Demonstrating that logical qubits outperform their physical counterparts represents a milestone in achieving reliable quantum computation. Here, we propose to protect logical qubits with a dynamical decoupling scheme that implements iswap gates on nearest-neighbor physical qubits, and we experimentally demonstrate this scheme on superconducting transmon qubits. In our scheme, each logical qubit requires only two physical qubits. A universal set of quantum gates on the logical qubits can be achieved such that each logical gate comprises only one or two physical gates. Our experiments reveal that the coherence time of a logical qubit is extended by up to 357% compared to the better-performing physical qubit. Moreover, we demonstrate—to the best of our knowledge, for the first time—that multiple logical qubits can outperform their physical superconducting counterparts. We illustrate a set of universal gates through a logical Ramsey experiment and the creation of a logical Bell state. Given its scalable nature, our scheme holds promise as a component for future reliable quantum computation.

We propose a method for generating optical cat states in propagating pulses based on cavity quantum electrodynamics (QED). This scheme uses multiple four-level systems (FLSs) inside an optical cavity as a light source. Time-modulating driving stimulates it to produce a superposition of coherent states entangled with the FLSs. The postselection of an appropriate state of the FLSs leads to a multicomponent cat state in a propagating pulse. Taking atomic decay and cavity loss into account, we optimize the cavity external coupling rate to maximize the fidelity. We find that its optimum value is formulated similarly to those of other generation methods for propagating states, suggesting a universal property of cavity-QED systems interacting with fields outside the cavity.

High-fidelity quantum nondemolition qubit measurement is critical to error correction and rapid qubit feedback in large-scale quantum computing. Maximizing high-fidelity dispersive readout in superconducting qubits commonly requires passing a short and strong pulse through the qubit’s readout resonator, which is then processed by a sufficiently high bandwidth, high saturation power, and quantum-limited amplifier. We have developed an amplifier design pipeline that combines time-domain simulation of the untruncated device Hamiltonian, fabrication constraints, and maximization of saturation power. We have realized an amplifier based on a modified trilayer (Nb-Al-${\mathrm{Al}\mathrm{O}}_x$-Nb) fabrication suite at NIST, which utilizes an array of 25 rf superconducting quantum interference devices embedded within a low-$Q$ resonator powered by a high-power voltage pump delivered via a diplexer on the signal port. We show that, despite the intensity of the pump, the device is quantum efficient and capable of high-fidelity measurement limited by state transitions in the transmon. We present experimental data demonstrating up to $-91.2\mathrm{dBm}$ input saturation power with 20-$dB$ gain, up to 28-MHz instantaneous bandwidth, and phase-preserving qubit measurements with $62\mathrm{\%}$ quantum efficiency.

In quantum information processing, a tension between two different tasks occurs: while qubits’ states can be preserved by one isolating them, quantum gates can be realized only through qubit-qubit interactions. In arrays of qubits, weak coupling leads to states being spatially localized and strong coupling leads to delocalized states. Here we study the average energy level spacing and the relative entropy of the distribution of the level spacings (Kullback-Leibler divergence from Poisson distribution and Gaussian orthogonal ensemble) to analyze the crossover between localized and delocalized (chaotic) regimes in linear arrays of superconducting qubits. We consider both transmons and capacitively shunted flux qubits, which enables us to tune the qubit anharmonicity. Arrays with uniform anharmonicity, comprising only transmons or flux qubits, display remarkably similar dependencies of level statistics on the coupling strength. In systems with alternating anharmonicity, for typical disorder in the qubit frequencies the localized regime is found to be more resilient to the increase in qubit-qubit coupling strength in comparison with arrays with a single qubit type. Our results, which we also confirm using generalized Bose-Hubbard models, support the design of devices that incorporate different qubit types to achieve higher performances.

Carrier transition is one of the major factors hindering the high-speed implementation of the Mølmer-Sørensen gates in trapped-ion quantum processors. We present an approach to design laser pulse shapes for the Mølmer-Sørensen gate in ion chains, which accounts for the effect of carrier transition on qubit-phonon dynamics. We show that the fast-oscillating carrier term effectively modifies the spin-dependent forces acting on ions, and this can be compensated by a simple nonlinear transformation of a laser pulse. Using numerical simulations for short ion chains and perturbation theory for longer chains up to 20 ions, we demonstrate that our approach allows suppression of the infidelity contribution from off-resonant carrier excitation from ${10}^{-3}$–${10}^{-2}$ to values below ${10}^{-4}$, whereas the gate duration remains of the order of tens of microseconds.

