The spectrally narrow, spin-dependent optical transitions of nitrogen-vacancy (N-$V$) center defects in diamond can be harnessed for quantum networking applications. Key to such networking schemes is the generation of indistinguishable photons. Two challenges limit scalability in such systems: defect-to-defect variations of the optical transition frequencies caused by local strain variation, and spectral diffusion of the optical frequencies on repeated measurement caused by photoexcitation of nearby charge traps. In this experimental study we undertake a group-theoretic approach to quantifying spectral diffusion and strain, decomposing each into components corresponding to Jahn-Teller symmetries of the N-$V$ center. We investigate correlations between the components of strain, spectral diffusion, and depth from the surface, finding that strain and spectral diffusion are each dominated by longitudinal perturbations. We also find a weak negative correlation between transverse static strain and total spectral diffusion, suggesting that transverse strain provides some degree of protection from spectral diffusion. Additionally, we find that spectral diffusion becomes more pronounced with increasing depth in the diamond bulk. Our symmetry-decomposed technique for quantifying spectral diffusion can be valuable for understanding how a given nanoscale charge trap environment influences spectral diffusion and for developing strategies of mitigation.

The advent of optical frequency standards with ultimate uncertainties in the low $1\times {10}^{-18}$ requires femtosecond frequency combs to support a similar level of resolution in the spectral transfer and the computation of optical frequency ratios. The related experimental challenges grow together with the number of optical frequencies to be measured simultaneously, as in many cases the comb’s optical power does not allow reliable beatnote counting or tracking in all the spectral regions of interest. Here we describe the use of a track-and-hold amplifier to implement the gated detection, a previously proposed technique for improving the signal-to-noise ratio of the beatnote between a low-power tooth of the frequency comb and a continuous-wave laser. We demonstrate a 12-dB improvement in the signal-to-noise ratio of beatnotes involving a broadband-spanning optical comb as compared to traditional detection schemes. Our approach enables reliable and cycle-slip-free spectral purity transfer and reduces the system sensitivity to power drops in the comb spectrum. Being based on a single chip, it is robust, versatile, and easily embedded in more complex experimental schemes.

We propose that photon blockade can be enabled by a two-photon absorption environment. The validity of this conclusion is confirmed by studying the blockade in a driven single-mode cavity. The feature of this environmentally induced photon blockade is that the nonlinear interaction in the system is not needed, which is entirely different from the conventional and the unconventional blockade mechanisms. We clearly give the physics behind this blockade mechanism. And the advantages of this environmentally induced photon blockade are discovered by comparing it with the conventional mechanism in a weakly driven region. In addition, we show that the proposal can be generalized straightforwardly to realize environmentally induced photon blockade in two coupled single-mode cavities.

Entanglement between distant massive mechanical oscillators is of particular interest in quantum-enabled devices due to its potential applications in distributed quantum information processing. Here we propose how to achieve nonreciprocal remote entanglement between two spatially separated mechanical oscillators within a cascaded optomechanical configuration, where the two optomechanical resonators are indirectly coupled through a telecommunication fiber. We show that by selectively spinning the optomechanical resonators, one can break the time-reversal symmetry of this compound system via the Sagnac effect, and more excitingly, enhance the indirect couplings between the mechanical oscillators via the individual optimizations of light-motion interaction in each optomechanical resonator. This ability allows us to generate and manipulate nonreciprocal entanglement between distant mechanical oscillators, that is, entanglement can be achieved only through driving the system from one specific input direction but not the other. Moreover, in the case of two frequency-mismatched mechanical oscillators, it is also found that the degree of the generated nonreciprocal entanglement is counterintuitively enhanced in comparison with its reciprocal counterparts, which are otherwise unattainable in static cascaded systems with a single-tone driving laser. Our work, which is well within the feasibility of current experimental capabilities, provides an enticing opportunity to explore the nonclassical correlations between distant massive objects and facilitates a variety of emerging quantum technologies ranging from quantum information processing to quantum sensing.

