We study information-theoretical security for space links between a satellite and a ground station. Quantum key distribution (QKD) is a well-established method for information-theoretical secure communication, giving the eavesdropper unlimited access to the channel and technological resources limited by only the laws of quantum physics. But QKD for space links is extremely challenging, the achieved key rates are extremely low, and daytime operating impossible. However, eavesdropping on a channel in free space without being noticed seems complicated, given the constraints imposed by orbital mechanics. If we also exclude the eavesdropper’s presence in a given area around the emitter and receiver, we can guarantee that he has access to only a fraction of the optical signal. In this setting, quantum keyless private (direct) communication based on the wiretap channel model is a valid alternative to provide information-theoretical security. Like for QKD, we assume that the legitimate users are limited by state-of-the-art technology, while the potential eavesdropper is only limited by physical laws: either by specifying her detection strategy (Helstrom detector) or by bounding her knowledge, assuming the most powerful strategy through the Holevo information. Nevertheless, we demonstrate information-theoretical secure communication rates (positive keyless private capacity) over a classical-quantum wiretap channel using on-off keying of coherent states. We present numerical results for a setting equivalent to the recent experiments with the Micius satellite and compare them to the fundamental limit for the secret key rate of QKD. We obtain much higher rates compared with QKD with an exclusion area of less than 13 m for low earth orbit satellites. Moreover, we show that the wiretap channel quantum keyless privacy is much less sensitive to noise and signal dynamics and daytime operation is possible.

Nitrogen-vacancy (N-$V$) centers in diamond have developed into a powerful solid-state platform for compact quantum sensors. However, high-sensitivity measurements usually come with additional constraints on the pumping intensity of the laser and the pulse control applied. Here, we demonstrate high-sensitivity N-$V$-ensemble-based magnetic field measurements with low-intensity optical excitation. Direct current magnetometry methods such as continuous-wave optically detected magnetic resonance and continuously excited Ramsey measurements combined with lock-in detection are compared to achieve an optimization. Gradiometry is also investigated as a step towards unshielded measurements of unknown gradients. The magnetometer demonstrates a minimum detectable field of 0.3–0.7 pT in a 73-s measurement when a flux guide with a sensing dimension of 2 mm is applied, corresponding to a magnetic field sensitivity of 2.6–6 $\mathrm{pT}/\sqrt{\mathrm{Hz}}$. Combined with our previous efforts on diamond ac magnetometry, the diamond magnetometer is promising for performing wide-bandwidth magnetometry with picotesla sensitivity and a cubic-millimeter sensing volume under ambient conditions.

Superconducting qubits on a circuit exhibit an always-on state-dependent phase error. This error is due to sub-MHz parasitic interaction that repels computational levels from noncomputational ones. We study a general theory to evaluate the “static” repulsion between seemingly idle qubits as well as the “dynamical” repulsion between entangled qubits under microwave driving gate. By combining qubits of either the same or opposite anharmonicity signs we find the characteristics of static and dynamical $ZZ$ freedoms. The latter universally eliminate the parasitic repulsion, leading us to mitigate high-fidelity gate operation. Our theory introduces the opportunities for making perfect entangled and unentangled states, which is extremely useful for quantum technology.

We propose a floating tunable coupler that does not rely on direct qubit-qubit coupling capacitances to achieve the zero-coupling condition. We show that the polarity of the qubit-coupler couplings can be engineered to offset the otherwise constant qubit-qubit coupling and attain the zero-coupling condition when the coupler frequency is above or below the qubit frequencies. We experimentally demonstrate these two operating regimes of the tunable coupler by implementing symmetric and asymmetric configurations of the superconducting pads of the coupler with respect to the qubits. Such a floating tunable coupler provides flexibility in designing large-scale quantum processors while reducing the always-on residual couplings.

A high-flux entangled-photon source is a key resource for quantum optical study and application. Here, it is realized in a lithium niobate on isolator (LNOI) chip, with 2.79 × ${10}^{11}$ Hz/mW photon-pair rate and 1.53 × ${10}^9$ Hz/(nm mW) spectral brightness. These data are boosted by over 2 orders of magnitude compared with existing technologies. A 160-nm-broad bandwidth is engineered for eight-channel multiplexed energy-time entanglement. Harnessed by high-extinction-frequency correlation and Franson interferences up to 99.17% visibility, such energy-time-entanglement multiplexing further enhances the high-flux data rate and warrants broad application in quantum-information processing on a chip.

