Robust Quantum Research Articles

π-SnS Colloidal Nanocrystals with Size-Dependent Band Gaps

Colloidal quantum dot (QD) solids are materials that exploit the quantum confinement properties of the constituent nanocrystals. Significant progress in QD optoelectronic devices has been made mainly using compounds with concerning high degree of toxicity for practical applications. Sn-chalcogenides are among the possible environmentally benign alternatives the bulks of which have demonstrated intriguing properties for energy devices. Nanostructuring these Sn-chalcogenides may create variations in their crystal structures and properties. Here, we demonstrate robust quantum confinement effects in small-diameter cubic SnS colloidal nanocrystals (NCs) and their potential exploitation for high-performance optoelectronic devices. Well-controlled production of cubic SnS colloidal NCs with a 4–10 nm diameter is established by developing a one-pot synthesis procedure in an inert condition that allows Sn complexation with only a single type of ligand molecule. The analyses of the X-ray diffractogram and transmission electron micrograph revealed that the structure of NCs is the so-called π-type chiral and cubic structures. These NCs behave as quantum dots, exhibiting a quantum confinement effect with strong variations of energy band gaps by size. The photodetectors of the SnS NCs exhibit high responsivity comparable to the other established QD systems, although the channels are made only from a single or few layer(s). This new compound will open new pathways for establishing environmentally safe and high-performance QD optoelectronic and photocatalysis devices.

Qopt: An Experiment-Oriented Software Package for Qubit Simulation and Quantum Optimal Control

Realistic modeling of qubit systems including noise and constraints imposed by control hardware is required for performance prediction and control optimization of quantum processors. We introduce qopt, a software framework for simulating qubit dynamics and robust quantum optimal control considering common experimental situations. To this end, we model open and closed qubit systems with a focus on the simulation of realistic noise characteristics and experimental constraints. Specifically, the influence of noise can be calculated using Monte Carlo methods, effective master equations or with the efficient filter function formalism, which enables the investigation and mitigation of autocorrelated noise. In addition, limitations of control electronics, including finite bandwidth effects as well as nonlinear transfer functions and drive-dependent noise, can be considered. The calculation of gradients based on analytic results is implemented to facilitate the efficient optimization of control pulses. The software easily interfaces with QuTiP, is published under an open-source license, is well tested, and features a detailed documentation.

Experimentally realizing efficient quantum control with reinforcement learning

We experimentally investigate deep reinforcement learning (DRL) as an artificial intelligence approach to control a quantum system. We verify that DRL explores fast and robust digital quantum controls with operation time analytically hinted by shortcuts to adiabaticity. In particular, the protocol’s robustness against both over-rotations and off-resonance errors can still be achieved simultaneously without any priori input. For the thorough comparison, we choose the task as single-qubit flipping, in which various analytical methods are well-developed as the benchmark, ensuring their feasibility in the quantum system as well. Consequently, a gate operation is demonstrated on a trapped 171Yb+ ion, significantly outperforming analytical pulses in the gate time and energy cost with hybrid robustness, as well as the fidelity after repetitive operations under time-varying stochastic errors. Our experiments reveal a framework of computer-inspired quantum control, which can be extended to other complicated tasks without loss of generality.

Solvent-assisted synthesis of CoC2O4 nanorods for enhanced charge transfer in dual catalytic activity: Photocatalysis and electrocatalytic oxygen evolution