Deterministic generation of single photons is essential for many quantum information technologies. A bulk optical nonlinearity emitting a photon pair, where the measurement of one of the photons heralds the presence of the other, is commonly used with the caveat that the single-photon emission rate is constrained due to a trade-off between multiphoton events and pair emission rate. Using an efficient and low noise photon-number-resolving superconducting nanowire detector we herald, in real time, a single photon at telecommunication wavelength. We perform a second-order photon correlation $g^2(0)$ measurement of the signal mode conditioned on the measured photon number of the idler mode for various pump powers and demonstrate an improvement of a heralded single-photon source. We develop an analytical model using a phase-space formalism that encompasses all multiphoton effects and relevant imperfections, such as loss and multiple Schmidt modes. We perform a maximum-likelihood fit to test the agreement of the model to the data and extract the best-fit mean photon number $\mu$ of the pair source for each pump power. A maximum reduction of $0.118\pm 0.012$ in the photon $g^2(0)$ correlation function at $\mu =0.327\pm 0.007$ is obtained, indicating a strong suppression of multiphoton emissions. For a fixed $g^2(0)=7\times {10}^{-3}$, we increase the single pair generation probability by 25%. Our experiment, built using fiber-coupled and off-the-shelf components, delineates a path to engineering ideal sources of single photons.

We investigate the stimulated emission of superradiant atoms coupled to a waveguide induced by a coherent-state photon pulse. We provide an analytical result when a short $\pi$ pulse is incident, which shows that the atoms emit photons coherently into the output pulse, which remains a coherent state in the short pulse limit. An incident pulse is amplified in a phase-preserving manner, where noise is added almost entirely in the phase direction in phase space. This property improves the ratio of the intensity signal to noise after the amplification for sufficiently short pulses. This is a unique feature different from general phase-preserving linear amplifiers, where the signal-to-noise ratio deteriorates in the amplification process. We also discuss the dependence of the photon-emission probability on pulse parameters, such as the pulse area and the duration.

Many precision laser applications require active frequency stabilization. However, such stabilization loops operate by pushing noise to frequencies outside their bandwidth, leading to large “servo bumps” that can have deleterious effects for certain applications. The prevailing approach to filtering this noise is to pass the laser through a high-finesse optical cavity, which places constraints on the system design. Here, we propose and demonstrate a different approach where a frequency error signal is derived from a beat note between the laser and the light that passes through the reference cavity. The phase noise derived from this beat note is fed forward to an electro-optic modulator after the laser, carefully accounting for relative delay, for real-time frequency correction. With a hertz-line-width laser, we show $\gtrsim 20\mathrm{dB}$ noise suppression at the peak of the servo bump (approximately $250\mathrm{kHz}$) and a noise-suppression bandwidth of approximately $5\mathrm{MHz}$—well beyond the servo bump. By simulating the Rabi dynamics of a two-level atom with our measured data, we demonstrate substantial improvements to the pulse fidelity over a wide range of Rabi frequencies. Our approach offers a simple and versatile method for obtaining a clean spectrum of a narrow-line-width laser, as required in many emerging applications of cold atoms, and is readily compatible with commercial systems that may even include wavelength conversion.

Nitrogen-vacancy centers in diamond have recently been established as effective sensors of the magnetization dynamics in vicinal ferromagnetic materials. We demonstrate sub-100-nm placement accuracy of nitrogen-vacancy-containing nanodiamonds and use these as local sensors that probe optically detected ferromagnetic resonance in mesoscopically patterned Permalloy islands. These measurements reveal variations in the ferromagnetic resonance signal at different sites on these structures with distinct behavior in the edge and the bulk of patterned features. These test measurements establish an easily implemented approach for spatially targeted measurements of spin dynamics in mesoscale ferromagnets. In principle, the methodology can also be extended to local studies of nanoscale ferromagnets, such as single magnetic nanowires and nanoparticles.

