Magnetoencephalography (MEG) is a noninvasive technique for investigating neuronal activity in the living human brain. The time resolution of the method is better than 1 ms and the spatial discrimination is, under favorable circumstances, 2-3 mm for sources in the cerebral cortex. In MEG studies, the weak 10 fT-1 pT magnetic fields produced by electric currents flowing in neurons are measured with multichannel SQUID (superconducting quantum interference device) gradiometers. The sites in the cerebral cortex that are activated by a stimulus can be found from the detected magnetic-field distribution, provided that appropriate assumptions about the source render the solution of the inverse problem unique. Many interesting properties of the working human brain can be studied, including spontaneous activity and signal processing following external stimuli. For clinical purposes, determination of the locations of epileptic foci is of interest. The authors begin with a general introduction and a short discussion of the neural basis of MEG. The mathematical theory of the method is then explained in detail, followed by a thorough description of MEG instrumentation, data analysis, and practical construction of multi-SQUID devices. Finally, several MEG experiments performed in the authors' laboratory are described, covering studies of evoked responses and of spontaneous activity in both healthy and diseased brains. Many MEG studies by other groups are discussed briefly as well.

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Magnetoencephalography—theory, instrumentation, and applications to noninvasive studies of the working human brain

Recent developments concerning the rapid single-flux-quantum (RSFQ) circuit family are reviewed. Elementary cells in this circuit family can generate, pass, memorize, and reproduce picosecond voltage pulses with a nominally quantized area corresponding to transfer of a single magnetic flux quantum across a Josephson junction. Functionally, each cell can be viewed as a combination of a logic gate and an output latch (register) controlled by clock pulses, which are physically similar to the signal pulses. Hand-shaking style of local exchange by the clock pulses enables one to increase complexity of the LSI RSFQ systems without loss of operating speed. The simplest components of the RSFQ circuitry have been experimentally tested at clock frequencies exceeding 100 GHz, and an increase of the speed beyond 300 GHz is expected as a result of using an up-to-date fabrication technology. This review includes a discussion of possible future developments and applications of this novel, ultrafast digital technology. >

RSFQ logic/memory family: a new Josephson-junction technology for sub-terahertz-clock-frequency digital systems

A 122-channel d.c. SQUID magnetometer with a helmet-shaped detector array covering the subject's head has been operational in the Low Temperature Laboratory of the Helsinki University of Technology since June 1992. The new system allows simultaneous recording of magnetic activity all over the head. The probe employs 122 planar first-order thin-film gradiometers in dual units with two exactly orthogonal channels at 61 measurement sites. The performance of the device is analyzed and compared with more conventional axial gradiometer arrays by considering signal-to-noise ratios, spatial sampling theory, confidence intervals for the estimated equivalent current dipole positions, and information-theoretical channel capacity. The signal-to-noise ratio and the resolution of the planar and axial arrays with the same number of channels are found practically equal. The number of channels and their spacing in our new Neuromag-122 system are found fully adequate for neuromagnetic measurements. An example of whole cortex recordings of auditory evoked brain activity is presented and analyzed.

https://iopscience.iop.org/article/10.1088/0031-8949/1993/T49A/033/pdf

122-channel squid instrument for investigating the magnetic signals from the human brain

An analytical formula is derived to calculate the inductance of a finite‐width superconducting strip transmission line. The formula gives an accurate inductance value when the aspect ratio (the ratio of the linewidth to the insulation thickness) exceeds about unity.

The inductance of a superconducting strip transmission line

We propose a model for 1/f flux noise in superconducting devices (f is frequency). The noise is generated by the magnetic moments of electrons in defect states which they occupy for a wide distribution of times before escaping. A trapped electron occupies one of the two Kramers-degenerate ground states, between which the transition rate is negligible at low temperature. As a result, the magnetic moment orientation is locked. Simulations of the noise produced by a plausible density of randomly oriented defects yield 1/f noise magnitudes in good agreement with experiments.

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Model for 1/f Flux noise in SQUIDs and Qubits.

A large number of highly robust and reliable thin film DC SQUIDs have been designed and fabricated which have excellent low frequency noise properties. Measurements performed on isolated devices have yielded a limit on the low frequency (1/f) flux noise component which is at least a factor of 60 below the average value reported for devices of this kind. The corresponding energy factor in the white noise region is 770 h at 0.1 Hz. The input coil inductance is 0.7 μH and the coupling efficiency α = 0.9. The substantial reduction of the low frequency noise in these SQUIDs demonstrates that improvements in the performance of these devices in the white noise region can be obtained without sacrificing the low frequency resolution.

Practical DC SQUIDs with extremely low 1/f noise

This letter describes the first system level test vehicle in Josephson technology. The experiment consists of four circuit chips assembled on two cards in a high density, 3-dimensional, card-on-board package. A data path, which is representative of a critical path of a future prototype processor, was successfully operated with a minimum cycle time of 3.7ns. The path simulates a jump control sequence and a cache access in each machine cycle. This experiment incorporates the essential components of the logic, power and package portions of a Josephson technology prototype.

A Josephson technology system level experiment

This paper describes the design, fabrication and characterization of damped three-junction interferometers suitable for LSI latching logic. The high speed and low power dissipation of Josephson interferometers make them ideal devices for logic applications. However, their inherent high Q gives rise to resonances which appear as current steps in the quasiparticle curve. If not controlled, these current steps can interfere with the proper operation of logic circuits. Studies have shown that these resonances can be effectively damped by shunting the interferometer inductance with a small resistance. In this work the interferometers were made by placing the interferometer loops parallel to the groundplane and clustering the Josephson junctions together. This minimized the loop inductance associated with the counterelectrode which in turn meant that adequate damping could be achieved with metallurgically compatible short resistors connected only to the base electrode. The resultant structure with two overlying control lines each 2.5μm wide had an area of 0.008mm2. The Josephson junctions were 5μm in diameter. The measured damped resonance characteristics and threshold characteristics (I g (V g ) and I m (I c ) respectively) of these structures agreed very well with theory.

Damped three-junction interferometers for latching logic

Experimental I-V curves of microwave-driven Josephson tunnel junctions with resistive shunts are reported. The results are in very good agreement with numerical calculations using the resistively shunted junction model. In the low-frequency regime there are three distinct regions in the I-V curves and two different types of Shapiro steps. It is demonstrated that the overall shape of the I-V curves can be explained by using an adiabatic interpretation of the junction response. The two different types of Shapiro steps are related to a compensation effect between dc and rf bias currents.

I-V characteristics of microwave-driven Josephson junctions in the low-frequency and high-damping regime

The cross‐sectional model (CSM) is an experiment designed to test the electrical and technological issues that are representative of a proposed Josephson Signal Processor. This paper describes the thin films aspects of the CSM. The existing Pb‐alloy technology for logic and memory circuits was developed further (for both chips and package parts) to meet the additional demands for the CSM. This included more extensive use of regulator junctions for on‐chip power distribution, more precise target value objectives for current levels on chip, and extension of the thin film technology to new package part structures, including additional thermal and mechanical processing. The vacuum depositions and materials, photolithography, package parts diagnostics, and chip diagnostic aspects of the CSM are described.

J. H. Greiner

Papers

Practical DC SQUIDs with extremely low 1/f noise

A Josephson technology system level experiment

Damped three-junction interferometers for latching logic

I-V characteristics of microwave-driven Josephson junctions in the low-frequency and high-damping regime

Thin film fabrication for the Josephson technology cross‐sectional model