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Rare-earth solid-state qubitsnano.org/07-02-001
Journal ref: Nature Nanotechnology 2, 39 - 42 (2007)
Posted:
05 February 2007, 17:37
Abstract: Quantum bits (qubits) are the basic building blocks of any quantum computer. Superconducting qubits have been created with a top-down approach that integrates superconducting devices into macroscopic electrical circuits, and electron-spin qubits have been demonstrated in quantum dots. The phase coherence time (t2) and the single qubit figure of merit (QM) of superconducting and electron-spin qubits are similar: a t2 ~ microseconds and QM ~ 10 - 1,000 below 100 mK and it should be possible to scale up these systems, which is essential for the development of any useful quantum computer. Bottom-up approaches based on dilute ensembles of spins have achieved much larger values of t2 (up to tens of milliseconds), but these systems cannot be scaled up, although some proposals for qubits based on two-dimensional nanostructures should be scalable. Here we report that a new family of spin qubits based on rare-earth ions demonstrates values of t2 (~50 microseconds) and QM (~1,400) at 2.5 K, which suggests that rare-earth qubits may, in principle, be suitable for scalable quantum information processing at 4He temperatures.
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John Morton posted 19:16 05/02/07 |
 Isn't this also unscalable?
In the abstract, the authors refer to pulsed EPR studies of phosphorous-doped silicon [Tyryshkin et al. Phys. Rev. B 68, 193207 (2003)] which yielded T2 times some orders of magnitude longer than those reported here. The authors describe this system as "unscalable" - perhaps they could clarify the ways in which their system of rare-earth ions randomly diluted within a CaWO4 matrix is more scalable?
The authors also describe as unscalable the work on N@C60 [Mehring et al. Phys. Rev. Lett. 93, 206603 (2004)]. N@C60 has been shown to have T2 times in excess of those demonstrated here, at room temperature. I am similarly doubtful that the rare-earth spin system described here is in any way more scalable than a molecular electron spin species (which can self-assemble into well-defined multi-spin structures). Perhaps I have not understood correctly the manner in which these samples have been synthesised, but it seems that this sample is isomorphic with a dilute sample of (S=1/2) molecular electron spins in frozen solution, for which a T2 of 50 microseconds would be quite unremarkable. |
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John Morton posted 10:49 07/02/07 |
 AUTOMATED POST: Change to thread
I have made a change to the article this thread is about. The reason for the change was:
Typo corrected in abstract (50 ms -> 50 microseconds) |
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Richard George posted 13:35 07/02/07 |
Rare-earth spins
I think an interesting thing about this paper is the concentration-dependent decay of the visibility of the Rabi oscillations.
The inset on Figure 2 could be re-plotted as   versus time (linear) - they're saying the decay is fits to and would find a straight line graph if they made the semi-log plot the other way round? - compare with figure 2 in Phys. Rev. A 59, 4087 - 4090 (1999) http://prola.aps.org/abstract/PRA/v59/i5/p4087_1 The inset of figure 3 is interesting, the g-factor being anisotropic means that they can try two different RF powers or pulse lengths (hence two different MW induced heatings), and find the same  . Would the equivalent measurement be possible in a higher symmetry crystal such as diamond or silicon, or if the centres were disolved in a liquid?They use:  pulse sequence. What happens with longer CP, CPMG sequences, etc? Are there other pulse sequences that could extract more information about (or refute) the proposed model of the Rabi-frequency dependent visibility decay as being due to dipolar interaction? |
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John Morton posted 00:14 08/02/07 |
Rabi oscillation decay Richard George wrote (13:35 07/02/07): I think an interesting thing about this paper is the concentration-dependent decay of the visibility of the Rabi oscillations. Is it all possible that the decay in the Rabi oscillations is in fact due to pulsed field inhomogeneity, as in [Phys. Rev. Lett. 95 200501 (2005)]? The fact that [eqn]\tau_R[/eqn]  decreases with increasing microwave power, and especially that N(c), the number of visible oscillations, is independent of power, is consistent with the decay being simply due to different spins being rotated at different rates (i.e. an inhomogeneous microwave field). Decay by this method goes as the rotation angle applied. What happens with longer CP, CPMG sequences, etc? This would also help clarify whether decay is due to field inhomogeneity. In CP, rotation angle errors are additive, whilst in CPMG, they cancel out over successive applications. Alternatively, the application of an error correcting sequence like BB1 might be useful? For example, the figure below shows Rabi oscillations decaying due to pulsed field inhomogeneity (solid line) which have then been correcting using the BB1 protocol (dashed line). |
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Matthew Webb posted 21:44 14/02/07 |
AUTOMATED POST: Change to thread
I have made a change to the article this thread is about. The reason for the change was:
transform to local article, courtesy of PI providing copy of paper with permission to upload |
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Richard George posted 00:10 15/02/07 |
Dipole Interaction
another question occured:
The average dipole interaction energy between adjacent spins keeps a constant magnitude for a given sample concentration, whereas the Zeeman energy could be changed by measuring at S, X, W bands. How would the ratio of the Zeeman and dipolar energies affect N(c)? |
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