Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2001-11-16
2004-02-10
Le, N. (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C505S846000
Reexamination Certificate
active
06690162
ABSTRACT:
The invention relates to a device for high-resolution measurement, in particular for high-resolution absolute measurement of magnetic fields.
PRIOR ART
The measuring principle is based on the physical effect of macroscopic quantum interference as it occurs in closed circuits made from superconducting materials which are coupled to one another by Josephson tunnel junctions or, in general, what are termed weak links.
It is known to be possible simply to use closed superconducting circuits which contain Josephson junctions or weak links to measure very small magnetic field changes down to the range of fT (10
−15
tesla). In the case of devices corresponding to the prior art, what are termed “SQUIDs” (Superconducting Quantum Interference Devices), use is simply made of closed superconducting current loops which usually contain two Josephson junctions, but also more in individual applications. If these current loops are driven by a current which is below a critical current, no voltage drops across the junctions. In modern SQUIDs, the current loops are, however, driven by a temporally constant supercritical current, such that a temporally quickly changing AC voltage drops across the two superconducting electrodes on both sides of the junctions. The frequency of this AC voltage depends on the strength of the driving current I
0
and the strength of the magnetic flux &PHgr;=B
⊥
F which penetrates the loop, B
⊥
denoting the component, perpendicular to the surface F of the SQUID, of the vector magnetic field {right arrow over (B)}. Serving as easily accessible measured quantity is the DC voltage according to <V({right arrow over (B)};I
0
)>, dropping across the current loop, which is produced by time averaging of the quickly changing AC voltage over one or more periods. The calibration curve <V({right arrow over (B)};I
0
)> of such a typical two-DC junctions SQUID is sketched in FIG.
12
. Whenever the flux &PHgr; penetrating the loop corresponds to an integral multiple of the elementary flux quantum
Φ
0
=
h
2
e
≅
2
×
10
-
15
Tm
2
,
the calibration curve assumes a minimum, whereas it assumes a maximum for half-integer multiples of the elementary flux quantum &PHgr;
0
. The calibration curves of all previously known SQUID systems have such a periodicity. For a known area F of the current loop, the component, perpendicular to this area, of the magnetic induction {right arrow over (B)} can be determined up to an integral multiple of &PHgr;
0
, that is to say it is therefore possible in principle to measure only &PHgr;mod&PHgr;
0
. Because of the periodicity of the calibration curve <V({right arrow over (B)};I
0
)>, it is therefore impossible to use conventional SQUIDs for absolute quantitative precision measurement of the magnetic induction {right arrow over (B)}. At present, this requires the very complicated and expensive combination of this with other physical measurement methods such as, for example, connection to optically pumped magnetometers. The commercial fields of application of SQUIDs are therefore limited to the detection of spatial or temporal, relative field changes such as occur, for example, in the case of material testing or when investigating metabolic processes in biological organisms. However, it is necessary in the case of these applications as well for the order of magnitude of the field changes to be known from the start if the measurement is to permit more than purely qualitative statements or rough estimates.
It is the object of the invention to create a simple device which permits highly precise absolute measurement of, in particular, even time-variant magnetic fields, and in the process can have recourse in full measure to the cryotechnology developed for conventional SQUIDS.
The invention proceeds from a device for high-resolution measurement, in particular for high-resolution absolute measurement of magnetic, in particular time-variant, magnetic fields, which comprises a network of transitions between superconductors which exhibit Josephson effects, called junctions below, the network having closed meshes, denoted by cells below, which in each case have at least two junctions, which junctions are connected in a superconducting fashion, and at least three of these cells being electrically connected in a superconducting and/or nonsuperconducting fashion. The core of the invention resides in the fact that the junctions of the at least three cells can be energized in such a way that a time-variant voltage drops in each case across at least two junctions of a cell, the time average of which voltage does not vanish, and in that the at least three cells are configured differently geometrically in such a way that the magnetic fluxes enclosed by the cells in the case of an existing magnetic field differ from one another in such a way that the frequency spectrum of the voltage response function has no significant &PHgr;
0
-periodic component with reference to the magnetic flux, or in that, if a discrete frequency spectrum exists, the contribution of the &PHgr;
0
-periodic component of the discrete frequency spectrum is not dominant by comparison with the non-&PHgr;
0
-periodic component of the discrete frequency spectrum.
With regard to the periodicity of the voltage response function, it is also possible to select the following functional approach: that the at least three cells are configured differently geometrically in such a way that the magnetic fluxes enclosed by the cells in the case of an existing magnetic field form a ratio to one another in such a way that the period of the voltage response function of the network with reference to the magnetic flux penetrating the network cells in their entirety is greater or very much greater than the value of an elementary flux quantum and/or the voltage response no longer has a &PHgr;
0
-periodic component. The invention is based on the finding that in the ideal case the voltage response function no longer has a period when the magnetic fluxes enclosed by the cells are not in a rational ratio to one another. In addition, the differences in area between the individual cells are preferably relatively large. In particular, cells connected in a superconducting fashion are superimposed in such a way that the voltage response function no longer has a period.
Consequently, according to the invention different cells are connected to one another specifically, and this is something the person skilled in the art would always want to avoid with conventional SQUID arrangements. This is expressed, for example, in the publication by HANSEN, BINSLEV J., LINDELOF P. E.: Static and dynamic interactions between Josephson junctions. In: Reviews of Modern Physics, Vol. 56, No. 3, July 1984, p. 431 to 459. On page 434, left-hand column, last paragraph and subsequently in the right-hand column, this publication favors a system with identical cells and identical junctions and, by contrast, classifies asymmetries as counterproductive for the functioning of the SQUID described in this regard.
Devices according to the invention (denoted below as superconducting quantum interference filters or SQIFs), by contrast, exhibit the physical effect of multiple macroscopic quantum interference in such a way that the ambiguity of the calibration curves of conventional SQUID magnetometers and SQUID gradiometers is removed.
In a superconducting quantum interference filter, the quantum mechanical wave functions which describe the state of the superconducting solid interfere in such a way that a unique microscopic calibration curve <V({right arrow over (B)};I
0
)> is produced. In the ideal case, the calibration curve <V({right arrow over (B)};I
0
)> of the superconducting quantum interference filter has no periodicity with the period &PHgr;
0
and is a function, rising monotonically in a specific measuring range, of the absolute value of the external magnetic field {right arrow over (B)} at the location of the SQIF.
The uniqueness of the calibration curve, and the high sensi
Haeussler Christoph
Oppenlaender Joerg
Schopohl Nils
Hogan & Hartson LLP
Kinder Darrell
Qest Quantenelektronische Systeme
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