WO2006043300A2

WO2006043300A2 - Superconductng magnetometer device, and related method of measuring

Info

Publication number: WO2006043300A2
Application number: PCT/IT2005/000606
Authority: WO
Inventors: Maria Gabriella Castellano; Fabio Chiarello; Guido Torrioli
Original assignee: Consiglio Nazionale Delle Ricerche
Priority date: 2004-10-20
Filing date: 2005-10-17
Publication date: 2006-04-27
Also published as: WO2006043300A3; ITRM20040515A1

Abstract

The invention concerns a method of measuring a magnetic flux ϕx, employing a superconducting magnetometer device (5) comprising a superconducting loop (2) apt to inductively couple to the magnetic flux ϕx to be measured , said loop being interrupted by at least a pair of Josephson junctions (3) having a hysteretic current-voltage characteristic, the method being characterised in that it comprises the following steps: biasing the superconducting loop (2) with a bias current Ib within a region of the current-voltage characteristic comprising at least one hysteresis cycle provided with a transition or switching, for a corresponding critical current Ic from a zero voltage state to a state of non zero voltage Vgap; and providing for a measure of the magnetic flux ϕx on the basis of an output pulsed voltage signal from the superconducting loop (2).

Description

SUPERCONDUCTING MAGNETOMETER DEVICE, AND RELATED METHOD OF MEASURING

The present invention concerns a superconducting magnetometer device, and the method of measuring using the same, that is simple, has small sized, extremely sensitive, and inexpensive, which allows in a precise, accurate, and efficient way, and with improved immunity to background noise, to measure a magnetic flux.

It is known that the most sensitive existing magnetometer is the superconducting quantum interference device or SQUID, which device has many applications in fields ranging from biomedical field to geomagnetism. Thanks to its features, such device is often also used as very low noise current amplifier.

As shown in Figure 1 , a SQUID 1 consists of a superconducting loop 2 interrupted by two Josephson junctions 3 (each one of which has a parasitic capacitance C), bias with a suitable continuous current I_b, typically of the order of tens or hundreds of microAmperes.

In the present applications of SQUIDs, junctions 3 are connected in parallel to resistors R, of the order of Ohms, also called shunt resistors, the Johnson noise of which may limit performance of the device 1 in some applications.

The operation of the device 1 is based on the shield currents generated in loop 2 owing to the application of a magnetic flux Φ, which currents do not decay since they are superconducting, and on the fact that the current-voltage characteristic of the junctions is not linear, but it presents a critical value I₀ for the current that may flow without a voltage appearing across it and that is periodically modulated by the applied magnetic flux Φ with a period given by the flux quantum Φ₀=h/2e=2.07 10^"15 Wb/m². In particular, in practical implementation, SQUID 1 is further provided with an input circuit comprising a coil 4, having inductance Li_nput, inductively coupled to the loop 2, whereby the flux Φ coupled to the loop is equal to that induced by the coil 4.

With reference to Figure 2a, it may be observed that the current-voltage characteristic of SQUID 1 of Figure 1 presents a critical value /_c for the current that may flow within the device without a voltage appearing across it: such value Ic depends on the applied magnetic flux Φ and ranges from a minimum value, obtained with bias flux equal to Φ_o/2 or half-integer multiples thereof, to a maximum value for bias flux equal to Φ_o or multiples thereof. Beyond these values, the characteristic becomes that of a non linear resistance.

With reference to Figure 2b, it may be observed the voltage-flux characteristic (wherein the flux Φ is applied to the loop 2 through the current flowing within the coupled coil 4) for three bias currents I_b-i, Ib2, lbs, with l_bi^<l_b2^<lb3: such characteristic has a periodicity of a flux quantum Φo, i.e. the output voltage V is a periodic function of the applied magnetic flux Φ, with period equal to a flux quantum Φo. Typical response capability is of the order of hundreds of microvolts per flux quantum Φo. Hence, a standard SQUID magnetometer behaves as an analog transducer that detects the magnetic flux transforming it in a voltage signal. In particular, present SQUIDs are usable provided that their current-voltage characteristic, as the one shown in Figure 2a, does not present hysteresis phenomena during transition from the supercurrent state (for l<l₀) and the non linear resistive state (for l>l₀). In order to make a non hysteretic SQUID, it is necessary to add a shunt resistor to the junction.

