HTS Qubits and JJ's using BSCCO Design and Fabrication of HTS Qubits using BSCCO Suzanne Gildert

HTS Qubits and JJ's using BSCCO Disclaimer! 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.

HTS Qubits and JJ's using BSCCO Theory of Qubits (I) ➔ 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 Ф 0 / 2 ➔ Energy band 'anticrossing' where classical system would be degenerate ➔ Resulting in a double well potential in the energy landscape....

HTS Qubits and JJ's using BSCCO Theory of Qubits (II) ➔ RF SQUID characteristic yields necessary Energy – Ф x 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

HTS Qubits and JJ's using BSCCO HTS Qubits 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 1 st 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 and JJ's using BSCCO Now I've got your attention... 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!

HTS Qubits and JJ's using BSCCO BSCCO Whisker surgery 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) Ref: M. Nagao et al, Physica C (2002) Ref: P. A. Warburton et al, PRB (2003)

HTS Qubits and JJ's using BSCCO Designing small things..... MQC can only occur in underdamped (hysteretic) DC SQUID systems. Hysteresis condition: 2лLI c / Ф 0 ~ 1 Need to minimise the product LI c to lower the energy barrier for quantum tunnelling, but not so much as to lose the hysteresis Inductance of a small loop ~ u 0 r. (v. approx) Lets make the loop 2um diameter L ~ 1x Therefore I c ~ 2x / 6x1x = 1x10 -5 A which is reasonable for a JJ. I c of the BSCCO JJ's will be measured and compared to this to establish their feasibility and required areas. Ref: Mooij et al. Science, Vol 285, (1999)

HTS Qubits and JJ's using BSCCO Measuring small things... 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 Ref: H Tanaka et al, Physica C (2002)

HTS Qubits and JJ's using BSCCO Noise in JJ's / Qubits - arrggh! 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 Ref: A. Wallraff et al, Rev. Sci. Instr. Vol 74 No. 8 (2003)

HTS Qubits and JJ's using BSCCO Conclusion 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