Portfolio optimization is an important problem in mathematical finance, and a promising target for quantum optimization algorithms. The use cases solved daily in financial institutions are subject to many constraints that arise from business objectives and regulatory requirements, which make these problems challenging to solve on quantum computers. We introduce a technique that uses quantum Zeno dynamics to solve optimization problems with multiple arbitrary constraints, including inequalities. We show that the dynamics of the quantum optimization can be efficiently restricted to the in-constraint subspace via repeated projective measurements, requiring only a small number of auxiliary qubits and no post-selection. Our technique has broad applicability, which we demonstrate by incorporating it into the quantum approximate optimization algorithm (QAOA) and variational quantum circuits for optimization. We analytically show that achieving a constant minimum success probability in QAOA requires a number of measurements that is independent of the problem size for a specific choice of mixer operator. We evaluate our method numerically on the problem of portfolio optimization with multiple realistic constraints, and observe better solution quality and higher in-constraint probability than the state-of-the-art technique of enforcing constraints by introducing a penalty into the objective. We demonstrate the proposed method on the Quantinuum H1-2 trapped-ion quantum processor, observing performance improvements from circuits with two-qubit gate depths of up to 148.

We study the time evolution of entanglement entropy of bosons in a one-dimensional optical lattice induced by a sudden quench of the hopping amplitude $J$. We consider the system being quenched into the deep Mott-insulating (MI) regime, i.e., $J/U\ll 1$ ($U$ is the strength of the on-site repulsive interaction), from the product state with individual boson isolated in each lattice site. The low-energy excited states in this regime can be effectively described by fermionic quasiparticles known as doublons and holons. Developing the effective theory, we analytically calculate the time evolution of the second-order R\'enyi entropy (RE) for a subsystem and propose a quasiparticle picture for the time evolution of the RE based on the obtained analytic expressions. Doublons and holons are excited by the quench as entangled pairs that propagate with the velocity $v_{\rm pair}=6J$. The RE reflects the population of doublon-holon pairs that span the boundary of the subsystem. In the short-time scale ($Jt/\hbar=\mathcal{O}(1)$), the RE exhibits the rapid oscillations with the frequency $U/\hbar$, while in the long-time scale ($Jt/\hbar\gg 1$) the RE grows linearly in time until the pair spreads beyond the size of the subsystem and the RE saturates to a constant. We analytically show that the constant value of the RE after the saturation obeys the volume-law scaling.

We study a three-qubit waveguide system in the presence of optical pumping, when the side qubits act as atomlike mirrors, manifesting in a strong light-matter coupling regime. The qubits are modelled as Fermionic two-level systems, where we account for important saturation effects and quantum nonlinearities. Optical pumping in this system is shown to lead to a rich manifold of dressed states that can be seen in the emitted spectrum, and we show two different theoretical solutions using a material master equation model in the Markovian limit, as well as using matrix products states without invoking any Markov approximations. We show how a rich nonlinear spectrum is obtained by varying the relative decay rates of the mirror qubits as well as their spatial separation, and demonstrate the limitations of using a Markovian master equation. Our model allows one to directly model giant atom phenomena, including important non-retardation effects and multi-photon nonlinearities.

Grover's search algorithm was a groundbreaking advancement in quantum algorithms, displaying a quadratic speed-up of querying for items. Since the creation of this algorithm it has been utilized in various ways, including in preparing specific states for the general circuit. However, as the number of desired items increases so does the gate complexity of the sub-process that conducts the query. To counter this complexity, an extension of Grover's search algorithm is derived where the focus of the query is on the undesirable items in order to suppress the amplitude of the queried items. To display the efficacy the algorithm is implemented as a sub-process into QAOA and applied to a traveling salesman problem. For a basis of comparison, the results are compared against QAOA.

Quantum simulation is a potentially powerful application of quantum computing, holding the promise to be able to emulate interesting quantum systems beyond the reach of classical computing methods. Despite such promising applications, and the increase in active research, there is little introductory literature or demonstrations of the topic at a graduate or undergraduate student level. This artificially raises the barrier to entry into the field which already has a limited workforce, both in academia and industry. Here we present an introduction to simulating quantum systems, starting with a chosen Hamiltonian, overviewing state preparation and evolution, and discussing measurement methods. We provide an example simulation by measuring the state dynamics of a tight-binding model with disorder by time evolution using the Suzuki-Trotter decomposition. Furthermore, error mitigation and noise reduction are essential to executing quantum algorithms on currently available noisy quantum computers. We discuss and demonstrate various error mitigation and circuit optimization techniques that significantly improve performance. All source code is freely available, and we encourage the reader to build upon it.

