Fabrication of Specimens for Atom Probe Tomography Using a Combined Gallium and Neon Focused Ion Beam Milling Approach.

We demonstrate a new focused ion beam sample preparation method for atom probe tomography. The key aspect of the new method is that we use a neon ion beam for the final tip-shaping after conventional annulus milling using gallium ions. This dual-ion approach combines the benefits of the faster milling capability of the higher current gallium ion beam with the chemically inert and higher precision milling capability of the noble gas neon ion beam. Using a titanium-aluminum alloy and a layered aluminum/aluminum-oxide tunnel junction sample as test cases, we show that atom probe tips prepared using the combined gallium and neon ion approach are free from the gallium contamination that typically frustrates composition analysis of these materials due to implantation, diffusion, and embrittlement effects. We propose that by using a focused ion beam from a noble gas species, such as the neon ions demonstrated here, atom probe tomography can be more reliably performed on a larger range of materials than is currently possible using conventional techniques.

Electromagnetically Induced Transparency in Circuit Quantum Electrodynamics with Nested Polariton States.

Quantum networks will enable extraordinary capabilities for communicating and processing quantum information. These networks require a reliable means of storage, retrieval, and manipulation of quantum states at the network nodes. A node receives one or more coherent inputs and sends a conditional output to the next cascaded node in the network through a quantum channel. Here, we demonstrate this basic functionality by using the quantum interference mechanism of electromagnetically induced transparency in a transmon qubit coupled to a superconducting resonator. First, we apply a microwave bias, i.e., drive, to the qubit-cavity system to prepare a Λ-type three-level system of polariton states. Second, we input two interchangeable microwave signals, i.e., a probe tone and a control tone, and observe that transmission of the probe tone is conditional upon the presence of the control tone that switches the state of the device with up to 99.73% transmission extinction. Importantly, our electromagnetically induced transparency scheme uses all dipole allowed transitions. We infer high dark state preparation fidelities of >99.39% and negative group velocities of up to -0.52±0.09 km/s based on our data.

Characterizing cryogenic amplifiers with a matched temperature-variable noise source.

We present a cryogenic microwave noise source with a characteristic impedance of 50 Ω, which can be installed in a coaxial line of a cryostat. The bath temperature of the noise source is continuously variable between 0.1 K and 5 K without causing significant back-action heating on the sample space. As a proof-of-concept experiment, we perform Y-factor measurements of an amplifier cascade that includes a traveling wave parametric amplifier and a commercial high electron mobility transistor amplifier. We observe system noise temperatures as low as 680-200 +20 mK at 5.7 GHz corresponding to 1.5-0.7 +0.1 excess photons. The system we present has immediate applications in the validation of solid-state qubit readout lines.

Cocaine activates Homer1 immediate early gene transcription in the mesocorticolimbic circuit: differential regulation by dopamine and glutamate signaling.

Homer proteins are intracellular scaffolding proteins that, among glutamate receptors, selectively bind to group1 metabotropic glutamate receptors and regulate their trafficking and intracellular signaling. Homer proteins have been implicated in synaptic and behavioral plasticity, including drug-seeking behavior after cocaine treatment. Homer1 gene activation leads to transcription of a variant mRNA (Homer1a), which functions as an immediate early gene. Homer1a competes with the constitutive Homer proteins (Homer1b/c/d, Homer2a/b, Homer3) for binding to group1 metabotropic glutamate and IP3 receptors. Binding of Homer1a to these proteins disrupts their association with the intracellular signaling scaffold and modulates receptor function. In this study, using RT-PCR, activation of Homer1a mRNA transcription in response to acute and repeated administration of cocaine was characterized in prefrontal cortex, nucleus accumbens, and ventral tegmental area, three mesocorticolimbic nuclei of the rat brain. Moreover, the dopaminergic and glutamatergic regulation of Homer1 gene activation by cocaine was investigated. Acute cocaine rapidly and transiently activated transcription of Homer1a mRNA in all three nuclei. However, repeated administration of cocaine was not effective in inducing the Homer1a mRNA transcription after various withdrawal times ranging from 2 h to 3 weeks. The acute cocaine-mediated activation of Homer1 gene was regulated by D1 but not D2 dopamine receptors. The blockade of AMPA or NMDA glutamate receptors did not prevent cocaine-mediated activation of Homer1 gene in the three mesocorticolimbic nuclei. These data indicate that acute administration of cocaine transiently activates Homer1 gene producing the immediate early gene Homer1a mRNA in the three mesocorticolimbic nuclei of the rat brain. Activation of Homer1 gene may contribute to the cocaine-mediated synaptic and behavioral plasticity.

Active protection of a superconducting qubit with an interferometric Josephson isolator.

