Hot Quantum Computing!

Normally when I think about quantum computers, I think about systems which are pretty cold, since a thermal equilibrium state at high temperature is a very mixed state. But is it really true that a quantum computer needs to be cold to quantum compute? I've often wondered (some would say pontificated) about this, and so I was excited when I found this Physical Review Letter describing quantum computing using plasmas.

The idea of this new approach, according to the paper, is to use modes in the Debye sheath as qubits. Because of the Child-Langmuir law, the current in this sheath is quantized and so we can perform single qubit gates in a manner not that similar to how we perform single qubit gates using flux based superconducting circuits. Two qubit gates are actually quite easy in this scheme, it seems, relying basically on the Coulomb interaction, mediated, of course, by Debye screened plasmons. The fact that two qubit gates are so simple is certainly one of the reasons why this paper was accepted into Physical Review Letters. Usually getting two qubits to talk to each other is like getting a cat to think its not the center of the universe. From the paper

Two qubit interactions mediated by plasmons offer a new sort of protection for faulty quantum operations. By taking advantage of the local, non-topological, properties of these states, we can remove the detrimental effects of decoherence, and cause quantum states to spontaneously recohere.

This proposal, of course, is a radical way to build a quantum computer. Normally, we think about quantum computers as requiring very cold, pure, quantum states. Since the technical requirements for this traditional approach to building a quantum computer are so insurmountable, it is often remarked by those less than qualified to do so that quantum computers will probably be built sometime around 2060. But, using the same technology that will fuel a future fusion reactor, it now seems to me that this number is not so far away at all. Indeed, the authors claim that this method of building a quantum computer will suffer from none of the benefits that merit fusion research, and thus, we can expect a quantum computer around the same time that we engineer a fusion reactor. How's that for an optimistic prediction for quantum computers!

Free energy and free quantum computation for everyone, my friends!

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Sweet: fusion, spaceflight, and quantum computers all in one invention. Now if only they could get the damn thing to room temperature superconduct.

Eh, we knew about this all along, the system is dual to N=3 SUSY black hole in 6 dimensions, when you compactify the rest on flux infused CY manifolds, with D-branes and orientifolds to boot. In fact it is likely to already be in operation somewhere in the multiverse (at least for some specific definition of "likely").

You had me going until "...so we can perform single qubit gates in a manner not that similar to how we perform single qubit gates using..."

It jarred me into thinking of this similar analogy:

"The ships hung in the sky in much the same way that bricks don't." -Douglas Adams

By Mike Magee (not verified) on 01 Apr 2008 #permalink

ScienceDaily (Apr. 5, 2008) A research team from the National Institute of Standards and Technology (NIST) and the University of Maryland has succeeded in cooling atoms of a rare-earth element, erbium, to within two millionths of a degree of absolute zero using a novel trapping and laser cooling technique. Their recent report* is a major step towards a capability to capture, cool and manipulate individual atoms of erbium, an element with unique optical properties that promises highly sensitive nanoscale force or magnetic sensors, as well as single-photon sources and amplifiers at telecommunications wavelengths. It also may have applications in quantum computing devices.
The strongly counterintuitive technique of "laser cooling" to slow down atoms to very low speeds--temperatures close to absolute zero--has become a platform technology of atomic physics. Laser cooling combined with specially arranged magnetic fields--a so-called magneto-optical trap (MOT)--has enabled the creation of Bose-Einstein condensates, the capture of neutral atoms for experiments in quantum computing and ultra-precise time-keeping and spectroscopy experiments.

The technique originally focused on atoms that were only weakly magnetic and had relatively simple energy structures that could be exploited for cooling, but two years ago a NIST team showed that the far more complex energy structures of erbium, a strongly magnetic element, also could be manipulated for laser cooling.

The typical MOT uses a combination of six tuned laser beams converging on a point that is in a low magnetic field but surrounded by stronger fields. Originally, the lasers were tuned near a strong natural energy oscillation or resonance in the atom, a condition that provides efficient cooling but to only moderately low temperatures. In the new work, the research team instead used much gentler forces applied through a very weak resonance in order to bring erbium atoms to within a few millionths of a degree of absolute zero.

Such weak resonances are only available in atoms with complex energy structures, and previously have been used only with a select group of non-magnetic atoms. When a strongly magnetic atom like erbium is used, the combination of strong magnetic forces and weak absorption of laser photons makes a traditional MOT unstable.

To beat this, the NIST/UM team turned classic MOT principles on their heads. Rather than shifting the laser frequency towards the red end of the spectrum--to impact fast, high-temperature atoms more than slow, cold ones--they shifted the laser towards the blue side to take advantage of the effects of the magnetic field on the highly magnetic erbium. Magnetism holds the atoms stably trapped while the lasers gently pushed them against the field, all the while extracting energy and cooling them.

The delicate balancing act not only cools and traps the elusive erbium atoms, it does it more efficiently. The team's modified trap design uses only a single laser and can cool erbium atoms to within two millionths of a degree of absolute zero. By contrast, a conventional MOT only brings rubidium atoms to about one ten-thousandth of a degree.

Erbium commonly is used in optical communications components for its convenient magneto-optical properties. The new trapping technique raises the possibility of using erbium and similar lanthanide elements for unique nanoscale magnetic field detectors, atomic resolution metrology, optical computing systems and quantum computing.

* A.J. Berglund, J.L. Hanssen and J.J. McClelland. Narrow-line magneto-optical cooling and trapping of strongly magnetic atoms. Physical Review Letters, V. 100, p. 113002 , March 18, 2008.