Because of its extremely high energy efficiency, adiabatic superconductor logic is one of the most promising candidates for the realization of a practical reversible computer. In a previous study, we proposed a logically and physically reversible logic gate using adiabatic superconductor logic, and numerically demonstrated reversible computing. In the numerical calculation, we assumed that the average energy dissipation at finite temperature corresponds to that at zero temperature. However, how the phase difference of a Josephson junction in adiabatic superconductor logic behaves at finite temperature is not yet well understood, and whether thermal noise can induce a nonadiabatic state change remains unclear. In the present study, we investigate energy dissipation in adiabatic superconductor logic at finite temperature through numerical analyses using the Monte Carlo method. We investigate the average and standard deviation of the energy dissipation through both numerical calculation and analytical estimation. Finally, we discuss the minimum energy dissipation required for adiabatic switching operations.

We analyze a design for a microwave circulator which could replace many of the commercial ferrite circulators that are ubiquitous in contemporary quantum superconducting microwave experiments. The lossless, lumped-element design is capable of being integrated on chip with other superconducting microwave devices, thus circumventing the many performance-limiting aspects of ferrite circulators. The design is based on the dynamic modulation of dc superconducting microwave quantum interference devices that function as nearly linear, tunable inductors. The connection to familiar ferrite-based circulators is a simple frame boost in the internal dynamics’ equation of motion. In addition to the general, schematic analysis, we also give an overview of many considerations necessary to achieve a practical design with a tunable center frequency in the 4–8-GHz frequency band, a bandwidth of 240 MHz, reflections at the $-20\text{−}\mathrm{dB}$ level, and a maximum signal power of approximately 100 microwave photons per inverse bandwidth.

We report on experiments with a microfabricated surface trap designed for confining a chain of ions in a ring. Uniform ion separation over most of the ring is achieved with a rotationally symmetric design and by measuring and suppressing undesired electric fields. After reducing stray fields, the ions are confined primarily by a radio-frequency pseudopotential and their mutual Coulomb repulsion. Approximately 400 ${{}^{40}\mathrm{Ca}}^{+}$ ions with an average separation of $9\mu \mathrm{m}$ comprise the ion crystal.

At strong pump powers, a semiconductor optical cavity passes through a Hopf bifurcation and undergoes self-oscillation. We simulate this device using semiclassical Langevin equations and assess the effect of quantum fluctuations on the dynamics. Below threshold, the cavity acts as a phase-insensitive linear amplifier, with noise approximately $5\times \text{larger}$ than the Caves bound. Above threshold, the limit cycle acts as an analog memory, and the phase diffusion is approximately $10\times \text{larger}$ than the bound set by the standard quantum limit. We also simulate entrainment of this oscillator and propose an optical Ising machine and classical controlled not (cnot) gate based on the effect.

A traveling-wave parametric amplifier (TWPA) composed of a transmission line made up of a chain of coupled asymmetric superconducting quantum interference devices (SQUIDs) is proposed. The unique nature of this transmission line is that its nonlinearity can be tuned with an external magnetic flux and can even change sign. This feature of the transmission line can be used to perform phase matching in a degenerate four-wave mixing process which can be utilized for the parametric amplification of a weak signal in the presence of a strong pump. Numerical simulations of the TWPA design show that, with tuning, phase matching can be achieved and an exponential gain as a function of the transmission-line length can be realized. The flexibility of the proposed design can realize: compact TWPAs with fewer than 211 unit cells, signal gains greater than 20 dB, 3-dB bandwidth greater than 5.4 GHz, and saturation powers up to $-98\mathrm{dBm}$. This amplifier design is well suited for the multiplexed readout of quantum circuits or astronomical detectors in a compact configuration which can foster on-chip implementations.

