| 1. |
- Anghel, D. V., et al.
(author)
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Cold-Electron Bolometer as a 1-cm-Wavelength Photon Counter
- 2020
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In: Physical Review Applied. - 2331-7019. ; 13:2
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Journal article (peer-reviewed)abstract
- We investigate theoretically the possibility of using the cold-electron bolometer (CEB) as a counter for 1-cm-wavelength (30-GHz) photons. To reduce the flux of photons from the environment that interact with the detector, the bath temperature is assumed to be below 50 mK. At such temperatures, the time interval between two subsequent photons of 30 GHz that hit the detector is more than 100 h, on average, for a frequency window of 1 MHz. Such temperatures allow the observation of the physically significant photons produced in rare events, such as axion conversion (or Primakoff conversion) in a magnetic field. We present the general formalism for the detector's response and noise, together with numerical calculations for proper experimental setups. We observe that the current-biased regime is favorable due to lower noise and allows for photon counting at least below 50 mK. For the experimental setups investigated here, the voltage-biased CEBs may also work as photon counters but with less accuracy and, eventually, may require smaller volumes of the normal-metal island.
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| 2. |
- Aurino, Pier Paolo, 1985, et al.
(author)
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Retention of Electronic Conductivity in LaAlO3/SrTiO3 Nanostructures Using a SrCuO2 Capping Layer
- 2016
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In: Physical Review Applied. - : American Physical Society. - 2331-7019. ; 6:2
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Journal article (peer-reviewed)abstract
- The interface between two wide band-gap insulators, LaAlO3 and SrTiO3 (LAO/STO) offers a unique playground to study the interplay and competitions between different ordering phenomena in a strongly correlated two- dimensional electron gas. Recent studies of the LAO/STO interface reveal the inhomogeneous nature of the 2DEG that strongly influences electrical-transport properties. Nanowires needed in future applications may be adversely affected, and our aim is, thus, to produce a more homogeneous electron gas. In this work, we demonstrate that nanostructures fabricated in the quasi-2DEG at the LaAlO3/SrTiO3 interface, capped with a SrCuO2 layer, retain their electrical resistivity and mobility independent of the structure size, ranging from 100 nm to 30 mu m. This is in contrast to noncapped LAO/STO structures, where the room-temperature electrical resistivity significantly increases when the structure size becomes smaller than 1 mu m. High-resolution intermodulation electrostatic force microscopy reveals an inhomogeneous surface potential with "puddles" of a characteristic size of 130 nm in the noncapped samples and a more uniform surface potential with a larger characteristic size of the puddles in the capped samples. In addition, capped structures show superconductivity below 200 mK and nonlinear currentvoltage characteristics with a clear critical current observed up to 700 mK. Our findings shed light on the complicated nature of the 2DEG at the LAO/STO interface and may also be used for the design of electronic devices.
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| 3. |
- Baghdadi, Reza, 1984, et al.
(author)
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Fabricating Nanogaps in YBa2Cu3O7-delta for Hybrid Proximity-Based Josephson Junctions
- 2015
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In: Physical Review Applied. - 2331-7019. ; 4:1
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Journal article (peer-reviewed)abstract
- The advances of nanotechnologies applied to high-critical-temperature superconductors (HTSs) have recently given a huge boost to the field, opening new prospectives for their integration in hybrid devices. The feasibility of this research goes through the realization of HTS nanogaps with superconductive properties close to the as-grown bulk material at the nanoscale. Here we present a fabrication approach allowing the realization of YBa2Cu3O7-delta (YBCO) nanogaps with dimensions as small as 35 nm. To assess the quality of the nanogaps, we measure, before and after an ozone treatment, the current-voltage characteristics and the resistance versus temperature of YBCO nanowires with various widths and lengths, fabricated by using different lithographic processes. The analysis of the superconducting transition with a thermally activated vortex-entry model allows us to determine the maximum damage the nanowires undergo during the patterning which relates to the upper bound for the dimension of the nanogap. We find that the effective width of the nanogap is of the order of 100 nm at the superconducting transition temperature while retaining the geometrical value of about 35 nm at lower temperatures. The feasibility of the nanogaps for hybrid Josephson devices is demonstrated by bridging them with thin Au films. We detect a Josephson coupling up to 85 K with an almost ideal magnetic-field response of the Josephson current. These results pave the way for the realization of complex hybrid devices, where tiny HTS nanogaps can be instrumental to study the Josephson effect through barriers such as topological insulators or graphene.