Quantum reservoir computing is a promising approach to quantum neural networks capable of solving hard learning tasks on both classical and quantum input data. However, current approaches with qubits are limited by low connectivity. We propose an implementation for quantum reservoir that obtains a large number of densely connected neurons by using parametrically coupled quantum oscillators instead of physically coupled qubits. We analyse a specific hardware implementation based on superconducting circuits. Our results give the coupling and dissipation requirements in the system and show how they affect the performance of the quantum reservoir. Beyond quantum reservoir computation, the use of parametrically coupled bosonic modes holds promise for realizing large quantum neural network architectures.

Classical shadow tomography is a powerful randomized measurement protocol for predicting many properties of a quantum state with few measurements. Two classical shadow protocols have been extensively studied in the literature: the single-qubit (local) Pauli measurement, which is well suited for predicting local operators but inefficient for large operators; and the global Clifford measurement, which is efficient for low-rank operators but infeasible on near-term quantum devices due to the extensive gate overhead. In this work, we demonstrate a scalable classical shadow tomography approach for generic randomized measurements implemented with finite-depth local Clifford random unitary circuits, which interpolates between the limits of Pauli and Clifford measurements. The method combines the recently proposed locally-scrambled classical shadow tomography framework with tensor network techniques to achieve scalability for computing the classical shadow reconstruction map and evaluating various physical properties. The method enables classical shadow tomography to be performed on shallow quantum circuits with superior sample efficiency and minimal gate overhead and is friendly to noisy intermediate-scale quantum (NISQ) devices. We show that the shallow-circuit measurement protocol provides immediate, exponential advantages over the Pauli measurement protocol for predicting quasi-local operators. It also enables a more efficient fidelity estimation compared to the Pauli measurement.

An indefinite causal order, where the causes of events are not necessarily in past events, is predicted by the process matrix framework. A fundamental question is how these non-separable causal structures can be related to the thermodynamic phenomena. Here, we approach this problem by considering the existence of two cooperating local Maxwell's demons which try to exploit the presence of global correlations and indefinite causal order to optimize the extraction of work. Thus, we prove that it is possible to have a larger probability to lower the local energy to zero if causal inequalities are violated, and that can be extracted more average work with respect to a definite causal order. However, for non-interacting parties, for the system considered the work extractable cannot be larger than the definite causal order bound.

It has long been known that there exists a coordinate transformation which exactly maps the quantum free particle to the quantum harmonic oscillator. Here we extend this result by reformulating it as a unitary operation followed by a time coordinate transformation. We demonstrate that an equivalent transformation can be performed for classical systems in the context of Koopman von-Neumann (KvN) dynamics. We further extend this mapping to dissipative evolutions in both the quantum and classical cases, and show that this mapping imparts an identical time-dependent scaling on the dissipation parameters for both types of dynamics. The derived classical procedure presents a number of opportunities to import squeezing dependent quantum procedures (such as Hamiltonian amplification) into the classical regime.

Impurity spins randomly distributed at the surfaces and interfaces of superconducting wires are known to cause flux noise in Superconducting Quantum Interference Devices, providing a mechanism for decoherence in superconducting qubits. While flux noise is well characterised experimentally, the microscopic model underlying spin dynamics remains unknown. First-principles theories are too computationally expensive to capture spin diffusion over large length scales, third-principles approaches lump spin dynamics into a single phenomenological spin-diffusion operator that is not able to describe the quantum noise regime and connect to microscopic models and disorder scenarios. Here we propose an intermediate "second principles" method to describe general spin dissipation and flux noise in the quantum regime. It leads to the interpretation that flux noise arises from the density of paramagnon excitations at the edge of the wire, with paramagnon-paramagnon interactions leading to spin diffusion, and interactions between paramagnons and other degrees of freedom leading to spin energy relaxation. At high frequency we obtain an upper bound for flux noise, showing that the (super)Ohmic noise observed in experiments does not originate from interacting spin impurities. We apply the method to Heisenberg models in two dimensional square lattices with random distribution of vacancies and nearest-neighbour spins coupled by constant exchange. Numerical calculations of flux noise show that it follows the observed power law $A/\omega^{\alpha}$, with amplitude $A$ and exponent $\alpha$ depending on temperature and inhomogeneities. These results are compared to experiments in niobium and aluminium devices. The method establishes a connection between flux noise experiments and microscopic Hamiltonians identifying relevant microscopic mechanisms and guiding strategies for reducing flux noise.

