Rabu, 14 Juli 2010

Squid Magnetometer

Hi guys, today i want to share about Squid Magnetometer!. Squid Magnetometer that include at least two SQUID loops, each of which is composed of at least two Josephson Junctions connected in parallel with superconducting wires, are provided. The SQUID loops are fabricated such that they share a common Josephson Junction. Devices and application that employ the multiple SQUID magnetometers are also provided.

FIELD OF THE INVENTION

[0001] This invention relates generally to SQUID magnetometers that include multiple SQUID loops with a shared Josephson Junction and to methods and applications that make use of such magnetometers.

BACKGROUND OF THE INVENTION

[0002] Magnetometers are devices used to measure magnetic fields. Magnetometers based on superconducting quantum interference devices (SQUIDs) are among the most sensitive devices for measuring small magnetic fields. A conventional DC SQUID consists of two parallel Josephson Junctions disposed along a superconducting loop. These devices convert magnetic flux threading the superconducting loop into a quantity (e.g., current or voltage) that may be measured by an associated electronic stage.

[0003] Recently, advances in the field of nanotechnology have driven the search for SQUID magnetometers capable of detecting magnetic flux changes associated with nanoscale molecules and objects. See John Gallop, Superconductor Science and Technology, 16, 1575-1582 (2003) and Ling Hao et al., Superconductor Science and Technology, 16, 1479-1482 (2003). In one instance, a SQUID loop smaller than 1 .mu.m has been used to measure the flipping of 1000 electron spins. See M. Jamet, W. Wernsdorfer, C. Thirion, D. Mailly, V. Dupuis, P. Melinon and A. Peres, Physical Review Letters, 86, 4676 (2001). Unfortunately, the sensitivity of SQUID magnetometers has been limited by the residual noise in these devices.

[0004] At low temperatures, the limiting noise is thought to be the circulating noise currents in the SQUID loop. These noise currents would prevent the noise energy in a SQUID from being smaller than /2, where h=2 .pi. is Planck's constant, as discussed in John Gallop, Superconductor Science and Technology, 16, 1575-1582 (2003) and Ling Hao et al., Superconductor Science and Technology, 16, 1479-1482 (2003). This means that a change in magnetic field associated with an energy change of 0.5.times.10.sup.-34 J is the minimum change that could be detected by a SQUID in a 1 Hz bandwidth. A bandwidth of 1 Hz corresponds approximately to a measurement time of 1 second. In practice that measurements longer than 1 second do not improve the sensitivity, as 1/f noise becomes larger.

[0005] In theory, an ideal SQUID magnetometer could be used to measure the change in magnetic field associated with a change in the nuclear spin of a single proton. In a strong magnetic field, the nuclear spin of a proton is in one of two states, separated by an energy difference of .DELTA.E=.gamma.B,

[0006] where B is the applied magnetic field and the proton has a gyromagnetic ratio of .gamma.=26.75.times.10.sup.7 rad.sup.-1 T.sup.-1. Increasing B increases the energy difference, .DELTA.E, but SQUIDs cannot function in magnetic fields that are too large. Niobium SQUIDs can be used at .about.0.01 T, where the energy difference of a proton spin flip is .DELTA.E=.apprxeq.2.7.times.10.sup.-28 J. See, for example, Tsuyoshi Tajima, Proceedings of 8.sup.th European Particle Accelerator Conference, http://apt.lanl.gov/documents/pdfLA-UR-02-3042.pdf; E. M. Forgan, S. J. Levett, P. G. Kealey, R. Cubitt, C. D. Dewhurst and D. Fort, Physical Review Letters, 88, 167003, 2002; and H. R. Kerchner, D. K. Christen and S. T. Sekula, Physical Review B, 21, 86 (1980). The highest coupling of a spin to the SQUID is achieved when the spin lies on the Josephson Junction. In this case, up to half of the magnetic flux is coupled, so the maximum energy detected by the SQUID as a result of the spin flip would fall to 1.3.times.10.sup.-28 J. Under these ideal conditions, a SQUID could be used to measure a single nuclear spin flip, with a signal to noise ratio (SNR) of >10.sup.6.

[0007] SQUIDs have been reported with a noise energy of 3 . See, for example, D. J. Van Harlingen, R. H. Koch and J. Clarke, App. Phys. Lett. 41, 197 (1982). However, a need exists for a SQUID magnetometer in which there is good coupling between the flux from a magnetic particle and the magnetometer. When this improved coupling is achieved, the need arises to increase the SQUID sensitivity further, for demanding applications such as single-molecule NMR.