Nonreciprocal microwave devices play critical roles in high-fidelity, quantum-nondemolition (QND) measurement schemes. They impose unidirectional routing of readout signals and protect the quantum systems from unwanted noise originated by the output chain. However, cryogenic circulators and isolators are disadvantageous in scalable superconducting architectures because they use magnetic materials and strong magnetic fields. Here, we realize an active isolator formed by coupling two nondegenerate Josephson mixers in an interferometric scheme and driving them with phase-shifted, same-frequency pumps. By incorporating our Josephson-based isolator into a superconducting qubit setup, we demonstrate fast, high-fidelity, QND measurements of the qubit while providing 20 dB of protection within a bandwidth of 10 MHz against amplified noise reflected off the Josephson amplifier in the output chain. A moderate reduction of 35% is observed in T2E when the Josephson-based isolator is turned on. Such a moderate degradation can be mitigated by minimizing heat dissipation in the pump lines.

Activator of G protein signaling 3: a gatekeeper of cocaine sensitization and drug seeking.

Chronic cocaine administration reduces G protein signaling efficacy. Here, we report that the expression of AGS3, which binds to GialphaGDP and inhibits GDP dissociation, was upregulated in the prefrontal cortex (PFC) during late withdrawal from repeated cocaine administration. Increased AGS3 was mimicked in the PFC of drug-naive rats by microinjecting a peptide containing the Gialpha binding domain (GPR) of AGS3 fused to the cell permeability domain of HIV-Tat. Infusion of Tat-GPR mimicked the phenotype of chronic cocaine-treated rats by manifesting sensitized locomotor behavior and drug seeking and by increasing glutamate transmission in nucleus accumbens. By preventing cocaine withdrawal-induced AGS3 expression with antisense oligonucleotides, signaling through Gialpha was normalized, and both cocaine-induced relapse to drug seeking and locomotor sensitization were prevented. When antisense oligonucleotide infusion was discontinued, drug seeking and sensitization were restored. It is proposed that AGS3 gates the expression of cocaine-induced plasticity by regulating G protein signaling in the PFC.

Neuroadaptations in cystine-glutamate exchange underlie cocaine relapse.

Repeated cocaine treatment and withdrawal produces changes in brain function thought to be involved in relapse to drug use. Withdrawal from repeated cocaine reduced in vivo extracellular glutamate in the nucleus accumbens of rats by decreasing the exchange of extracellular cystine for intracellular glutamate. In vivo restoration of cystine/glutamate exchange by intracranial perfusion of cystine or systemically administered N-acetylcysteine normalized the levels of glutamate in cocaine-treated subjects. To determine if the reduction in nonvesicular glutamate release is a mediator of relapse, we examined cocaine-primed reinstatement of drug seeking after cocaine self-administration was stopped. Reinstatement was prevented by stimulating cystine/glutamate exchange with N-acetylcysteine and restoring extracellular glutamate. Thus, withdrawal from repeated cocaine increases susceptibility to relapse in part by reducing cystine/glutamate exchange, and restoring exchanger activity prevents cocaine-primed drug seeking.

Observation of microwave absorption and emission from incoherent electron tunneling through a normal-metal-insulator-superconductor junction.

We experimentally study nanoscale normal-metal-insulator-superconductor junctions coupled to a superconducting microwave resonator. We observe that bias-voltage-controllable single-electron tunneling through the junctions gives rise to a direct conversion between the electrostatic energy and that of microwave photons. The measured power spectral density of the microwave radiation emitted by the resonator exceeds at high bias voltages that of an equivalent single-mode radiation source at 2.5 K although the phonon and electron reservoirs are at subkelvin temperatures. Measurements of the generated power quantitatively agree with a theoretical model in a wide range of bias voltages. Thus, we have developed a microwave source which is compatible with low-temperature electronics and offers convenient in-situ electrical control of the incoherent photon emission rate with a predetermined frequency, without relying on intrinsic voltage fluctuations of heated normal-metal components or suffering from unwanted losses in room temperature cables. Importantly, our observation of negative generated power at relatively low bias voltages provides a novel type of verification of the working principles of the recently discovered quantum-circuit refrigerator.

Quantum-circuit refrigerator.

Quantum technology promises revolutionizing applications in information processing, communications, sensing and modelling. However, efficient on-demand cooling of the functional quantum degrees of freedom remains challenging in many solid-state implementations, such as superconducting circuits. Here we demonstrate direct cooling of a superconducting resonator mode using voltage-controllable electron tunnelling in a nanoscale refrigerator. This result is revealed by a decreased electron temperature at a resonator-coupled probe resistor, even for an elevated electron temperature at the refrigerator. Our conclusions are verified by control experiments and by a good quantitative agreement between theory and experimental observations at various operation voltages and bath temperatures. In the future, we aim to remove spurious dissipation introduced by our refrigerator and to decrease the operational temperature. Such an ideal quantum-circuit refrigerator has potential applications in the initialization of quantum electric devices. In the superconducting quantum computer, for example, fast and accurate reset of the quantum memory is needed.

