EP1379887A2

EP1379887A2 - METHOD FOR READING OUT MAGNETIC FIELDS WITH A i DC /i SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE ( i DC /i -SQUID) AND CORRESPONDING DEVICE

Info

Publication number: EP1379887A2
Application number: EP02740234A
Authority: EP
Inventors: Yi Zhang; Ralph Otto; Norbert Wolters
Original assignee: Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 2001-04-19
Filing date: 2002-04-06
Publication date: 2004-01-14
Also published as: WO2002086524A3; WO2002086524A2; DE10119108A1

Abstract

ID (A) with a parallel-connected coupling transformer (B) comprises a step, wherein a non-constant <i>ac bias current (Ib) is supplied. Said suppl ied non-constant ac bias current compensate s the current (Ic) induced by the transformer in such a way that the current flowing through the SQUID has a periodic rectangular function. This makes it possible to advantageously achieve low-noise operation of up to high reversal frequencies of over 100 kHz.

Description

Method for reading magnetic fields with a de superconducting quantum interference device

(dc-SQUID) and a suitable device

The invention relates to a method and a device for reading magnetic fields with the aid of a de-SQUID.

Superconducting quantum interference devices (SQUIDs) are currently the most sensitive magnetic field sensors. The development of improved SQUIDs and circuits to minimize noise and increase sensitivity are becoming increasingly important.

The basic principles of today's SQUIDs and circuit technology for coupling and reading out the weakest magnetic fields are dealt with below.

A dc-SQUID consists of a superconducting ring that is interrupted by two Josephson contacts. If a bias current I _{B is} fed into the SQUID, a voltage drops across it. To measure magnetic fields, the external magnetic flux to be measured is coupled into a SQUID. The voltage drop across the SQUID is modulated by the coupled magnetic flux due to the quantum mechanical effects on the Josephson contacts. The voltage U (φ) dropping at the SQUID is a periodic function of the magnetic flux φ, the periodicity being determined by the flux quantum φ ₀ .

There are two basic variants for feeding a current.

In the first variant, a constant (de) current is fed into the SQUID, which is also described as a "de bias" current. The second variant uses "ac bias" alternating current, also known as "bias reversal" current. This bias reversal current changes periodically between positive and negative (plus and minus = reversal) states. The amount of both states is the same. The frequency of the AC voltage is also called the reversal frequency.

The advantage of using ac bias current is that the 1 / f noise of the dc-SQUID can be significantly reduced. The reduction is a statistical effect, however, the higher the reversal frequency, the more effectively the l / f noise is suppressed.

When using ac bias current, there are three basic options for the circuit electronics for reading. This includes a direct connection of a dc-SQUID to a preamplifier, a connection via a transformer and a so-called bridge circuit. The disadvantages of a direct connection are the high noise and the large drift. A bridge circuit disadvantageously results in a bad one Adjustability. The connection via a transformer basically has the best l / f noise behavior and the smallest drift of these three variants. The disadvantage, however, is that the transformer variant is only operated up to a low reversal frequency of 2 kHz. Above 2 kHz there is a strong increase in white noise, which increases with increasing frequency. However, the limitation of the reversal frequency to 2 kHz leads disadvantageously to only a small bandwidth in this method.

The additional noise can be explained as follows using FIG. 1. When the SQUID is connected to an amplifier device (preamplifier), the SQUID is connected in parallel with an inductance (coupling transformer). The ac bias current I _b impressed into this SQUID arrangement corresponds to a rectangular function, but the current I _s through the SQUID does not. With each polarity reversal, I _s receives an additional peak, which is generated by the inductance of the transformer. The additional noise is caused by the non-constant current I _s . As shown in Figure 1, the ratios of t / T and A '/ A determine the amount of additional noise. Here t is the length of time that the non-constant current I _s lasts, and T is the period of the alternating current (bias reversal current), A 'is the maximum value of the current (the height of the peak) and A is the size of the constant ac bias current ,

In the event that the ratio t / T is <1%, there is no significant difference for the noise with or without bias reversal. By shortening the pe During period t, the value of t / T becomes ever greater at constant t, ie the time period in which the SQUID is operated under the non-constant current I _s in a period is becoming increasingly important. This is the reason for the additional noise.

Task and solution

The object of the invention is to provide a method for reading out small magnetic fields with the aid of a

To create SQUIDs in which the reversal frequency, and thus the bandwidth, can be significantly increased without the aforementioned disadvantages of the prior art occurring. Furthermore, it is an object of the invention to provide a device which is suitable for carrying out this method.

