We develop new pulse schemes to significantly speed up adiabatic state transfer protocols. Our general strategy involves adding corrections to an initial control Hamiltonian which harness nonadiabatic transitions. These corrections define a set of dressed states that the system follows exactly during the state transfer. We apply this approach to STIRAP protocols and show that a suitable choice of dressed states allows one to design fast protocols that do not require additional couplings, while simultaneously minimizing the occupancy of the "intermediate" level.Introduction -The general goal of moving quantum states between two different systems finds numerous applications in quantum information processing [1,2]. It has generated intense theoretical interest, with numerous approaches developed to allow high fidelity state transfer that are robust against dissipation and noise. Among the more powerful and interesting strategies are adiabatic transfer protocols [3]. These generically involve adiabatically evolving an eigenstate of a composite quantum system, such that the state is initially localized in the "source" system and ends up being localized in the "target" system (see Fig. 1(a)). The adiabatic evolution thus corresponds to a state transfer, with the initial state of the source system "riding" the adiabatic eigenstates, and ending up in the target system. The most famous examples of such approaches are the STIRAP [4] and CTAP [5] protocols, well known in atomic physics.

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We theoretically model a nuclear-state preparation scheme that increases the coherence time of a two-spin qubit in a double quantum dot. The two-electron system is tuned repeatedly across a singlet-triplet level anticrossing with alternating slow and rapid sweeps of an external bias voltage. Using a Landau-Zener-Stückelberg model, we find that in addition to a small nuclear polarization that weakly affects the electron spin coherence, the slow sweeps are only partially adiabatic and lead to a weak nuclear spin measurement and a nuclear-state narrowing which prolongs the electron spin coherence. This resolves some open problems brought up by a recent experiment [D. J. Reilly, Science 321, 817 (2008).10.1126/science.1159221]. Based on our description of the weak measurement, we simulate a system with up to n=200 nuclear spins per dot. Scaling in n indicates a stronger effect for larger n.

We study Landau-Zener dynamics in a double quantum dot filled with two electrons, where the spin states can become correlated with charge states, and the level velocity can be tuned in a timedependent fashion. We show that a correct interpretation of experimental data is only possible when finite-time effects are taking into account. In addition, our formalism allows the study of partial adiabatic dynamics in the presence of phonon-mediated hyperfine relaxation and charge noise induced dephasing. Our findings demonstrate that charge noise severely impacts the visibility of LZSM interference fringes. This indicates that charge coherence must be treated on an equal footing with spin coherence.

We present a systematic, perturbative method for correcting quantum gates to suppress errors that take the target system out of a chosen subspace. It addresses the generic problem of nonadiabatic errors in adiabatic evolution and state preparation, as well as general leakage errors due to spurious couplings to undesirable states. The method is based on the Magnus expansion: by correcting control pulses, we modify the Magnus expansion of an initially-given, imperfect unitary in such a way that the desired evolution is obtained. Applications to adiabatic quantum state transfer, superconducting qubits and generalized Landau-Zener problems are discussed.

We theoretically demonstrate that nuclear spins can be harnessed to coherently control two-electron spin states in a double quantum dot. Hyperfine interactions lead to an avoided crossing between the spin singlet state and the ms = +1 triplet state, T_+ . We show that a coherent superposition of singlet and triplet states can be achieved using finite-time Landau-Zener-St\"uckelberg interferometry. In this system the coherent rotation rate is set by the Zeeman energy, resulting in ~1 nanosecond single spin rotations. We analyze the coherence of this spin qubit by considering the coupling to the nuclear spin bath and show that T_2^* ~ 16 ns, in good agreement with experimental data. Our analysis further demonstrates that efficient single qubit and two qubit control can be achieved using Landau-Zener-St\"uckelberg interferometry.Comment: 8 pages, 6 figure

Microwave squeezing represents the ultimate sensitivity frontier for superconducting qubit measurement. However, measurement enhancement has remained elusive, in part because integration with standard dispersive readout pollutes the signal channel with antisqueezed noise. Here we induce a stroboscopic light-matter coupling with superior squeezing compatibility, and observe an increase in the final signal-to-noise ratio of 24%. Squeezing the orthogonal phase slows measurement-induced dephasing by a factor of 1.8. This scheme provides a means to the practical application of squeezing for qubit measurement.

We study finite-time Landau-Zener transitions at a singlet-triplet level crossing in a GaAs double quantum dot, both experimentally and theoretically. Sweeps across the anticrossing in the high driving speed limit result in oscillations with a small visibility. Here we demonstrate how to increase the oscillation visibility while keeping sweep times shorter than T * 2 using a tailored pulse with a detuning dependent level velocity. Our results show an improvement of a factor ∼ 2.9 for the oscillation visibility. In particular, we were able to obtain a visibility of ∼ 0.5 for Stückelberg oscillations, which demonstrates the creation of an equally weighted superposition of the qubit states.