Recent milestone experiments establishing satellite-to-ground quantum communication are paving the way for the development of the quantum Internet, a network interconnected by quantum channels. Here, we employ network theory to study the properties of the photonic networks that can be generated by satellite-based quantum communication and compare them with those of their optical-fiber counterpart. We predict that satellites can generate small-world networks, implying that physically distant nodes are actually near from a network perspective. We also analyze the connectivity properties of the network and show, in particular, that they are robust against random failures. This positions satellite-based quantum communication as the most promising technology to distribute entanglement across large distances in quantum networks of growing size and complexity.

Quantum computation relies on accurate measurements of qubits not only for reading the output of the calculation, but also to perform error correction. Most proposed scalable silicon architectures utilize Pauli blockade of triplet states for spin-to-charge conversion. In recent experiments there have been instances when instead of conventional triplet blockade readout, Pauli blockade is sustained only between parallel spin configurations, with $|T_0⟩$ relaxing quickly to the singlet state and leaving $|T_{+}⟩$ and $|T_{-}⟩$ states blockaded—which we call parity readout. Both types of blockade can be used for readout in quantum computing, but it is crucial to maximize the fidelity and understand in which regime the system operates. We devise and perform an experiment in which the crossover between parity and singlet-triplet readout can be identified by investigating the underlying physics of the $|T_0⟩$ relaxation rate. This rate is tunable over 4 orders of magnitude by controlling the Zeeman energy difference between the dots induced by spin-orbit coupling, which in turn depends on the direction of the applied magnetic field. We suggest a theoretical model incorporating charge noise and relaxation effects that explains quantitatively our results. Investigating the model both analytically and numerically, we identify strategies to obtain on demand either singlet-triplet or parity readout consistently across large arrays of dots. We also discuss how parity readout can be used to perform full two-qubit state tomography and its impact on quantum error-detection schemes in large-scale silicon quantum computers.

We present a theoretical model and experimental characterization of a microwave kinetic inductance traveling-wave (KIT) amplifier, whose noise performance, measured by a shot-noise tunnel junction (SNTJ), approaches the quantum limit. Biased with a dc current, this amplifier operates in a three-wave mixing fashion, thereby reducing by several orders of magnitude the power of the microwave pump tone and associated parasitic heating compared to conventional four-wave mixing KIT amplifier devices. It consists of a $50\mathrm{\Omega }$ artificial transmission line whose dispersion allows for a controlled amplification bandwidth. We measure ${16.5}_{-1.3}^{+1}$ dB of gain across a 2 GHz bandwidth with an input 1 dB compression power of $-63$ dBm, in qualitative agreement with theory. Using a theoretical framework that accounts for the SNTJ-generated noise entering both the signal and idler ports of the KIT amplifier, we measure the system-added noise of an amplification chain that integrates the KIT amplifier as the first amplifier. This system-added noise, $3.1\pm 0.6$ quanta (equivalent to $0.66\pm 0.15$ K) between 3.5 and 5.5 GHz, is the one that a device replacing the SNTJ in that chain would see. This KIT amplifier is therefore suitable to read large arrays of microwave kinetic inductance detectors and promising for multiplexed superconducting qubit readout.

Accurate time-delay measurement is at the core of many modern technologies. We present a temporal-mode demultiplexing scheme that achieves the ultimate quantum precision for the simultaneous estimation of the temporal centroid, the time offset, and the relative intensities of an incoherent mixture of ultrashort pulses at the single-photon level. We experimentally resolve temporal separations 10 times smaller than the pulse duration, as well as imbalanced intensities differing by a factor of ${10}^2$. This represents an improvement of more than an order of magnitude over the best standard methods based on intensity detection.

Conference key agreement (CKA) is a cryptographic effort of multiple parties to establish a shared secret key. In future quantum networks, generating secret keys in an anonymous way is of tremendous importance for parties that want to keep their shared key secret and at the same time protect their own identity. We provide a definition of anonymity for general protocols and present a CKA protocol that is provably anonymous under realistic adversarial scenarios. We base our protocol on shared Greenberger-Horne-Zeilinger states, which have been proposed as more efficient resources for CKA protocols, compared to bipartite entangled resources. The existence of secure and anonymous protocols based on multipartite entangled states provides a new insight on their potential as resources and paves the way for further applications.