The object of the invention is achieved by a method with the entirety of the features of the main claim and by a device with the features of the auxiliary claim. Advantageous refinements of the method and of the device can be found in the dependent claims, which are dependent thereon.

Subject of the invention

The method according to the invention for operating a dc-SQUID arrangement, which comprises two Josephson contacts and a transformer coil connected in parallel, has the step that is essential to the invention that a non-constant ac bias current I _{b is} fed into the SQUID arrangement. The non-constant ac bias current pens the current I _c induced by the transformer such that the current I _s flowing through the SQUID has a rectangular function.

For an optimal operation of a dc-SQUID, the current I _{s should have} a rectangular function. The injected current I _b is selected in a suitable manner so that the injected current I _b and the current I _c induced by the transformer compensate each other in such a way that a compensation of these two currents results in a rectangular function for the SQUID current I _a .

In an advantageous embodiment of the method, the current I _b fed in has the following function:

I _b (t) = I _R - 2 I _R x R _N / (R „+ R) xe ^{_t τ} with I _b = ^injected ac bias current;

I _R = maximum flow of I _b ; R _N line resistance of the SQUID;

R = series resistance of the transformer t = time τ = L / R (L = inductance of the transformer, R = adjustable resistance)

Such a current completely compensates for the current induced by the transformer, so that the SQUID can work under optimal conditions. This also means that it is advantageous to work with a high reversal frequency and thus a high bandwidth is possible. The method according to claim 3 is characterized in that only two settings have to be made for the selected SQUID arrangement in order to achieve the desired function of the supplied, non-constant bias reversal current I _b (t) at the current source:

1. a one-time setting of the constant τ = L / R

(regardless of the SQUID),

2. an adjustment of the ratio R / R _N after each exchange of the SQUID.

In a further advantageous embodiment of the method according to claim 4, the fed-in ac bias current has a reversal frequency of more than 10 kHz, in particular of more than 100 kHz. The high reversal frequency also has the advantage of a high bandwidth and thus enables the measurement of magnetic fields that change quickly over time with the highest sensitivity.

The inventive device for performing the

The method has a dc-SQUID A and a coupling transformer B. The dc-SQUID is characterized by the line resistance R _N. The primary winding of the transformer has inductance L.

The non-constant current feed of the ac bias current takes place via a current source with a generator C, which supplies a square-wave voltage VI, a subtractor D, a capacitor C _t and four resistors R _lr R ₂ , R _t and R _b . The square-wave voltage VI generated by the generator is divided by the resistors Ri and R ₂ (voltage divider) and by the capacitor C _t and the resistor R _t corresponding to R ₂ / (R _x + R ₂ ) x 2e ^{"t / τ} with τ = R _t xC after subtraction of this voltage from the square-wave voltage VI by the subtractor D, the bias voltage is obtained according to V _b = VI - VlxR ₂ / (Rχ + R ₂ ) x 2e ^{"t / τ} . The voltage V _{b is converted} into the current I by the resistor R _b .

If Rι / R ₂ = R / R _N and R _t xC _t = L / R, the bias current I _b (t) corresponds exactly to the desired behavior over time.

The invention is explained in more detail below using equations and figures. Show it

Figure 1 schematic dc-SQUID reading according to the prior art with constant ac bias current feed I _b at R = R _N

FIG. 2 shows a schematic readout method according to the invention with a non-constant ac bias current feed I _b at R = R _N

Figure 3 I _b (t) diagrams for different ratios of R to R _N.

FIG. 4 embodiment of the device according to the invention with means for the non-constant ac bias current feed with A = dc-SQUID, B = coupling transformer (primary inductance L), C = square wave generator D = subtractor

Figure 5 Simulation of the currents I and I _s with conventional ac bias current feed I _b at R = R _N

FIG. 6 simulation of the currents I _c and I _s with a non-constant ac bias current feed I _b at R = R _N

According to the invention, the current of the dc-SQUID I _{s should have} a rectangular function (see also FIG. 2). According to the Kirchhoff law, the following applies to the circuit shown in FIG. 2:

= Is + I _£ (1)

A differential equation is set up to describe the relationships of the three currents after the polarity reversal.

Since the SQUID and the inductance L of the transformer are connected in parallel, the voltage drop across the two branches is the same:

Lxdlc / dt + I _c xR = I _S XR _N (2) where R _{N is} the normally conductive resistance of the SQUID (contacts) and R, as can be seen in FIGS. 1, 2 and 4, is a small ohmic series resistance.

