Scalable Quantum Technologies Research Articles

Enhanced Superconductivity in 2H-TaS2 Devices through in Situ Molecular Intercalation.

The intercalation of guest species into the gap of van der Waals materials often leads to the emergence of intriguing phenomena such as superconductivity. While intercalation-induced superconductivity has been reported in several bulk crystals, reaching a zero-resistance state in flakes remains challenging. Here, we show a simple method for enhancing the superconducting transition in tens-of-nanometers thick 2H-TaS2 crystals contacted by gold electrodes through in situ intercalation. Our approach enables measuring the electrical characteristics of the same flake before and after intercalation, permitting us to precisely identify the effect of the guest species on the TaS2 transport properties. We find that the intercalation of amylamine molecules into TaS2 flakes causes a suppression of the charge density wave and an increase in the superconducting transition with an onset temperature above 3 K. Additionally, we show that a fully developed zero-resistance state can be achieved in flakes by engineering the conditions of the chemical intercalation. Our findings pave the way for the integration of chemically tailored intercalation compounds in scalable quantum technologies.

Purifying Photon Indistinguishability through Quantum Interference.

Indistinguishability between photons is a key requirement for scalable photonic quantum technologies. We experimentally demonstrate that partly distinguishable single photons can be purified to reach near-unity indistinguishability by the process of quantum interference with ancillary photons followed by heralded detection of a subset of them. We report on the indistinguishability of the purified photons by interfering two purified photons and show improvements in the photon indistinguishability of 2.774(3)% in the low-noise regime, and as high as 10.2(5)% in the high-noise regime.

Radiatively cooled quantum microwave amplifiers

Superconducting microwave amplifiers are essential for sensitive signal readout in superconducting quantum processors. Typically based on Josephson junctions, these amplifiers require operation at milli-Kelvin temperatures to achieve quantum-limited performance. Here, we demonstrate a quantum microwave amplifier that employs radiative cooling to operate at elevated temperatures and maintain near quantum-limited added noise. This kinetic-inductance-based parametric amplifier, patterned from a single layer of relatively high-Tc NbN thin film, maintains a high gain and meanwhile enables low added noise of 1.3 quanta when operated at 1.5 K. Remarkably, this represents only a 0.2 quanta increase compared to the performance at a base temperature of 0.1 K. Based on our findings, we also discuss the practicality of such an operating scheme for various quantum applications. By uplifting the parametric amplifiers from the mixing chamber without compromising readout efficiency, this work represents an important step toward more scalable microwave quantum technologies.

Exciton–photocarrier interference in mixed lead-halide-perovskite nanocrystals

The use of semiconductor nanocrystals in scalable quantum technologies requires characterization of the exciton coherence dynamics in an ensemble of electronically isolated crystals in which system–bath interactions are nevertheless strong. In this communication, we identify signatures of Fano-like interference between excitons and photocarriers in the coherent two-dimensional photoluminescence excitation spectral lineshapes of mixed lead-halide perovskite nanocrystals in dilute solution. Specifically, by tuning the femtosecond-pulse spectrum, we show such interference in an intermediate coupling regime, which is evident in the coherent lineshape when simultaneously exciting the exciton and the free-carrier band at higher energy. We conclude that this interference is an intrinsic effect that will be consequential in the quantum dynamics of the system and will thus dictate decoherence dynamics, with consequences in their application in quantum technologies.

Increasing quantum communication rates using hyperentangled photonic states

Quantum communication is based on the generation of quantum states and exploitation of quantum resources for communication protocols. Currently, photons are considered as the optimal carriers of information, because they enable long-distance transition with resilience to decoherence and they are relatively easy to create and detect. Entanglement is a fundamental resource for quantum communication and information processing, and it is of particular importance for quantum repeaters. Hyperentanglement, a state where parties are entangled with two or more degrees of freedom (DoFs) simultaneously, provides an important additional resource because it increases data rates and enhances error resilience. However, in photonics, the channel capacity, i.e., the ultimate throughput, is fundamentally limited when dealing with linear elements. We propose a technique for achieving higher transmission rates for quantum communication by using hyperentangled states, based on multiplexing multiple DoFs on a single photon, transmitting the photon, and eventually demultiplexing the DoFs to different photons at the destination, using Bell state measurements. Following our scheme, one can generate two entangled qubit pairs by sending only a single photon. The proposed transmission scheme lays the groundwork for novel quantum communication protocols with higher transmission rates and refined control over scalable quantum technologies.