However, in certain applications, as those of quantum- computing, such shunt resistors connected in parallel to the Josephson junctions cause a dissipation that may be unacceptable.

Moreover, they cause a size increase of the final device, that make it sometimes not applicable in those applications where a great miniaturisation of the magnetometer is required.

Still, since the shunt resistors must have rather small values, this make the implementation of SQUID devices more complex, owing to the fact that rather strict requirements on the tolerances of manufacturing parameters have to be satisfied.

A further drawback not directly linked to the presence of the shunt resistors is due to the not very high noise immunity of the small signal output by the device.

Some solutions have been developed for overcoming the mentioned drawbacks, among which digital SQUIDs combine a high sensitivity with a high response rate and a virtually unlimited range of application. As described, for instance, by S.V. Rylov in "Analysis of high- performance counter-type A/D converters using RSFQ logic/memory elements", IEEE Trans, on Magn. 27, pp. 2431-2434, 1991 , and by K.K. Likharev and V.K. Semenov in "RSFQ logic/memory family: A new Josephson-junction technology for sub-terahertz-clock-frequency digital systems", IEEE Trans, on Appl. Supercond. 1, pp. 3-28, 1991 , digital SQUIDs are made by using Rapid Single Flux Quantum or RSFQ circuits. In particular, the output signal coming from the digital SQUID does or does not comprise a pulse depending on whether the applied bias current is higher or lower than the critical current. Such output signal is used for increasing or decreasing the count of a counter depending on the presence or the lack of such pulse, and for increasing or decreasing the feedback flux applied to the digital SQUID by a flux quantum.

However, even such digital SQUIDs entail some drawbacks, substantially due to the complexity of the additional feedback electronics.

It is therefore an object of the present invention to provide a measuring method that uses a superconducting magnetometer device which is simple, has small size, with substantially neglectable dissipation, extremely sensitive, and inexpensive, so that the process measures a magnetic flux in a precise, accurate, and efficient way, and with improved immunity to background noise.

It is specific subject matter of the present invention a method of measuring of measuring a magnetic flux Φ_x , employing a superconducting magnetometer device comprising a superconducting loop apt to inductively couple to the magnetic flux Φ_x to be measured, said loop being interrupted by at least a pair of Josephson junctions, having a hysteretic current-voltage characteristic, the method being characterised in that it comprises the following steps:

- biasing the superconducting loop with a bias current I_b within a region of the current-voltage characteristic comprising at least one hysteresis cycle provided with a transition or switching, for a corresponding critical current Ic, from a zero voltage state to a state of non zero voltage V_sαp; and

- providing for a measure of the magnetic flux Φ^ on the basis of an output pulsed voltage signal from the superconducting loop.

Preferably according to the invention, the bias current I_b is a periodic current ranging from a minimum value and a maximum value. Always according to the invention, said minimum value and/or said maximum value may be variable.

Still according to the invention, the bias current l_b may be periodic with an amplitude offset I_Δ.

Furthermore according to the invention, said amplitude offset I_Δ may be variable.

Always according to the invention, the bias current I_b may have a sawtooth waveform.

Still according to the invention, the bias current I_b may be sinusoidal.

Furthermore according to the invention, the bias current I_b may be a square wave. ' Always according to the invention, said region of the current- voltage characteristic within which the bias current I_b biases the superconducting loop may comprise only one hysteresis cycle.

