Controlling quasiparticle dynamics can improve the performance of superconducting devices. For example, it has been demonstrated effective in increasing the lifetime and stability of superconducting qubits. Here we study how to optimize the placement of normal-metal traps in transmon-type qubits. When the trap size increases beyond a certain characteristic length, the details of the geometry and trap position, and even the number of traps, become important. We discuss for some experimentally relevant examples how to shorten the decay time of the excess quasiparticle density. Moreover, we show that a trap in the vicinity of a Josephson junction can significantly reduce the steady-state quasiparticle density near that junction, thus, suppressing the quasiparticle-induced relaxation rate of the qubit. Such a trap also reduces the impact of fluctuations in the generation rate of quasiparticles, rendering the qubit more stable.

By contrasting the performance of two quantum annealers operating at different temperatures, we address recent questions related to the role of temperature in these devices and their function as “Boltzmann samplers.” Using a method to reliably calculate the degeneracies of the energy levels of large-scale spin-glass instances, we are able to estimate the instance-dependent effective temperature from the output of annealing runs. Our results corroborate the “freeze-out” picture which posits two regimes, one in which the final state corresponds to a Boltzmann distribution of the final Hamiltonian with a well-defined “effective temperature” determined at a freeze-out point late in the annealing schedule, and another regime in which such a distribution is not necessarily expected. We find that the output distributions of the annealers do not, in general, correspond to a classical Boltzmann distribution for the final Hamiltonian. We also find that the effective temperatures at different programing cycles fluctuate greatly, with the effect worsening with problem size. We discuss the implications of our results for the design of future quantum annealers to act as more-effective Boltzmann samplers and for the programing of such annealers.

We report on high-resolution photoluminescence (PL) and photocurrent (PC) spectroscopies of a single self-assembled InAs/GaAs quantum dot (QD) embedded in an $n\text{−}i$-Schottky device with an applied magnetic field in Faraday and Voigt geometries. The single-QD PC spectrum of neutral exciton ($X^0$) is obtained by sweeping the bias-dependent $X^0$ transition energy to achieve resonance with a fixed narrow-bandwidth laser through the quantum-confined Stark effect. With a magnetic field applied in Faraday geometry, the diamagnetic effect and the Zeeman splitting of $X^0$ are observed both in PL and PC spectra. When the magnetic field is applied in Voigt geometry, the mixture of bright and dark states results in an observation of dark-exciton states, which are confirmed by the polarization-resolved PL and PC spectra.

The realization of an elementary quantum network that is intrinsically secure and operates over long distances requires the interconnection of several quantum modules performing different tasks. In this work, we report the realization of a communication network functioning in a quantum regime, consisting of four different quantum modules: (i) a random polarization qubit generator, (ii) a free-space quantum-communication channel, (iii) an ultralow-noise portable quantum memory, and (iv) a qubit decoder, in a functional elementary quantum network possessing all capabilities needed for quantum-information distribution protocols. We create weak coherent pulses at the single-photon level encoding polarization states $|H⟩$, $|V⟩$, $|D⟩$, and $|A⟩$ in a randomized sequence. The random qubits are sent over a free-space link and coupled into a dual-rail room-temperature quantum memory and after storage and retrieval are analyzed in a four-detector polarization analysis akin to the requirements of the BB84 protocol. We also show ultralow noise and fully portable operation, paving the way towards memory-assisted all-environment free-space quantum cryptographic networks.

In this work, we propose a flexible architecture of microwave resonators with tunable couplings to perform quantum simulations of problems from the field of molecular chemistry. The architecture builds on the experience of the D-Wave design, working with nearly harmonic circuits instead of qubits. This architecture, or modifications of it, can be used to emulate molecular processes such as vibronic transitions. Furthermore, we discuss several aspects of these emulations, such as dynamical ranges of the physical parameters, quenching times necessary for diabaticity, and, finally, the possibility of implementing anharmonic corrections to the force fields by exploiting certain nonlinear features of superconducting devices.

