In this work, we theoretically demonstrate that the ultimate detectivities in multi-stage interband cascade infrared photodetector configurations are higher than what can be achieved in a conventional single-absorber detector structure in any circumstance even including where carrier diffusion length is very long. Detailed derivations are provided to analytically show that in the limit of an infinite diffusion length, the ultimate detectivity of a multiple-stage device is about 11% higher than that of an optimized single-stage device. Furthermore, how the sequence of absorber thicknesses is optimized for maximizing the detectivity is described and discussed. Insights and explanations to the results are presented to improve the understanding, clarify possible confusion with different perspectives in the technical community, and to generate more interest in the development of advanced multi-stage interband cascade infrared photodetectors for various applications.

We demonstrate, for the first time, that a quantum flux parametron (QFP) is capable of acting as both isolator and amplifier in the readout circuit of a capacitively shunted flux qubit (CSFQ). By treating the QFP like a tunable coupler and biasing it such that the coupling is off, we show that $T_1$ of the CSFQ is not impacted by Purcell loss from its low-Q readout resonator ($Q_e = 760$) despite being detuned by only $40$ MHz. When annealed, the QFP amplifies the qubit's persistent current signal such that it generates a flux qubit-state-dependent frequency shift of $85$ MHz in the readout resonator, which is over $9$ times its linewidth. The device is shown to read out a flux qubit in the persistent current basis with fidelities surpassing $98.6\%$ with only $80$ ns integration, and reaches fidelities of $99.6\%$ when integrated for $1$ $\mu$s. This combination of speed and isolation is critical to the readout of high-coherence quantum annealers.

We demonstrate experimentally the operation of a double-balanced mixer, a phase shifter, and an I/Q modulator, built with Josephson junction active elements in a superconducting integrated ciruit. The devices operate at cryogenic temperatures with no power dissipation on chip, making them suitable for qubit control.

In the quest to reboot computing, quantum annealing (QA) is an interesting candidate for a new capability. While it has not demonstrated an advantage over classical computing on a real-world application, many important regions of the QA design space have yet to be explored. In IARPA's Quantum Enhanced Optimization (QEO) program, we have opened some new lines of inquiry to get to the heart of QA, and are designing testbed superconducting circuits and conducting key experiments. In this paper, we discuss recent experimental progress related to one of the key design dimensions: qubit coherence. Using MIT Lincoln Laboratory's qubit fabrication process and extending recent progress in flux qubits, we are implementing and measuring QA-capable flux qubits. Achieving high coherence in a QA context presents significant new engineering challenges. We report on techniques and preliminary measurement results addressing two of the challenges: crosstalk calibration and qubit readout. This groundwork enables exploration of other promising features and provides a path to understanding the physics and the viability of quantum annealing as a computing resource.

We demonstrate Josephson junction based double-balanced mixer and phase shifter circuits operating at 6–10 GHz and integrate these components to implement both a monolithic amplitude/phase vector modulator and an I/Q quadrature mixer. The devices are actuated by flux signals, dissipate no power on chip, exhibit input saturation powers in excess of 1 nW, and provide cryogenic microwave modulation solutions for integrated control of superconducting qubits.

We report our recent studies of interband cascade infrared photodetectors (ICIPs) with 100% cutoff wavelengths of 16.0μm and 9.2μm at 78K. The very-long-wavelength infrared (VLWIR) detectors were able to operate at temperatures up to 143K. Relatively high dark current densities were observed from these VLWIR detectors. This was attributed to the very narrow bandgap and possibly to defects in the materials. In addition, the photo-response of these detectors was strongly dependent on bias, indicating that further research and device optimization are necessary. Two-stage ICIPs in the long-wavelength infrared (LWIR) spectrum (8–12μm) were able to operate at higher temperatures (up to 250K) with an extended cutoff wavelength of ∼12μm. At 78K, these LWIR detectors had a bias-independent photocurrent, with an R0A of 115Ωcm2. This corresponded to a Johnson-noise-limited D∗ of 3.7×1010cmHz1/2/W at 8.0μm.

Interband cascade (IC) photovoltaic (PV) device structures, consisting of multiple discrete InAs/GaSb superlattice absorbers sandwiched between electron and hole barriers, were grown by molecular beam epitaxy. Details of the molecular beam epitaxy growth and material characterization of the structures are presented. The discrete absorber architecture enables certain advantages, such as high open-circuit voltage, high collection efficiency, high operating temperature, and smooth integration of cascade stages with different bandgaps. The two- and three-stage ICPV devices presented in this article operate at room temperature with substantial open-circuit voltages at a cutoff wavelength of 5.3 μm (corresponding to a bandgap of 0.23 eV), the longest ever reported for room temperature PV devices. The device characteristics indicate a high level of current matching and demonstrate the advantages of the interband cascade approach in thermophotovoltaic cell design.

Multi-stage Interband Cascade Photovoltaic (ICPV) devices with a room-temperature cutoff wavelength of ~3 μm were investigated. These devices were characterized at various temperatures under illumination from both a blackbody source and an interband cascade (IC) laser (emission wavelength of ~2.86 μm). Under laser illumination (8.0 W/cm 2 ) an open-circuit voltage (V oc ) of 447 mV with a short-circuit current density (J sc ) of 1.5 A/cm 2 was obtained from a three-stage device at room temperature. Studies of size dependence with different devices found significant surface leakage current, resulting in low fill factors in these devices. These results suggest a great potential of ICPV devices to attain higher V oc at lower J sc compared to conventional single-absorber thermophotovoltaic (TPV) cells.