This work presents an in-depth experimental and theoretical investigation into the morphological and optoelectronic properties of cobalt oxalate nanorods for applications in both photocatalysis and electrocatalysis. The morphological tunability of anhydrous cobalt oxalate nanostructures has been demonstrated by developing cobalt oxalate nanorods of varying dimensions in 2 different organic solvents, namely Tetrahydrofuran (THF) and Dimethylformamide (DMF). Varying solvation kinetics have led to different rates of nucleation and growth of different crystallographic planes, resulting in much shorter nanorods in the samples prepared in THF (CoX-THF), compared with long anisotropic nanorods in the samples prepared in DMF (CoX-DMF). Various structural and electrochemical analyses have revealed higher crystallinity and enhanced charge transfer kinetics in the CoX-THF samples, resulting in more sites for catalytic activity along their consistent nanoscale dimensions. Spectroscopic analyses have revealed visible light activity in both the samples, with CoX-THF samples having a more visible light active band gap and lesser electron hole recombination. Such optoelectronic properties have been theoretically verified by DFT calculations on an optimized unit cell geometry of monoclinic cobalt oxalate, which were found to be well in agreement with experimental findings. TDDFT calculations have revealed the presence of robust quantum transitions in the CoO6 octahedra, which have played a key role in their photoactivity. In accordance with their enhanced characteristics, CoX-THF samples showed higher photocatalytic activity, achieving higher photocatalytic degradation and higher photocurrent generation than the longer nanorods in CoX-DMF. The CoX-THF samples also showed superior oxygen evolution kinetics, with their LSV curves exhibiting a remarkably low overpotential of 240 mV to achieve 10 mA/cm2 current density. Thus rigorous experimental analyses complemented by theoretical calculations have been conducted to establish the tunable dual catalytic properties of cobalt oxalate nanorods.

Robust Quantum Metrology With Explicit Symmetric States

Quantum metrology is a promising practical use case for quantum technologies, where physical quantities can be measured with unprecedented precision. In lieu of quantum error correction procedures, near term quantum devices are expected to be noisy, and we have to make do with noisy probe states. We prove that, for a set of carefully chosen symmetric probe states that lie within certain quantum error correction codes, quantum metrology exhibits an advantage over classical metrology even after the probe states are corrupted by a constant number of erasure and dephasing errors. These probe states prove useful for robust metrology not only in the NISQ regime, but also in the asymptotic setting where they achieve Heisenberg scaling. This brings us closer towards making robust quantum metrology a technological reality.

A Primer on Path Integrals, Aharonov–Bohm Effect and the Geometric Phase

This work reports an effort to introduce a few advanced concepts in quantum physics to undergraduate students who have only a minimal familiarity with wave mechanics. The geometric phase is not introduced in most introductory courses on quantum mechanics. It is of pivotal significance for the understanding of topological phase transitions which are becoming increasingly important in a large number of physical phenomena, including those that involve platforms for robust quantum computing. This article would provide a gentle introduction to the geometric phase. Toward this goal, Feynman’s path integral approach to quantum mechanics and its application in the interpretation of the wondrous Aharonov–Bohm effect is introduced in a gentle manner. A computer simulation of this interesting phenomenology is presented to help gain an early insight into some advanced concepts.

Challenges to magnetic doping of thin films of the Dirac semimetal Cd3As2

Magnetic doping of topological quantum materials provides an attractive route for studying the effects of time-reversal symmetry breaking. Thus motivated, we explore the introduction of the transition metal Mn into thin films of the Dirac semimetal Cd3As2 during growth by molecular beam epitaxy. Scanning transmission electron microscopy measurements show the formation of a Mn-rich phase at the top surface of Mn-doped Cd3As2 thin films grown using both uniform doping and delta doping. This suggests that Mn acts as a surfactant during epitaxial growth of Cd3As2, resulting in phase separation. Magnetometry measurements of such samples indicate a ferromagnetic phase with out-of-plane magnetic anisotropy. Electrical magneto-transport measurements of these films as a function of temperature, magnetic field, and chemical potential reveal a lower carrier density and higher electron mobility compared to pristine Cd3As2 films grown under similar conditions. This suggests that the surfactant effect might also serve to getter impurities. We observe robust quantum transport (Shubnikov-de Haas oscillations and an incipient integer quantum Hall effect) in very thin (7 nm) Cd3As2 films despite being in direct contact with a structurally disordered surface ferromagnetic overlayer.

Designing robust quantum refrigerators in disordered spin models

We explore a small quantum refrigerator in which the working substance is made of paradigmatic nearest-neighbor quantum spin models, the XYZ and the XY model with Dzyaloshinskii-Moriya interactions, consisting of two and three spins, each of which is in contact with a bosonic bath. We identify a specific range of interaction strengths which can be tuned appropriately to ensure a cooling of the selected spin in terms of its local temperature in the weak-coupling limit. Moreover, we report that in this domain, when one of the interaction strengths is disordered, the performance of the thermal machine operating as a refrigerator remains almost unchanged instead of degradation, thereby establishing the flexibility of this device. However, to obtain a significant amount of cooling via ordered as well as disordered spin models, we observe that one has to go beyond the weak-coupling limit and compute the figures of merits by using global master equations.