Precise Characterization of a Waveguide Fiber Interface in Silicon Carbide.

Spin-active optical emitters in silicon carbide are excellent candidates toward the development of scalable quantum technologies. However, efficient photon collection is challenged by undirected emission patterns from optical dipoles, as well as low total internal reflection angles due to the high refractive index of silicon carbide. Based on recent advances with emitters in silicon carbide waveguides, we now demonstrate a comprehensive study of nanophotonic waveguide-to-fiber interfaces in silicon carbide. We find that across a large range of fabrication parameters, our experimental collection efficiencies remain above 90%. Further, by integrating silicon vacancy color centers into these waveguides, we demonstrate an overall photon count rate of 181 kilo-counts per second, which is an order of magnitude higher compared to standard setups. We also quantify the shift of the ground state spin states due to strain fields, which can be introduced by waveguide fabrication techniques. Finally, we show coherent electron spin manipulation with waveguide-integrated emitters with state-of-the-art coherence times of T 2 ∼ 42 μs. The robustness of our methods is very promising for quantum networks based on multiple orchestrated emitters.

Efficient characterizations of multiphoton states with an ultra-thin optical device

Metasurface enables the generation and manipulation of multiphoton entanglement with flat optics, providing a more efficient platform for large-scale photonic quantum information processing. Here, we show that a single metasurface optical device would allow more efficient characterizations of multiphoton entangled states, such as shadow tomography, which generally requires fast and complicated control of optical setups to perform information-complete measurements, a demanding task using conventional optics. The compact and stable device here allows implementations of general positive operator valued measures with a reduced sample complexity and significantly alleviates the experimental complexity to implement shadow tomography. Integrating self-learning and calibration algorithms, we observe notable advantages in the reconstruction of multiphoton entanglement, including using fewer measurements, having higher accuracy, and being robust against experimental imperfections. Our work unveils the feasibility of metasurface as a favorable integrated optical device for efficient characterization of multiphoton entanglement, and sheds light on scalable photonic quantum technologies with ultra-thin optical devices.

Taming the Rotating Wave Approximation

The interaction between light and matter is one of the oldest research areas of quantum mechanics, and a field that just keeps on delivering new insights and applications. With the arrival of cavity and circuit quantum electrodynamics we can now achieve strong light-matter couplings which form the basis of most implementations of quantum technology. But quantum information processing also has high demands requiring total error rates of fractions of percentage in order to be scalable (fault-tolerant) to useful applications. Since errors can also arise from modelling, this has brought into center stage one of the key approximations of quantum theory, the Rotating Wave Approximation (RWA) of the quantum Rabi model, leading to the Jaynes-Cummings Hamiltonian. While the RWA is often very good and incredibly useful to understand light-matter interactions, there is also growing experimental evidence of regimes where it is a bad approximation. Here, we ask and answer a harder question: for which experimental parameters is the RWA, although perhaps qualitatively adequate, already not good enough to match the demands of scalable quantum technology? For example, when is the error at least, and when at most, 1%? To answer this, we develop rigorous non-perturbative bounds taming the RWA. We find that these bounds not only depend, as expected, on the ratio of the coupling strength and the oscillator frequency, but also on the average number of photons in the initial state. This confirms recent experiments on photon-dressed Bloch-Siegert shifts. We argue that with experiments reporting controllable cavity states with hundreds of photons and with quantum error correcting codes exploring more and more of Fock space, this state-dependency of the RWA is increasingly relevant for the field of quantum computation, and our results pave the way towards a better understanding of those experiments.