SUMMARY OF THE INVENTION

[0008] The present invention provides SQUID magnetometers having increased sensitivity to changes in the magnetic fields associated with very small objects and, in particular, objects having dimensions smaller than the width of the semiconductor wires of their SQUID loops. Such objects include nanoscale objects, such as nanoparticles and biomolecules (e.g., proteins).

[0009] In their basic embodiment the SQUID magnetometers include at least two SQUID loops, each of which is composed of at least two Josephson Junctions connected in parallel with superconducting wires. The SQUID loops are fabricated to share a common Josephson Junction and superconducting wire. In this construction the circulating noise currents in each SQUID loop are different, but the loops can be synchronized such that the signal from an event along the common superconducting wire may be recorded at the same time by both loops. The correlated signal from the simultaneous measurements taken by both SQUID loops provides a measure of the magnetic field on this shared wire with a sensitivity that is much greater than the sensitivity provided by either SQUID loop operating independently. These devices may be used to study the magnetic properties of very small systems, particularly systems having dimensions smaller than the superconducting wires. In some embodiments, the multiple SQUID magnetometers may be used to measure the nuclear magnetic resonance (NMR) of a single molecule, such as a protein. The ability to measure the magnetic properties of a single molecule or particle is important because it avoids complications due to distributions of particles shapes, sizes and orientations that are present in larger samples.

[0010] A number of variations may be developed from this basic construction. For example, in some embodiments the SQUID magnetometers may include a plurality of SQUID loops (e.g., three or more) with each SQUID loop sharing a common Josephson Junction and superconducting wire. In these embodiments the sensitivity of the correlated signal may be improved by adding additional SQUID loops. In some embodiments the SQUID loops will include three Josephson Junctions. In such embodiments, independent current biasing of the SQUID loops may be accomplished by applying a separate bias current to each SQUID loop.

[0011] In another embodiment the present invention provides an array of SQUID loops. In this array, each SQUID loop shares a common Josephson Junction and superconducting wire with each of its neighbouring SQUID loops in the array. By coupling a sample that acts as a qubit to each common Josephson Junction in the array, the array of SQUID loops may be used as quantum computer. This can be understood as follows: A molecule, such as an endohedral fullerene (e.g., N@C.sub.60--a nitrogen atom encapsulated in a C.sub.60 Buckyball), on a common Josephson Junction between two SQUID loops should behave as an electron-spin qubit. The spin state of the qubit may be initialized and subsequently manipulated with a magnetic field applied across the SQUID loops. Qubit manipulation would be controlled by the application of electromagnetic pulses. Finally the change in magnetic flux associated with the change in spin state may be measured with the SQUID magnetometer. Interactions mediated by the normal electrons in the superconductor may provide coupling between the qubits along the array and make it possible to scale the system up to a quantum computer.

[0012] Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1A is a circuit diagram showing the design of a double SQUID magnetometer. The noise is different for both SQUID loops, but the signal from a sample (represented by a small dark circle) on the common Josephson Junction is the same for both loops.

[0014] FIG. 1B is a circuit diagram showing the design of a double SQUID magnetometer with independent current biasing of each SQUID loop.

[0015] FIG. 2 is a schematic diagram showing the voltage measured by an event (e.g., a spin flip) generated by a sample coupled to the common Josephson Junction of the double SQUID magnetometer shown in FIG. 1. The upper signal represents the voltage measured across the first SQUID loop. The middle signal represents the voltage measured across the second SQUID loop. The lower signal represents the correlated signal, which may be found by adding or multiplying the voltage signals from the first and second SQUID loops.

[0016] FIG. 3A. is a circuit diagram showing the design of a four loop SQUID magnetometer.

[0017] FIG. 3B is a circuit diagram showing the design of a four loop SQUID magnetometer with independent current biasing of each SQUID loop.

[0018] FIG. 4A is a circuit diagram showing the design of a six loop SQUID magnetometer. (The non-superconducting wires and other circuitry are not shown for clarity.)

[0019] FIG. 4B is a circuit diagram showing the design of a six SQUID magnetometer adapted for independent current biasing of each SQUID loop. (The non-superconducting wires and other circuitry are not shown for clarity.)

[0020] FIG. 5. a) Flux lines of a magnetic dipole in free space. b) Flux can be trapped if the dipole is inside a bulk superconductor. c) A small hole in the superconductor may allow the flux to escape. The dipole is shown schematically in the centre of the hole, but it could be on the inner surface of the hole.

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