Extract the Degradation Information in Squeezed States with Machine Learning.

In order to leverage the full power of quantum noise squeezing with unavoidable decoherence, a complete understanding of the degradation in the purity of squeezed light is demanded. By implementing machine-learning architecture with a convolutional neural network, we illustrate a fast, robust, and precise quantum state tomography for continuous variables, through the experimentally measured data generated from the balanced homodyne detectors. Compared with the maximum likelihood estimation method, which suffers from time-consuming and overfitting problems, a well-trained machine fed with squeezed vacuum and squeezed thermal states can complete the task of reconstruction of the density matrix in less than one second. Moreover, the resulting fidelity remains as high as 0.99 even when the antisqueezing level is higher than 20dB. Compared with the phase noise and loss mechanisms coupled from the environment and surrounding vacuum, experimentally, the degradation information is unveiled with machine learning for low and high noisy scenarios, i.e., with the antisqueezing levels at 12dB and 18dB, respectively. Our neural network enhanced quantum state tomography provides the metrics to give physical descriptions of every feature observed in the quantum state with a single scan measurement just by varying the local oscillator phase from 0 to 2π and paves a way of exploring large-scale quantum systems in real time.

Nuclear spin-wave quantum register for a solid-state qubit.

Solid-state nuclear spins surrounding individual, optically addressable qubits1,2 are a crucial resource for quantum networks3-6, computation7-11 and simulation12. Although hosts with sparse nuclear spin baths are typically chosen to mitigate qubit decoherence13, developing coherent quantum systems in nuclear-spin-rich hosts enables exploration of a much broader range of materials for quantum information applications. The collective modes of these dense nuclear spin ensembles provide a natural basis for quantum storage14; however, using them as a resource for single-spin qubits has thus far remained elusive. Here, by using a highly coherent, optically addressed 171Yb3+ qubit doped into a nuclear-spin-rich yttrium orthovanadate crystal15, we develop a robust quantum control protocol to manipulate the multi-level nuclear spin states of neighbouring 51V5+ lattice ions. Via a dynamically engineered spin-exchange interaction, we polarize this nuclear spin ensemble, generate collective spin excitations, and subsequently use them toimplement a quantum memory. We additionally demonstrate preparation and measurement of maximally entangled 171Yb-51V Bell states. Unlike conventional, disordered nuclear-spin-based quantum memories16-24, our platform is deterministic and reproducible, ensuring identical quantum registers for all 171Yb3+ qubits. Our approach provides a framework for utilizing the complex structure of dense nuclear spin baths, paving the way towards building large-scale quantum networks using single rare-earth ion qubits15,25-28.

Optimal control of stimulated Raman adiabatic passage in a superconducting qudit

Stimulated Raman adiabatic passage (STIRAP) is a widely used protocol to realize high-fidelity and robust quantum control in various quantum systems. However, further application of this protocol in superconducting qubits is limited by population leakage caused by the only weak anharmonicity. Here, we introduce an optimally controlled shortcut-to-adiabatic (STA) technique to speed-up the STIRAP protocol in a superconducting qudit. By modifying the shapes of the STIRAP pulses, we experimentally realize a fast (32 ns) and high-fidelity (0.996 ± 0.005) quantum state transfer. In addition, we demonstrate that our protocol is robust against control parameter perturbations. Our stimulated Raman shortcut-to-adiabatic passage transition provides an efficient and practical approach for quantum information processing.

Robust Quantum Optimal Control with Trajectory Optimization

The ability to engineer high-fidelity gates on quantum processors in the presence of systematic errors remains the primary barrier to achieving quantum advantage. Quantum optimal control methods have proven effective in experimentally realizing high-fidelity gates, but they require exquisite calibration to be performant. We apply robust trajectory optimization techniques to suppress gate errors arising from system parameter uncertainty. We propose a derivative-based approach that maintains computational efficiency by using forward-mode differentiation. Additionally, the effect of depolarization on a gate is typically modeled by integrating the Lindblad master equation, which is computationally expensive. We employ a computationally efficient model and utilize time-optimal control to achieve high-fidelity gates in the presence of depolarization. We apply these techniques to a fluxonium qubit and suppress simulated gate errors due to parameter uncertainty below ${10}^{\ensuremath{-}7}$ for static parameter deviations of the order of 1%.