We demonstrate that small quantum memories, realized via quantum error correction in multiqubit devices, can benefit substantially by choosing a quantum code that is tailored to the relevant error model of the system. For a biased noise model, with independent bit and phase flips occurring at different rates, we show that a single code greatly outperforms the well-studied Steane code across the full range of parameters of the noise model, including for unbiased noise. In fact, this tailored code performs almost optimally when compared with 10 000 randomly selected stabilizer codes of comparable experimental complexity. Tailored codes can even outperform the Steane code with realistic experimental noise, and without any increase in the experimental complexity, as we demonstrate by comparison in the observed error model in a recent seven-qubit trapped ion experiment.

Statistics of photons emitted by mobile excitons in individual carbon nanotubes are investigated. Photoluminescence spectroscopy is used to identify the chiralities and suspended lengths of air-suspended nanotubes, and photon-correlation measurements are performed at room temperature on telecommunication-wavelength nanotube emission with a Hanbury-Brown-Twiss setup. We obtain zero-delay second-order correlation $g^{(2)}(0)$ less than 0.5, indicating single-photon generation. Excitation power dependence of the photon antibunching characteristics is examined for nanotubes with various chiralities and suspended lengths, where we find that the minimum value of $g^{(2)}(0)$ is obtained at the lowest power. The influence of exciton diffusion and end quenching is studied by Monte Carlo simulations, and we derive an analytical expression for the minimum value of $g^{(2)}(0)$. Our results indicate that mobile excitons in micrometer-long nanotubes can in principle produce high-purity single photons, leading to new design strategies for quantum photon sources.

Nonreciprocal transmission and isolation at microwave frequencies are important in many practical applications. In particular, compact isolators are useful in protecting sensitive quantum circuits operating at cryogenic temperatures from amplifier backaction and other environmental noise such as black-body radiation from higher temperature stages. However, the size of commercial cryogenic isolators limits the ability to measure multiple quantum circuits because of space constraints in typical dilution refrigerator systems. Furthermore, isolators usually require the use of ferrite components that cannot be integrated at the chip level and, since they also need large biasing magnetic fields, are incompatible with superconducting quantum circuits. In this work we show one way to accomplish isolation in a superconducting chip-scale device, a traveling-wave unidirectional frequency converter based on a parametrically pumped superconducting Josephson-junction transmission line, demonstrating better than 4.8 dB of inferred signal isolation from 6.6 to 11.4 GHz, with a maximum of 12 dB at 9.5 GHz. By using frequency diplexing techniques a conventional isolator could be implemented over this bandwidth.

Single-mode Josephson-junction-based parametric amplifiers are often modeled as perfect amplifiers and squeezers. We show that in practice, the gain, quantum efficiency, and output field squeezing of these devices are limited by usually neglected higher-order corrections to the idealized model. To arrive at this result, we derive the leading corrections to the lumped-element Josephson parametric amplifier of three common pumping schemes: monochromatic current pump, bichromatic current pump, and monochromatic flux pump. We show that the leading correction for the last two schemes is a single Kerr-type quartic term, while the first scheme contains additional cubic terms. In all cases, we find that the corrections are detrimental to squeezing. In addition, we show that the Kerr correction leads to a strongly phase-dependent reduction of the quantum efficiency of a phase-sensitive measurement. Finally, we quantify the departure from the ideal Gaussian character of the filtered output field from numerical calculation of third- and fourth-order cumulants. Our results show that while a Gaussian output field is expected for an ideal Josephson parametric amplifier, higher-order corrections lead to non-Gaussian effects which increase with both gain and nonlinearity strength. This theoretical study is complemented by experimental characterization of the output field of a flux-driven Josephson parametric amplifier. In addition to a measurement of the squeezing level of the filtered output field, the Husimi $Q$ function of the output field is imaged by the use of a deconvolution technique and compared to numerical results. This work establishes nonlinear corrections to the standard degenerate parametric amplifier model as an important contribution to the Josephson parametric amplifier’s squeezing and noise performance.

