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.

Valley splitting is a key feature of silicon-based spin qubits. Quantum dots in $\mathrm{Si}$/${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ heterostructures reportedly suffer from a relatively low valley splitting, limiting the operation temperature and the scalability of such qubit devices. Here, we demonstrate a robust and large valley splitting exceeding 200 $\mu \mathrm{eV}$ in a gate-defined single quantum dot, hosted in molecular-beam-epitaxy-grown ${}^{28}\mathrm{Si}/{\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$. The valley splitting is monotonically and reproducibly tunable up to 15% by gate voltages, originating from a 6-nm lateral displacement of the quantum dot. We observe static spin relaxation times $T_1>1$ s at low magnetic fields in our device containing an integrated nanomagnet. At higher magnetic fields, $T_1$ is limited by the valley hotspot and by phonon noise coupling to intrinsic and artificial spin-orbit coupling, including phonon bottlenecking.

High accuracy recognition of the orbital angular momentum (OAM) of light based on petal interference patterns is demonstrated using a convolutional neural network (CNN) approach with an improved Alexnet structure. A type of hybrid beam carrying OAM is utilized to provide more controllable degrees of freedom to recognize the OAM of light. The relationship between the training sample resolution (or the number associated with the accuracy) and the training time of the model, is presented. The recognition accuracy is closely related with the quantum number l of OAM, the angular ratio n of the spire phase over the hybrid phase in one modulation period, and the propagation distance z. Our studies show that when l ranges from 1 to 10, and n varies from 0.02 to 0.99, the recognition accuracy rate of OAM is nearly 100%. The minimum interval of n recognized at the OAM modes decreases to 0.01, which shows the super-high bandwidth of the generation and detection of OAM modes. Such results suggest great potential for the next generation of CNN-based OAM optical communication applications.

There is an increasing and demanding interest in the realization of secure communication schemes. Here, we conceive and realize an approach for a message-encoding scheme between two parties, Alice and Bob, by exploiting the infinite number of polarization states of unpolarized thermal light on the Poincaré sphere together with their correlation properties to camouflage and recover a message, thus realizing ghost polarization communication. At first, we investigate the second-order correlation coefficient $g^{(2)}(\tau )$ of classical unpolarized broadband amplified-spontaneous-emission light emitted by an erbium-doped fiber amplifier at 1550 nm by manipulating its instantaneous polarization state utilizing various polarization optics in the beam paths of a Hanbury-Brown and Twiss like ghost polarimetry setup using ultra-fast two-photon absorption in a photomultiplier tube. The observed polarization state modifications are in excellent agreement with our developed model based on the Stokes vector dynamics and a Glauber protocol for $g^{(2)}(\tau )$. On the basis of these results we then proceed towards the realization of a message-encoding scheme. By changing the instantaneous polarization state using a half-wave plate, Alice encodes and subsequently transmits a message. The camouflaged message can be recovered uniquely by Bob measuring the second-order correlations of the modified instantaneous polarization state with his reference beam. By using the agreed keypad of the communication scheme, he is able to retrieve the bit values of the encoded message. Finally, we address real-world implementation of this first proof-of-principle demonstration and discuss its security issues. This scheme demonstrates a method of establishing a secure communication link between two parties directly on the physical layer based on polarization correlations of classical light. It is expected that by the realization of this correlated photon modality not only avenues for ghost modalities, in general, are opened but also insight into polarization, even more than 175 years after Stokes will be achieved.

We propose simple protocols for performing quantum noise spectroscopy based on the method of transfer tensor maps (TTMs) [Cerrillo and Cao, Phys. Rev. Lett. 112, 110401 (2014)]. The TTM approach is a systematic way to deduce the memory kernel of a time-nonlocal quantum master equation via quantum process tomography. With access to the memory kernel, it is possible to (1) assess the non-Markovianity of a quantum process, (2) reconstruct the noise spectral density beyond pure-dephasing models, and (3) investigate collective decoherence in multiqubit devices. We illustrate the usefulness of TTM spectroscopy on the IBM Quantum Experience platform, and demonstrate that the qubits in the IBM device are subject to mild non-Markovian dissipation with spatial correlations.

Very-high-quality-factor superconducting radio-frequency cavities developed for accelerators can enable fundamental physics searches with orders of magnitude higher sensitivity, and they can also offer a path to a 1000-fold increase in the achievable coherence times for cavity-stored quantum states in three-dimensional circuit QED architecture. Here we report measurements of multiple accelerator cavities of resonant frequencies of $f_0=1.3$, 2.6, 5 GHz down to temperatures of about 10 mK and field levels down to a few photons, which reveal very long photon lifetimes up to 2 s, while also further exposing the role of the two-level systems (TLS) in niobium oxide. We also demonstrate how the TLS contribution can be greatly suppressed by vacuum heat treatments at 340–450 ${}^{\circ }\mathrm{C}$.