We propose and analyze a family of periodic braiding protocols in systems with multiple localized Majorana modes (majoranas) for the purposes of Hamiltonian engineering. The protocols rely on double braids—draids—which flip the signs of both majoranas, as one is taken all the way around the other. Rapid draiding can be used to dynamically suppress some or all intermajorana couplings. Suppressing all couplings can drastically reduce residual majorana dynamics, producing a more robust computational subspace. Nontrivial topological models can be obtained by selectively applying draids to some of the overlapping (imperfect) majoranas. Remarkably, draids can be implemented without having to physically braid majoranas or performing projective measurements. For instance, we show that draids can be performed by periodically modulating the coupling between a quantum dot and a topological superconducting wire to dynamically suppress the hybridization of majoranas in the quantum wire. In current experimental setups, this could lead to suppression of this coupling by a few orders of magnitude. The robustness of this protocol can be shown to parallel the topological robustness of physically braided majoranas. We propose an architecture that implements draids between distant majorana modes within a quantum register using a setup with multiple quantum dots and also discuss measurement-based ways of implementing the same.

Rydberg-atom quantum simulators are of keen interest because of their possibilities towards high-dimensional qubit architectures. Here we report continuous tuning of quantum Ising Hamiltonians of Rydberg atoms in three-dimensional arrangements. Various connected graphs of Rydberg atoms constructed with vertices and edges respectively representing atoms and Rydberg-blockaded atom pairs, and their eigenenergies are probed along with their geometric intermediates during structural transformations. Conformation spectra of star, complete, cyclic, and diamond graphs are probed for four interacting atoms and antiprism structures for six atoms. The energy level shifts and merges of the tested structural transformations are clearly observed with Fourier-transform spectroscopy, in good agreement with the model few-body quantum Ising Hamiltonian. This result demonstrates the possibility of continuous geometry tuning and thus programming of many-body spin-Hamiltonian systems.

Control of quantum systems is a central element of high-precision experiments and the development of quantum technological applications. Control pulses that are typically temporally or spatially modulated are often designed based on theoretical simulations. As we gain control over larger and more complex quantum systems, however, we reach the limitations of our capabilities of theoretical modeling and simulations, and learning how to control a quantum system based exclusively on experimental data can help us to exceed those limitations. Because of the intrinsic probabilistic nature of quantum mechanics, it is fundamentally necessary to repeat measurements on individual quantum systems many times in order to estimate the expectation value of an observable with good accuracy. Control algorithms requiring accurate data can thus imply an experimental effort that negates the benefits of avoiding theoretical modeling. We present a control algorithm that finds optimal control solutions in the presence of large measurement shot noise and even in the limit of single-shot measurements. The algorithm builds up on Bayesian optimization that is well suited to handle noisy data; but since the commonly used assumption of Gaussian noise is not appropriate for projective measurements with a low number of repetitions, we develop Bayesian inference that respects the binomial nature of shot noise. With several numerical and experimental examples we demonstrate that this method is capable of finding excellent control solutions with minimal experimental effort.

Understanding the nature of confinement, as well as its relation with the spontaneous breaking of chiral symmetry, remains one of the long-standing questions in high-energy physics. The difficulty of this task stems from the limitations of current analytical and numerical techniques to address nonperturbative phenomena in non-Abelian gauge theories. In this work, we show how similar phenomena emerge in simpler models, and how these can be further investigated using state-of-the-art cold-atom quantum simulators. More specifically, we introduce the rotor Jackiw-Rebbi model, a ($1+1$)-dimensional quantum field theory where interactions between Dirac fermions are mediated by quantum rotors. Starting from a mixture of ultracold atoms in an optical lattice, we show how this quantum field theory emerges in the long-wavelength limit. For a wide and experimentally relevant parameter regime, the Dirac fermions acquire a dynamical mass via the spontaneous breakdown of chiral symmetry. We study the effect of both quantum and thermal fluctuations, and show how they lead to the phenomenon of chiral symmetry restoration. Moreover, we uncover a confinement-deconfinement quantum phase transition, where mesonlike fermions fractionalize into quarklike quasiparticles bound to topological solitons of the rotor field. The proliferation of these solitons at finite chemical potentials again serves to restore the chiral symmetry, yielding a clear analogy with the quark-gluon plasma in quantum chromodynamics, where the restored symmetry coexists with the deconfined fractional charges. Our results indicate how the interplay between these phenomena could be analyzed in more detail in realistic atomic experiments.

