Suppressing Qubit Dephasing Using Real-Time Hamiltonian Estimation

October 21, 2014
ΔBz oscillations

ΔBz oscillations.: (a) The pulse sequence used to estimate ΔBz. (b) Using nuclear feedback, ΔBz oscillations decay in a coherence time due to residual slow fluctuations in ΔBz. (c) The Ramsey sequence used to operate the S-T*2 ≈ qubit in the rotating frame. (d) The Ramsey contrast (blue dots) decays in a characteristic time (solid line fit ) similar to the oscillations in b due to the same residual slow fluctuations in ΔBz. (e) The Rabi pulse sequence used to drive the qubit in the rotating frame. (f) The rotating frame S−T*2 ≈ qubit exhibits the typical behaviour when sweeping drive frequency and time (top). When driven on resonance (bottom), the qubit undergoes Rabi oscillations, demonstrating control in the rotating frame. [M.D. Shulman, S.P. Harvey, J.M. Nichol, S.D. Bartlett, A.C. Doherty, V. Umansky & A. Yacoby,"Suppressing qubit dephasing using real-time Hamiltonian estimation,"Nature Communications 5 | doi:10.1038/ncomms6156]

Unwanted interaction between a quantum system and its fluctuating environment leads to decoherence and is the primary obstacle to establishing a scalable quantum information processing architecture. Strategies such as environmental and materials engineering, quantum error correction, and dynamical decoupling can mitigate decoherence, but generally increase experimental complexity. In a recent Nature Communications article, a group of researchers led by Prof. Yacoby described their ersearch on improving coherence in a qubit using real-time Hamiltonian parameter estimation. Using a rapidly converging Bayesian approach, the physicists precisely measure the splitting in a singlet-triplet spin qubit faster than the surrounding nuclear bath fluctuates. They continuously adjust qubit control parameters based on this information, thereby improving the inhomogenously broadened coherence time (T*2) from tens of nanoseconds to >2 μs. Because the technique is compatible with arbitrary qubit operations, it is a natural complement to quantum error correction and can be used to improve the performance of a wide variety of qubits in both meteorological and quantum information processing applications.