We propose Qubit4Sync, a synchronization method for quantum-key-distribution (QKD) setups, based on the same qubits exchanged during the protocol and without requiring additional hardware other than that necessary to prepare and measure the quantum states, in a similar fashion to the clock recovery used in classical communications. Our approach introduces a cross-correlation algorithm with low computational complexity for high channel losses. We test the robustness of our scheme in a real QKD implementation, and we believe it may find application in other quantum communication protocols.

To ascertain the feasibility of large-scale quantum computing, the performance and quantum resource of a practical quantum-computing situation needs to be analyzed. However, most of the analyses reported so far have focused on the statistical examination that simply calculates the performance and resource based on individual quantum-computing components. In this work, we propose an integrated analysis methodology that models a large-scale fault-tolerant quantum-computing system based on three components: algorithm, error correction, and device. Furthermore, to implement the proposed methodology, we develop a quantum-computing software platform composed of three functional layers: compile, system, and building block. By using our platform, we observe that it takes $8.78\times {10}^5\mathrm{h}$, which is much longer than previous estimations, for factoring a 512-bit integer with Shor’s factoring algorithm. We also discuss whether the proposed platform can play a significant role in finding an optimal concatenation level and/or code distance of quantum error-correcting code.

Quantum key distribution (QKD) networks can provide unconditional secure communications among many remote users. In real QKD networks, with the dynamic access and quit of nodes, qualities of various user links and the link lengths among different users, such as transmission loss, are different and changing. Customized optimization that aims at various channel environments offers a network performance benefit. In this paper, we propose including the detection efficiency, dark-count probability, and afterpulse rate of the single-photon avalanche photodiode (SPAD) into the parameter optimization by controlling its voltage and temperature to optimize the secure key rates in dynamic QKD networks. The result shows that our method improves the network adaptability to the variation and asymmetry of channel and profits the establishment of QKD networks in a complex channel environment. This method is suitable for arbitrary SPAD-based QKD system and network architecture.

Electrostatically-defined semiconductor quantum dot arrays offer a promising platform for quantum computation and quantum simulation. However, crosstalk of gate voltages to dot potentials and interdot tunnel couplings complicates the tuning of the device parameters. To date, crosstalk to the dot potentials is routinely and efficiently compensated using so-called virtual gates, which are specific linear combinations of physical gate voltages. However, due to exponential dependence of tunnel couplings on gate voltages, crosstalk to the tunnel barriers is currently compensated through a slow iterative process. In this work, we show that the crosstalk on tunnel barriers can be efficiently characterized and compensated for, using the fact that the same exponential dependence applies to all gates. We demonstrate efficient calibration of crosstalk in a quadruple quantum dot array and define a set of virtual barrier gates, with which we show orthogonal control of all interdot tunnel couplings. Our method marks a key step forward in the scalability of the tuning process of large-scale quantum dot arrays.

Silicon-vacancy qubits in silicon carbide ($\mathrm{Si}\mathrm{C}$) are emerging tools in quantum-technology applications due to their excellent optical and spin properties. In this paper, we explore the effect of temperature and strain on these properties by focusing on the two silicon-vacancy qubits, V1 and V2, in $4H$-$\mathrm{Si}\mathrm{C}$. We apply density-functional theory beyond the Born-Oppenheimer approximation to describe the temperature-dependent mixing of electronic excited states assisted by phonons. We obtain a polaronic gap of around 5 and 22 meV for the V1 and V2 centers, respectively, which results in a significant difference in the temperature-dependent dephasing and zero-field splitting of the excited states, which explains recent experimental findings. We also compute how crystal deformations affect the zero-phonon line of these emitters. Our predictions are important ingredients in any quantum applications of these qubits sensitive to these effects.

Discrimination of coherent states beyond the standard quantum limit (SQL) is an important task not only for quantum information processing but also for optical coherent communication. To optimize long-distance optical fiber networks, it is of practical importance to develop a quantum receiver beating the SQL and approaching the quantum bound at telecom wavelength. In this paper, we experimentally demonstrate a receiver beating the conventional SQL at telecom wavelength. Our receiver is composed of a displacement operation, a single-photon counter, and a real-time adaptive-feedback operation. By using a high-performance single-photon detector operating at the telecom wavelength, we achieve a discrimination error beyond the SQL. The demonstration in the telecom band is an enabler for quantum and classical communication beyond the SQL using a coherent state alphabet, and we envision that the technology can be used for long-distance quantum-key distribution, effective quantum-state preparation, and quantum estimation.

We construct optimal protocols for verifying qubit and qudit Greenberger-Horne-Zeilinger (GHZ) states using local projective measurements. When the local dimension is a prime, an optimal protocol is constructed from Pauli measurements only. Our protocols provide a highly efficient way to estimate the fidelity and to certify genuine multipartite entanglement. In particular, they enable the certification of genuine multipartite entanglement using only one test when the local dimension is sufficiently large. By virtue of adaptive local projective measurements, we then construct protocols for verifying GHZ-like states that are optimal over all protocols based on one-way communication. The efficiency can be improved further if additional communications are allowed. Finally, we construct optimal protocols for verifying GHZ states and nearly optimal protocols for GHZ-like states in the adversarial scenario.

The precise and automated calibration of quantum gates is a key requirement for building a reliable quantum computer. Unlike errors from decoherence, systematic errors can in principle be completely removed by tuning experimental parameters. Here, we present an iterative calibration routine, which can remove systematic gate errors on several qubits. A central ingredient is the construction of pulse sequences that extract independent indicators for every linearly independent error generator. We show that decoherence errors only moderately degrade the achievable infidelity due to systematic errors. Furthermore, we investigate the convergence properties of our approach by performing simulations for a specific qubit encoded in a pair of spins. Our results indicate that a gate set with 230 gate parameters can be calibrated in about ten iterations, after which incoherent errors limit the gate fidelity.