From equations (1) and (2) it follows:

Lxd (Ib - I _s ) / dt + (I _b - I _s ) xR = I _S xR _N (3) Since the current I _{s is} to remain constant immediately after the polarity reversal, the differential equation (3) is rewritten:

Lxdl _b / dt + Rxl _b - I _s x (R + R _N ) = 0 (4)

The desired bias current I _b results from the solution of equation (4). The current I _s thus remains constant immediately after each polarity reversal. This can advantageously drastically increase the reversal frequency.

With the initial condition I ₀ = I _R x (R _N -R) / (R _N + R) you get the solution:

I _b = I _R - 2I _R xR _N / (R _N + R) xe ^{"t / τ} (5)

where I _{R is} the maximum amplitude of the ac bias current and the time constant τ = L / R.

This solution gives two very interesting results: 1) only the ratio of R / R _N has to be adjusted; 2) τ is independent of R _N.

The first result means that only one parameter for all SQUIDs with different R _N is set. The second result means that τ is only dependent on the constant quantities L and R.

FIG. 3 shows three different current curves I _b as a function of time (t) for the corresponding ratios of: a) R> R _N , b) R = R _N , c) R <R _N. FIG. 4 shows the advantageous embodiment of a device with which the method according to the invention is carried out. In addition to the dc-SQUID A and the read-out unit B, a possible circuit for the ac bias power supply is also entered in the figure.

This circuit comprises a square-wave voltage generator C, four resistors Ri, R ₂ , R _t and R _b , a capacitor C and a subtractor D. Via the resistors Ri and R ₂ , the ratio of R (series resistance of the coupling transformer B) to R _{N can be determined} Set (line resistance of the dc-SQUID). The appropriate time constant is obtained via the capacitor C _t and the downstream resistor R _t according to R _t x C _t = L / R. The desired output voltage is generated by the subtractor D. The high-resistance resistor R _b then converts the voltage into the ac bias current.

Since the current induced by the transformer is compensated in a suitable manner by the non-constant ac bias current I _b , the current I _s remains constant immediately after each polarity reversal. This advantageously causes a very low noise even at reversible frequencies of the ac bias current of more than 10 kHz, in particular at more than 100 kHz.

Claims

Patent claims

1. Method for operating a dc-SQUID arrangement which comprises two Josephson contacts and a transformer coil connected in parallel, a non-constant ac bias current I _{b being} fed into the SQUID arrangement.

2. The method according to the preceding claim, in which the non-constant ac bias current I _{b of} the function:

I _b (t) = I _R - 2 I _R x R _N / (R _N + R) xe ^"t with I _b = injected ac bias current;

I _R = maximum flow of I _b ;

R _N = line resistance of the SQUID;

R = series resistance of the transformer t = time τ L / R (L = inductance of the transformer, R = adjustable resistance).

3. The method according to any one of the preceding claims, characterized in that the supplied non-constant ac bias current I _b and the current induced by the transformer I _c compensate such that there is a rectangular function for the SQUID current I _s .

4. The method according to any one of the preceding claims, wherein the injected ac bias current has a reversal frequency of more than 10 kHz, in particular of more than 100 kHz.

5. Device for performing the method according to one of the preceding claims 1 to 4, comprising a dc-SQUID (A), characterized by a series resistor R _N and a coupling transformer (B) with the primary inductance L, characterized by a means for generating and feeding a non-constant ac bias current.

6. Device according to the preceding claim 5, characterized by a circuit with

a generator (C) for generating a square-wave voltage VI,

- Two resistors Ri and R ₂ as voltage dividers, - a capacitor C _t

- a subtractor (D)

- And two further resistors R _t and R _b as a means for generating the non-constant ac bias current.

7. Device according to one of the preceding claims 5 to 6, characterized in that the square-wave voltage VI generated by the generator is divided by the resistors Ri and R ₂ and by the capacitor C and

Resistance R _t corresponding to R ₂ / (Rι + R ₂ ) x 2e ^{"t τ} with τ = R _t xC _{t is} influenced in time that after subtracting this voltage from the square wave voltage VI by the subtractor D, the bias voltage according to V _b = VI - VlxR ₂ / (Rι + R ₂ ) x 2e ^{"t / τ} is obtained and by the

Resistor R _b the voltage V _{b is converted} into the current I _b .