Theoretical projections of the power enhancement of photovoltaic (PV) devices offered by the multiple-stage interband cascade (IC) architecture are presented. This work focuses on the improvement this architecture offers for the case of single-bandgap PV devices with negligible series resistance and shunting effects. It is shown that the power efficiency enhancement offered by a multiple-stage device, compared to a simple single-absorber device, is roughly equal to the improvement in the particle conversion efficiency (PCE). In conventional single-absorber PV devices, the PCE is fundamentally limited by the parameters of the absorber material, i.e. the absorption coefficient and the minority carrier diffusion length. The IC architecture circumvents the diffusion length limitation and should be useful for narrow-bandgap semiconductors that are used for thermophotovoltaic applications. The ability to adjust both the absorber thickness in each stage and the number of cascade stages should enable IC PV devices to achieve optimized power conversion efficiency by achieving current matching in each stage.

Quantum-engineered multiple stage photovoltaic (PV) devices are explored based on InAs/GaSb/AlSb interband cascade (IC) structures. These ICPV devices employ multiple discrete absorbers that are connected in series by wide-bandgap unipolar barriers using type-II heterostructure interfaces for facilitating carrier transport between cascade stages similar to IC lasers. The discrete architecture is beneficial for improving the collection efficiency and for spectral splitting by utilizing absorbers with different bandgaps. As such, the photo-voltages from each individual cascade stage in an ICPV device add together, creating a high overall open-circuit voltage, similar to conventional multi-junction tandem solar cells. Furthermore, photo-generated carriers can be collected with nearly 100% efficiency in each stage. This is because the carriers travel over only a single cascade stage, designed to be shorter than a typical diffusion length. The approach is of significant importance for operation at high temperatures where the diffusion length is reduced. Here, we will present our recent progress in the study of ICPV devices, which includes the demonstration of ICPV devices at room temperature and above with narrow bandgaps (e.g. 0.23 eV) and high open-circuit voltages.

A theoretical framework for studying signal and noise in multiple-stage interband infrared photovoltaic devices is presented. The theory flows from a general picture of electrons transitioning between thermalized reservoirs. Making the assumption of bulk-like absorbers, we show how the standard semiconductor transport and recombination equations can be extended to the case of multiple-stage devices. The electronic noise arising from thermal fluctuations in the transition rates between reservoirs is derived using the Shockley-Ramo and Wiener-Khinchin theorems. This provides a unified noise treatment accounting for both the Johnson and shot noise. Using a Green's function formalism, we derive consistent analytic expressions for the quantum efficiency and thermal noise in terms of the design parameters and macroscopic material properties of the absorber. The theory is then used to quantify the potential performance improvement from the use of multiple stages. We show that multiple-stage detectors can achieve higher sensitivities for applications requiring a fast temporal response. This is shown by deriving an expression for the optimal number of stages in terms of the absorption coefficient and absorber thicknesses for a multiple-stage detector with short absorbers. The multiple-stage architecture may also be useful for improving the sensitivity of high operating temperature detectors in situations where the quantum efficiency is limited by a short diffusion length. The potential sensitivity improvement offered by a multiple-stage architecture can be judged from the product of the absorption coefficient, α, and diffusion length, Ln, of the absorber material. For detector designs where the absorber lengths in each of the stages are equal, the multiple-stage architecture offers the potential for significant detectivity improvement when αLn ≤ 0.2. We also explore the potential of multiple-stage detectors with photocurrent-matched absorbers. In this architecture, the absorbers are designed to absorb and collect an equal number of carriers in each stage. It is shown that for zero-bias operation, this design has a higher ultimate detectivity than a single-absorber device. Such improvements in detectivity are significant for material with αLn ≤ 0.5. Using the results derived for general values of αLn, we offer an outlook for multiple-stage detectors that utilize InAs/GaSb superlattice absorbers.

Narrow-bandgap (<0.25 eV) photovoltaic (PV) devices are demonstrated at room temperature and above. These PV devices are based on interband cascade (IC) structures and can achieve a high open-circuit voltage (∼0.65 V at 300 K) that significantly exceeds the single bandgap limited value. This work demonstrates the capabilities and advantages of ICPV devices designed to effectively convert long wavelength (>5 μm) infrared photons from relatively low-temperature radiation sources (<1000 K) into electricity. Detailed characteristics of these PV devices are presented and discussed.

Interband cascade (IC) lasers take advantage of the broken band-gap alignment in type-II quantum wells to reuse injected electrons in cascade stages for photon generation with high quantum efficiency, while retaining interband transitions for photon emission without involving fast phonon scattering. As such, the threshold current density can be significantly lowered with high voltage efficiency, resulting in low power consumption. After about 18 years of exploration and development, IC lasers have now been proven to be capable of continuous wave operation at room temperature and above for a wide wavelength range of 2.9 to 5.7 μm in the mid-infrared spectral region. Here, we present our recent progress in InAs-based IC lasers, which use plasmon cladding layers to replace superlattice cladding layers, resulting in improved thermal dissipation and extended lasing wavelengths.