Quantum key distribution (QKD) enables information-theoretically secure communication against eavesdropping. However, phase instability remains a challenge across many QKD applications, particularly in schemes such as twin-field QKD and measurement-device-independent QKD. The most dominant source of phase fluctuation arises from the frequency offset between independent lasers. To overcome this limitation, we introduce a classical photodiode-based active compensation scheme for laser frequency mismatches and incorporate it into a mode-pairing QKD protocol. We have achieved an error rate approaching the theoretical limit and surpassing the linear key-rate bound over a fiber distance of 296.8 km. This approach provides a practical solution for frequency matching between independent lasers and can be extended to other fields requiring precise phase stabilization.

The readout speed of qubits is a major limitation for error correction in quantum information science. We show simulations of a proposed device that gives readout of a fluxonium qubit using a ballistic fluxon with an estimated readout time of less than 1 nanosecond, without the need for an input microwave tone. This contrasts the prevalent readout based on circuit quantum electrodynamics, but is related to previous studies where a fluxon moving in a single long Josephson junction (LJJ) can exhibit a time delay depending on the state of a coupled qubit. Our readout circuit contains two LJJs and a qubit coupled at their interface. We find that the device can exhibit single-shot readout of a qubit—one qubit state leads to a single dynamical bounce at the interface and fluxon reflection, and the other qubit state leads to a couple of bounces at the interface and fluxon transmission. Dynamics are initially computed with a separate degree of freedom for all Josephson junctions of the circuit. However, a collective coordinate model reduces the dynamics to 3 degrees of freedom: one for the fluxonium Josephson junction and one for each LJJ. The large mass imbalance in this model allows us to simulate the mixed quantum-classical dynamics, as an approximation for the full quantum dynamics. Calculations give backaction on the qubit at $\le 0.1\mathrm{\%}$.

In this study, we employed Fourier-based quantum phase estimation (QPE) to calculate x-ray absorption spectroscopy (XAS) spectra. The primary focus of this study is the calculation of the XAS spectra of transition metal $L_{2,3}$ edges, which are dominated by strong correlation effects. First, the $\mathrm{Fe}$ $L_{2,3}$-edge x-ray absorption near-edge structure of ${\mathrm{Fe}\mathrm{PO}}_4$ is calculated using a noiseless simulator. The present computation involves a comparison of three types of input states: a uniform superposition state, optimal entangled input state, and Slater function state. Subsequently, we investigated the resolution error of the QPE and statistical error attributed to the measurements. It was revealed that postprocessing to introduce Lorentzian broadening reduces the statistical error, which becomes a significant problem for a large number of qubits. Finally, we implemented QPE on a trapped-ion quantum computer, encompassing three orbitals within the active space. To this end, we implemented QPE using dynamic circuits to reduce ancilla qubits and $\text{⟦}k+2,k,2\text{⟧}$ quantum error-detection code to mitigate the quantum noise inherent in current quantum computers. As a result, it was demonstrated that hardware noise was reduced, and spectra close to the noiseless ones were obtained.

Device-independent certification of quantum states enables the characterization of states within a device under minimal physical assumptions. A major problem in this regard is to certify quantum states using minimal resources. Aiming to address this problem, we consider a multipartite quantum steering scenario involving an arbitrary number of parties, of which only one is trusted, meaning that the measurements performed by this party are known. Consequently, the self-testing scheme is almost device-independent. Importantly, all the parties can only perform two measurements each, which is the minimal number of measurements required to observe any form of quantum nonlocality. Then, we propose steering inequalities that are maximally violated by three major classes of genuinely multipartite entangled (GME) states: graph states of arbitrary local dimension, Schmidt states of arbitrary local dimension, and $N$-qubit generalized $W$ states. Using the proposed inequalities, we then provide an almost device-independent certification of the above GME states. Restricting to qubits, we also lift our almost device-independent scheme to device-independent self-testing.

We consider a network of users connected by pairwise quantum key distribution (QKD) links. Using these pairwise secret keys and public classical communication, the users want to generate a common (conference) secret key at the maximal rate. We propose an algorithm based on spanning-tree packing (a known problem in graph theory) and prove its optimality. This algorithm enables optimal conference-key generation in modern quantum networks of arbitrary topology. Additionally, we discuss how it can guide the optimal placement of new bipartite links in the network design.

Tunable coupling architectures consisting of frequency-tunable qubits and couplers are widely employed in superconducting quantum processors. Magnetic flux crosstalk between these components poses a significant challenge to platform scalability, while quantum crosstalk induced by strong qubit-coupler interactions further complicates the compensation of flux crosstalk. To address these issues, we propose and experimentally validate a spin-echo-based method that effectively separates quantum and flux crosstalk, enabling accurate characterization of flux crosstalk. Furthermore, we integrate learning-based algorithm with a high-parallelism measurement scheme to improve efficiency. This approach achieves the stabilization of frequency-shift fluctuations at a noise baseline of approximately 20 kHz, with the accuracy of the crosstalk coefficient reaching an order of ${10}^{-5}$ after compensation. The method provides a robust and efficient framework for mitigating crosstalk, paving the way for high-fidelity control of large-scale quantum processors.

In this work, we propose a robust control protocol for achieving high-fidelity control of cat-state qubits. The cat-state qubits are generated in a Kerr nonlinear cavity by a single-photon drive. For high-fidelity control of these qubits, we apply a polychromatic pulse train (PPT), which consists of a sequence of $N$ pulses with different driving amplitudes. By optimizing the parameters of individual pulses in the PPT, errors in the single-photon drive can be suppressed. The ability to suppress such errors can be further enhanced by increasing the pulse-sequence length $N$, albeit at the cost of an extended bit-flip time. Numerical simulations demonstrate that the protocol exhibits strong robustness not only against single-photon drive-strength errors but also against errors in the tunable parameter itself. Furthermore, it maintains high fidelity even under the influence of single-photon losses. We expect that this protocol may provide an effective approach for the robust control of cat-state qubits.