High-fidelity coherent control of quantum systems is critical to building quantum devices and quantum computers. We provide a general optimal control framework for designing control sequences that account for hardware control distortions while maintaining robustness to environmental noise. We demonstrate the utility of our algorithm by presenting examples of robust quantum gates optimized in the presence of nonlinear distortions. We show that nonlinear classical controllers do not necessarily incur additional computational cost to pulse-optimization, enabling more powerful quantum devices.

Here we demonstrate quantum interference of photons on a silicon chip produced from a single-ring-resonator photon source. The source is seamlessly integrated with a Mach-Zehnder interferometer, which path entangles degenerate biphotons produced via spontaneous four-wave mixing in the silicon ring resonator. The resulting entangled N00N state is controlled by varying the relative phase of the integrated Mach-Zehnder interferometer, resulting in high two-photon interference visibilities of $V\sim 96\%$. Furthermore, we show that the interference can be produced using pump wavelengths tuned to all of the ring resonances accessible with our tunable lasers ($C+L$ band). This work is a key demonstration towards the simplified integration of multiple photon sources and quantum circuits together on a monolithic chip, in turn, enabling quantum-information chips with much greater complexity and functionality.

The suitability of the spin dynamics of paramagnetic silicon dangling bonds ($E^{\prime }$ centers) in high-$E^{\prime }$-density amorphous silicon dioxide (${\mathrm{SiO}}_2$) as probe spins for single-spin tunneling force microscopy (SSTFM) is studied. SSTFM is a spin-selection-rule-based scanning-probe single-spin readout concept. Following the synthesis of ${\mathrm{SiO}}_2$ thin films on (111)-oriented crystalline-silicon substrates with room-temperature stable densities of $[E^{\prime }]>5\times {10}^{18}{\mathrm{cm}}^{-3}$ throughout the 60-nm thin film, pulsed electron paramagnetic resonance spectroscopy is conducted on the $E^{\prime }$ centers at temperatures between $T=5\mathrm{K}$ and $T=70\mathrm{K}$. The measurements reveal that the spin coherence (the transverse spin-relaxation time $T_2$) of these centers is significantly shortened compared to low-$E^{\prime }$-density ${\mathrm{SiO}}_2$ films and within error margins not dependent on temperature. In contrast, the spin-flip times (the longitudinal relaxation times $T_1$) are dependent on the temperature but with much weaker dependence than low-density ${\mathrm{SiO}}_2$, with the greatest deviations from low-density ${\mathrm{SiO}}_2$ seen for $T=5\mathrm{K}$. These results, discussed in the context of the spin-relaxation dynamics of dangling-bond states of other silicon-based disordered solids, indicate the suitability of $E^{\prime }$ centers in high-density ${\mathrm{SiO}}_2$ as probe spins for SSTFM.

We demonstrate fast readout of a double quantum dot (DQD) that is coupled to a superconducting resonator. Utilizing a parametric amplifier beyond its range of linear amplification, we improve the signal-to-noise ratio (SNR) by a factor of 2000 compared to the situation with the parametric amplifier turned off. With an integration time of 400 ns comparable to the inverse effective bandwidth, we achieve a SNR of 76. By measuring the SNR as a function of the integration time, we extract an equivalent charge sensitivity of $8\times {10}^{-5}e/\sqrt{\mathrm{Hz}}$. The high SNR allows us to acquire a DQD charge-stability diagram in just 20 ms. At such a high data rate, it is possible to acquire charge-stability diagrams in a live “video mode,” enabling real-time tuning of the DQD confinement potential.

We demonstrate a promising fiber architecture for generating strong photon-photon interactions. Exposed-core silica optical fibers possess low-loss guidance between 400 and 1700 nm crucial for quantum-logic applications. The potential of this fiber is demonstrated by exciting a two-photon transition within a rubidium vapor using an exposed-core silica optical fiber. Transit-time broadened spectral features enable measurement of the evanescent-field scale length of $(120\pm 20)\mathrm{nm}$ which shows excellent agreement with the characteristics of the modeled fiber mode $(118\pm 2)\mathrm{nm}$. We observe a two-photon absorption coefficient of $8.3{\mathrm{cm}}^{-1}$ for one optical mode in response to a transmitted power of 1.3 mW in the second mode. A clear pathway to an exposed-core fiber exhibiting substantial absorption mediated by a single photon is identified.

