Computational reversibility is necessary for quantum computation and inspires the development of computing systems, in which information carriers are conserved as they flow through a circuit. While conservative logic provides an exciting vision for reversible computing with no energy dissipation, the large dimensions of information carriers in previous realizations detract from the system efficiency, and nanoscale conservative logic remains elusive. We therefore propose a nonvolatile reversible computing system in which the information carriers are magnetic skyrmions, topologically-stable magnetic whirls. These nanoscale quasiparticles interact with one another via the spin Hall and skyrmion Hall effects as they propagate through ferromagnetic nanowires structured to form cascaded conservative logic gates. These logic gates can be directly cascaded in large-scale systems that perform complex logic functions, with signal integrity provided by clocked synchronization structures. The feasibility of the proposed system is demonstrated through micromagnetic simulations of Boolean logic gates, a Fredkin gate, and a cascaded full adder. As skyrmions can be transported in a pipelined and nonvolatile manner at room temperature without the motion of any physical particles, this skyrmion logic system has the potential to deliver scalable high-speed low-power reversible Boolean and quantum computing.

We present measurements of the capacitive coupling energy and the interdot capacitances in a linear quadruple-quantum-dot array in undoped $\mathrm{Si}$/$\mathrm{Si}\mathrm{Ge}$. With the device tuned to a regime of strong ($>1\mathrm{GHz}$) intra-double-dot tunnel coupling, as is typical for double-dot qubits, we measure a capacitive coupling energy of $20.9\pm 0.3\mathrm{GHz}$. In this regime, we demonstrate a fitting procedure to extract all the parameters in the four-dimensional Hamiltonian for two capacitively coupled charge qubits from a two-dimensional slice through the quadruple-dot charge-stability diagram. We also investigate the tunability of the capacitive coupling energy, using interdot barrier gate voltages to tune the inter- and intra-double-dot capacitances, and change the capacitive coupling energy of the double dots over a range of 15–32 GHz. We provide a model for the capacitive coupling energy based on the electrostatics of a network of charge nodes joined by capacitors, which shows how the coupling energy should depend on inter-double-dot and intra-double-dot capacitances in the network, and find that the expected trends agree well with the measurements of coupling energy.

Hybrid quantum registers consisting of different types of qubit offer a range of advantages as well as challenges. The main challenge is that some types of qubit react only slowly to external control fields, thus considerably slowing down the information-processing operations. One promising approach that has been tested in a number of cases is to use indirect control, where external fields are applied only to qubits that interact strongly with resonant excitation pulses. Here, we use this approach to indirectly control the nuclear spins of a nitrogen-vacancy center, using microwave pulses to drive the electron spin, combined with free precession periods optimized for generating logical gate operations on the nuclear spins. The scheme provides universal control and we present two typical applications: polarizing the nuclear spin and measuring nuclear-spin free-induction-decay signals, both without applying radio-frequency pulses. This scheme is versatile, as it can be implemented over a wide range of magnetic field strengths and at any temperature.

Alignment error is an important element, which affects the secure key rate of quantum key distribution (QKD). With the large-scale application of weak coherent states in practical QKD systems, the model of misalignment error reflects, however, the behavior of a single photon. Here, based on the recent afterpulse model, we develop a realistic model to include the propagation of multiphoton pulses in the misalignment-error analysis. Moreover, double-click events are also included. The former narrow the gap between the model and real systems and the latter secure the model. We present that the double-click events can be depicted precisely in our model, which is underestimated in previous works. Also, the compatibility of multiphoton pulses in our model almost does not amplify the influence of afterpulse. Besides, the results show that other factors of real-life QKD systems such as the modulation accuracy and the system’s misalignment have a similar impact on the secure key rate.