We describe an experimental effort designing and deploying error-robust single-qubit operations using a cloud-based quantum computer and analog-layer programming access. We design numerically optimized pulses that implement target operations and exhibit robustness to various error processes including dephasing noise, instabilities in control amplitudes, and crosstalk. Pulse optimization is performed using a flexible optimization package incorporating a device model and physically relevant constraints (e.g., bandwidth limits on the transmission lines of the dilution refrigerator housing IBM Quantum hardware). We present techniques for conversion and calibration of physical Hamiltonian definitions to pulse waveforms programmed via Qiskit Pulse and compare performance against hardware default derivative removal by adiabatic gate (DRAG) pulses on a five-qubit device. Experimental measurements reveal default DRAG pulses exhibit coherent errors an order of magnitude larger than tabulated randomized-benchmarking measurements; solutions designed to be robust against these errors outperform hardware-default pulses for all qubits across multiple metrics. Experimental measurements demonstrate performance enhancements up to: about a $10$ times single-qubit gate coherent-error reduction; about a $5$ times average coherent-error reduction across a five-qubit system; about a $10$ times increase in calibration window to one week of valid pulse calibration; about a $12$ times reduction gate-error variability across qubits and over time; and up to about a $9$ times reduction in single-qubit gate error (including crosstalk) in the presence of fully parallelized operations. Randomized benchmarking reveals error rates for Clifford gates constructed from optimized pulses consistent with tabulated $T_1$ limits, and demonstrates a narrowing of the distribution of outcomes over randomizations associated with suppression of coherent errors.

Hybrid circuit QED involves the study of coherent quantum physics in solid-state systems via their interactions with superconducting microwave circuits. Here we present a crucial step in the implementation of a hybrid superconducting qubit that employs a carbon nanotube as a Josephson junction. We realize the junction by contacting a carbon nanotube with a superconducting Pd/Al bilayer, and implement voltage tunability of the quantum circuit’s frequency using a local electrostatic gate. We demonstrate a strong dispersive coupling to a coplanar waveguide resonator by investigating the gate-tunable resonator frequency. We extract qubit parameters from spectroscopy using dispersive readout and find qubit relaxation and coherence times in the range of $10$–$200\mathrm{ns}$.

We develop an ultracompact high-resolution imaging system for cold atoms. Its only in-vacuum element is a multimode optical fiber with a diameter of 230 $\mu \mathrm{m}$, which simultaneously collects light and guides it out of the vacuum chamber. External adaptive optics allow us to image cold $\mathrm{Rb}$ atoms with approximately 1-$\mu \mathrm{m}$ resolution over a $100\times 100\mu {\mathrm{m}}^2$ field of view. These optics can be easily rearranged to switch between fast absorption imaging and high-sensitivity fluorescence imaging. This system is particularly suited for hybrid quantum-engineering platforms where cold atoms are combined with optical cavities, superconducting circuits, or optomechanical devices restricting optical access.

Silicon photonics offers a stable, miniaturized platform for quantum key distribution (QKD). As such, the chip-based system is vital for a wider adoption of QKD in the near future. However, the practical security issues presented in this alternative platform have not been fully studied. Here, we theoretically and experimentally evaluate the security of a chip-based QKD system under a powerful source attack, namely, Trojan-horse attack. We analyze the finite-key security of two prominent QKD protocols—BB84 and measurement-device-independent QKD—with the information leakage from both the polarization modulator and the intensity modulator integrated on a silicon photonic chip. Furthermore, we prove that our chip-based measurement-device-independent QKD system is feasibly resistant to the Trojan-horse attack by adding 232-dB isolation to the chip. In this sense, our work provides a guideline for the secure implementation of a leaky transmitter chip in the QKD system.

Quantum cryptography provides the inherent security for transmitting confidential information across free space or a fiber link. However, a high secure-key rate is still a challenge for a quantum-cryptography system. High-dimensional quantum cryptography, which can tolerate much higher channel noise, is a prospective way to share a higher secure-key rate between legitimate users, and has received substantial attention over the last decade. In particular, orbital angular momentum (OAM) can provide an abundant resource for high-dimensional quantum cryptography. Furthermore, combining spin angular momentum (SAM) with OAM can increase the encoding alphabet. Here we verify a prepare-and-measure quantum-cryptography scheme based on four-dimensional SAM-OAM hybrid states over kilometer-scale ring-core fibers. The measured quantum-bit error rates are $4.3\mathrm{\%}$ for 4 km of fiber and $16.3\mathrm{\%}$ for 25 km of fiber. The scheme simplifies the process of state preparation and measurement, with a compact and scalable setup.

Reading out the state of superconducting artificial atoms typically relies on dispersive coupling to a readout resonator. For a given system noise temperature, increasing the circulating photon number $\overline{n}$ in the resonator enables a shorter measurement time and is therefore expected to reduce readout errors caused by spontaneous atom transitions. However, increasing $\overline{n}$ is generally observed to also monotonously increase these transition rates. Here we present a fluxonium artificial atom in which, despite the fact that the measured transition rates show nonmonotonous fluctuations within a factor of 6, for photon numbers up to $\overline{n}\approx 200$, the signal-to-noise ratio continuously improves with increasing $\overline{n}$. Even without the use of a parametric amplifier, at $\overline{n}=74$, we achieve fidelities of 99% and 93% for feedback-assisted ground and excited state preparations, respectively. At higher $\overline{n}$, leakage outside the qubit computational space can no longer be neglected and it limits the fidelity of quantum state preparation.