We show that uniaxial color centers in silicon carbide with hexagonal lattice structure can be used to measure not only the strength but also the polar angle of the external magnetic field with respect to the defect axis with high precision. The method is based on the optical detection of multiple spin resonances in the silicon vacancy defect with quadruplet ground state and suggests significant improvement of the angle sensitivity compared to spin-1 color centers. We demonstrate a good agreement between the experimental and calculated spin resonance spectra, providing angle resolution better than 1° per ${\mathrm{Hz}}^{1/2}$ in submillitesla magnetic fields. Our approach is suitable for ensemble as well as single spin-$3/2$ color centers, allowing for angle-resolved magnetometry on the nanoscale at ambient conditions.

We report on transport in the second Landau level in in situ back-gated two-dimensional electron gases in $\mathrm{GaAs}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ quantum wells. Minimization of gate leakage is the primary heterostructure design consideration. Leakage currents resulting in dissipation as small as $\sim 10\mathrm{pW}$ can cause noticeable heating of the electrons at 10 mK, limiting the formation of novel correlated states. We show that when the heterostructure design is properly optimized, gate voltages as large as 4 V can be applied with negligible gate leakage, allowing the density to be tuned over a large range from depletion to over $4\times {10}^{11}{\mathrm{cm}}^{-2}$. As a result, the strength of the $\nu =5/2$ state can be continuously tuned from onset at $n\sim 1.2\times {10}^{11}{\mathrm{cm}}^{-2}$ to a maximum ${\mathrm{\Delta }}_{5/2}=625\mathrm{mK}$ at $n=3.35\times {10}^{11}{\mathrm{cm}}^{-2}$. An unusual evolution of the reentrant-integer quantum Hall states as a function of density is also reported. These devices can be expected to be useful in experiments aimed at proving the existence of non-Abelian phases useful for topological quantum computation.

The generation of random numbers via quantum processes is an efficient and reliable method to obtain true indeterministic random numbers that are of vital importance to cryptographic communication and large-scale computer modeling. However, in realistic scenarios, the raw output of a quantum random-number generator is inevitably tainted by classical technical noise. The integrity of the device can be compromised if this noise is tampered with or even controlled by some malicious party. To safeguard against this, we propose and experimentally demonstrate an approach that produces side-information-independent randomness that is quantified by min-entropy conditioned on this classical noise. We present a method for maximizing the conditional min entropy of the number sequence generated from a given quantum-to-classical-noise ratio. The detected photocurrent in our experiment is shown to have a real-time random-number generation rate of $14(\mathrm{Mb}\mathrm{i}\mathrm{t}/\mathrm{s})/\mathrm{MHz}$. The spectral response of the detection system shows the potential to deliver more than $70\mathrm{Gbit}/\mathrm{s}$ of random numbers in our experimental setup.

A precise measurement of dephasing over a range of time scales is critical for improving quantum gates beyond the error correction threshold. We present a metrological tool based on randomized benchmarking capable of greatly increasing the precision of Ramsey and spin-echo sequences by the repeated but incoherent addition of phase noise. We find our superconducting-quantum-interference-device-based qubit is not limited by $1/f$ flux noise at short time scales but instead observe a telegraph noise mechanism that is not amenable to study with standard measurement techniques.

Detuning a superconducting qubit from its rotating frame is one means to implement a $Z$-gate operation. In this work, we implement a $Z$ gate by pulsing a current through the qubit’s readout dc SQUID. While the dc SQUID acts as a magnetic flux sensor for qubit readout, we in turn may use it as a flux actuator with tunable strength to impose a qubit frequency shift. Using this approach, we demonstrate Ramsey-type free-induction experiments with time constants as long as 280 ns and rotation frequencies as high as 1.4 GHz. We experimentally demonstrate an inferred $Z$-gate fidelity of approximately 90%, limited largely by the bandwidth of our system. In the absence of this limitation, we argue that the inferred fidelity may be improved to as high as 99%.