Tunable quantum dots in monolithic Fabry-Perot microcavities for high-performance single-photon sources

Cavity-enhanced single quantum dots (QDs) are the main approach towards ultra-high-performance solid-state quantum light sources for scalable photonic quantum technologies. Nevertheless, harnessing the Purcell effect requires precise spectral and spatial alignment of the QDs’ emission with the cavity mode, which is challenging for most cavities. Here we have successfully integrated miniaturized Fabry-Perot microcavities with a piezoelectric actuator, and demonstrated a bright single-photon source derived from a deterministically coupled QD within this microcavity. Leveraging the cavity-membrane structures, we have achieved large spectral tunability via strain tuning. On resonance, a high Purcell factor of ~9 is attained. The source delivers single photons with simultaneous high extraction efficiency of 0.58, high purity of 0.956(2) and high indistinguishability of 0.922(4). Together with its compact footprint, our scheme facilitates the scalable integration of indistinguishable quantum light sources on-chip, therefore removing a major barrier to the development of solid-state quantum information platforms based on QDs.

Highly-excited Rydberg excitons in synthetic thin-film cuprous oxide

Cuprous oxide (hbox {Cu}{}_2hbox {O}) has recently emerged as a promising material in solid-state quantum technology, specifically for its excitonic Rydberg states characterized by large principal quantum numbers (n). The significant wavefunction size of these highly-excited states (proportional to n^2) enables strong long-range dipole-dipole (proportional to n^4) and van der Waals interactions (proportional to n^{11}). Currently, the highest-lying Rydberg states are found in naturally occurring hbox {Cu}_2hbox {O}. However, for technological applications, the ability to grow high-quality synthetic samples is essential. The fabrication of thin-film hbox {Cu}{}_2hbox {O} samples is of particular interest as they hold potential for observing extreme single-photon nonlinearities through the Rydberg blockade. Nevertheless, due to the susceptibility of high-lying states to charged impurities, growing synthetic samples of sufficient quality poses a substantial challenge. This study successfully demonstrates the CMOS-compatible synthesis of a hbox {Cu}{}_2hbox {O} thin film on a transparent substrate that showcases Rydberg excitons up to n = 8 which is readily suitable for photonic device fabrications. These findings mark a significant advancement towards the realization of scalable and on-chip integrable Rydberg quantum technologies.

Shared control of a 16 semiconductor quantum dot crossbar array

The efficient control of a large number of qubits is one of the most challenging aspects for practical quantum computing. Current approaches in solid-state quantum technology are based on brute-force methods, where each and every qubit requires at least one unique control line—an approach that will become unsustainable when scaling to the required millions of qubits. Here, inspired by random-access architectures in classical electronics, we introduce the shared control of semiconductor quantum dots to efficiently operate a two-dimensional crossbar array in planar germanium. We tune the entire array, comprising 16 quantum dots, to the few-hole regime. We then confine an odd number of holes in each site to isolate an unpaired spin per dot. Moving forward, we demonstrate on a vertical and a horizontal double quantum dot a method for the selective control of the interdot coupling and achieve a tunnel coupling tunability over more than 10 GHz. The operation of a quantum electronic device with fewer control terminals than tunable experimental parameters represents a compelling step forward in the construction of scalable quantum technology.