Optically addressable solid-state spin defects are promising candidates for storing and manipulating quantum information using their long coherence ground-state manifold; individual defects can be entangled using photon-photon interactions, offering a path toward large-scale quantum photonic networks. Quantum computing protocols place strict limits on the acceptable photon losses in the system. These low-loss requirements cannot be achieved without photonic engineering, but are attainable if combined with state-of-the-art nanophotonic technologies. However, most materials that host spin defects are challenging to process: as a result, the performance of quantum photonic devices is orders of magnitude behind that of their classical counterparts. Silicon carbide ($\mathrm{Si}\mathrm{C}$) is well suited to bridge the classical-quantum photonics gap, since it hosts promising optically addressable spin defects and can be processed into $\mathrm{Si}\mathrm{C}$-on-insulator for scalable, integrated photonics. In this paper, we discuss recent progress toward the development of scalable quantum photonic technologies based on solid-state spins in silicon carbide, and discuss current challenges and future directions.

A core concept in condensed matter physics is geometric frustration that leads to emergent spin phases in magnetic materials. These distinct phases, which depart from the conventional ferromagnet or the antiferromagnet, require unique computational techniques to decipher. In this study, we use the canonical Ising Shastry-Sutherland lattice to demonstrate new techniques for solving frustrated Hamiltonians using a quantum annealer of programmable superconducting qubits. This Hamiltonian can be tuned to produce a variety of intriguing ground states ranging from short- and long-range orders and fractional order parameters. We show that a large-scale finite-field quantum annealing experiment is possible on 468 logical spins of this model embedded into the quantum hardware. We determine microscopic spin configurations using an iterative quantum annealing protocol and develop mean-field boundary conditions to attenuate finite-size effects and defects. We not only recover all phases of the Shastry-Sutherland Ising model—including the well-known fractional magnetization plateau in a longitudinal field—but also predict the spin behavior at the critical points with significant ground-state degeneracy and in the presence of defects. The results lead us to establish the connection to the diffuse neutron scattering experiments by calculation of the static structure factors.

Quantum variational algorithms are one of the most promising applications of near-term quantum computers; however, recent studies have demonstrated that unless the variational quantum circuits are configured in a problem-specific manner, optimization of such circuits will most likely fail. In this paper, we focus on a special family of quantum circuits called the Hamiltonian variational ansatz (HVA), which its takes inspiration from the quantum approximate optimization algorithm and adiabatic quantum computation. Through the study of its entanglement spectrum and energy-gradient statistics, we find that the HVA exhibits favorable structural properties such as mild or entirely absent barren plateaus and a restricted state space that eases their optimization in comparison to the well-studied “hardware-efficient ansatz.” We also numerically observe that the optimization landscape of the HVA becomes almost trap free, i.e., there are no suboptimal minima, when the ansatz is overparametrized. We observe a size-dependent “computational phase transition” as the number of layers in the HVA circuit is increased where the optimization crosses over from a hard to an easy region in terms of the quality of the approximations and the speed of convergence to a good solution. In contrast to the analogous transitions observed in the learning of random unitaries, which occur at a number of layers that grows exponentially with the number of qubits, our variational-quantum-eigensolver experiments suggest that the threshold to achieve the overparametrization phenomenon scales at most polynomially in the number of qubits for the transverse-field Ising and XXZ models. Lastly, as a demonstration of its entangling power and effectiveness, we show that the HVA can find accurate approximations to the ground states of a modified Haldane-Shastry Hamiltonian on a ring, which has long-range interactions and has a power-law entanglement scaling.

We present an improvement to the cross resonance gate realized with the addition of resonant, target rotary pulses. These pulses, applied directly to the target qubit, are simultaneous to and in phase with the echoed cross resonance pulses. Using specialized Hamiltonian error amplifying tomography, we confirm a reduction of error terms with target rotary—directly translating to improved two-qubit gate fidelity. Beyond improvement in the control-target subspace, the target rotary reduces entanglement between target and target spectators caused by residual quantum interactions. We further characterize multiqubit performance improvement enabled by target rotary pulsing using unitarity benchmarking and quantum volume measurements, achieving a new record quantum volume for a superconducting qubit system.

Quantum teleportation is essential for many quantum information technologies, including long-distance quantum networks. Using fiber-coupled devices, including state-of-the-art low-noise superconducting nanowire single-photon detectors and off-the-shelf optics, we achieve conditional quantum teleportation of time-bin qubits at the telecommunication wavelength of 1536.5 nm. We measure teleportation fidelities of $\ge 90$% that are consistent with an analytical model of our system, which includes realistic imperfections. To demonstrate the compatibility of our setup with deployed quantum networks, we teleport qubits over 22 km of single-mode fiber while transmitting qubits over an additional 22 km of fiber. Our systems, which are compatible with emerging solid-state quantum devices, provide a realistic foundation for a high-fidelity quantum Internet with practical devices.