Solid-state qubits have recently advanced to the level that enables them, in principle, to be scaled up into fault-tolerant quantum computers. As these physical qubits continue to advance, meeting the challenge of realizing a quantum machine will also require the development of new supporting devices and control architectures with complexity far beyond the systems used in today’s few-qubit experiments. Here, we report a microarchitecture for controlling and reading out qubits during the execution of a quantum algorithm such as an error-correcting code. We demonstrate the basic principles of this architecture using a cryogenic switch matrix implemented via high-electron-mobility transistors and a new kind of semiconductor device based on gate-switchable capacitance. The switch matrix is used to route microwave waveforms to qubits under the control of a field-programmable gate array, also operating at cryogenic temperatures. Taken together, these results suggest a viable approach for controlling large-scale quantum systems using semiconductor technology.

Since the introduction of the decoy-state technique, phase-randomized weak coherent light pulses have been the key to increase the practicality of quantum-based communications. Their ultrafast generation was accomplished via compact gain-switched (GS) lasers, leading to high key rates in quantum key distribution (QKD). Recently, the question arose whether the same laser could be employed to achieve high-speed measurement-device-independent QKD, a scheme that promises long-haul quantum communications immune to all detector attacks. For that, a challenging high-visibility interference between independent picosecond optical pulses is required. Here, we answer the above question in the affirmative by demonstrating high-visibility interference from two independent GS lasers triggered at 1 GHz. The result is obtained through a careful characterization of the laser frequency chirp and time jitter. By relating these quantities to the interference visibility, we obtain a parameter-free verification of the experimental data and a numerical simulation of the achievable key rates. These findings are beneficial to other applications making use of GS lasers, including random number generation and standard QKD.

We describe the coherent manipulation of harmonic oscillator and qubit modes using resonant trains of single flux quantum pulses in place of microwaves. We show that coherent rotations are obtained for pulse-to-pulse spacing equal to the period of the oscillator. We consider a protocol for preparing bright and dark harmonic oscillator pointer states. Next, we analyze rotations of a two-state qubit system. We calculate gate errors due to timing jitter of the single flux quantum pulses and due to weak anharmonicity of the qubit. We show that gate fidelities in excess of 99.9% are achievable for sequence lengths of order 20 ns.

In this work, the on-chip quantum interference of two single surface plasmons is achieved using dielectric-loaded surface-plasmon-polariton waveguides. The high visibility (greater than 90%) proves the bosonic nature of single plasmons and emphasizes the feasibility of achieving basic quantum logic gates for linear optical quantum computation. The effect of intrinsic losses in plasmonic waveguides with regard to quantum-information processing is also discussed. Although the influence of this effect is negligible in the current experiment, our studies reveal that such losses can dramatically reduce quantum-interference visibility in certain cases; thus, quantum coherence must be carefully considered when designing quantum photonic integrated circuit devices.

We introduce an electrically driven scheme to tune the polariton condensate energy in a high-finesse GaAs microcavity. In contrast to the conventional redshift observed in semiconductor quantum wells (QWs) under applied electrical bias arising from the quantum-confined Stark effect (QCSE), we report here the blueshift of a polariton condensate caused by controlled reduction of the Rabi splitting due to tunneling-induced charge buildup and fractional bleaching of QWs. At larger electrical bias, the QCSE becomes dominant, leading to a redshift in the linear regime, while in the nonlinear regime to the eventual quenching of the condensate emission. This ability to tune the polariton condensate energy brings within reach the realization of voltage-controlled polariton condensate devices and variable-wavelength sources of coherent light.