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Design and Fabrication of HTS Qubits using BSCCO

Suzanne Gildert. Design and Fabrication of HTS Qubits using BSCCO.

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Design and Fabrication of HTS Qubits using BSCCO

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  1. Suzanne Gildert Design and Fabrication of HTS Qubits using BSCCO

  2. I am not talking about quantum computing (bit of a buzzword at the moment) as I believe this is a good 10 years away at least. Although I am very interested in applications of the devices for future systems, the main aims of this project is to use the devices to gain a better understanding of the physics principles behind them. Disclaimer!

  3. Chris talked about this – so I'm not going to go into detail! Starting point is a charge / phase / flux system Opposing currents circulate round loop when biased at Ф0 / 2 Energy band 'anticrossing' where classical system would be degenerate Resulting in a double well potential in the energy landscape.... Theory of Qubits (I)

  4. RF SQUID characteristic yields necessary Energy – Фx double well. Barrier height depends on SQUID parameters (more later) Want barrier to be small enough to see MQC But BIG enough to prevent thermal excitation..... So MUST keep kT small, e.g. T<20mK. This is why dilution fridges required for all these experiments Theory of Qubits (II)

  5. HTS is anisotropic d-wave -> nodes in the order parameter and quasiparticles down to T=0! (or so) - ISSUES!!!! How to get around this: Make devices smaller! (You may have guessed I was going to do this) Quantum system – compare particle in a box – energy of 1st excited state moves away from ground state when size of system decreases Other advantages: Less decoherence due to noise (will explain more later...) Isolation of devices within single grain (no messy grain boundaries) Scalability! Better Junction Parameters (see later) HTS Qubits

  6. So now you think I'm going to build a HTS Quantum Computer........well, no. Sorry. Firstly, we must find out if HTS qubits are feasible. For this we must observe Macroscopic Quantum Coherence (MQC) in a HTS junction device. Why not just use LTS? MQC has been observed, and even manipulated in LTS Qubits (advanced techniques are about 5/6 years in progress) HTS is novel, and very few groups are working on this! If it is possible, applications would perhaps be easier to implement?? We are a HTS group - (!) JJ's do not need to be fabricated – nature has given them to us...... We need JJ's in HTS! Now I've got your attention...

  7. BSCCO is a HTS with Intrinsic JJ's between the S/C unit cell layers We believe we can make whiskers: They are Single Xtal & Low Impurity Levels. Therefore very high quality Intrinsic JJ's And (conveniently) they grow to the right dimensions!! Grow whiskers from precursory powders Use conventional Lithography FIB to create junction stacks Ar+ Ion mill to isolate 1 junction (difficult) BSCCO Whisker surgery Ref: P. A. Warburton et al, PRB 67 184513 (2003) Ref: M. Nagao et al, Physica C 377 260-266 (2002)

  8. MQC can only occur in underdamped (hysteretic) DC SQUID systems. Hysteresis condition: 2лLIc / Ф0 ~ 1 Need to minimise the product LIc to lower the energy barrier for quantum tunnelling, but not so much as to lose the hysteresis Inductance of a small loop ~ u0r. (v. approx) Lets make the loop 2um diameter L ~ 1x10-11 Therefore Ic ~ 2x10-15 / 6x1x10-11 = 1x10-5 A which is reasonable for a JJ. Ic of the BSCCO JJ's will be measured and compared to this to establish their feasibility and required areas. Designing small things..... Ref: Mooij et al. Science, Vol 285, 1036-1039 (1999)

  9. In situ characterisation: Ion beam mill modification In situ normal state / 77K measurements Observe IV characteristics of 1 junction Ex-situ custom built low-noise electronics setup Ramp current in a sawtooth fashion Produces stochastic switching results. Info about thermal activation / MQT Or you can do RF Hysteresis Measurements (better) This is all fine until you start making qubits - Then of course you have the real measurement problem!! This is overcome by using a surrounding readout system e.g. DC SQUID Measuring small things... Ref: H Tanaka et al, Physica C 386 300-304 (2002)

  10. Intrinsic junction noise: Thermally induced Flux Noise - 'Telegraph' noise (PINK) Coulomb field noise (PINK) Johnson noise across an inductor (WHITE) In HTS, also quasiparticle states! External noise sources: Thermal and electrical substrate fluctuations External flux noise / interference pick up Therefore need very good electronics to shield from noise Noise in JJ's / Qubits - arrggh! Ref: A. Wallraff et al, Rev. Sci. Instr. Vol 74 No. 8 (2003)

  11. Short term goals: Design In situ milling / measuring technique for normal state / 77K Make BSCCO tunnel junctions and measure their properties Apply RF to demonstrate MQC Collaborate with theorists to discuss HTS as Qubits Long term goals: Design low noise electronics system for use with dilution fridge Design a HTS Qubit structure with 1 or 3 JJ's on the same whisker Implement a readout technique for the qubit Investigate current state of BSCCO thin films Conclusion

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