In the traditional approach to controlling superconducting qubits using microwave pulses, the field of pulse shaping has emerged to assist in the removal of leakage and increase gate fidelity. However, the challenge of scaling microwave control electronics has created an opportunity to explore alternative methods such as single-flux quantum (SFQ) pulses. For qubits controlled by SFQ pulses, high fidelity gates can be achieved by optimization of the binary control sequence. We extend the notion of the derivative removal by adiabatic gate framework to a transmon qubit controlled by SFQ drivers. The proposed implementation of SFQ pulse sequences can be stored in 22 bits or fewer, with gate fidelities exceeding 99.99%. This modest memory requirement could help reduce the footprint of the SFQ coprocessors and power dissipation while preserving their inherent advantages of scalability and cost-effectiveness.

The ability to execute a large number of quantum gates in parallel is a fundamental requirement for quantum error correction, allowing an error threshold to exist under the finite coherence time of physical qubits. Recently, two-dimensional ion crystals have been demonstrated as a plausible approach to scale up the qubit number in a trapped ion quantum computer. However, although the long-range Coulomb interaction between the ions enables their strong connectivity, it also complicates the design of parallel gates and leads to intrinsic crosstalk errors. Here we examine the effects of crosstalk errors on a rotated surface code. We show that, instead of the distance-3 code considered in previous works, a distance-5 code is necessary to correct the two-qubit crosstalk error. We numerically calculate the logical error rates and coherence times under various crosstalk errors, gate infidelities, and coherence times of the physical qubits, and we optimize the parallelism level according to the competition between different error sources. We show that a break-even point can be reached under realistic parameters. We further analyze the spatial dependence of the crosstalk, and discuss the scaling of the logical error rate versus the code distance for the long-term goal of a logical error rate below ${10}^{-10}$.

Reservoir computation is a recurrent framework for learning and predicting time-series data, that benefits from extremely simple training and interpretability, often as the dynamics of a physical system. In this paper, we will study the impact of noise on the learning capabilities of analog reservoir computers. Recent work on reservoir computation has shown that the information processing capacity (IPC) is a useful metric for quantifying the degradation of the performance due to noise. We further this analysis and demonstrate that this degradation of the IPC limits the possible features that can be meaningfully constructed in an analog reservoir computing setting. We borrow a result from quantum complexity theory that relates the circuit model of computation to a continuous-time model, and demonstrate an exponential reduction in the accessible volume of reservoir configurations. We conclude by relating this degradation in the IPC to the fat-shattering dimension of a family of functions describing the reservoir dynamics, which allows us to express our result in terms of a classification task. We conclude that any physical, analog reservoir computer that is exposed to noise can only be used to perform a polynomial amount of learning, despite the exponentially large latent space, even with an exponential amount of postprocessing.

We use spectral-hole burning to investigate spin dynamics within the electronic Zeeman sublevels of the ground state, ${}^4{I}_{15/2}$, of the erbium (${\mathrm{Er}}^{3+}$) ions in erbium-doped fibers (EDFs). Conducted at ultralow temperatures and under varying magnetic fields, our study reveals distinct changes in spin-relaxation dynamics across different conditions. We identified three decay components at approximately $7$ mK, with one achieving spin lifetimes of over 9 h under optimal conditions, while two components were observed at higher temperatures. The fairly stable relative weights of the decay components across conditions suggest distinct ion populations contributing to the observed relaxation dynamics. While earlier studies struggled to account for all decay components at higher temperatures, our approach successfully models spin dynamics across all observed decay components, using a consistent set of underlying mechanisms—spin flip-flop interactions, direct coupling to two-level systems, and Raman-type processes—and distinguishing the decay components by the strengths with which these mechanisms contribute. These results suggest EDFs’ potential as a promising candidate for quantum memory applications, with further room for optimization.

We investigate nonreciprocal scattering within the modes of a microwave frequency comb. Adjusting the pump frequencies, amplitudes, and phases of a Josephson parametric oscillator, we control constructive interference for the $m⟶\ell$ scattering processes, while concurrently achieving destructive interference for the inverse process $\ell ⟶m$. We outline the methodology for realizing nonreciprocity in the context of two-mode isolation and a three-mode circulation, which we extend to multiple modes. We find good agreement between the experiments and a linearized theoretical model. Nonreciprocal scattering expands the toolset for parametric control, with the potential to engineer alternative quantum correlations.