Quantum key distribution (QKD) based on the laws of quantum physics allows the secure distribution of secret keys over an insecure channel. Unfortunately, imperfect implementations of QKD compromise its information-theoretical security. Measurement-device-independent quantum key distribution (MDI QKD) is a promising approach to remove all side channels from the measurement unit, which is regarded as the “Achilles’ heel” of QKD. An essential assumption in MDI QKD is, however, that the sources are trusted. Here we experimentally demonstrate that a practical source based on a semiconductor laser diode is vulnerable to a laser-seeding attack, in which light injected from the communication line into the laser results in an increase of the intensities of the prepared states. The unnoticed increase of intensity may compromise the security of QKD, as we show theoretically for the prepare-and-measure decoy-state BB84 and MDI QKD protocols. Our theoretical security analysis is general and can be applied to any vulnerability that increases the intensity of the emitted pulses. Moreover, a laser-seeding attack might be launched as well against decoy-state-based quantum cryptographic protocols beyond QKD.

Fermion-to-qubit mappings that preserve geometric locality are especially useful for simulating lattice fermion models (e.g., the Hubbard model) on a quantum computer. They avoid the overhead associated with geometric nonlocal parity terms in mappings such as the Jordan-Wigner transformation and the Bravyi-Kitaev transformation. As a result, they often provide quantum circuits with lower depth and gate complexity. In such encodings, fermionic states are encoded in the common $+1$ eigenspace of a set of stabilizers, akin to stabilizer quantum error-correcting codes. Here, we discuss several known geometric locality-preserving mappings and their abilities to correct and detect single-qubit errors. We introduce a geometric locality-preserving map, whose stabilizers correspond to products of Majorana operators on closed paths of the fermionic hopping graph. We show that our code, which we refer to as the Majorana loop stabilizer code (MLSC) can correct all single-qubit errors on a two-dimensional square lattice, while previous geometric locality-preserving codes can only detect single-qubit errors on the same lattice. Compared to existing codes, the MLSC maps the relevant fermionic operators to lower-weight qubit operators despite having higher code distance. Going beyond lattice models, we demonstrate that the MLSC is compatible with state-of-the-art algorithms for simulating quantum chemistry, and can offer those simulations the same error-correction properties without additional asymptotic overhead. These properties make the MLSC a promising candidate for error-mitigated quantum simulations of fermions on near-term devices.

Dispersive coupling based on the Rabi model with large detuning is widely used for qubit quantum nondemolition (QND) readout in quantum computation. However, the measurement speed and fidelity are usually significantly limited by the Purcell effects (i.e., Purcell decay, critical photon numbers) and qubit-dependent Kerr nonlinearity. To avoid these effects, we propose how to realize an ideal QND readout of a gradiometric flux qubit with a tunable gap via its nonperturbative dispersive coupling (NPDC) to a frequency-tunable measurement resonator. We show that this NPDC-based readout mechanism is free of dipole-field interactions and that the qubit QND measurement is not deteriorated by intracavity photons. Both qubit-readout speed and qubit-readout fidelity can avoid the Purcell limitations. Moreover, NPDC can be conveniently turned on and off via an external control flux. We show how to extend this proposal to a multiqubit architecture for a joint qubit readout.

Various protocols implementing a quantum computer are being pursued, one of which is the adiabatic quantum computer. Natural interactions in electromagnetic environments are only two-body local interactions, but the construction or simulation of higher-order couplers is indispensable for a universal adiabatic quantum computer using conventional flux qubits (no nonstoquastic interactions). Here we show that in a specific flux-qubit coupler design without ancilla qubits, four-body stoquastic interactions are induced by virtual coupler excitations. For specific parameter regimes they are the leading effect and can be tuned up to the gigahertz range.

We propose a squeezed-light source meeting the stringent requirements of continuous-variable quantum sampling. Using the time-dependent effective second-order nonlinear interaction induced by a strong driving beam in the presence of the ${\chi }_3$ response in a microresonator, our approach is compatible with established nanophotonic platforms. With typical realistic parameters, squeezed states with a mean photon number of 10 or higher can be generated in a single consistent temporal mode at repetition rates of over 100 MHz. Over 15 dB of squeezing is expected in existing ultralow-loss platforms.