At high temperature, thermal decoherence dominates so that the entanglement of quantum states is difficult to preserve. Realizing high-temperature entanglement is, therefore, a challenge to the current quantum technologies. Here, we demonstrate that considerable degrees of continuous-variable entanglement between two macroscopic objects placed in an environment of high temperature can be created through the medium of properly prepared light fields coupled to them. There are two steps to make such entanglement. First, by pumping an optical cavity field pressuring on a mechanical resonator as a macroscopic object with a blue-detuned drive field, the competition between the induced squeezing effect due to the blue-detuned drive and the existing thermal decoherence leads to a stable entanglement between the cavity field and mechanical resonator. A condition for realizing field-resonator entanglement is determined at any temperature and for any given optomechanical system. The second step is to entangle two distant mechanical resonators through a procedure of entanglement swapping. A detailed example of illustrating this entanglement swapping shows that a considerable degree of entanglement between the two mechanical resonators can be created. The current study proposes a route toward high-temperature entanglement in a realistic physical system.

Entanglement offers substantial advantages in quantum information processing, but loss and noise hinder its application in practical scenarios. Although it has been well known for decades that the classical communication capacity of lossy and noisy bosonic channels can be significantly enhanced by entanglement, no practical encoding and decoding schemes are available to realize any entanglement-enabled advantage. Here, we report structured encoding and decoding schemes for such an entanglement-assisted communication scenario. Specifically, we show that phase encoding on an entangled two-mode squeezed vacuum state saturates the entanglement-assisted classical communication capacity of a very noisy channel and overcomes the fundamental limit on covert communication that exists without the assistance of entanglement. We then construct receivers for optimum hypothesis-testing protocols with discrete phase modulation and for optimum noisy phase-estimation protocols with continuous phase modulation. Our results pave the way for entanglement-assisted communication and sensing in the radio-frequency and microwave spectral ranges.

Although the main loss channel of planar microwave superconducting resonators has been identified to be related to an external coupling to a two-level system (TLS) bath, the behavior of such resonators in the presence of an off-resonant pump has yet to be fully understood. Alongside the well-known power-dependent damping, we observe a frequency shift with a conspicuous maximum for intermediate pump power that is attributed to a spectrally asymmetric saturation of the TLSs. We derive a semiclassical model that describes both of these effects quantitatively. The model is validated experimentally by performing a two-tone spectroscopy of several resonators fabricated on various substrates. Together with the provided analytic formulas, the technique proposed here is a simple yet powerful tool to unambiguously identify the presence of a limiting TLS bath, and to characterize various properties thereof, such as its average dephasing rate.

Many quantum key distribution systems employ a laser followed by an optical attenuator to prepare weak coherent states in the source. Their mean photon number must be precalibrated to guarantee the security of key distribution. Here we experimentally show that this calibration can be broken with a high-power laser attack. We test four fiber-optic attenuator types used in quantum key distribution systems, and find that two of them exhibit a permanent decrease in attenuation after laser damage. This results in higher mean photon numbers in the prepared states and may allow an eavesdropper to compromise the key.

Time-bin qubits, where information is encoded in a single photon at different times, have been widely used in optical-fiber- and waveguide-based quantum communications. With the recent developments in distributed quantum computation, it is logical to ask whether time-bin encoded qubits may be useful in that context. We have recently realized a time-bin qubit controlled-phase (C-phase) gate using a $2\times 2$ optical switch based on a lithium-niobate waveguide, with which we demonstrated the generation of an entangled state. However, the experiment was performed with only a pair of input states and thus the functionality of the C-phase gate was not fully verified. In this research, we use quantum process tomography to establish a process fidelity of $97.1\mathrm{\%}$. Furthermore, we demonstrate the controlled-not gate operation with a process fidelity greater than $94\mathrm{\%}$. This study confirms that typical two-qubit logic gates used in quantum computational circuits can be implemented with time-bin qubits and thus it is a significant step forward for the realization of distributed quantum computation based on time-bin qubits.