Flux-tunable phase shifter for microwaves.

We introduce a magnetic-flux-tunable phase shifter for propagating microwave photons, based on three equidistant superconducting quantum interference devices (SQUIDs) on a transmission line. We experimentally implement the phase shifter and demonstrate that it produces a broad range of phase shifts and full transmission within the experimental uncertainty. Together with previously demonstrated beam splitters, this phase shifter can be utilized to implement arbitrary single-qubit gates for qubits based on propagating microwave photons. These results complement previous demonstrations of on-demand single-photon sources and detectors, and hence assist in the pursuit of an all-microwave quantum computer based on propagating photons.

Quantum-limited heat conduction over macroscopic distances.

The emerging quantum technological apparatuses1, 2, such as the quantum computer3-6, call for extreme performance in thermal engineering7. Cold distant heat sinks are needed for the quantized electric degrees of freedom due to the increasing packaging density and heat dissipation. Importantly, quantum mechanics sets a fundamental upper limit for the flow of information and heat, which is quantified by the quantum of thermal conductance8-10. However, the short distance between the heat-exchanging bodies in the previous experiments11-14 hinders their applicability in quantum technology. Here, we present experimental observations of quantum-limited heat conduction over macroscopic distances extending to a metre. We achieved this improvement of four orders of magnitude in the distance by utilizing microwave photons travelling in superconducting transmission lines. Thus, it seems that quantum-limited heat conduction has no fundamental distance cutoff. This work establishes the integration of normal-metal components into the framework of circuit quantum electrodynamics15-17 which provides a basis for the superconducting quantum computer18-21. Especially, our results facilitate remote cooling of nanoelectronic devices using far-away in-situ-tunable heat sinks22, 23. Furthermore, quantum-limited heat conduction is important in contemporary thermodynamics24, 25. Here, the long distance may lead to ultimately efficient mesoscopic heat engines with promising practical applications26.

GABA transmission in the nucleus accumbens is altered after withdrawal from repeated cocaine.

Repeated cocaine causes enduring changes in dopamine and glutamate transmission in the nucleus accumbens, and dopamine and glutamate terminals synapse on GABAergic accumbens neurons. The present study demonstrates that there are changes in GABA transmission in the accumbens at 3 weeks after discontinuing daily cocaine injections. No-net flux microdialysis revealed a significant increase in the basal levels of extracellular GABA in the accumbens of cocaine-treated rats. The elevated extracellular GABA was normalized by blocking voltage-dependent Na+ channels and provided increased tone on GABA(B) presynaptic autoreceptors and heteroreceptors because blocking GABA(B) receptors produced a greater elevation in extracellular GABA, dopamine, and glutamate in cocaine-treated compared with control subjects. For many G-protein-coupled receptors, increased agonist can cause receptor desensitization. Consistent with GABA(B) receptor desensitization, baclofen-stimulated GTPgammaS binding was reduced, and the reduction in G-protein coupling was accompanied by reduced Ser phosphorylation of the GABA(B2) receptor subunit. No effect by repeated cocaine was found in the levels of total GABA(B1) or GABA(B2) protein. Together, these data demonstrate that withdrawal from repeated cocaine treatment produces an increase in the basal levels of extracellular GABA in the accumbens that depends on neuronal activity. The increase may be mediated in part by functional desensitization of GABA(B) receptors, likely the result of diminished Ser phosphorylation of the GABA(B2) receptor.

Homer1 proteins and AMPA receptors modulate cocaine-induced behavioural plasticity.

Homer proteins form functional assemblies in the excitatory postsynaptic density, and withdrawal from repeated cocaine administration reduces the expression of Homer1b/c in the nucleus accumbens. To determine if the reduction in Homer1b/c may be contributing to cocaine-induced behavioural sensitization, antisense oligonucleotides were infused over two weeks into the nucleus accumbens of rats to reduce Homer1 gene expression by approximately 35%. Infusion of antisense sequences (AS1 and AS2) caused a sensitization-like augmentation in the motor response to acute cocaine administration in naive rats. One of the sequences (AS1) also prevented the development of sensitization to repeated cocaine treatment, while AS2 was without effect. A panel of immunoblots for other proteins in the excitatory postsynaptic density revealed that AS1, but not AS2 reduced the level of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit GluR1 protein. This posed the possibility that altered AMPA signalling may mediate the inhibitory effect of AS1 on the development of sensitization. To examine this possibility, rats were pretreated in the accumbens with drugs to block AMPA/kainate, N-methyl-d-aspartate, group 1 metabotropic glutamate or dopamine receptors prior to each daily injection of cocaine. Only AMPA/kainate receptor blockade prevented the development of behavioural sensitization to cocaine. These data indicate that the expression of behavioural sensitization arises in part from a reduction in Homer1 gene products in the accumbens, while the development of sensitization requires stimulation of AMPA/kainate receptors.