Category Archives: Publications

Publication: Solid-State Qubits with Current-Controlled Coupling

As some of you know, we recently had a paper accepted to Science. The paper appears in the latest issue, and is now available online.
I will try to post something in the next few days that explains these results for the non-physicists in the audience. In the meantime, there’s this post from March about these experiments (from before we had the major findings), and here’s the abstract:

Solid-State Qubits with Current-Controlled Coupling
T. Hime, P. A. Reichardt, B. L. T. Plourde, T. L. Robertson, C.-E. Wu, A. V. Ustinov, John Clarke
The ability to switch the coupling between quantum bits (qubits) on and off is essential for implementing many quantum-computing algorithms. We demonstrated such control with two flux qubits coupled together through their mutual inductances and through the dc superconducting quantum interference device (SQUID) that reads out their magnetic flux states. A bias current applied to the SQUID in the zero-voltage state induced a change in the dynamic inductance, reducing the coupling energy controllably to zero and reversing its sign.

Publication: Quantum theory of three-junction flux qubit with non-negligible loop inductance: Towards scalability

Here’s the latest publication on Clarke group qubit research, which appeared in Physical Review B at the end of May. Normally I give a non-technical explanation in these posts, but this paper is entirely devoted to working out gory technical details. It essentially goes through how to calculate a priori the properties of the flux qubits that I’ve written about previously. This calculation had been done for “small” qubit loops—small being defined in terms of the loop inductance but corresponding to a few microns on a side—our qubits are much larger than this (100 microns) and so we needed to figure out the more general solution.
The vast majority of the work in this paper was done by T. L. Robertson; my primary contribution was checking the math and the Mathematica code.
Quantum theory of three-junction flux qubit with non-negligible loop inductance: Towards scalability
T. L. Robertson, B. L. T. Plourde, P. A. Reichardt, T. Hime, C.-E. Wu, and John Clarke
Phys. Rev. B 73, 174526 (2006)

The three-junction flux qubit (quantum bit) consists of three Josephson junctions connected in series on a superconducting loop. We present a numerical treatment of this device for the general case in which the ratio betaQ of the geometrical inductance of the loop to the kinetic inductance of the Josephson junctions is not necessarily negligible. Relatively large geometric inductances allow the flux through each qubit to be controlled independently with on-chip bias lines, an essential consideration for scalability. We derive the three-dimensional potential in terms of the macroscopic degrees of freedom, and include the possible effects of asymmetry among the junctions and of stray capacitance associated with them. To find solutions of the Hamiltonian, we use basis functions consisting of the product of two plane wave states and a harmonic oscillator eigenfunction to compute the energy levels and eigenfunctions of the qubit numerically. We present calculated energy levels for the relevant range of betaQ. As betaQ is increased beyond 0.5, the tunnel splitting between the ground and first excited states decreases rapidly, and the device becomes progressively less useful as a qubit.

2006 March Meeting Abstract

Less than two months remain before the APS March Meeting, which in terms of blogging means more short posts at odd hours, when I’m not in the lab trying to gather lots of last-minute data. Here’s the abstract for my talk:

Abstract: K40.00012 : Variable Coupling of Two Flux Qubits
5:06 PM–5:18 PM
T. Hime, P.A. Reichardt, B.L.T. Plourde, T.L. Robertson, C.-E. Wu, A.V. Ustinov, John Clarke
We report observations of variable coupling of two flux qubits. The qubits are coupled inductively to each other and to a readout Superconducting QUantum Interference Device (SQUID). By applying microwave radiation to the device, we observed resonant absorption in each of the qubits when the level splitting in the qubit matched the energy of the microwave photons. Using the two on-chip flux bias lines we adjusted the bias of each qubit so that the energy levels of the two qubits were equal; we then observed a splitting of the resulting absorption peak characteristic of coupling between the qubits. We varied the coupling between the qubits by changing the current bias in the SQUID in the zero voltage state, thereby changing its dynamic inductance and thus modifying the effective mutual inductance between the qubits. We compare the resulting changes in splitting with our predictions. This controllable coupling should be extendable to many qubits.

I’ll do a post explaining this in more detail around the time of my talk; some of this work is still, uh, “in progress”. (In fact we have performed all the experiments mentioned in the abstract, but we are working on collecting more/better data.) The talk immediately before mine covers some other results from these experiments.

SQUID Handbook sighting

This item is a bit dated, but apparently there’s a prize for “oddest book title” awarded every year:

Rick Pelicano and Lauren Tjaden’s extremely serious manual on how to Bombproof Your Horse is today hailed as runaway winner of the prize for the oddest book title of the past year.
It takes what the Bookseller magazine describes as a staggering 46% of the vote in a poll of publishers and booksellers.
Runners-up in a shortlisted international field of six are Detecting Foreign Bodies in Food, with 27%, followed by The Aesthetics of the Japanese Lunchbox, with 15%.
The British-based Diagram prize – a magnum of champagne awarded by the Bookseller since 1978 – reflects the book trade’s unceasing bafflement and delight at the highly specialised titles which some of its members in Britain and further afield produce.

Also on the 2004 shortlist were Applications of High Tech Squids (VCH Verlagsgesellschaft), Equids in Time and Space (Oxbow Books) and Sexual Health at Your Fingertips (Class Publishing).

(Emphasis mine.) Actually, that’s Applications of High Tech SQUIDs by one J. Clarke. Although it’s nice to see my advisor’s book getting publicity, I think the title is not so odd if the acronym is written properly.

Clarke group research in ScienceMatters@Berkeley

My advisor was profiled in the latest issue of ScienceMatters@Berkeley, an online UCB publication written by Boing Boing’s David Pescovitz. Most of you know about my work on the qubit project; the ScienceMatters article also covers some of the other research in the group.
UPDATE: It’s pretty cool to see one of our figures on Boing Boing, even if it is from the (admittedly more photogenic) MRI project rather than the qubit research.

Publication: Flux qubits and readout device with two independent flux lines

This paper contains the major results of my graduate research so far, compressed into four pages. Instead of the abstract I’m posting something closer to a layman’s explanation, which is below the fold since it got a bit long.
Flux qubits and readout device with two independent flux lines
B. L. T. Plourde, T. L. Robertson, P. A. Reichardt, T. Hime, S. Linzen, C.-E. Wu, and John Clarke
Phys. Rev. B 72, 060506(R) (2005)

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