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SQUBIT Résumé de rapport

Project ID: IST-1999-10673
Financé au titre de: FP5-IST
Pays: France

NMR-style operation of the quantronium qubit

Although coherent super-positions of quantum states have already been demonstrated in qubit circuits, the manipulation methods used were still rather primitive and mainly limited to Rabi precession.

The aim of the result #21684
"NMR-style operation of the Quantronium qubit" is to precisely demonstrate that sophisticated NMR methods can be applied for manipulating the quantum state of a qubit and obtain detailed information on decoherence processes. Proving that arbitrary and robust transformations of the qubit state can be performed is indeed an essential issue. The present result proves that NMR methods can be successfully used in order to achieve this goal, even if the accuracy and fidelity of presently achieved operation are still insufficient.

The main results are described in the first document:
- "NMR-like Control of a Quantum Bit Super-conducting Circuit" (to appear in Phys. Rev. Lett.). The other documents describe some parts of the result more in detail.

We have demonstrated in particular that:
1) rotations of the effective spin representing the qubit can be combined and do yield to the predicted results. This was achieved by measuring the qubit state after a two-pulse sequence with different rotation axes, as a function of the time delay between pulses. This Ramsey interference experiment allows probing the combined rotation performed by the two rotations. We have found good agreement with theoretical predictions.
2) arbitrary transformations of the qubit state can thus be achieved by combining three pulses with orthogonal rotation axes, which is achieved by using microwave pulses with phases differing by pi/2. Although this result was expected, its proof was essential.
3) robust rotations can be performed by using the composite pulse methods developed in NMR. We have demonstrated improved robustness respectively to frequency detuning using a Corpse pulse sequence. Quantum Computing requires an accuracy that has been marginally reached in NMR, despite 50 years of continuous development. Composite pulses provide the most accurate method to implement robust rotations in NMR. Demonstrating that these methods also apply to qubits paves the way to more sophisticated qubit operations.
4) Various other NMR methods can be used. We have in particular exploited the NMR spin-locking technique in order to measure relaxation and decoherence of the qubit in presence of a driving microwave field. We have found that relaxation in the driven situation is slower than during free evolution in the lab frame. This result rises the question of actively fighting decoherence. Since the Quantum Error Correcting Codes are to difficult to implement in circuits presently, we look for "analog" methods able tio fight decoherence. We found that driving the qubit can indeed increase the effective relaxation and coherence times. Note that a similar result has been recently obtained for the flux qubit.
5) the spin-echo technique, used to suppress inhomogeneous broadening in NMR, can be used to suppress the effect of low frequency temporal fluctuations of the qubit parameters. We have demonstrated that a significant increase of the coherence time can be obtained with this method. We have found in particular that the charge noise spectrum decreases faster than the expected 1/f law at high frequency, above 1MHz. This unexpected result raises the interest of the Cooper pair box, which suffers from charge noise.

Our partners already benefit from the result, which is presently the state of the art for qubit manipulation in super-conducting circuits. Other teams involved in qubit circuits will also certainly use the methods that we have applied to the quantronium. The published documents provide the necessary information for using the result, which has been communicated in international conferences, and in three international Quantum Computing schools during year 2004.


Daniel ESTEVE, (head of Quantronics)
Tél.: +33-1-69085529
Fax: +33-1-69087442