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| 4. |
- Bejanin, J. H., et al.
(author)
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Three-Dimensional Wiring for Extensible Quantum Computing: The Quantum Socket
- 2016
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In: Physical Review Applied. - 2331-7019. ; 6:4
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Journal article (peer-reviewed)abstract
- Quantum computing architectures are on the verge of scalability, a key requirement for the implementation of a universal quantum computer. The next stage in this quest is the realization of quantum error-correction codes, which will mitigate the impact of faulty quantum information on a quantum computer. Architectures with ten or more quantum bits (qubits) have been realized using trapped ions and superconducting circuits. While these implementations are potentially scalable, true scalability will require systems engineering to combine quantum and classical hardware. One technology demanding imminent efforts is the realization of a suitable wiring method for the control and the measurement of a large number of qubits. In this work, we introduce an interconnect solution for solid-state qubits: the quantum socket. The quantum socket fully exploits the third dimension to connect classical electronics to qubits with higher density and better performance than two-dimensional methods based on wire bonding. The quantum socket is based on spring-mounted microwires-the three-dimensional wires-that push directly on a microfabricated chip, making electrical contact. A small wire cross section (approximately 1 mm), nearly nonmagnetic components, and functionality at low temperatures make the quantum socket ideal for operating solid-state qubits. The wires have a coaxial geometry and operate over a frequency range from dc to 8 GHz, with a contact resistance of approximately 150 m Omega, an impedance mismatch of approximately 10 Omega, and minimal cross talk. As a proof of principle, we fabricate and use a quantum socket to measure high-quality superconducting resonators at a temperature of approximately 10 mK. Quantum error-correction codes such as the surface code will largely benefit from the quantum socket, which will make it possible to address qubits located on a two-dimensional lattice. The present implementation of the socket could be readily extended to accommodate a quantum processor with a (10 x 10)-qubit lattice, which would allow for the realization of a simple quantum memory.
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| 5. |
- Bengtsson, Andreas, 1991, et al.
(author)
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Improved Success Probability with Greater Circuit Depth for the Quantum Approximate Optimization Algorithm
- 2020
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In: Physical Review Applied. - 2331-7019. ; 14:3
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Journal article (peer-reviewed)abstract
- Present-day, noisy, small or intermediate-scale quantum processors-although far from fault tolerant-support the execution of heuristic quantum algorithms, which might enable a quantum advantage, for example, when applied to combinatorial optimization problems. On small-scale quantum processors, validations of such algorithms serve as important technology demonstrators. We implement the quantum approximate optimization algorithm on our hardware platform, consisting of two superconducting transmon qubits and one parametrically modulated coupler. We solve small instances of the NP (nondeterministic polynomial time)-complete exact-cover problem, with 96.6% success probability, by iterating the algorithm up to level two.
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| 6. |
- Bylander, Jonas, 1978
(author)
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Z-Gate Operation on a Superconducting Flux Qubit via its Readout SQUID
- 2015
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In: Physical Review Applied. - 2331-7019. ; 3:3, s. 034004-
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Journal article (peer-reviewed)abstract
- 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%.
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| 7. |
- Chang, C. W. Sandbo, et al.