Quantum register of fermion pairs.

Quantum control of motion is central for modern atomic clocks1 and interferometers2. It enables protocols to process and distribute quantum information3,4, and allows the probing of entanglement in correlated states of matter5. However, the motional coherence of individual particles can be fragile to maintain, as external degrees of freedom couple strongly to the environment. Systems in nature with robust motional coherence instead often involve pairs of particles, from the electrons in helium, to atom pairs6, molecules7 and Cooper pairs. Here we demonstrate long-lived motional coherence and entanglement of pairs of fermionic atoms in an optical lattice array. The common and relative motion of each pair realize a robust qubit, protected by exchange symmetry. The energy difference between the two motional states is set by the atomic recoil energy, is dependent on only the mass and the lattice wavelength, and is insensitive to the noise of the confining potential. We observe quantum coherence beyond ten seconds. Modulation of the interactions between the atoms provides universal control of the motional qubit. The methods presented here will enable coherently programmable quantum simulators of many-fermion systems8, precision metrology based on atom pairs and molecules9,10 and, by implementing further advances11-13, digital quantum computation using fermion pairs14.

Light detection and ranging with entangled photons.

Single-photon light detection and ranging (LiDAR) is a key technology for depth imaging through complex environments. Despite recent advances, an open challenge is the ability to isolate the LiDAR signal from other spurious sources including background light and jamming signals. Here we show that a time-resolved coincidence scheme can address these challenges by exploiting spatio-temporal correlations between entangled photon pairs. We demonstrate that a photon-pair-based LiDAR can distill desired depth information in the presence of both synchronous and asynchronous spurious signals without prior knowledge of the scene and the target object. This result enables the development of robust and secure quantum LiDAR systems and paves the way to time-resolved quantum imaging applications.

One-dimensional quantum channel in bent honeycomb nanoribbons.

The directionality of steering charge carriers is of great importance for the application of two-dimensional (2D) materials. Using the generalized Bloch theorem coupled with the self-consistent charge density-functional tight-binding method, we theoretically propose an approach to construct a one-dimensional (1D) quantum channel in honeycomb nanoribbons (NR) via in-plane bending deformation. Bending-induced pseudo-magnetic fields lead to Landau quantization and localize the electronic states along both edges of bent NR. These localized states form robust 1D quantum channels, whose energies can be linearly modulated through the bending angle. Our findings give new inspiration for the realization of transverse magnetic focusing (TMF) under zero magnetic fields and pave the way for the design of 2D material-based nano-devices via strain-engineering.

Building Topological Superconductors for Quantum Computers

Despite the numerous opportunities presented by quantum computers, their practical use has so far been hindered by their vulnerability to the surrounding environments. Dr Badih Assaf, Dr Xinyu Liu and their colleagues at the University of Notre Dame, Indiana, show how these problems could be overcome, through the use of an exotic class of hybridised materials named ‘topological superconductors’. Through the optimised fabrication of these materials, and a detailed analysis of their quantum properties, the team’s results could pave the way for the widespread use of topological superconductors in robust and practical quantum computers.

One-pot synthesis of robust dendritic sulfur quantum dots for two-photon fluorescence imaging and "off-on" detection of hydroxyl radicals and ascorbic acid.