In the gate model of quantum computing, a program is typically decomposed into a sequence of 1- and 2-qubit gates that are realized as control pulses acting on the system. A key requirement for a scalable control system is that the qubits are addressable—that control pulses act only on the targeted qubits. The presence of control crosstalk makes this addressability requirement difficult to meet. To provide metrics that can drive the requirements for decreasing crosstalk, we present three measurements that directly quantify the dc and ac flux crosstalk present between tunable transmons, with sensitivities as fine as 0.001%. We develop the theory to connect ac-flux-crosstalk measures to the infidelity of a parametrically activated two-qubit gate. We use quantum process tomography in the presence of crosstalk to provide an empirical study of the effects of crosstalk on two-qubit-gate fidelity.

Quantum computation requires the precise control of the evolution of a quantum system, typically through application of discrete quantum-logic gates on a set of qubits. Here, we use the cross-resonance interaction to implement a gate between two superconducting transmon qubits with a direct static dispersive coupling. We demonstrate a practical calibration procedure for the optimization of the gate, combining continuous and repeated-gate Hamiltonian tomography with stepwise reduction of dominant two-qubit coherent errors through mapping to microwave control parameters. We show experimentally that this procedure can enable a ${\widehat{ZX}}_{-\pi /2}$ gate with a fidelity $F=97.0(7)\mathrm{\%}$, measured with interleaved randomized benchmarking. We show this in an architecture with out-of-plane control and readout that is readily extensible to larger-scale quantum circuits.

We consider an approach to fault tolerant quantum computing based on a simple error-detecting code operating as the substrate for a conventional surface code. We anticipate that the approach will be efficient when the noise in the hardware is skewed towards one channel (e.g., phase), or alternatively when the hardware is modular in such a fashion that the local error-detect operations have higher fidelity. We employ a customized decoder where information about the likely location of errors, obtained from the error-detection code, is translated into an advanced variant of the minimum-weight perfect-matching algorithm. A threshold gate-level error rate of 1.42% is found for the concatenated code given highly asymmetric noise. This is superior to the standard surface code and remains so as we introduce a significant component of depolarizing noise; specifically, until the latter is 70% the strength of the former. Moreover, given the asymmetric noise case, the threshold rises to 6.24% if we additionally assume that local operations have 20 times higher fidelity than long-range gates. Thus, for systems that are both modular and prone to asymmetric noise our code structure can be very advantageous.

Nitrogen-vacancy centers (NVCs) in diamond show promise for quantum computing, communication, and sensing. However, the best current method for entangling two NVCs requires that each one is in a separate cryostat, which is not scalable. We show that single NVCs can be laser written 6–15-µm deep inside of a diamond with spin coherence times that are an order of magnitude longer than previous laser-written NVCs and at least as long as naturally occurring NVCs. This depth is suitable for integration with solid immersion lenses or optical cavities and we present depth-dependent ${\mathrm{T}}_2$ measurements. 200 000 of these NVCs would fit into one diamond.

We demonstrate the tunable quantum beat of single photons through the co-development of core nonlinear nanophotonic technologies for frequency-domain manipulation of quantum states in a common physical platform. Spontaneous four-wave mixing in a nonlinear resonator is used to produce nondegenerate, quantum-correlated photon pairs. One photon from each pair is then frequency shifted, without degradation of photon statistics, using four-wave-mixing Bragg scattering in a second nonlinear resonator. Fine tuning of the applied frequency shift enables tunable quantum interference of the two photons as they are impinged on a beam splitter, with an oscillating signature that depends on their frequency difference. Our work showcases the potential of nonlinear nanophotonic devices as a valuable resource for photonic quantum-information science.

Off-axis vortex beams are generated by superposing a Gaussian beam onto a symmetric optical vortex beam of unit topological charge in a single-path interferometer with a control of their relative intensities and phases. The radial displacement of the point vortex from the center of the beam is controlled by varying the relative intensity of the superposed beams, while the azimuthal displacement of the vortex is controlled by the phase difference between the superposed beams. This phase difference is employed through the Pancharatnam-Berry geometric phase by different cyclic evolutions of the polarization states of the superposed beams on the Poincaré sphere. Interferometric field reconstruction of the resultant beams from experiment, simulation, and numerical calculations are used to obtain the transverse linear momentum density. The net transverse linear momentum vector and the resulting extrinsic orbital angular momentum in an off-axis vortex beam is demonstrated to be related to the radial and azimuthal position of the vortex across the beam. Controlling the Pancharatnam-Berry geometric phase and intensity ratio of the component beams is thus proposed as an effective and robust technique to tune the extrinsic orbital angular momentum of off-axis vortex beams. The presented results can be useful in applications ranging from optical manipulation of trapped microparticles, controlling micromachines using light with orbital angular momentum to enabling more flexibility in superresolution microscopy and controlled asymmetric interaction of light with atom, molecule, and Bose-Einstein condensate.

