Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fiber-optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into one-dimensional optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realize QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs ‘‘droplet-epitaxy’’ of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at $\sim 900\mathrm{nm}$, and metal-organic vapor-phase epitaxy on InP emitting at $\sim 1550\mathrm{nm}$, with mean FSS $4\times$ smaller than for Stranski-Krastanow QDs.

Quantum annealing is an optimization technique which potentially leverages quantum tunneling to enhance computational performance. Existing quantum annealers use superconducting flux qubits with short coherence times limited primarily by the use of large persistent currents $I_{\mathit{p}}$. Here, we examine an alternative approach using qubits with smaller $I_{\mathit{p}}$ and longer coherence times. We demonstrate tunable coupling, a basic building block for quantum annealing, between two flux qubits with small (approximately 50-nA) persistent currents. Furthermore, we characterize qubit coherence as a function of coupler setting and investigate the effect of flux noise in the coupler loop on qubit coherence. Our results provide insight into the available design space for next-generation quantum annealers with improved coherence.

Using frequency-modulated electrically detected magnetic resonance (EDMR), we show that signals measured from lightly doped ($1.2–5\times 10^{15}{\mathrm{cm}}^{-3}$) silicon devices vary significantly with the wavelength of the optical excitation used to generate the mobile carriers. We measure EDMR spectra at 4.2 K as a function of modulation frequency and applied microwave power using a 980-nm laser, a 405-nm laser, and a broadband white-light source. EDMR signals are observed from the phosphorus donor and two distinct defect species in all of the experiments. With near-infrared irradiation, we find that the EDMR signal primarily arises from donor-defect pairs, while, at higher photon energies, there are significant additional contributions from defect-defect pairs. The contribution of spins from different spatial regions to the EDMR signal is seen to vary as the optical penetration depth changes from about 120 nm at 405-nm illumination to $100\mu \mathrm{m}$ at 980-nm illumination. The modulation frequency dependence of the EDMR signal shows that the energy of the optical excitation strongly modulates the kinetics of the underlying spin-dependent recombination (SDR) process. Careful tuning of the optical photon energy could therefore be used to control both the subset of spin pairs contributing to the EDMR signal and the dynamics of the SDR process.

Color centers in silicon carbide have increasingly attracted attention in recent years owing to their excellent properties such as single-photon emission, good photostability, and long spin-coherence time even at room temperature. As compared to diamond, which is widely used for hosting nitrogen-vacancy centers, silicon carbide has an advantage in terms of large-scale, high-quality, and low-cost growth, as well as an advanced fabrication technique in optoelectronics, leading to prospects for large-scale quantum engineering. In this paper, we report an experimental demonstration of the generation of a single-photon-emitter array through ion implantation. $V_{\mathrm{Si}}$ defects are generated in predetermined locations with high generation efficiency (approximately $19\%\pm 4\%$). The single emitter probability reaches approximately $34\%\pm 4\%$ when the ion-implantation dose is properly set. This method serves as a critical step in integrating single $V_{\mathrm{Si}}$ defect emitters with photonic structures, which, in turn, can improve the emission and collection efficiency of $V_{\mathrm{Si}}$ defects when they are used in a spin photonic quantum network. On the other hand, the defects are shallow, and they are generated about 40 nm below the surface which can serve as a critical resource in quantum-sensing applications.

Accurate quantum gates are basic elements for building quantum computers. Recently, there has been great interest in designing quantum logic gates by using the blockade effect of Rydberg atoms. The fidelity and operation speed of these gates, however, are fundamentally limited by an intrinsic blockade error. Here we propose a type of quantum gate, which is based on the Rydberg blockade effect, yet free from any blockade error. In contrast to the “blocking” method in previous schemes, we use the Rydberg energy shift to realize a rational generalized Rabi frequency so that a $\pi$ phase for one input state of the gate emerges. This leads to an accurate Rydberg quantum logic gate that can operate on a $0.1\text{−}\mu \mathrm{s}$ time scale or faster because it works by a Rabi frequency which is comparable to the blockade shift.

Optical isolation, nonreciprocal phase transmission, and topological phases for light based on synthetic gauge fields have been raising significant interest in the recent literature. Cavity-optomechanical systems that involve two optical modes coupled to a common mechanical mode form an ideal platform to realize these effects, providing the basis for various recent demonstrations of optomechanically induced nonreciprocal light transmission. Here, we establish a unifying theoretical framework to analyze optical nonreciprocity and the breaking of time-reversal symmetry in multimode optomechanical systems. We highlight two general scenarios to achieve isolation, relying on either optical or mechanical losses. Depending on the loss mechanism, our theory defines the ultimate requirements for optimal isolation and the available operational bandwidth in these systems. We also analyze the effect of sideband resolution on the performance of optomechanical isolators, highlighting the fact that nonreciprocity can be preserved even in the unresolved sideband regime. Our results provide general insights into a broad class of parametrically modulated nonreciprocal devices, paving the way towards optimal nonreciprocal systems for low-noise integrated nanophotonics.