Still according to the invention, in said region of the current- voltage characteristic within which the bias current I_b biases the superconducting loop, the voltage V of the superconducting loop may not exceed the value assumed in said state of non null voltage V_gap.

Furthermore according to the invention, said minimum value of the bias current I_b may be lower, in absolute value, than the critical current Ic corresponding to said transition. Always according to the invention, said minimum value of the bias current I_b may be equal to zero.

Still according to the invention, said maximum value of the bias current I_b may be higher, in absolute value, than the critical current Ic corresponding to said transition. Furthermore according to the invention, said maximum value of the bias current I_b may be substantially equal to the critical current Ic corresponding to said transition.

Always according to the invention, said measure of the magnetic flux Φ^ may be provided through a low-pass filtering of the output pulsed voltage signal from the superconducting loop.

Still according to the invention, said low-pass filtering may be carried out with a cut-off frequency not higher than 1/2, preferably not higher than 1/10, more preferably not higher than 1/20, still more preferably not higher than 1/100, of the frequency of the bias current I_b. Furthermore according to the invention, said measure of the magnetic flux Φ., may be provided through a time integration of the output pulsed voltage signal from the superconducting loop. 6

5

Always according to the invention, said time integration may be carried out over a range not shorter than the half period of the bias current Ib-

Still according to the invention, said time integration may be carried out over a range not shorter than the period of the bias current I_b.

Furthermore according to the invention, the superconducting loop of the superconducting magnetometer device may have a whole inductance equal to L that satisfies the following relationship:

2I₀L « Φ₀ /2π , where:

I₀ is the critical current of each one of the Josephson junctions,

Φ_o is the flux quantum equal to

Φ₀=h/2e=2.07 10^"15 WbZm², and the symbol "«" stands for "much less than". Always according to the invention, the whole inductance L of the superconducting loop may satisfy the following relationship:

2I₀L ≤ 0,\ - Φ₀ /2π , or, preferably, the following relationship:

2/₀I < 0,05 . Φ₀ /2^ , or, more preferably, the following relationship:

2/₀Z < 0,01. Φ₀ /2π .

Still according to the invention, the superconducting magnetometer device may further comprise coil means apt to inductively couple to the superconducting loop. Furthermore according to the invention, the superconducting magnetometer device may further comprise amplifier means for amplifying an output voltage signal from the superconducting loop.

Always according to the invention, the superconducting magnetometer device may further comprise low-pass filtering electronic means for filtering an output voltage signal from the superconducting loop.

Still according to the invention, the superconducting magnetometer device may further comprise integrator electronic means for integrating an output voltage signal from the superconducting loop.

Furthermore according to the invention, the superconducting magnetometer device may further comprise feedback coil means, towards which said integrator electronic means generates a current so as to stabilise the operating point of the device. The present invention will now be described, by way of illustration and not by way of limitation, according to its preferred embodiments, by particularly referring to the Figures of the enclosed drawings, in which: Figure 1 schematically shows a SQUID device according to the prior art;

Figure 2a shows the current-voltage characteristic of the SQUID of Figure 1;

Figure 2b shows the voltage-flux characteristic of the SQUID of Figure 1 for three bias currents;

Figure 3 schematically shows a superconducting magnetometer device used by a first embodiment of the method according to the invention;

Figure 4 shows the current-voltage characteristic of the device of Figure 3;

Figures 5a and 5b show, respectively, the behaviour of a bias current I_b and the behaviour of the corresponding voltage V sensed across the device of Figure 3;

Figure 6 schematically shows a superconducting magnetometer device used by a second embodiment of the method according to the invention;

Figure 7a shows the behaviour of a magnetic flux applied to the device of Figure 6;

Figure 7b shows the behaviours of a bias current I_b and of the critical current Ic corresponding to the flux of Figure 7a;

Figure 7c shows the behaviour of the voltage across the superconducting loop of the device of Figure 6; and

Figure 7d shows the behaviour of the time integration carried out by the device of Figure 6 of the voltage of Figure 7c. In the Figures, alike elements are indicated by same reference numbers.