Unconditionally secure communication, which has been pursued for thousands of years hasn’t, however, yet been reached due to a continuous competition between encryption and hacking. Quantum key distribution (QKD), harnessing the quantum mechanical nature of superposition and noncloning, may promise unconditional security by incorporating the one-time pad algorithm rigorously proved by Claude Shannon. Massive efforts have been made in building practical and commercial QKD systems; in particular, decoy states have been employed to detect photon-number splitting attacks against a single-photon source loophole, and measurement-device-independent (MDI) QKD has further closed all loopholes on the detection side, which leads to a seemingly real-life application. Here, we propose and experimentally demonstrate a MDI-QKD hacking strategy on the trusted-source assumption by using an injection-locking technique. An eaves-dropper (Eve) injects near off-resonance photons with randomly chosen polarization into a sender’s laser, where injection locking in a shifted frequency can happen only when Eve’s choice matches the sender’s state. By setting a shifted window and switching the frequency of photons back afterwards, Eve in principle can obtain all the keys without terminating the real-time QKD. We observe the dynamics of a semiconductor laser with injected photons, and obtain a hacking success rate reaching $60.0\mathrm{\%}$ of raw keys. Our results suggest that the spear-and-shield competition of unconditional security may continue until all potential loopholes are discovered and closed ultimately.

Single-photon sources are of great interest because they are key elements in different promising applications of quantum technologies. Here we demonstrate a highly efficient tunable on-demand microwave single-photon source based on a transmon qubit with an intrinsic emission efficiency above $98\mathrm{\%}$. The high efficiency ensures a negligible pure dephasing rate and the necessary conditions for generation of indistinguishable photons. We provide an extended discussion and analysis of the efficiency of the photon generation. To further experimentally confirm the single-photon property of the source, correlation functions of the emission field are also measured using linear detectors with a GPU-enhanced signal-processing technique. Our results experimentally demonstrate that frequency tunability and negligible pure dephasing rate can be achieved simultaneously and show that such a tunable single-photon source can be good for various practical applications in quantum communication, simulations and information processing in the microwave regime.

A powerful control method in experimental quantum computing is the use of spin echoes, employed to select a desired term in the system’s internal Hamiltonian, while refocusing others. Here, we address a more general problem, describing a method to not only turn on and off particular interactions but also to rescale their strengths so that we can generate any desired effective internal Hamiltonian. We propose an algorithm based on linear programming for achieving time-optimal rescaling solutions in fully coupled systems of tens of qubits, which can be modified to obtain near-time-optimal solutions for rescaling systems with hundreds of qubits.

An experimental platform for ultralow-temperature pulsed ENDOR spectroscopy is constructed for general bulk materials. Coherent properties of the coupled electron and nuclear spins of rare-earth dopants in a crystal (${}^{143}{\mathrm{Nd}}^{3+}:{\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$) are investigated from 100 mK to 6 K. At the lowest working temperatures, two-pulse-echo coherence times exceeding 2 and 40 ms are achieved for the electron and nuclear spins, while the electronic Zeeman and hyperfine population lifetimes are more than 15 s and 10 min. With the aid of the near-unity electron spin polarization at 100 mK, the complete hyperfine-level structure with 16 energy levels is measured by the ENDOR technique without the assistance of the reconstructed spin Hamiltonian. These results demonstrate the suitability of deeply cooled paramagnetic rare-earth ions for memory components aimed at quantum communication and quantum computation. The experimental platform developed is expected to be a powerful tool for study of paramagnetic materials in various research fields.

We present a method that combines continuous and pulsed microwave-radiation patterns to achieve robust interactions among hyperfine trapped ions placed in a magnetic field gradient. More specifically, our scheme displays continuous microwave drivings with modulated phases, phase flips, and $\pi$ pulses. This leads to high-fidelity entangling gates that are resilient against magnetic field fluctuations, changes in the microwave amplitudes, and crosstalk effects. Our protocol runs with arbitrary values of microwave power, which includes the technologically relevant case of low microwave intensities. We demonstrate the performance of our method with detailed numerical simulations that take into account the main sources of decoherence.

Monolithic optical cavities employing total internal reflections are appealing compact platforms for a wide range of applications from fundamental physics to industry applications. We characterize the transverse patterns of a millimeter-size monolithic nonplanar ring vortex laser pumped by a Gaussian beam. Vortex beams carrying orbital angular momentum (OAM) up to $30$ together with vortex crystals and petal patterns are observed. The rigid nature of the monolithic cavity brings along its low-noise lasing performance for these beams. A frequency noise floor as low as $0.1\mathrm{Hz}/{\mathrm{Hz}}^{1/2}$ corresponding to sub-Hz instantaneous linewidth is measured. Furthermore, we demonstrate low-noise dual-frequency lasing with phase noise as low as $-118\mathrm{dBc}/\mathrm{Hz}$ at 10-kHz offset for the beatnote at 8.2 GHz. We expect that such versatile and compact OAM laser sources can facilitate multiple applications including optical communications, optical tweezers, and laser metrology such as dual-frequency Doppler lidar.