Low-loss deposited dielectrics will benefit superconducting devices such as integrated superconducting spectrometers, superconducting qubits, and kinetic inductance parametric amplifiers. Compared with planar structures, multilayer structures such as microstrips are more compact and eliminate radiation loss at high frequencies. Multilayer structures are most easily fabricated with deposited dielectrics, which typically exhibit higher dielectric loss than crystalline dielectrics. We measure the subkelvin and low-power microwave and millimeter-submillimeter-wave dielectric loss of hydrogenated amorphous silicon carbide ($a$-SiC:H), using superconducting chips with Nb-Ti-$\mathrm{N}/a$-SiC:H/Nb-Ti-N microstrip resonators. We deposit the $a$-SiC:H by plasma-enhanced chemical vapor deposition at a substrate temperature of $400{}^{\circ }\mathrm{C}$. The $a$-SiC:H has a millimeter-submillimeter loss tangent ranging from $0.9\times {10}^{-4}$ at 270 GHz to $1.5\times {10}^{-4}$ at 385 GHz. The microwave loss tangent is $3.1\times {10}^{-5}$. These are the lowest low-power subkelvin loss tangents that have been reported for microstrip resonators at millimeter-submillimeter and microwave frequencies. The $a$-SiC:H films are free of blisters and have low stress: $-20$ MPa compressive at 200-nm thickness to 60 MPa tensile at 1000-nm thickness.

Hole spin qubits in semiconductor quantum dots (QDs) are promising candidates for quantum information processing due to their weak hyperfine coupling to nuclear spins, and to the strong spin-orbit coupling, which allows for rapid operation time. We propose a coherent control on two heavy-hole spin qubits in a double QD by a fast adiabatic driving protocol, which helps to achieve higher fidelities than other experimentally commonly used protocols as linear ramping, $\pi$ pulses or Landau-Zener passages. Using fast quasiadiabatic driving via spin-orbit coupling, it is possible to reduce charge noise significantly for qubit manipulation and achieve high robustness for the qubit initialization. We also implement one- and two-qubit gates, in particular, not, cnot, and swaplike gates, of hole spins in a double QD achieving fidelities above $99\mathrm{\%}$, exhibiting the capability of hole spins to implement universal gates for quantum computing.

The gate-all-around (GAA) $\mathrm{Si}$ nanowire (NW) field-effect transistor (FET) is considered one of the most promising successors of the current mainstream $\mathrm{Si}$ fin FET (FinFET) owing to its better electrostatic gate control. Experimentally, the diameter of $\mathrm{Si}$ NWs has been scaled down to 1 nm. In this paper, the performance limit of the GAA $\mathrm{Si}$ NWFET with a 1-nm diameter is investigated by utilizing ab initio quantum transport simulations. We prove that the electrical conduction is concentrated in the core of the ultranarrow wire channel. The minimum gate length ($L_g$) at which the n- and p-type GAA $\mathrm{Si}$ NWFET can satisfy the high-performance application requirements (on-state current, gate capacitance, delay time, and power dissipation) of the International Technology Roadmap for Semiconductors is 3 nm. The best-performing 5-nm-$L_g$ n-type GAA $\mathrm{Si}$ NWFET exhibits an energy-delay product comparable with typical monolayer two-dimensional FETs. Compared with the similar-sized trigate $\mathrm{Si}$ NW FinFET, an approximately 200% increase in the on-state current and about 15% decrease in the subthreshold swing are witnessed in GAA $\mathrm{Si}$ NWFET at the same 5-nm $L_g$. Through strain engineering, about an 80% increase of on-state current is observed in the 5-nm-$L_g$ p-type GAA $\mathrm{Si}$ NWFET. Our research demonstrates the vast potential of the GAA $\mathrm{Si}$ NWFET in the sub-3-nm gate-length region.