The inventors have developed a measuring method that employs a superconducting magnetometer device usable as it were a SQUID switcher, with performance comparable to that of a standard SQUID magnetometer and some further advantage.

With reference to Figure 3, the device used by the preferred embodiment of the method according to the invention is substantially a SQUID 5 having a superconducting loop 2 interrupted by two Josephson junctions 3 (each one of which has a parasitic capacitance C) which, with respect to the standard SQUID of Figure 1, are devoid of shunt resistors. The whole inductance of the loop 2 is equal to L. The device 5 according to the invention may advantageously comprise an input circuit comprising a coil 4, having inductance Lj_nput, inductively coupled to the loop 2.

With reference to Figure 4, the current-voltage characteristic of the device 5 according to the invention is hysteretic, whereby biasing it with an increasing current Ib it is obtained a voltage V across it that is initially null along the superconducting branch a, up to reaching a critical value I₀, modulated by the applied magnetic flux Φ^ according to the relationship

/_c = 2/₀|cos(M⁾, /Φ₀)|

Beyond such threshold value Ic occurs a transition b to the state of voltage V_gap different from zero, equal to the gap of the superconductor

(that, for the used junctions, is equal to 2.7 mV). The arrows of Figure 4 show the direction of course of the hysteretic current-voltage characteristic of the device 5 according to the invention.

Advantageously, the device 5 is built so that the whole inductance L of the loop 2 satisfies the following relationship:

2I₀L « Φ₀ /2π , where Io is the critical current of each junction, and the symbol "«" stands for "much less than". In particular, 2I₀L is preferably not larger than 10%, more preferably not larger than 5%, still more preferably not larger than 1% of Φ_o /2π . The value of the whole inductance L of the loop 2 must be in any case sufficient to allow an appreciable coupling to the flux Φ_x to be measured (and also to the input coil 4).

The method of measuring according to the invention substantially comprises the encoding of the value of the measured magnetic flux Φ_x by monitoring the transition or switching (represented by the branch b of Figure 4) from the zero voltage state to the state of non zero voltage V.

With reference to Figure 5, it may be observed that the first embodiment of the method of measuring according to the invention provides the application of a bias periodic current Ib. For the sake of simplicity, the bias periodic current I_b of Figure 5a is a sawtooth wave starting from zero and arriving to exceed the critical current Ic, however it T/IT2005/000606

8 has to be understood that other embodiments of the method according to the present invention may comprise bias currents I_b having any periodic wave forms, ranging from a minimum value, equal to or lower than zero (that is such that it resets to zero the voltage across the squid, and preferably of module lower than the critical current /_c, so as to work only along the positive hysteretic cycle of the characteristic of Figure 4), to a maximum value, higher than the critical current Ic-

Figure 5b shows the curve of the voltage V detected across the device 5 of Figure 3 when the coupled magnetic flux changes. It may be observed that the device 5 remains in the zero voltage superconducting state (branch a of Figure 4) until the value of the bias current I_b remains lower than the critical value /_c, that, as shown in Figure 5a, changes depending on the externally applied magnetic flux Φ_x. Beyond such value Ic, there is a transition to the state of non zero voltage V. When the bias current I_b returns to zero (along the hysteretic branches c and d of Figure 4) the voltage is reset to zero too. Consequently, a sequence of voltage pulses is obtained across the device of Figure 3. Preferably, the bias periodic current I_b is such that it makes the device 5 work only over one hysteresis cycle, either positive or negative, in order to have only either positive or negative, respectively, transitions and consequent voltage pulses.