Entangling remote qubits is an essential technological element in the distributed quantum information processing. Here, we propose a deterministic scheme to generate maximal entanglement between remote superconducting atoms, using a propagating microwave photon as a flying qubit. The building block of this scheme is an atom-photon two-qubit gate, in which the photon qubit is encoded on its carrier frequencies. The gate operation completes deterministically upon reflection of a photon, and the gate type can be continuously varied (including swap, $\sqrt{\mathrm{SWAP}}$, and identity gates) through in situ control of the drive field. Applying such atom-photon gates sequentially, we can perform various gate operations between remote superconducting atoms.

The silicon-on-chip photonic circuit is a very promising platform for scalable quantum-information technology for its low loss, small footprint, and its compatibility with CMOS as well as telecom techniques. Multiplexed multiple entanglement sources, including energy-time, time-bin, and polarization-entangled sources based on 1-cm-length single silicon nanowire, are all compatible with the (100-GHz) dense-wave-division-multiplexing system. Different methods, such as two-photon interference as well as Bell-inequality and quantum-state tomography, are used to characterize the quality of these entangled sources. Multiple entanglements are generated over more than five channel pairs with high raw (net) visibilities of around 97% (100%). The emission spectral brightness of these entangled sources reaches $4.2\times {10}^5/(\mathrm{s}\mathrm{nm}\mathrm{mW})$. The quality of the photon pair generated in the continuous and pulse-pump regimes are compared. The high quality of these multiple entanglement sources makes them very promising for use in minimized quantum-communication and computation systems.

We present the “trimon,” a multimode superconducting circuit implementing three qubits with all-to-all longitudinal coupling. This always-on interaction enables simple implementation of generalized controlled-not gates which form a universal set. Furthermore, two of the three qubits are protected against Purcell decay while retaining measurability. We demonstrate high-fidelity state swapping operations between two qubits and characterize the coupling of all three qubits to a neighboring transmon qubit. Our results offer a different paradigm for multiqubit architecture with applications in quantum error correction, quantum simulations, and quantum annealing.

Implementing holonomic quantum computation is a challenging task as it requires complicated interaction among multilevel systems. Here we propose to implement nonadiabatic holonomic quantum computation based on dressed-state qubits in circuit QED. An arbitrary holonomic single-qubit gate can be conveniently achieved using external microwave fields and tuning their amplitudes and phases. Meanwhile, nontrivial two-qubit gates can be implemented in a coupled-cavities scenario assisted by a grounding superconducting quantum-interference device (SQUID) with tunable interaction, where the tuning is achieved by modulating the ac flux threaded through the SQUID. In addition, our proposal is directly scalable, up to a two-dimensional lattice configuration. In the present scheme, the dressed states involve only the lowest two levels of each transmon qubit, and the effective interactions exploited are all of resonant nature. Therefore, we release the main difficulties for physical implementation of holonomic quantum computation on superconducting circuits.

We show that a weak external magnetic field affects significantly nonequilibrium quasiparticle (QP) distributions under the conditions of the inverse proximity effect, using the single-electron hybrid turnstile as a generic example. Inverse proximity suppresses the superconducting gap in superconducting leads in the vicinity of turnstile junctions, thus, trapping hot QPs in this region. An external magnetic field creates additional QP traps in the leads in the form of vortices or regions with a reduced superconducting gap resulting in the release of QPs away from the junctions. We present clear experimental evidence of the interplay of the inverse proximity effect and magnetic field revealing itself in the superconducting gap enhancement and significant improvement of the turnstile characteristics. The observed interplay and its theoretical explanation in the context of QP overheating are important for various superconducting and hybrid nanoelectronic devices, which find applications in quantum computation, photon detection, and quantum metrology.

The speed of quantum gates and measurements is a decisive factor for the overall fidelity of quantum protocols when performed on physical qubits with a finite coherence time. Reducing the time required to distinguish qubit states with high fidelity is, therefore, a critical goal in quantum-information science. The state-of-the-art readout of superconducting qubits is based on the dispersive interaction with a readout resonator. Here, we bring this technique to its current limit and demonstrate how the careful design of system parameters leads to fast and high-fidelity measurements without affecting qubit coherence. We achieve this result by increasing the dispersive-interaction strength, by choosing an optimal linewidth of the readout resonator, by employing a Purcell filter, and by utilizing phase-sensitive parametric amplification. In our experiment, we measure 98.25% readout fidelity in only 48 ns, when minimizing readout time, and 99.2% in 88 ns, when maximizing the fidelity, limited predominantly by the qubit lifetime of $7.6\mu \mathrm{s}$. The presented scheme is also expected to be suitable for integration into a multiplexed readout architecture.

An approach to quantum random number generation based on unambiguous quantum state discrimination is developed. We consider a prepare-and-measure protocol, where two nonorthogonal quantum states can be prepared, and a measurement device aims at unambiguously discriminating between them. Because the states are nonorthogonal, this necessarily leads to a minimal rate of inconclusive events whose occurrence must be genuinely random and which provide the randomness source that we exploit. Our protocol is semi-device-independent in the sense that the output entropy can be lower bounded based on experimental data and a few general assumptions about the setup alone. It is also practically relevant, which we demonstrate by realizing a simple optical implementation, achieving rates of $16.5\mathrm{Mbits}/\mathrm{s}$. Combining ease of implementation, a high rate, and a real-time entropy estimation, our protocol represents a promising approach intermediate between fully device-independent protocols and commercial quantum random number generators.