(author)
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Generating Multimode Entangled Microwaves with a Superconducting Parametric Cavity
- 2018
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In: Physical Review Applied. - 2331-7019. ; 10:4
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Journal article (peer-reviewed)abstract
- We demonstrate the generation of multimode entangled states of propagating microwaves. The entangled states are generated by our parametrically pumping a multimode superconducting cavity. By combining different pump frequencies, applied simultaneously to the device, we can produce different entanglement structures in a programable fashion. The Gaussian output states are fully characterized by our measuring the full covariance matrices of the modes. The covariance matrices are absolutely calibrated by our using an in situ microwave calibration source, a shot-noise tunnel junction. Applying a variety of entanglement measures, we demonstrate both full inseparability and genuine tripartite entanglement of the states. Our method is easily extensible to more modes.
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| 8. |
- Chen, X, et al.
(author)
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Modulated Photoluminescence Mapping of Long-Wavelength Infrared InAs / GaSb Type-II Superlattice: In-Plane Optoelectronic Uniformity
- 2021
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In: Physical Review Applied. - 2331-7019. ; 15:4
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Journal article (peer-reviewed)abstract
- In-plane uniformity of narrow-gap semiconductor InAs/GaSb type-II superlattice (T2SL) wafer is a crucial yet hard-to-evaluate prerequisite for high-performance long-wavelength infrared optoelectronic device applications of, e.g., focal-plane-array (FPA) photodetectors. In this work, we report a modulated photoluminescence-mapping (PL-mapping) study of InAs/GaSb T2SL in long-wavelength infrared range with a spatial resolution of a typical FPA-pixel scale. Spatial distributions are analyzed of PL-peak energy, linewidth, and integral intensity, which indicate a high in-plane uniformity of effective band gap but a considerable fluctuation of radiative recombination. The in-plane distributions of effective carrier lifetime and Shockley-Read-Hall defect concentration are evaluated, with the aid of a model that takes into account the pumping power dependence of the PL integral intensity. The results reveal a considerable in-plane nonuniformity of the optoelectronic response that may restrict the performance of the derivative FPA photodetector, and indicate the modulated PL mapping of a good pathway particularly for uniformity analysis of long-wavelength infrared FPA semiconductors.
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| 9. |
- Dashti, Nastaran, 1988, et al.
(author)
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Probing charge- and heat-current noise by frequency-dependent fluctuations in temperature and potential
- 2018
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In: Physical Review Applied. - 2331-7019. ; 10:2
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Journal article (peer-reviewed)abstract
- The energetic properties of electron transport in mesoscopic and nanoscale conductors are of considerable interest at present. Here, we theoretically investigate the possibility of probing charge- and heat-current fluctuations as well as their mixed correlations via the temperature and electrochemical potential fluctuations of a probe coupled to the conductor. Our particular interest is devoted to the charge and energy noise stemming from time-dependently-driven nanoelectronic systems designed for the controlled emission of single electrons, even though our setup is appropriate for more general ac-driving schemes. We employ a Boltzmann-Langevin approach in order to relate the bare charge- and energy-current fluctuations emitted from the electron source to frequency-dependent electrochemical potential and temperature fluctuations, which the former induce in the probe. We apply our findings to the prominent example of an on-demand single-electron source, realized by a driven mesoscopic capacitor in the quantum Hall regime. We show that neither the background fluctuations of the probe in the absence of the working source, nor the fluctuations induced by the probe hinder the access to the sought-after direct source noise for a large range of parameters.
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| 10. |
- Gasparinetti, S., et al.
(author)
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Fast Electron Thermometry for Ultrasensitive Calorimetric Detection
- 2015
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In: Physical Review Applied. - 2331-7019. ; 3:1
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Journal article (peer-reviewed)abstract
- We demonstrate radio-frequency thermometry on a micrometer-sized metallic island below 100 mK. Our device is based on a normal-metal-insulator-superconductor tunnel junction coupled to a resonator with transmission readout. In the first generation of the device, we achieve 90 mu K/root Hz noise-equivalent temperature with 10 MHz bandwidth. We measure the thermal relaxation time of the electron gas in the island, which we find to be of the order of 100 mu s. Such a calorimetric detector, upon optimization, can be seamlessly integrated into superconducting circuits, with immediate applications in quantum-thermodynamics experiments down to single quanta of energy.
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