In particular, by biasing the device so that it operates within the region of the current-voltage characteristic wherein the module of the voltage V does not substantially exceed the value V_gap of the superconductor gap (that is, in case of positive hysteresis cycle, within the non resistive operation region, i.e. within the region comprising branches a, b, c, d, and e of Figure 4, i.e. for 0 < V ≤ V_gap), it is further possible to obtain voltage pulses with constant amplitude and variable duration depending on the critical current, and hence on the input magnetic flux, but variable duration depending on the critical current Ic, and hence on the externally applied magnetic flux Φ_x. Consequently, as shown in Figure 5b, the output voltage signal of the device according to the invention is substantially a digital signal encoded according to a pulse width modulation or PWM dependent on the externally applied magnetic flux Φ_x. In other words, the first embodiment of the method of measuring according to the invention performs an analog-to-digital conversion according to a pulse duration modulation (PWM) scheme:, the input magnetic flux Φ_x, to be measured, is encoded into the duration of the output voltage pulses of the device 5.

With reference to Figure 6, such digital signal V may be converted back into an analog signal by using standard techniques, for example by filtering the voltage pulse sequence through a low-pass circuit 6 with cut-off frequency much less than the frequency of the periodic signal; in particular, the low-pass circuit 6 is advantageously preceded by an amplifier circuit 7. With a feedback technique is then possible to use the device according to the invention in a either closed loop or flux-locked loop configuration, so that the output voltage F_out generates, through an integrator circuit 8, a current flowing in a feedback coil 4', still inductively coupled to the loop 2, that creates a flux which adds to the flux to be measured, so as to stabilise the operating point and so increase the range of values of the flux to be measured for which there is a hysteretic operation region of the current-voltage characteristic of the device according to the invention.

A second embodiment of the method of measuring according to the invention uses the region of the characteristic of Figure 4 around the superconductor-non zero voltage state transition (i.e. around the branch a of Figure 4). Such transition does not occur for a well fixed value of the current, but it is randomly distributed in a zone below the critical value due to thermal effects, with jump probability as much high as the current is close to the critical value Ic. In this second embodiment of the method of measuring a periodic current biasing is used, with a signal I_b that for the sake of simplicity is assumed as having rectangular pulses from zero to a value close to the critical one Ic, but it may be of any other type. The probability to have a transition is linked to the proximity of the bias current I_b to the critical value /_c, depending on the applied magnetic flux Φ_x. At the output, voltage pulses (digital signal) may be hence obtained with frequency depending on the applied magnetic flux Φ_x. In other words, such second embodiment of the method of measuring according to the invention carries out a stochastic jump encoding of the flux to be measured. This different type of digital encoding may be converted to an analog signal always through a filtering and integrating process, by still using a feedback technique for increasing the linear operation region. In particular, such scheme entails a very high responsivity of the magnetometer device 5. T2005/000606

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The two described embodiments of the method of measuring may be combined with each other according to specific needs for optimising the measure, by appropriately designing the bias periodic signal

Ib. By way of example, with reference to Figure 7, a sinusoidal signal I_b, with a proper offset I_Δ and maximum value close to the critical one Ic, as shown in Figure 7b, may be used in the case when a very large bandwidth is required, allowing to reach high operating frequencies, even of the order of at least fractions of GHz. Other waveforms of I_b may equally arrive at similar frequencies.

The operating point may be fixed by modifying offset and signal amplitude. The operating mode is fixed by the maximum value l_b_m_ax of the bias signal: when the latter is higher than the critical one Ic (lb_max > Ic), the device 5 operates in PWM mode; when the maximum value l_b_max of the bias signal is close to the critical one /_c (l_b_max « Ic), the device 5 operates in stochastic jump encoding mode.