The interplay between thermal machines and quantum correlations is of great interest in both quantum thermodynamics and quantum information science. Recently, a quantum Szil\'ard engine has been proposed, showing that the quantum steerability between a Maxwell's demon and a work medium can be beneficial to a work extraction task. Nevertheless, this type of quantum-fueled machine is usually fragile in the presence of decoherence effects. We provide an example of the pure dephasing process, showing that the engine's quantumness can be degraded. Therefore, in this work, we tackle this question by introducing a second demon who can access a control system and make the work medium pass through two dephasing channels in a manner of quantum superposition. Furthermore, we provide a quantum circuit to simulate our proposed concept and test it on IBMQ and IonQ quantum computers.

Author(s): Lewis Wooltorton, Peter Brown, and Roger Colbeck

Two parties sharing entangled quantum systems can generate correlations that cannot be produced using only shared classical resources. These *nonlocal* correlations are a fundamental feature of quantum theory but also have practical applications. For instance, they can be used for *device-independent* r…

[Phys. Rev. Lett. 129, 150403] Published Wed Oct 05, 2022

Author(s): Evan Meyer-Scott, Nidhin Prasannan, Ish Dhand, Christof Eigner, Viktor Quiring, Sonja Barkhofen, Benjamin Brecht, Martin B. Plenio, and Christine Silberhorn

A new experiment generates entanglement between many photons with a much higher probability than available methods, which could be a boon for quantum information applications.

[Phys. Rev. Lett. 129, 150501] Published Wed Oct 05, 2022

Author(s): Ruslan Shaydulin and Stefan M. Wild

Quantum kernel methods are considered a promising avenue for applying quantum computers to machine learning problems. Identifying hyperparameters controlling the inductive bias of quantum machine learning models is expected to be crucial given the central role hyperparameters play in determining the…

[Phys. Rev. A 106, 042407] Published Wed Oct 05, 2022

Author(s): Ya-Li Mao, Zheng-Da Li, Sixia Yu, and Jingyun Fan

While Bell nonlocality of a bipartite system is counterintuitive, multipartite nonlocality in our many-body world turns out to be even more so. Recent theoretical study reveals in a theory-agnostic manner that genuine multipartite nonlocal correlations cannot be explained by any causal theory involv…

[Phys. Rev. Lett. 129, 150401] Published Tue Oct 04, 2022

Author(s): Huan Cao, Marc-Olivier Renou, Chao Zhang, Gaël Massé, Xavier Coiteux-Roy, Bi-Heng Liu, Yun-Feng Huang, Chuan-Feng Li, Guang-Can Guo, and Elie Wolfe

Quantum theory predicts the existence of genuinely tripartite-entangled states, which cannot be obtained from local operations over any bipartite-entangled states and unlimited shared randomness. Some of us recently proved that this feature is a fundamental signature of quantum theory. The state |GH…

[Phys. Rev. Lett. 129, 150402] Published Tue Oct 04, 2022

Author(s): Amrita Mandal, Rohit Sarma Sarkar, Shantanav Chakraborty, and Bibhas Adhikari

In this article, we undertake a detailed study of the limiting behavior of a three-state discrete-time quantum walk on a one-dimensional lattice with generalized Grover coins. Two limit theorems are proved and consequently we show that the quantum walk exhibits localization at its initial position f…

[Phys. Rev. A 106, 042405] Published Tue Oct 04, 2022

Author(s): Justin Provazza and Roel Tempelaar

In discrete quantum systems, quantum decoherence is the disappearance of simple phase relations as a result of interactions with an environment. For many applications, the question is not necessarily how to avoid (inevitable) system-environment interactions, but rather how to design environments tha…

[Phys. Rev. A 106, 042406] Published Tue Oct 04, 2022

Author(s): Joe H. Jenne and David R. M. Arvidsson-Shukur

Several tasks in quantum-information processing involve quantum learning. For example, quantum sensing, quantum machine learning, and quantum-computer calibration involve learning and estimating unknown parameters θ=(θ1,⋯,θM) from measurements of many copies of a quantum state ρ̂θ. This type of metr…

[Phys. Rev. A 106, 042404] Published Tue Oct 04, 2022

Thermalization (generalized thermalization) in nonintegrable (integrable) quantum systems requires two ingredients, equilibration and an agreement with the predictions of the Gibbs (generalized Gibbs) ensemble. We prove that observables that exhibit eigenstate thermalization in single-particle sector equilibrate in many-body sectors of quantum-chaotic quadratic models. Remarkably, the same observables do not exhibit eigenstate thermalization in many-body sectors (we establish that there are exponentially many outliers). Hence, the generalized Gibbs ensemble is generally needed to describe their expectation values after equilibration, and it is characterized by Lagrange multipliers that are smooth functions of single-particle energies.