Nonlinear processes in the quantum regime are essential for many applications, such as quantum-limited amplification, measurement, and control of quantum systems. In particular, the field of quantum error correction relies heavily on high-order nonlinear interactions between various modes of a quantum system. However, the required order of nonlinearity is often not directly available or weak compared to dissipation present in the system. Here, we experimentally demonstrate a route to obtain higher-order nonlinearity by combining more easily available lower-order nonlinear processes, using a generalization of the Raman transition. In particular, we show a transformation of four photons of a high-$Q$ superconducting resonator into two excitations of a superconducting transmon mode and two pump photons, and vice versa. The resulting eight-wave mixing process is obtained by cascading two fourth-order nonlinear processes through a virtual state. We expect this type of process to become a key component of hardware-efficient quantum error correction using continuous-variable error-correction codes.

Graph states and hypergraph states are of wide interest in quantum information processing and foundational studies. Efficient verification of these states is a key to various applications. Here we propose a simple method for verifying hypergraph states, which requires only two distinct Pauli measurements for each party, yet its efficiency is comparable to the best strategy based on entangling measurements. For a given state, the overhead is bounded by the chromatic number and degree of the underlying hypergraph. Our protocol is dramatically more efficient than all previous protocols based on local measurements, including tomography and direct-fidelity estimation. It enables the verification of hypergraph states and genuine multipartite entanglement of thousands of qubits. The protocol can also be generalized to the adversarial scenario, while achieving almost the same efficiency. This merit is particularly appealing to demonstrating blind measurement-based quantum computation and quantum supremacy.

The successful testing of non-Abelian statistics not only serves as a milestone in fundamental physics but also provides a quantum-gate operation in topological quantum computation. An accurate and efficient readout scheme of a topological qubit is an essential step toward the experimental confirmation of non-Abelian statistics. In the current work, we propose a protocol to read out the quantum state of a Majorana vortex qubit on a topological superconductor island. The protocol consists of four Majorana zero modes trapped in spatially well-separated vortex cores on the two-dimensional surface of a Coulomb blockaded topological superconductor. Our proposed measurement is implemented by a pair of weakly coupled Majorana modes separately in touch with two normal-metal leads and the readout is realized by observing the conductance-peak location in terms of the gate voltage. Using this protocol, we can further test the non-Abelian statistics of Majorana zero modes in the two-dimensional platform. A successful readout of a Majorana qubit is a crucial step toward the future application of topological quantum computation. In addition, this Coulomb-blockaded setup can distinguish Majorana zero modes from Caroli–de Gennes–Matricon modes in vortex cores.

We present a simple protocol for which Alice and Bob must only decide to either send out a coherent state or not send out a coherent state to Charlie. No switching of bases occurs. We demonstrate that this protocol is both source-state side-channel free and measurement-device independent. We do not have to exactly control the whole space of the state of the light pulse, which is an impossible task in practice. The protocol is immune to all adverse effects caused by quantum-state imperfections in the side-channel space, such as the photon frequency spectrum, emission time, propagation direction, spatial angular momentum, and so on. Numerical simulation shows that our scheme can reach a side-channel-free result for quantum key distribution over distances exceeding 200 km, given the single-photon interference-misalignment error rate of 20%.

Integrated quantum photonics is a promising strand of research that involves guiding light in submicron-scale structures and utilizing the quantum properties of the fundamental particle of light—the photon. One of these quantum properties, the indistinguishability of the photons, is used to achieve high-visibility quantum interference, which is one of the key resources in photonic quantum information processing. Here, we demonstrate that the faster and simpler heralded and unheralded second-order correlation functions ($g^{(2)}$) estimate the visibility (indistinguishability) well without the need to perform laborious and time-consuming quantum-interference measurements.