Counterdiabatic driving (CD) offers a fast and robust strategy to manipulate and prepare the quantum state in multibody systems. However, the exact CD term involving the spectral properties of the system is difficult to calculate and generally takes a complicated form for higher-dimensional complex systems. Recently, Sels and Polkovnikov [Proc. Natl. Acad. Sci. U. S. A. 114, E3909 (2017)] proposed a variational method to optimally approximate the CD term with no need for the full spectral information of the original Hamiltonian. Using a nuclear-magnetic-resonance setup, here we report an experimental demonstration of this method to construct available shortcuts to adiabaticity in a nonintegrable spin-$1/2$ chain via only local controls. In this process, the ground state of such a system is rapidly prepared and the final state fidelity is significantly increased compared with the traditional adiabatic method. Our experiments prove the feasibility of this variational method to design CD in practice, and thus it is a promising strategy for fast, high-fidelity quantum-state manipulation in complex and noisy quantum systems.

Irradiation-induced lattice defects in silicon carbide ($\mathrm{Si}\mathrm{C}$) have already exceeded their previous reputation as purely performance inhibiting. With their remarkable quantum properties, such as long room-temperature spin coherence and the possibility of downscaling to single-photon-source level, they have proven to be promising candidates for a multitude of quantum-information applications. One of the most crucial parameters of any quantum system is how long its quantum coherence can be preserved. By using the pulsed optically detected magnetic resonance (ODMR) technique, we investigate the spin-lattice relaxation time ($T_1$) and spin-coherence time ($T_2$) of silicon vacancies in $4H$-$\mathrm{Si}\mathrm{C}$ created by neutron, electron, and proton irradiation in a broad range of fluences. We also examine the effect of irradiation energy and sample annealing. We establish a robustness of the $T_1$ time against all types of irradiation and reveal a universal scaling of the $T_2$ time with the emitter density. Our results can be used to optimize the coherence properties of silicon-vacancy qubits in $\mathrm{Si}\mathrm{C}$ for specific tasks.

Rare-earth-doped crystals have numerous applications ranging from frequency metrology to quantum information processing. To fully benefit from their exceptional coherence properties, the effect of mechanical strain on the energy levels of the dopants—whether it is a resource or perturbation—needs to be considered. We demonstrate that by applying uniaxial stress to a rare-earth-doped crystal containing a spectral hole, we can shift the hole by a controlled amount that is larger than the width of the hole. We deduce the sensitivity of $\mathrm{Eu}$${}^{3+}$ ions in an $\mathrm{Y}$${}_2$$\mathrm{Si}\mathrm{O}$${}_5$ matrix as a function of the crystal site and the crystalline axis along which the stress is applied.

Entanglement is a fundamental resource for quantum information science. However, bipartite entanglement is destroyed when one particle is observed via projective (sharp) measurements, as is typically the case in most experiments. Here, we show experimentally that, if instead of sharp measurements, one performs many sequential unsharp measurements on one particle that are suitably chosen depending on the previous outcomes, then entanglement is preserved and it is possible to reveal quantum correlations through measurements on the second particle at any step of the sequence. Specifically, we observe that pairs of photons entangled in polarization maintain their entanglement when one particle undergoes three sequential measurements and that each of these can be used to violate a Clauser-Horne-Shimony-Holt inequality. This proof-of-principle experiment demonstrates the possibility of repeatedly harnessing two crucial resources, entanglement and Bell nonlocality, that, in most quantum protocols, are destroyed after a single measurement. The protocol we use, which in principle works for an unbounded sequence of measurements, can be useful for randomness extraction.

The measurement of a quantum system is often performed by encoding its state in a single observable of a light field. The measurement efficiency of this observable can be reduced by loss or excess noise on the way to the detector. Even a quantum-limited detector that simultaneously measures a second noncommuting observable would double the output noise, therefore limiting the efficiency to $50\mathrm{\%}$. At microwave frequencies, an ideal measurement efficiency can be achieved by noiselessly amplifying the information-carrying quadrature of the light field but this has remained an experimental challenge. Indeed, while state-of-the-art Josephson-junction-based parametric amplifiers can perform an ideal single-quadrature measurement, they require lossy ferrite circulators in the signal path, drastically decreasing the overall efficiency. In this paper, we present a nonreciprocal parametric amplifier that combines single-quadrature measurement and directionality without the use of strong external magnetic fields. We extract a measurement efficiency of ${62}_{-9}^{+17}\mathrm{\%}$ that exceeds the quantum limit and that is not limited by fundamental factors. The amplifier can be readily integrated with superconducting devices, creating a path for ideal measurements of quantum bits and mechanical oscillators.

The output randomness from a random number generator can be certified by observing the violation of quantum contextuality inequalities based on the Kochen-Specker theorem. Contextuality can be tested in a single quantum system, which significantly simplifies the experimental requirements to observe the violation comparing to the ones based on nonlocality tests. However, it is not yet resolved as to how to ensure compatibilities for sequential measurements that is required in contextuality tests. Here, we employ a modified Klyachko-Can-Binicioğlu-Shumovsky contextuality inequality, which can ease the strict compatibility requirement on measurements. On a trapped single ${}^{138}{\mathrm{Ba}}^{+}$ ion system, we experimentally demonstrate violation of the contextuality inequality and realize quantum random number expansion by closing detection loopholes. We perform $1.29\times {10}^8$ trials of experiments and extract a randomness of $5.28\times {10}^5$ bits with a speed of $270\mathrm{bits}{\mathrm{s}}^{-1}$. Our demonstration paves the way for practical high-speed spot-checking quantum random number expansion and other secure information processing applications.