Paramount for performances of quantum network applications are the structure and quality of distributed entanglement. Here we propose a scalable and efficient approach to reveal the topological information of unknown quantum networks, and quantify entanglement simultaneously. The scheme exploits entropic uncertainty, an operationally meaningful measure of correlation, by performing only two local measurements on each qubit. Moreover, when measurement outcomes in each node are collectively evaluated, integrating uncertainty and mutual information enables a direct count of the number of bipartite sources between any two nodes. This surpasses what is possible via applying either approach solely. Moreover, quantum error-mitigation techniques, including probabilistic error cancellation (PEC) and virtual distillation (VD), which have been widely applied to suppress biases in single expectation value, are further incorporated to mitigate errors in entropic quantities. We find that PEC successfully removes deviations in correlation estimations. Meanwhile, VD extends the depolarizing noise strength that allows for valid bipartite entanglement certification from 8.8% to 26.4%, thus substantially enhancing robustness against bias-inducing noise in practical situations. The proposal is applicable to a broad variety of platforms and helps to spur future studies toward harnessing the advantages of quantum networks.

Information reconciliation plays a crucial role in continuous-variable quantum key distribution (CVQKD), directly affecting the secret key rate and the maximum secure transmission distance. As the demand for CVQKD in both long-distance and numerous-user access network scenarios increases, high-performance error-correction codes tailored for low-SNR environments become essential for efficient reconciliation. However, traditional approaches face challenges such as complex construction and high system overhead, which hinder further development. To address these issues, we propose an error-correction scheme for CVQKD based on low-density parity-check (LDPC)-Hadamard codes, which incorporates multidimensional reconciliation and greatly simplifies the construction of low-rate codes, enabling efficient error correction at a code rate on the order of 0.001 without requiring multiple interactions. Results show that the LDPC-Hadamard-based reconciliation protocol achieves a code rate as low as 0.001 with a reconciliation efficiency of 95.98% at a signal-to-noise ratio of $-28.40$ dB, pushing the error-correction code rate formally into the thousandths regime while maintaining code lengths in the megabit range. The proposed method provides a practical technological foundation for point-to-point long-distance QKD and large-scale downstream CVQKD networks that support numerous users.

Rapid and accurate initialization of qubits, known as reset, is a crucial building block for various tasks in quantum information processing, such as quantum error correction and estimation of the statistics of noisy quantum devices with many qubits. We demonstrate unconditional reset of a frequency-tunable transmon qubit that simultaneously resets multiple excited states using a metamaterial waveguide engineered to provide a cold bath over a wide spectral range, while providing strong protection against Purcell decay of the qubit. We report a reset error below 0.13% (0.16%) when prepared in the first (second) excited state of the transmon within 88 ns. In addition, through the sharp roll-off in the density of states of the metamaterial waveguide, we implement a leakage-reduction unit that selectively resets the transmon’s second excited state to 0.285(3)% residual population within 44 ns while acting trivially in the computational subspace as an identity operation that preserves encoded information with an infidelity of 0.72(1)%.

In this paper, we present a method for security analysis of Trojan-horse attacks (THAs) launched on a practical fiber-based quantum key distribution system across a wide spectral range. To achieve this, we use optical time-domain reflectometry (OTDR) for spectral reflectance analysis in the near-infrared range λ = 1100–1800 nm with centimeter-level resolution and a noise floor down to −80 dB. A total theoretical security analysis of the THA side channel considering the spectral reflectance and transmittance data over this spectral range is conducted. To the best of our knowledge, our OTDR setup and the corresponding results are the first of their kind to be obtained across a wide spectral range.

Quantum random-number generators (QRNGs), which exploit quantum mechanical effects to produce truly random numbers, are extensively used in information security and cryptography. Despite their inherent randomness, practical implementations of QRNGs, particularly those based on CMOS sensors, face security challenges due to device imperfections and external interference. This paper investigates the security vulnerabilities of QRNG systems against source-signal-injection attacks, specifically focusing on electrical signal fluctuations that modulate the intensity of the light source. This modulation induces subtle classical fluctuations in the photon-detection process, which are misinterpreted by legitimate users as quantum noise. This leads to an overestimation of secure entropy, compromising the security of the system. We demonstrate the effectiveness of the attack through experimental design and implementation, simulating practical-attack scenarios and assessing the impact on the entropy of generated random numbers. Our experiments reveal that while the altered random numbers can still pass National Institute of Standards and Technology (NIST) tests, the attacker can extract additional entropy without significantly affecting the statistical properties of the random numbers. To mitigate these risks, we introduce a defense mechanism that sets an entropy-threshold range and continuously monitors entropy variations, effectively preventing classical fluctuations from being misinterpreted as secure entropy. These findings hopefully provide critical insights for enhancing the reliability and security of QRNG systems in future large-scale quantum security applications.