in enabling the numerical optimization over sets of quantum objects and allowing simpler connections between different concepts and objects. Here, we provide a general framework to deduce these properties in a direct and easy to use way. Additionally, while primarily guided by practical quantum mechanical considerations, we extend our analysis to mappings between \textit{general} linear/affine spaces and derive their properties, opening the possibility for analysing sets which are not explicitly forbidden by quantum theory, but are still not much explored. Together, these results yield versatile and readily applicable tools for all tasks that require the characterization of linear transformations, in quantum mechanics and beyond. As an application of our methods, we discuss the emergence of indefinite causality in higher-order quantum transformation.

6.Quantum Circuit Implementation and Resource Analysis of LBlock and LiCi

Authors:XiaoYu Jing, YanJu Li, GuangYue Zhao, Huiqin Xie

Abstract: Due to Grover's algorithm, any exhaustive search attack of block ciphers can achieve a quadratic speed-up. To implement Grover,s exhaustive search and accurately estimate the required resources, one needs to implement the target ciphers as quantum circuits. Recently, there has been increasing interest in quantum circuits implementing lightweight ciphers. In this paper we present the quantum implementations and resource estimates of the lightweight ciphers LBlock and LiCi. We optimize the quantum circuit implementations in the number of gates, required qubits and the circuit depth, and simulate the quantum circuits on ProjectQ. Furthermore, based on the quantum implementations, we analyze the resources required for exhaustive key search attacks of LBlock and LiCi with Grover's algorithm. Finally, we compare the resources for implementing LBlock and LiCi with those of other lightweight ciphers.

7.Classification of real and complex 3-qutrit states

Authors:Sabino Di Trani, Willem A. de Graaf, Alessio Marrani

Abstract: In this paper we classify the orbits of the group SL(3,F)^3 on the space F^3\otimes F^3\otimes F^3 for F=C and F=R. This is known as the classification of complex and real 3-qutrit states. We also give an overview of physical theories where these classifications are relevant.

8.Design and Analysis of Genuine Entanglement Access Control for the Quantum Internet

Authors:Jessica Illiano, Marcello Caleffi, Michele Viscardi, Angela Sara Cacciapuoti

Abstract: Multipartite entanglement plays a crucial role for the design of the Quantum Internet, due to its peculiarities with no classical counterpart. Yet, for entanglement-based quantum networks, a key open issue is constituted by the lack of an effective entanglement access control (EAC) strategy for properly handling and coordinating the quantum nodes in accessing the entangled resource. In this paper, we design a quantum-genuine entanglement access control (EAC) to solve the contention problem arising in accessing a multipartite entangled resource. The proposed quantum-genuine EAC is able to: i) fairly select a subset of nodes granted with the access to the contended resource; ii) preserve the privacy and anonymity of the identities of the selected nodes; iii) avoid to delegate the signaling arising with entanglement access control to the classical network. We also conduct a theoretical analysis of noise effects on the proposed EAC. This theoretical analysis is able to catch the complex noise effects on the EAC through meaningful parameters.

9.Adiabatic ground state preparation of fermionic many-body systems from a two-body perspective

Authors:Dyon van Vreumingen, Kareljan Schoutens

Abstract: A well-known method to prepare ground states of fermionic many-body hamiltonians is adiabatic state preparation, in which an easy to prepare state is time-evolved towards an approximate ground state under a specific time-dependent hamiltonian. However, which path to take in the evolution is often unclear, and a direct linear interpolation, which is the most common method, may not be optimal. In this work, we explore new types of adiabatic paths based on an eigendecomposition of the coefficient tensor in the second quantised representation of the difference between the final and initial hamiltonian (the residual hamiltonian). Since there is an equivalence between this tensor and a projection of the residual hamiltonian onto the subspace of two particles, this approach is essentially a two-body spectral decomposition. We show how for general hamiltonians, the adiabatic time complexity may be upper bounded in terms of the number of one-body modes $L$ and a minimal gap $\Delta$ along the path. Our finding is that the complexity is determined primarily by the degree of pairing in the two-body states. As a result, systems whose two-body eigenstates are uniform superpositions of distinct fermion pairs tend to exhibit maximal complexity, which scales as $O(L^4/\Delta^3)$ in direct interpolation and $O(L^6/\Delta^3)$ in an evolution that follows a path along the corners of a hypercube in parameter space. The usefulness of our method is demonstrated through a few examples involving Fermi-Hubbard models where, due to symmetries, level crossings occur in direct interpolation. We show that our method of decomposing the residual hamiltonian and thereby deviating from a direct path appropriately breaks the relevant symmetries, thus avoiding level crossings and enabling an adiabatic passage.