All-JJ Logic Based on Bistable JJs

The all-JJ logic family is a promising area and power efficient, scalable single flux quantum (SFQ) technology for application to exascale supercomputers. All-JJ superconductive logic is based on a superconductor-ferromagnet-superconductor (SFS) bistable JJ, enabling nanometer feature sizes in VLSI complexity superconductive systems. In this paper, a mechanical analogy is proposed to describe the dynamic behavior of these bistable JJs. Novel all-JJ logic gates, such as TFF, DFF, OR, AND, and NOT gates, are presented here. All-JJ circuits are composed of bistable JJs, standard JJs, and bias currents, requiring no large inductors within the storage loops. All-JJ logic cells exhibit less delay and power than standard SFQ cells with the same critical current density. All-JJ systems can operate at high frequencies due to the small capacitance of the SFS JJ. A complex all-JJ circuit from the suite of ISCAS'85 benchmark circuits is also characterized. This complex all-JJ circuit exhibits less delay and power as compared to standard SFQ logic. The bias current in a conventional benchmark circuit and all-JJ benchmark circuit is, respectively, 22 mA and 13 mA. The delay of each cell within the conventional benchmark circuit and all-JJ benchmark circuit is, respectively, approximately 20 and 8 ps. A parasitic inductance in series with the JJs disturbs the current distribution within the all-JJ circuits while degrading the margins. To suppress the effects of this parasitic inductance on SFS JJs, small linear inductors are added to manage the current distribution and improve parameter margins.

Error Rate Reduced for Scalable Quantum Technology

Error Rate Reduced for Scalable Quantum Technology

High-speed thin-film lithium niobate quantum processor driven by a solid-state quantum emitter.

Scalable photonic quantum computing architectures pose stringent requirements on photonic processing devices. The needs for low-loss high-speed reconfigurable circuits and near-deterministic resource state generators are some of the most challenging requirements. Here, we develop an integrated photonic platform based on thin-film lithium niobate and interface it with deterministic solid-state single-photon sources based on quantum dots in nanophotonic waveguides. The generated photons are processed with low-loss circuits programmable at speeds of several gigahertz. We realize a variety of key photonic quantum information processing functionalities with the high-speed circuits, including on-chip quantum interference, photon demultiplexing, and reprogrammability of a four-mode universal photonic circuit. These results show a promising path forward for scalable photonic quantum technologies by merging integrated photonics with solid-state deterministic photon sources in a heterogeneous approach to scaling up.

Single-Photon Emission from Two-Dimensional Materials, to a Brighter Future

Single photons, often called flying qubits, have enormous promise to realize scalable quantum technologies ranging from an unhackable communication network to quantum computers. However, finding an ideal single-photon emitter (SPE) is a great challenge. Recently, two-dimensional (2D) materials have shown great potential as hosts for SPEs that are bright and operate under ambient conditions. This Perspective enumerates the metrics required for an SPE source and highlights that 2D materials, because of reduced dimensionality, exhibit interesting physical effects and satisfy several metrics, making them excellent candidates to host SPEs. The performance of SPE candidates discovered in 2D materials, hexagonal boron nitride and transition metal dichalcogenides, will be assessed based on the metrics, and the remaining challenges will be highlighted. Lastly, strategies to mitigate such challenges by developing design rules to deterministically create SPE sources will be presented.

Observation of large spontaneous emission rate enhancement of quantum dots in a broken-symmetry slow-light waveguide

Quantum states of light and matter can be manipulated on the nanoscale to provide a technological resource for aiding the implementation of scalable photonic quantum technologies. Experimental progress relies on the quality and efficiency of the coupling between photons and internal spin states of quantum emitters. Here we demonstrate a nanophotonic waveguide platform with embedded quantum dots (QDs) that enables both Purcell-enhanced emission and strong chiral coupling. The design uses slow-light effects in a glide-plane photonic crystal waveguide with QD tuning to match the emission frequency to the slow-light region. Simulations were used to map the chirality and Purcell enhancement depending on the position of a dipole emitter relative to the air holes. The highest Purcell factors and chirality occur in separate regions, but there is still a significant area where high values of both can be obtained. Based on this, we first demonstrate a record large radiative decay rate of 17 ± 2 ns−1 (60 ± 6 ps lifetime) corresponding to a 20 ± 2 fold Purcell enhancement. This was achieved by electric-field tuning of the QD to the slow-light region and quasi-resonant phonon-side band excitation. We then demonstrate a 5 ± 1 fold Purcell enhancement for a dot with high degree of chiral coupling to waveguide modes, substantially surpassing all previous measurements. Together these demonstrate the excellent prospects for using QDs in scalable implementations of on-chip spin-photonics relying on chiral quantum optics.