In particular, Figure 7a shows a magnetic flux Φ_x to be measured that is linearly increasing with time, Figure 7b shows the bias signal l_b applied to the device 5 and the value of critical current /_c variable with the flux Φ_x to be measured, while Figure 7c shows PWM code obtained as output from the device 5 (where it may be observed that the offset IA and the oscillation amplitude of the bias signal I_b are selected so as to make the device 5 work in the positive hysteresis cycle region). Figure 7d . shows the curve of the output voltage signal F_0Ut from a low- pass filter (as for instance circuit 6 of Figure 6) that filters the pulse sequence with a cut-off frequency much less than the periodic signal frequency. In the case when the stochastic jump encoding is also used in combination with the PWM pulses, some of the pulses of Figure 7c could be missing with probability proportional to the value of the magnetic flux Φ_x to be measured. It should be noticed in Figure 7 the effect of the periodicity of the current-voltage characteristic of the device 5 with the value of the applied magnetic flux Φ_x (with periodicity equal to the flux quantum Φ_o).

Advantageously, the device 5 according to the invention may be made through micromachining techniques, by using the Niobium, Aluminium Oxide, and Niobium (Nb/AIOx/Nb) trilayer technology. Other micromachining technologies, such as the so called aluminium based "Shadow Evaporation", may be also used.

The obtainable results in characterisation of the magnetic behaviour of other devices, for example the applied flux ~ flux response characteristic of a rf-SQUID, are very good, not less than those of standard SQUID magnetometers.

The advantages obtainable with the device according to the invention with respect to a standard SQUID magnetometer are significant. First of all, the elimination of the shunt resistors allows to simplify the manufacturing process, and entails great advantages in cases where the resistor is an unacceptable dissipative source, such as in quantum computing applications.

Moreover, the high responsivity of the device used by the method according to the invention is of the order of mV/Φ_Q , allowing the use of a direct coupling reading electronics, hence very simple, reliable, and inexpensive.

Furthermore, the size of the device used by the method according to the invention is very reduced, since the required inductance must be small, of the order of few picoHenrys.

Still, the fact that the output signal is of digital nature makes the method according to the invention more insensitive to background noise.

Finally, the used device requires an additional electronics extremely simplex than that of the conventional digital SQUID.

Therefore, the method according to the invention makes a superconductor magnetometer simple, compact, and very sensitive, that in some applications has performance higher than those of a standard SQUID magnetometer, allowing a magnetic flux to be measured in a precise, accurate, and efficient way, and with improved immunity to background noise.

The preferred embodiments have been above described and some modifications of this invention have been suggested, but it should be understood that those skilled in the art can make other variations and changes, without so departing from the related scope of protection, as defined by the following claims.

Claims

612 CLAIMS

1. Method of measuring a magnetic flux Φ_x , employing a superconducting magnetometer device (5) comprising a superconducting loop (2) apt to inductively couple to the magnetic flux Φ_x to be measured, said loop being interrupted by at least a pair of Josephson junctions (3), having a hysteretic current-voltage characteristic, the method being characterised in that it comprises the following steps:

- biasing the superconducting loop (2) with a bias current I_b within a region of the current-voltage characteristic comprising at least one hysteresis cycle provided with a transition or switching, for a corresponding critical current Ic, from a zero voltage state to a state of non zero voltage V_gap\ and

- providing for a measure of the magnetic flux Φ^ on the basis of an output pulsed voltage signal from the superconducting loop (2).

2. Method according to claim 1 , characterised in that the bias current I_b is a periodic current ranging from a minimum value and a maximum value.

3. Method according to claim 2, characterised in that said minimum value and/or said maximum value are variable.

4. Method according to claim 2 or 3, characterised in that the bias current I_b is periodic with an amplitude offset IΛ.

5. Method according to claim 4, characterised in that said amplitude offset I_Δ is variable.

6. Method according to any one of claims 2 to 5, characterised in that the bias current I_b has a sawtooth waveform.

7. Method according to any one of claims 2 to 5, characterised in that the bias current I_b is sinusoidal.

8. Method according to any one of claims 2 to 5, characterised in that the bias current I_b is a square wave.

9. Method according to any one of the preceding claims, characterised in that said region of the current-voltage characteristic within which the bias current l_b biases the superconducting loop (2) comprises only one hysteresis cycle.