Pauli spin blockade (PSB) is a key paradigm in semiconductor nanostructures that gives access to the spin physics. We report the direct observation of PSB using gate-dispersive reflectometry on double quantum dots in the presence of a finite source-drain bias voltage. The reservoir charge transitions are strongly modulated, turning on and off when entering and leaving the blockaded region, consistent with a simple model. Observed in hole double quantum dots in a $\mathrm{Ge}$/$\mathrm{Si}$ core/shell nanowire and a $\mathrm{Si}$ fin field-effect transistor, the effects are enhanced at larger bias voltage and suppressed by a magnetic field. This work establishes the fundamental dispersive signature of PSB under bias as an emerging tool for probing quantum dot devices.

Error mitigation has enabled quantum computing applications with more than a hundred qubits and deep circuits. Many error mitigation methods are noise-aware, relying on a faithful characterization of the noise channels of the hardware. However, fundamental limitations lead to unlearnable degrees of freedom of the underlying noise models when considering qubits. Here, we show how to leverage noncomputational states as an additional resource to learn state-preparation errors in superconducting qubits. This allows one to fully constrain the noise models. We can thus independently and accurately mitigate state-preparation errors, gate errors, and measurement errors. Our proposed method is also applicable to dynamic circuits with midcircuit measurements. This work opens the door to improved error mitigation for measurements, both at the end of the circuit and midcircuit.

Quantum computers may one day enable the efficient simulation of strongly coupled plasmas that lie beyond the reach of classical computation in regimes where quantum effects are important and the scale separation is large. In this article, we take a first step toward efficient simulation of quantum plasmas by demonstrating linear plasma wave propagation on a superconducting quantum chip. Using high-fidelity and highly expressive device-native gates, combined with an error-mitigation technique, we simulate the scattering of laser pulses from inhomogeneous plasmas. Our approach is made feasible by the identification of a suitable local spin model whose excitations mimic plasma waves, and whose circuit implementation requires a lower gate count than other proposed approaches that would require a future fault-tolerant quantum computer. This work opens avenues to study more complicated phenomena that cannot be simulated efficiently on classical computers, such as nonlinear quantum dynamics when strongly coupled plasmas are driven out of equilibrium.

We propose a universal approach based on Hamiltonian inverse engineering to realize a set of parameterized two-qubit gates. This method possesses unique advantages to simultaneous control of transitions among four energy levels, providing a simpler and effective way to construct composite two-qubit gates with fewer operations than traditional methods. Applied to silicon double quantum dots (DQDs), one can realize a one-step fSim gate and a $B$ gate with only one pulse switch. Of note, the method can be further integrated with various optimization theories to enhance gate performance. Based on quantum optimal control theory, we develop a high-fidelity fSim gate scheme with experimentally feasible pulse shapes, featuring an average gate time of 50 ns and a theoretical fidelity of 99.95% in the presence of decoherence and approximation error. By incorporating geometric quantum gate principles, we propose a combined geometric and dynamic fSim gate scheme. Numerical simulations demonstrate that this hybrid scheme exhibits stronger robustness against systematic errors compared to the purely dynamic approach. Our method is generalizable to arbitrary two-qubit physical systems, offering a feasible pathway for rapidly and robustly constructing composite two-qubit gates.

In hybrid microwave structures based on magnonics (so-called magnon cavities), the energy of the electromagnetic field can be transduced into the energy of magnetostatic spin excitations of a nanomagnet (Kittel magnon). This paper proposes a mechanism for transfer of coherent long-range spin excitation between nanomagnets, showing how Kittel magnons can be used for communication between magnon cavities. Analytical expressions for the magnon transport rate are obtained, and key parameters controlling off-resonant and resonant magnon tunneling regimes are indicated. Physical conditions are determined under which peak values of the rate characterizing the resonant transmission of the Kittel magnon arise either at a fixed tunneling energy with an arbitrary number of units in the ferromagnetic chain or at a fixed number of chain units with a varying tunneling energy. The features of resonant tunneling of the Kittel magnon formed in a magnon cavity at frequencies of about 10 GHz are analyzed. It is also shown that due to the strong photon-magnon coupling in the magnon cavity, at a certain magnetic field, the tunneling flow of magnons can reach an additional peak value, which reflects the process of resonant photon-to-magnon conversion.

A strongly coupled cavity-quantum-electrodynamics (CQED) system can serve as a powerful platform for quantum optics and quantum information processing; however, the excitation of high-order optical modes can lead to linewidth broadening and diminished mode-matching efficiency, constraining the system’s performance. A degenerate optical cavity, in which all supported modes resonate at the same frequency, provides a means to break through these limitations, facilitating unified interactions between high-order modes and atoms. In this work, we theoretically and experimentally investigate the coupling between high-order optical orbital angular momentum modes and atoms in a degenerate cavity. The cavity consists of a $4f$ system, which possesses a stable self-imaging property that effectively suppresses atom-induced cross-mode coupling. The mode volume of the degenerate cavity and the coupling strength are analytically calculated. The interaction strength between high-order modes and atoms is measured using mode-splitting spectroscopy. The results demonstrate that the collective coupling strength $g\sqrt{N}$ of the atomic ensemble remains independent of the transverse mode distribution. This study establishes a foundation for further exploration of multimode light-matter interactions in CQED systems.