Highly Uniform Array of Hexagonally Symmetric Micro‐Pyramid Structures for Scalable and Single Quantum Dot Emitters

AbstractControlling the site, size, and shape of group III‐nitride quantum dots (QDs) is critical for the development of mass‐producible single‐photon sources for scalable quantum technologies operable at room temperature. Herein, a methodology is proposed for fabricating high‐purity single QD emitters by controlling site‐controlled GaN micro‐pyramid structures with a high degree of uniformity and symmetry. To achieve a uniformly grown, hexagonally symmetric micro‐pyramid array, the H2/N2 carrier gas ratio, growth temperature, and V/III ratio are controlled to attain self‐limited growth regime and self‐limited width at the GaN pyramid apex. A thin InGaN layer is consecutively grown on a pyramid array under the growth condition for enhancing the growth rate anisotropy to hinder the growth of InGaN quantum wells (QWs) at semi‐polar facets. As a result, single‐photon emission is observed from apex QD with suppressed background side QW emission while maintaining more than 90% high hexagonal QD symmetry over the large area of the wafer.

Coherence Properties of Electron-Beam-Activated Emitters in Hexagonal Boron Nitride Under Resonant Excitation

Lattice defects in two-dimensional materials are compelling sources of single photons for applications in quantum information processing. However, the range of emission wavelengths is broad, the defect types vary and the atomic structures of most are unknown, and the inability to fabricate defects systematically on demand has limited progress. This work leverages a robust, site-specific fabrication technique to produce arrays of blue quantum emitters for study via cryogenic spectroscopy. The authors characterize processes that limit emitter coherence and observe Rabi oscillations, demonstrating the potential of these defects for scalable quantum technologies.

Diamond Integrated Quantum Nanophotonics: Spins, Photons and Phonons

Integrated photonic devices in diamond have tremendous potential for many quantum applications, including long-distance quantum communication, quantum information processing, and quantum sensing. These devices benefit from diamond's combination of exceptional thermal, optical, and mechanical properties. Its wide electronic bandgap makes diamond an ideal host for a variety of optical active spin qubits that are key building blocks for quantum technologies. In landmark experiments, diamond spin qubits have enabled demonstrations of remote entanglement, memory-enhanced quantum communication, and multi-qubit spin registers with fault-tolerant quantum error correction, leading to the realization of multinode quantum networks. These advances put diamond at the forefront of solid-state material platforms for quantum information processing. Recent developments in diamond nanofabrication techniques provide a promising route to further scaling of these landmark experiments towards real-life quantum technologies. In this paper, we focus on the recent progress in creating integrated diamond quantum photonic devices, with particular emphasis on spin-photon interfaces, cavity optomechanical devices, and spin-phonon transduction. Finally, we discuss prospects and remaining challenges for the use of diamond in scalable quantum technologies.

Front Cover: Symmetry‐Protected Two‐Level System in the H3 Center Enabled by a Spin–Photon Interface: A Competitive Qubit Candidate for the NISQ Technology (Adv. Quantum Technol. 11/2022)

Investigation of promising spin-qubit candidates in solid materials is vital for scalable noisy intermediate scale quantum (NISQ) technologies. As demonstrated by Hailong Wang, Heng Yuan, and colleagues in article number 2200044, a symmetry-protected two-level system (qubit) based on the H3 center in diamond with C2v symmetry is characterized, which is capable of applications in quantum information processing. The figure depicts an N–V–N geometry (H3 center) in the diamond acting as a spin–photon interface, which can interrogate the spin information of the qubit in the form of fluorescence. The green beam is fluorescence, the oscillation at the bottom represents the evolution of spin states, and the surrounding “0” and “1” elements indicate that this work provides a very competitive processor for quantum computing.