10. Method according to claim 9, characterised in that, in said region of the current-voltage characteristic within which the bias current I_b biases the superconducting loop (2), the voltage V of the superconducting loop (2) does not exceed the value assumed in said state of non null voltage V_sap.

11. Method according to claim 9 or 10, when depending on claim 2, characterised in that said minimum value of the bias current l_b is lower, in absolute value, than the critical current Ic corresponding to said transition.

12. Method according to claim 11 , characterised in that said minimum value of the bias current I_b is equal to zero.

13. Method according to claim 11 or 12, characterised in that said maximum value of the bias current I_b is higher, in absolute value, than the critical current Ic corresponding to said transition.

14. Method according to claim 11 or 12, characterised in that said maximum value of the bias current I_b is substantially equal to the critical current /c corresponding to said transition.

15. Method according to any one of the preceding claims, characterised in that said measure of the magnetic flux Φ_x is provided through a low-pass filtering of the output pulsed voltage signal from the superconducting loop (2).

16. Method according to claim 15, when depending on claim 2, characterised in that said low-pass filtering is carried out with a cut-off frequency not higher than 1/2 of the frequency of the bias current I_b.

17. Method according to claim 16, characterised in that said cut-off frequency is not higher than 1/10 of the frequency of the bias current Ib.

18. Method according to claim 17, characterised in that said cut-off frequency is not higher than 1/20 of the frequency of the bias current I_b.

19. Method according to claim 18, characterised in that said cut-off frequency is not higher than 1/100 of the frequency of the bias current l_b.

20. Method according to any one of the preceding claims, characterised in that said measure of the magnetic flux Φ_x is provided through a time integration of the output pulsed voltage signal from the superconducting loop (2).

21. Method according to claim 20, when depending on claim 2, characterised in that said time integration is carried out over a range not shorter than the half period of the bias current Ib-

22. Method according to claim 21, characterised in that said time integration is carried out over a range not shorter than the period of the bias current I_b.

23. Method according to any one of the preceding claims, characterised in that the superconducting loop (2) of the superconducting magnetometer device (5) has a whole inductance equal to L that satisfies the following relationship:

2I₀L « Φ₀ /2π , where: I₀ is the critical current of each one of the Josephson junctions (3), Φ_o is the flux quantum equal to

Φ₀=h/2e=2.07 KT¹⁵ Wb/m², and the symbol "«" stands for "much less than".

24. Method according to claim 23, characterised in that the whole inductance L of the superconducting loop (2) satisfies the following relationship:

2I₀L ≤ 0,l - Φ₀ /2π .

25. Method according to claim 24, characterised in that the whole inductance L of the superconducting loop (2) satisfies the following relationship:

2I₀L ≤ 0,05 - Φ₀ /2π .

26. Method according to claim 25, characterised in that the whole inductance^' L of the superconducting loop (2) satisfies the following relationship: 2J₀I ≤ 0,01 - Φ₀ /2ff .

27. Method according to any one of the preceding claims, characterised in that the superconducting magnetometer device (5) further comprises coil means (4) apt to inductively couple to the superconducting loop (2).

28. Method according to any one of the preceding claims, characterised in that the superconducting magnetometer device (5) further comprises amplifier means (7) for amplifying an output voltage signal from the superconducting loop (2).

29. Method according to any one of the preceding claims, characterised in that the superconducting magnetometer device (5) further comprises low-pass filtering electronic means (6) for filtering an output voltage signal from the superconducting loop (2). T2005/000606

15

30. Method according to any one of the preceding claims, characterised in that the superconducting magnetometer device (5) further comprises integrator electronic means (8) for integrating an output voltage signal from the superconducting loop (2).

31. Method according to claim 30, characterised in that the superconducting magnetometer device (5) further comprises feedback coil means (4'), towards which said integrator electronic means (8) generates a current so as to stabilise the operating point of the device (5).