Carbon nanotube quantum dots allow accurate control of electron charge, spin, and valley degrees of freedom in a material which is atomically perfect and can be grown isotopically pure. These properties underlie the unique potential of carbon nanotubes for quantum information processing, but developing nanotube charge, spin, or spin-valley qubits requires efficient readout techniques as well as understanding and extending quantum coherence in these devices. Here, we report on microwave spectroscopy of a carbon nanotube charge qubit in which quantum information is encoded in the spatial position of an electron. We combine radio-frequency reflectometry measurements of the quantum capacitance of the device with microwave manipulation to drive transitions between the qubit states. This approach simplifies charge-state readout and allows us to operate the device at an optimal point where the qubit is first-order insensitive to charge noise. From these measurements, we are able to quantify the degree of charge noise experienced by the qubit and obtain an inhomogeneous charge coherence of 5 ns. We use a chopped microwave signal whose duty-cycle period is varied to measure the decay of the qubit states, yielding a charge relaxation time of 48 ns.

Two-pulse photon-echo-based quantum-memory applications require a precise control over the echo strength and appearance time. We describe a numerical investigation of observation and control of photon echo in a room-temperature $\mathrm{InAs}/\mathrm{InP}$–based quantum-dot (QD) semiconductor optical amplifier (SOA). We address an important case where the spectral excitation is narrower than the inhomogeneous broadening of the SOA. It is revealed that, in such a QD SOA, the amplitude of the echo pulse depends not only on the excitation-to-rephasing pulse temporal separation but also on the interference among the rephrasing pulse and the echo pulses generated during the propagation along the amplifier. More importantly, the appearance time and amplitude of the echo pulse can be precisely controlled by shaping the first (excitation) pulse. We also assert that deviations in the echo pulse stemming from the SOA gain inhomogeneity can be compensated for so as to be utilized in quantum coherent information processing.

We demonstrate hybrid photoelectric pulse protocols for reading the spin states of nitrogen-vacancy ($\mathrm{N}\text{−}V$) centers in diamond, compatible with coherent spin control and performed on shallow nitrogen-implanted electronic grade diamond. The measurements are carried out on spin ensembles from 1000 to just five $\mathrm{N}\text{−}V$ centers as a first step towards the fabrication of scalable photoelectric quantum chips. Specific microwave protocols are developed that suppress background photocurrent related to ionization of ${{\mathrm{N}}_S}^0$ defects and provide a high contrast and SNR. The technique is demonstrated on Rabi and Ramsey sequences.

We present the generation of continuous-wave optical squeezing from a titanium–in-diffused lithium niobate waveguide resonator. We directly measure $2.9\pm 0.1\mathrm{dB}$ of single-mode squeezing, which equates to a produced level of $4.9\pm 0.1\mathrm{dB}$ after accounting for detection losses. This device showcases the current capabilities of this waveguide architecture and precipitates more complicated integrated continuous-wave quantum devices in the continuous-variable regime.

We present a tuneup protocol for qubit gates with tenfold speedup over traditional methods reliant on qubit initialization by energy relaxation. This speedup is achieved by constructing a cost function for Nelder-Mead optimization from real-time correlation of nondemolition measurements interleaving gate operations without pause. Applying the protocol on a transmon qubit achieves 0.999 average Clifford fidelity in one minute, as independently verified using randomized benchmarking and gate-set tomography. The adjustable sensitivity of the cost function allows the detection of fractional changes in the gate error with a nearly constant signal-to-noise ratio. The restless concept demonstrated can be readily extended to the tuneup of two-qubit gates and measurement operations.

We present a device demonstrating a lithographically patterned transmon integrated with a micromachined cavity resonator. Our two-cavity, one-qubit device is a multilayer microwave-integrated quantum circuit (MMIQC), comprising a basic unit capable of performing circuit-QED operations. We describe the qubit-cavity coupling mechanism of a specialized geometry using an electric-field picture and a circuit model, and obtain specific system parameters using simulations. Fabrication of the MMIQC includes lithography, etching, and metallic bonding of silicon wafers. Superconducting wafer bonding is a critical capability that is demonstrated by a micromachined storage-cavity lifetime of $34.3\mu \mathrm{s}$, corresponding to a quality factor of $2\times {10}^6$ at single-photon energies. The transmon coherence times are $T_1=6.4\mu \mathrm{s}$, and $T_2^{\mathrm{echo}}=11.7\mu \mathrm{s}$. We measure qubit-cavity dispersive coupling with a rate ${\chi }_{q\mu }/2\pi =-1.17\mathrm{MHz}$, constituting a Jaynes-Cummings system with an interaction strength $g/2\pi =49\mathrm{MHz}$. With these parameters we are able to demonstrate circuit-QED operations in the strong dispersive regime with ease. Finally, we highlight several improvements and anticipated extensions of the technology to complex MMIQCs.