We report the development of niobium nitride (NbN)-based ferromagnetic $\pi$ Josephson junctions. For fabricated $\mathrm{NbN}/\mathrm{copper}\text{−}\mathrm{nickel}$ $(\mathrm{CuNi})/\mathrm{NbN}$ junctions, we measure the ferromagnetic CuNi thickness and temperature dependences of the Josephson critical current and find the cusp structure indicating the transition between the 0 and $\pi$ states. We also observe the characteristic temperature dependences for the junctions with CuNi thicknesses near the $0\text{−}\pi$ transition thickness, which originate from the interplay between superconductivity and magnetism. The experimental results are consistently explained by the microscopic theory with reasonable physical parameters. The developed NbN-based $\pi$ junctions are compatible with and provide advantages for superconducting logic circuits and/or quantum computers based on nitride superconductors.

We report on the microwave characterization of a novel one-dimensional Josephson metamaterial composed of a chain of asymmetric superconducting quantum interference devices with nearest-neighbor coupling through common Josephson junctions. This metamaterial demonstrates a strong Kerr nonlinearity, with a Kerr constant tunable over a wide range, from positive to negative values, by a magnetic flux threading the superconducting quantum interference devices. The experimental results are in good agreement with the theory of nonlinear effects in Josephson chains. The metamaterial is very promising as an active medium for Josephson traveling-wave parametric amplifiers; its use facilitates phase matching in a four-wave-mixing process for efficient parametric gain.

We demonstrate the full functionality of a circuit that generates single microwave photons on demand, with a wave packet that can be modulated with a near-arbitrary shape. We achieve such a high tunability by coupling a superconducting qubit near the end of a semi-infinite transmission line. A dc superconducting quantum interference device shunts the line to ground and is employed to modify the spatial dependence of the electromagnetic mode structure in the transmission line. This control allows us to couple and decouple the qubit from the line, shaping its emission rate on fast time scales. Our decoupling scheme is applicable to all types of superconducting qubits and other solid-state systems and can be generalized to multiple qubits as well as to resonators.

We study the generation of Greenberger-Horne-Zeilinger (GHZ) states of three path-encoded photons. Inspired by the seminal work of Bouwmeester et al. [Phys. Rev. Lett. 82, 1345 (1999)] on polarization-entangled GHZ states, we find a corresponding path representation for the photon states of an optical circuit, identify the elements required for the state generation, and propose a possible implementation of our strategy. Besides the practical advantage of employing an integrated system that can be fabricated with proven lithographic techniques, our example suggests that it is possible to enhance the generation efficiency by using microring resonators.

An ideal preamplifier for qubit measurement must not only provide high gain and near quantum-limited noise performance, but also isolate the delicate quantum circuit from noisy downstream measurement stages while producing negligible backaction. Here, we use a superconducting low-inductance undulatory galvanometer (SLUG) microwave amplifier to read out a superconducting transmon qubit, and we characterize both reverse isolation and measurement backaction of the SLUG. For appropriate dc bias, the SLUG achieves reverse isolation that is better than that of a commercial cryogenic isolator. Moreover, SLUG backaction is dominated by thermal emission from dissipative elements in the device. When the SLUG is operated in pulsed mode, it is possible to characterize the transmon qubit using a measurement chain that is free from cryogenic isolators or circulators with no measurable degradation of qubit performance.

We report on the realization of a bonded-bridge on-chip superconducting coil and its use in impedance matching a highly ohmic quantum dot (QD) to a 3-GHz measurement setup. The coil, modeled as a lumped-element $LC$ resonator, is more compact and has a wider bandwidth than resonators based on coplanar transmission lines (e.g., $\lambda /4$ impedance transformers and stub tuners), at potentially better signal-to-noise ratios. Specifically, for measurements of radiation emitted by the device, such as shot noise, the $50\times$-larger bandwidth reduces the time to acquire the spectral density. The resonance frequency, close to 3.25 GHz, is 3 times higher than that of the one previously reported, a wire-bonded coil. As a proof of principle, we fabricate an $LC$ circuit that achieves impedance matching to an approximately $15\text{−}\mathrm{k}\mathrm{\Omega }$ load and validate it with a load defined by a carbon nanotube QD, whose shot noise we measure in the Coulomb-blockade regime.

