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 / 2
0
➔ 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 – 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
➔
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 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 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: P. A. Warburton et al, PRB 67 184513 (2003)
Ref: M. Nagao et al, Physica C 377 260-266 (2002)
HTS Qubits and JJ's using BSCCO
Designing small things.....
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.
Ref: Mooij et al. Science, Vol 285, 1036-1039 (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 386 300-304 (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
➔