Fast discrimination between quantum states of superconducting artificial atoms is an important ingredient for quantum information processing. In circuit quantum electrodynamics, increasing the signal-field amplitude in the readout resonator, dispersively coupled to the artificial atom, improves the signal-to-noise ratio and increases the measurement strength. Here, we employ this effect over 2 orders of magnitude in readout power, made possible by the unique combination of a dimer-Josephson-junction-array amplifier with a large dynamic range and the fact that the readout of our granular aluminum fluxonium artificial atom remains quantum nondemolition (QND) at relatively large photon numbers in the readout resonator, up to $\overline{n}=110$. Using Bayesian inference, this allows us to detect quantum jumps faster than the readout-resonator response time $2/\kappa$, where $\kappa$ is the bandwidth of the readout resonator.

Ensembles of erbium dopants can realize quantum memories and frequency converters that operate in the minimal-loss wavelength band of fiber optical communication. Their operation requires the initialization, coherent control, and readout of the electronic spin state. In this work, we use a split-ring microwave resonator to demonstrate such control in both the ground and optically excited state. The presented techniques can also be applied to other combinations of dopant and host and may facilitate the further development of quantum memory protocols and sensing schemes.

Noise in quantum operations often negates the advantage of quantum computation. However, most classical simulations of quantum computers calculate the ideal probability amplitudes by either storing full state vectors or using sophisticated tensor-network contractions. Here we investigate sampling-based classical simulation methods for noisy quantum circuits. Specifically, we characterize the simulation costs of two major schemes, stabilizer-state sampling of magic states and Heisenberg propagation, for quantum circuits subject to stochastic Pauli noise, such as depolarizing and dephasing noise. To this end, we introduce several techniques for the stabilizer-state sampling to reduce the simulation costs under such noise. It is revealed that in the low-noise regime, stabilizer-state sampling results in a smaller sampling cost, while Heisenberg propagation is better in the high-noise regime. Furthermore, for a high depolarizing noise rate of approximately 10%, these methods provide better scaling than that given by the low-rank stabilizer decomposition. We believe that this knowledge of classical simulation costs is useful to achieve possible quantum advantage on near-term noisy quantum devices as well as efficient classical simulation methods.

Silicon carbide is a promising host material for spin-defect-based quantum sensors owing to its commercial availability and established techniques for electrical and optical microfabricated device integration. The negatively charged silicon vacancy is one of the leading spin defects studied in silicon carbide owing to its near-telecom photoemission, high spin number, and nearly temperature-independent ground-state zero-field splitting. We report the realization of nanotesla shot-noise-limited ensemble magnetometry based on optically detected magnetic resonance with the silicon vacancy in $4H$ silicon carbide. By coarsely optimizing the anneal parameters and minimizing power broadening, we achieve a sensitivity of $50\mathrm{nT}/\sqrt{\mathrm{Hz}}$ and a theoretical shot-noise-limited sensitivity of $3.5\mathrm{nT}/\sqrt{\mathrm{Hz}}$. This is accomplished without utilizing complex photonic engineering, control protocols, or applying excitation powers greater than a watt. This work demonstrates that the silicon vacancy in silicon carbide provides a low-cost and simple approach to quantum sensing of magnetic fields.

Spontaneous parametric down-conversion (SPDC) in a laser pumped optical nonlinear medium can produce heralded single photons with a high purity but a very low yield. Improving the yield by increasing the pump power in SPDC inevitably reduces the purity due to excitation of multiphoton events. We propose a scheme to overcome this purity-yield trade-off by suppressing multiphoton events in a cavity-enhanced SPDC via the photon-blockade effect. By introducing a strong photon-photon interaction into the intracavity medium and increasing the pump power, we can improve the available single-photon yield to larger than $90\mathrm{\%}$, while keeping the purity high, towards on-demand generation of single photons through the SPDC process. Our quasi-on-demand SPDC sources may boost single-photon-based quantum-information technology.

Reference-frame-independent (RFI) measurement-device-independent (MDI) quantum key distribution (QKD) is immune to all detector side-channel attacks and is insensitive to slow drifts of reference frames; thus, it is a promising candidate for the realization of long-distance quantum secure communications. However, its practical performance is predominantly limited by the finite-data-size effect compared with standard MDI QKD. By combining the collective constraint and joint-study strategy with our parameter estimation processes for the RFI-MDI-QKD protocol, we reduce the influence of the finite-data-size effect and substantially improve the protocol’s performance under realistic experimental conditions. Furthermore, a proof-of-principle experimental demonstration is conducted, and a transmission distance of up to 200 km is achieved. Therefore, this study represents a further step toward long-distance quantum secure communications.

We propose a design to realize integrated broadband nonreciprocal microwave isolators and circulators using superconducting circuit elements without any magnetic materials. To obtain a broadband response, we develop a waveguide-based design by temporal modulations. The corresponding compact traveling-wave structure is implemented with integrated superconducting composite right-/left-handed transmission lines. The calculations show that the bandwidth of 580 MHz can be realized over a nonreciprocal isolation of 20 dB in reflections. Such on-chip isolators and circulators are useful for cryogenic integrated microwave connections and measurements, such as protecting qubits from the amplified reflected signal in multiplexed readout.