Trapped-ion quantum computers have demonstrated high-performance gate operations in registers of about ten qubits. However, scaling up and parallelizing quantum computations with long one-dimensional (1D) ion strings is an outstanding challenge due to the global nature of the motional modes of the ions, which mediate qubit-qubit couplings. Here, we devise methods to implement scalable and parallel entangling gates by using engineered localized phonon modes. We propose to tailor such localized modes by tuning the local potential of individual ions with programmable optical tweezers. Localized modes of small subsets of qubits form the basis to perform entangling gates on these subsets in parallel. We demonstrate the inherent scalability of this approach by presenting analytical and numerical results for long 1D ion chains and even for infinite chains of uniformly spaced ions. Furthermore, we show that combining our methods with optimal coherent control techniques allows realization of maximally dense universal parallelized quantum circuits.

Microwave photonics lends the advantages of fiber optics to electronic sensing and communication systems. In contrast to nonlinear optics, electro-optic devices so far require classical modulation fields whose variance is dominated by electronic or thermal noise rather than quantum fluctuations. Here we demonstrate bidirectional single-sideband conversion of $X$ band microwave to $C$ band telecom light with a microwave mode occupancy as low as $0.025\pm 0.005$ and an added output noise of less than or equal to $0.074$ photons. This is facilitated by radiative cooling and a triply resonant ultra-low-loss transducer operating at millikelvin temperatures. The high bandwidth of $10.7\mathrm{MHz}$ and total (internal) photon conversion efficiency of 0.03% (0.67%) combined with the extremely slow heating rate of 1.1 added output noise photons per second for the highest available pump power of 1.48 mW puts near-unity efficiency pulsed quantum transduction within reach. Together with the non-Gaussian resources of superconducting qubits this might provide the practical foundation to extend the range and scope of current quantum networks in analogy to electrical repeaters in classical fiber optic communication.

We demonstrate, for the first time, that a quantum flux parametron (QFP) is capable of acting as both isolator and amplifier in the readout circuit of a capacitively shunted flux qubit (CSFQ). By treating the QFP like a tunable coupler and biasing it such that the coupling is off, we show that $T_1$ of the CSFQ is not impacted by Purcell loss from its low-$Q$ readout resonator ($Q_e=760$) despite being detuned by only 40 MHz. When annealed, the QFP amplifies the qubit’s persistent current signal such that it generates a flux qubit-state-dependent frequency shift of 85 MHz in the readout resonator, which is over 9 times its linewidth. The device is shown to read out a flux qubit in the persistent current basis with fidelities surpassing $98.6\mathrm{\%}$ with only 80 ns integration, and reaches fidelities of $99.6\mathrm{\%}$ when integrated for 1 $\mu \mathrm{s}$. This combination of speed and isolation is critical to the readout of high-coherence quantum annealers.

We demonstrate, for the first time, that a quantum flux parametron (QFP) is capable of acting as both isolator and amplifier in the readout circuit of a capacitively shunted flux qubit (CSFQ). By treating the QFP like a tunable coupler and biasing it such that the coupling is off, we show that $T_1$ of the CSFQ is not impacted by Purcell loss from its low-$Q$ readout resonator ($Q_e=760$) despite being detuned by only 40 MHz. When annealed, the QFP amplifies the qubit’s persistent current signal such that it generates a flux qubit-state-dependent frequency shift of 85 MHz in the readout resonator, which is over 9 times its linewidth. The device is shown to read out a flux qubit in the persistent current basis with fidelities surpassing $98.6\mathrm{\%}$ with only 80 ns integration, and reaches fidelities of $99.6\mathrm{\%}$ when integrated for 1 $\mu \mathrm{s}$. This combination of speed and isolation is critical to the readout of high-coherence quantum annealers.

The second quantum revolution has been built on a foundation of fundamental research at the intersection of physics and information science, giving rise to the discipline we now call quantum information science (QIS). The quest for new knowledge and understanding drove the development of new experimental tools and rigorous theory, which defined the road map for second-wave quantum technologies, including quantum computers, quantum-enhanced sensors, and communication systems. As technology has matured, the race to develop and commercialize near-term applications has accelerated. In the current regime of Noisy Intermediate Scale Quantum (NISQ) devices [J. Preskill, Quantum 2, 79 (2018)], the continued necessity of basic research is manifest. Under what conditions can we truly harness quantum complexity and what are its implications for potential useful applications? These questions remain largely unanswered and as the QIS industry ramps up, a continuous feedback between basic science and technology is essential. In this Perspective, I review how curiosity-driven research has led to radical new technologies and why the quest for basic understanding is essential for further progress.