The topological quantum field-effect transition in buckled two-dimensional-Xenes can potentially enable subthermionic transistor operation coupled with a dissipationless on-state conduction. We investigate realistic device structures that exploit the quantum field-effect transition between the dissipationless topological phase and the band-insulator phase. We find that the previously considered dual-gate structure is disadvantageous, leading to a near doubling of the subthreshold swing. However, we identify a single-gate strategy capable of overcoming the thermionic limit at the cost of sacrificing the dissipationless on-state conduction. By introducing an out-of-plane antiferromagnetic exchange in the material via proximity coupling, we exploit transitions between the quantum spin-valley Hall and the spin quantum anomalous Hall phase, which restore the topological robustness of the on state while simultaneously surpassing the thermionic limit. Our work thus outlines a realistic pathway to topological transistors that can overcome Boltzmann’s tyranny while preserving topological robustness.

In recent years, multifunctional materials have attracted increasing interest for magnetic memories and energy-harvesting applications. Magnetic insulating materials are of special interest for this purpose, since they allow the design of more efficient devices due to the lower Joule heat losses. In this context, ${\text{Ga}}_{0.6}{\text{Fe}}_{1.4}{\text{O}}_3$ (GFO) is a good candidate for spintronics applications, since it can exhibit multiferroicity and presents a spin Hall magnetoresistance similar to the one observed in a yttrium-iron-garnet (YIG)/$\mathrm{Pt}$ bilayer. Here, we explore GFO utilizing thermospin measurements in an on-chip approach. By carefully considering the geometry of our thermospin devices we are able to quantify the spin Seebeck effect and the spin-current generation in a GFO/$\mathrm{Pt}$ bilayer, obtaining a value comparable to that of YIG/$\mathrm{Pt}$. This further confirms the promises of an efficient spin-current generation with the possibility of an electric field manipulation of the magnetic properties of the system in an insulating ferrimagnetic material.

One route to create tunable metamaterials is through integration with “on-demand” dynamic quantum materials, such as vanadium dioxide (${\mathrm{VO}}_2$). This enables modalities to create high-performance devices for historically challenging applications. Indeed, dynamic materials have often been integrated with metamaterials to imbue artificial structures with some degree of tunability. Conversely, metamaterials can be used to enhance and extend the natural tuning range of dynamic materials. Utilizing a complementary split-ring resonator array deposited on a ${\mathrm{VO}}_2$ film, we demonstrate enhanced terahertz transmission modulation upon traversing the insulator-to-metal transition (IMT) at approximately $340$ K. Our complementary metamaterial increases the modulation amplitude of the original ${\mathrm{VO}}_2$ film from 0.42 to 0.68 at 0.47 THz upon crossing the IMT, corresponding to an enhancement of 62%. Moreover, temperature-dependent transmission measurements reveal a significant redshift of the resonant frequency in a narrow temperature range where phase coexistence is known to occur. Neither Maxwell-Garnett nor Bruggeman effective medium theory adequately describes the observed frequency shift and amplitude decrease. However, a Drude model incorporating a significant increase of the effective permittivity does describe the experimentally observed redshift. Our results highlight that symbiotic integration of metamaterial arrays with quantum materials provides a powerful approach to engineer emergent functionality.

The stochastic magnetic tunnel junction (MTJ), the resistance of which fluctuates in time between low and high states under thermal fluctuation, is expected to serve as a key element in probabilistic computers. For reliable operation and versatile application, one needs to address the challenge that the relaxation time in each state should be virtually independent of external magnetic fields in the range of, for example, several mT. In this research, we fabricate in-plane easy-axis elliptical stochastic MTJs with a synthetic antiferromagnetic (SAF) free layer and examine the robustness of their properties against external fields while comparing them with stochastic MTJs with a conventional single ferromagnetic structure. We show that the MTJ with a SAF free layer shows more robust relaxation times against fields applied along the easy- and hard-axis directions. The results are reproduced with a macrospin simulation, from which design guidelines for robust stochastic MTJs targeting probabilistic computers are discussed.