Single-electron pumps based on semiconductor quantum dots are promising candidates for the emerging quantum standard of electrical current. They can transfer discrete charges with part-per-million (ppm) precision in nanosecond time scales. Here, we employ a metal-oxide-semiconductor silicon quantum dot to experimentally demonstrate high-accuracy gigahertz single-electron pumping in the regime where the number of electrons trapped in the dot is determined by the thermal distribution in the reservoir leads. In a measurement with traceability to primary voltage and resistance standards, the averaged pump current over the quantized plateau, driven by a 1-GHz sinusoidal wave in the absence of a magnetic field, is equal to the ideal value of $ef$ within a measurement uncertainty as low as 0.27 ppm.

Quantum key distribution (QKD) is a quantum-proof key-exchange scheme which is fast approaching the communication industry. An essential component in QKD is the information reconciliation step, which is used for correcting the quantum-channel noise errors. The recently suggested blind-reconciliation technique, based on low-density parity-check codes, offers remarkable prospectives for efficient information reconciliation without an a priori quantum bit error rate estimation. We suggest an improvement of the blind-information-reconciliation protocol promoting a significant increase in the efficiency of the procedure and reducing its interactivity. The proposed technique is based on introducing symmetry in operations of parties, and the consideration of results of unsuccessful belief-propagation decodings.

We propose and demonstrate a technique for coupling phonons out of an optomechanical crystal cavity. Using a perturbation that breaks a symmetry in the elastic structure, we induce phonon leakage without affecting the optical properties. It is shown experimentally via cryogenic measurements that the proposed cavity perturbation causes a loss of phonons into mechanical waves on the surface of silicon while leaving photon lifetimes unaffected. This result demonstrates that phonon leakage can be engineered in on-chip optomechanical systems. We experimentally observe large fluctuations in leakage rates that we attribute to fabrication disorder and verify this finding using simulations. Our technique opens the way to engineering more complex on-chip phonon networks utilizing guided mechanical waves to connect quantum systems.

We study the impact of low-frequency magnetic flux noise upon superconducting transmon qubits with various levels of tunability. We find that qubits with weaker tunability exhibit dephasing that is less sensitive to flux noise. This insight is used to fabricate qubits where dephasing due to flux noise is suppressed below other dephasing sources, leading to flux-independent dephasing times $T_2^{*}\sim 15\mu \mathrm{s}$ over a tunable range of approximately 340 MHz. Such tunable qubits have the potential to create high-fidelity, fault-tolerant qubit gates and to fundamentally improve scalability for a quantum processor.

We experimentally investigate superconducting coplanar waveguide resonators in external magnetic fields and present two strategies to reduce field-induced dissipation channels and resonance frequency shifts. One of our approaches is to significantly reduce the superconducting ground-plane areas, which leads to reduced magnetic-field focussing and thus to lower effective magnetic fields inside the waveguide cavity. By this measure, the field-induced losses can be reduced by more than one order of magnitude in mT out-of-plane magnetic fields. When these resonators are additionally coupled inductively instead of capacitively to the microwave feed lines, an intrinsic closed superconducting loop is effectively shielding the heart of the resonator from magnetic fields by means of flux conservation. In total, we achieve a reduction of the field-induced resonance frequency shift by up to 2 orders of magnitude. We combine systematic parameter variations on the experimental side with numerical magnetic-field calculations to explain the effects of our approaches and to support our conclusions. The presented results are relevant for all areas, where high-performance superconducting resonators need to be operated in magnetic fields, e.g., for quantum hybrid devices with superconducting circuits or electron spin resonance detectors based on coplanar waveguide cavities.