10.Performance Analysis of Quantum Error-Correcting Codes via MacWilliams Identities

Authors:Diego Forlivesi, Lorenzo Valentini, Marco Chiani

Abstract: One of the main challenges for an efficient implementation of quantum information technologies is how to counteract quantum noise. Quantum error correcting codes are therefore of primary interest for the evolution towards quantum computing and quantum Internet. We analyze the performance of stabilizer codes, one of the most important classes for practical implementations, on both symmetric and asymmetric quantum channels. To this aim, we first derive the weight enumerator (WE) for the undetectable errors of stabilizer codes based on the quantum MacWilliams identities. The WE is then used to evaluate the error rate of quantum codes under maximum likelihood decoding or, in the case of surface codes, under minimum weight perfect matching (MWPM) decoding. Our findings lead to analytical formulas for the performance of generic stabilizer codes, including the Shor code, the Steane code, as well as surface codes. For example, on a depolarizing channel with physical error rate $\rho \to 0$ it is found that the logical error rate $\rho_\mathrm{L}$ is asymptotically $\rho_\mathrm{L} \to 16.2 \rho^2$ for the $[[9,1,3]]$ Shor code, $\rho_\mathrm{L} \to 16.38 \rho^2$ for the $[[7,1,3]]$ Steane code, $\rho_\mathrm{L} \to 18.74 \rho^2$ for the $[[13,1,3]]$ surface code, and $\rho_\mathrm{L} \to 149.24 \rho^3$ for the $[[41,1,5]]$ surface code.

11.Experimental free-space quantum key distribution over a turbulent high-loss channel

Authors:Md Mehdi Hassan, Kazi Reaz, Adrien Green, Noah Crum, George Siopsis

Abstract: Free-space quantum cryptography plays an integral role in realizing a global-scale quantum internet system. Compared to fiber-based communication networks, free-space networks experience significantly less decoherence and photon loss due to the absence of birefringent effects in the atmosphere. However, the atmospheric turbulence contributes to deviation in transmittance distribution, which introduces noise and channel loss. Several methods have been proposed to overcome the low signal-to-noise ratio. Active research is currently focused on establishing secure and practical quantum communication in a high-loss channel, and enhancing the secure key rate by implementing bit rejection strategies when the channel transmittance drops below a certain threshold. By simulating the atmospheric turbulence using an acousto-optical-modulator (AOM) and implementing the prefixed-threshold real-time selection (P-RTS) method, our group performed finite-size decoy-state Bennett-Brassard 1984 (BB84) quantum key distribution (QKD) protocol for 19 dB channel loss. With better optical calibration and efficient superconducting nano-wire single photon detector (SNSPD), we have extended our previous work to 40 dB channel loss characterizing the transmittance distribution of our system under upper moderate turbulence conditions.

12.Aharonov-Bohm effect in Presence of Superconductors

Authors:L. O'Raifeartaigh, N. Straumann, A. Wipf

Abstract: The analysis of a previous paper, in which it was shown that the energy for the Aharonov-Bohm effect could be traced to the interaction energy between the magnetic field of the electron and the background magnetic field, is extended to cover the case in which the magnetic field of the electron is shielded from the background magnetic field by superconducting material. The paradox that arises from the fact that such a shielding would apparently preclude the possibility of an interaction energy is resolved and, within the limits of the ideal situation considered, the observed experimental result is derived.

13.Efficient estimation of quantum state k-designs with randomized measurements

Authors:Lorenzo Versini, Karim Alaa El-Din, Florian Mintert, Rick Mukherjee

Abstract: Random ensembles of pure states have proven to be extremely important in various aspects of quantum physics such as benchmarking the performance of quantum circuits, testing for quantum advantage, providing novel insights for many-body thermalization and studying black hole information paradox. Although generating a fully random ensemble is almost impossible and experimentally challenging, approximations of it are just as useful and are known to emerge naturally in a variety of physical models, including Rydberg setups. These are referred to as approximate quantum state designs, and verifying their degree of randomness can be an expensive task, similar to performing full quantum state tomography on many-body systems. In this theoretical work, we efficiently validate the character of approximate quantum designs with respect to data size acquisition when compared to conventional frequentist approach. This is achieved by translating the information residing in the complex many-body state into a succinct representation of classical data using a random projective measurement basis, which is then processed, using methods of statistical inference including neural networks. Our scheme of combining machine learning methods for postprocessing the data obtained from randomized measurements for efficient characterisation of (approximate) quantum state k designs is applicable to any noisy quantum platform that can generate quantum designs.

14.Exploring the synergistic potential of quantum annealing and gate model computing for portfolio optimization

Authors:Naman Jain, M Girish Chandra

Abstract: Portfolio optimization is one of the most studied problems for demonstrating the near-term applications of quantum computing. However, large-scale problems cannot be solved on today's quantum hardware. In this work, we extend upon a study to use the best of both quantum annealing and gate-based quantum computing systems to enable solving large-scale optimization problems efficiently on the available hardware. The existing work uses a method called Large System Sampling Approximation (LSSA) that involves dividing the large problem into several smaller problems and then combining the multiple solutions to approximate the solution to the original problem. This paper introduces a novel technique to modify the sampling step of LSSA. We divide the portfolio optimization problem into sub-systems of smaller sizes by selecting a diverse set of assets that act as representatives of the entire market and capture the highest correlations among assets. We conduct tests on real-world stock data from the Indian stock market on up to 64 assets. Our experimentation shows that the hybrid approach performs at par with the traditional classical optimization methods with a good approximation ratio. We also demonstrate the effectiveness of our approach on a range of portfolio optimization problems of different sizes. We present the effects of different parameters on the proposed method and compare its performance with the earlier work. Our findings suggest that hybrid annealer-gate quantum computing can be a valuable tool for portfolio managers seeking to optimize their investment portfolios in the near future.