The straightforward preparation of fluorescent sulfur quantum dots (SQDs) with good photostability and biocompatibility and multifunction remains a challenge. Herein, a simple method to improve the performance of SQDs is reported, that is, using hyperbranched polyglycerol (HPG) as a ligand to direct the synthesis of dendritic HPG-SQD nanocomposites from cheap elemental sulfur. Thanks to the protection of HPG, the HPG-SQDs show much better biocompatibility and photostability as compared with the widely reported polyethylene glycol (PEG) ligand-capped SQDs (PEG-SQDs). In addition, the HPG-SQDs also present excellent aqueous solubility, stable fluorescence against environmental variation, good cell uptake capability, and strong single- and two-photon fluorescence. Moreover, the HPG-SQDs display sensitive and selective fluorescence “off–on” behavior to hydroxyl radicals (˙OH) and ascorbic acid (AA), respectively, and thereby hold potential as a fluorescent switch to detect ˙OH and AA. For the first time, the utilization of two-photon fluorescence of HPG-SQDs to monitor ˙OH and AA in cells is demonstrated in this study.

Design and Implementation of the Illinois Express Quantum Metropolitan Area Network

The Illinois Express Quantum Network (IEQNET) is a program to realize metropolitan scale quantum networking over deployed optical fiber using currently available technology. IEQNET consists of multiple sites that are geographically dispersed in the Chicago metropolitan area. Each site has one or more quantum nodes (Q-nodes) representing the communication parties in a quantum network. Q-nodes generate or measure quantum signals such as entangled photons and communicate the measurement results via standard, classical signals and conventional networking processes. The entangled photons in IEQNET nodes are generated at multiple wavelengths, and are selectively distributed to the desired users via transparent optical switches. Here we describe the network architecture of IEQNET, including the Internet-inspired layered hierarchy that leverages software-defined networking (SDN) technology to perform traditional wavelength routing and assignment between the Q-nodes. Specifically, SDN decouples the control and data planes, with the control plane being entirely implemented in the classical domain. We also discuss the IEQNET processes that address issues associated with synchronization, calibration, network monitoring, and scheduling. An important goal of IEQNET is to demonstrate the extent to which the control plane classical signals can co-propagate with the data plane quantum signals in the same fiber lines (quantum-classical signal "coexistence"). This goal is furthered by the use of tunable narrow-band optical filtering at the receivers and, at least in some cases, a wide wavelength separation between the quantum and classical channels. We envision IEQNET to aid in developing robust and practical quantum networks by demonstrating metro-scale quantum communication tasks such as entanglement distribution and quantum-state teleportation.

High photoluminescence quantum yield near-infrared emission from a lead-free ytterbium-doped double perovskite.

When excited by photons with energies greater than 2.2 eV, the bandgap energy, Yb-doped Cs2AgBiBr6 thin films synthesized via physical vapor deposition emit strong near-infrared luminescence centered at ∼1.24 eV via the Yb3+ 2F5/2 → 2F7/2 electronic transition. Robust, reproducible, and stable photoluminescence quantum yields (PLQY) as high as 82.5% are achieved with Cs2AgBiBr6 films doped with 8% Yb. This high PLQY indicates facile and efficient energy transfer from the perovskite host, Cs2AgBiBr6, to Yb, making Cs2AgBiBr6 the most promising lead-free down-conversion material.

Hybrid CV-DV Quantum Communications and Quantum Networks

Quantum information processing (QIP) opens new opportunities for high-performance computing, high-precision sensing, and secure communications. Among various QIP features, the entanglement is a unique one. To take full advantage of quantum resources, it will be necessary to interface quantum systems based on different encodings of information both discrete and continuous. The goal of this paper is to lay the groundwork for the development of a robust and efficient hybrid continuous variable-discrete variable (CV-DV) quantum network, enabling the distribution of a large number of entangled states over hybrid DV-CV multi-hop nodes in an arbitrary topology. The proposed hybrid quantum communication network (QCN) can serve as the backbone for a future quantum Internet, thus providing extensive long-term impacts on the economy and national security through QIP, distributed quantum computing, quantum networking, and distributed quantum sensing. By employing the photon addition and photon subtraction modules we describe how to generate the hybrid DV-CV entangled states and how to implement their teleportation and entanglement swapping through entangling measurements. We then describe how to extend the transmission distance between nodes in hybrid QCN by employing macroscopic light states, noiseless amplification, and reconfigurable quantum LDPC coding. We further describe how to enable quantum networking and distributed quantum computing by employing the deterministic cluster state concept introduced here. Finally, we describe how the proposed hybrid CV-DV states can be used in an entanglement-based hybrid QKD.