15.International time transfer between precise timing facilities secured with a quantum key distribution network

Authors:Francesco Picciariello, Francesco Vedovato, Davide Orsucci, Pablo Nahuel Dominguez, Thomas Zechel, Marco Avesani, Matteo Padovan, Giulio Foletto, Luca Calderaro, Daniele Dequal, Amita Shrestha, Ludwig Blumel, Johann Furthner, Giuseppe Vallone, Paolo Villoresi, Tobias D. Schmidt, Florian Moll

Abstract: Global Navigation Satellite Systems (GNSSs), such as GPS and Galileo, provide precise time and space coordinates globally and constitute part of the critical infrastructure of modern society. To reliably operate GNSS, a highly accurate and stable system time is required, such as the one provided by several independent clocks hosted in Precise Timing Facilities (PTFs) around the world. Periodically, the relative clock offset between PTFs is measured to have a fallback system to synchronize the GNSS satellite clocks. The security and integrity of the communication between PTFs is of paramount importance: if compromised, it could lead to disruptions to the GNSS service. Therefore, it is a compelling use-case for protection via Quantum Key Distribution (QKD), since this technology provides information-theoretic security. We have performed a field trial demonstration of such use-case by sharing encrypted time synchronization information between two PTFs, one located in Oberpfaffenhofen (Germany) and one in Matera (Italy) - more than 900km apart as the crow flies. To bridge this large distance, a satellite-QKD system is required, plus a "last-mile" terrestrial link to connect the optical ground station (OGS) to the actual location of the PTF. In our demonstration we have deployed two full QKD systems to protect the last-mile connection at both the locations and have shown via simulation that upcoming QKD satellites will be able to distribute keys between Oberpfaffenhofen and Matera exploiting already existing OGSs.

16.Learning Hard Distributions with Quantum-enhanced Variational Autoencoders

Authors:Anantha Rao, Dhiraj Madan, Anupama Ray, Dhinakaran Vinayagamurthy, M. S. Santhanam

Abstract: An important task in quantum generative machine learning is to model the probability distribution of measurements of many-body quantum systems. Classical generative models, such as generative adversarial networks (GANs) and variational autoencoders (VAEs), can model the distributions of product states with high fidelity, but fail or require an exponential number of parameters to model entangled states. In this paper, we introduce a quantum-enhanced VAE (QeVAE), a generative quantum-classical hybrid model that uses quantum correlations to improve the fidelity over classical VAEs, while requiring only a linear number of parameters. We provide a closed-form expression for the output distributions of the QeVAE. We also empirically show that the QeVAE outperforms classical models on several classes of quantum states, such as 4-qubit and 8-qubit quantum circuit states, haar random states, and quantum kicked rotor states, with a more than 2x increase in fidelity for some states. Finally, we find that the trained model outperforms the classical model when executed on the IBMq Manila quantum computer. Our work paves the way for new applications of quantum generative learning algorithms and characterizing measurement distributions of high-dimensional quantum states.

17.Probing critical states of matter on a digital quantum computer

Authors:Reza Haghshenas, Eli Chertkov, Matthew DeCross, Thomas M. Gatterman, Justin A. Gerber, Kevin Gilmore, Dan Gresh, Nathan Hewitt, Chandler V. Horst, Mitchell Matheny, Tanner Mengle, Brian Neyenhuis, David Hayes, Michael Foss-Feig

Abstract: Although quantum mechanics underpins the microscopic behavior of all materials, its effects are often obscured at the macroscopic level by thermal fluctuations. A notable exception is a zero-temperature phase transition, where scaling laws emerge entirely due to quantum correlations over a diverging length scale. The accurate description of such transitions is challenging for classical simulation methods of quantum systems, and is a natural application space for quantum simulation. These quantum simulations are, however, not without their own challenges \textemdash~representing quantum critical states on a quantum computer requires encoding entanglement of a large number of degrees of freedom, placing strict demands on the coherence and fidelity of the computer's operations. Using Quantinuum's H1-1 quantum computer, we address these challenges by employing hierarchical quantum tensor-network techniques, creating the ground state of the critical transverse-field Ising chain on 128-sites with sufficient fidelity to extract accurate critical properties of the model. Our results suggest a viable path to quantum-assisted tensor network contraction beyond the limits of classical methods.