[Show abstract][Hide abstract] ABSTRACT: Tailoring spin coupling to electric fields is central to spintronics and
spin-based quantum information processing. We present an optimal micromagnet
design that produces appropriate stray magnetic fields to mediate fast
electrical spin manipulations in nanodevices. We quantify the practical
requirements for spatial field inhomogeneity and tolerance for misalignment
with spins, and propose a design scheme to improve the spin-rotation frequency
(to exceed 50MHz in GaAs nanostructures). We then validate our design by
experiments in separate devices. Our results will open a route to rapidly
control solid-state electron spins with limited lifetimes and to study coherent
spin dynamics in solids.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate fast universal electrical spin manipulation with inhomogeneous
magnetic fields. With fast Rabi frequency up to 127 MHz, we leave the
conventional regime of strong nuclear-spin influence and observe a spin-flip
fidelity > 96%, a distinct chevron Rabi pattern in the spectral-time domain,
and spin resonance linewidth limited by the Rabi frequency, not by the
dephasing rate. In addition, we establish fast z-rotations up to 54 MHz by
directly controlling the spin phase. Our findings will significantly facilitate
tomography and error correction with electron spins in quantum dots.
[Show abstract][Hide abstract] ABSTRACT: Tunneling in a quantum coherent structure is not restricted to only nearest neighbors. Hopping between distant sites is possible via the virtual occupation of otherwise avoided intermediate states. Here we report the observation of long-range transitions in the transport through three quantum dots coupled in series. A single electron is delocalized between the left and right quantum dots, while the center one remains always empty. Superpositions are formed, and both charge and spin are exchanged between the outermost dots. The delocalized electron acts as a quantum bus transferring the spin state from one end to the other. Spin selection is enabled by spin correlations. The process is detected via the observation of narrow resonances which are insensitive to Pauli spin blockade.
[Show abstract][Hide abstract] ABSTRACT: Tunneling in a quantum coherent structure is not restricted to only nearest
neighbours. Hopping between distant sites is possible via the virtual
occupation of otherwise avoided intermediate states. Here we report the
observation of long range transitions in the transport through three quantum
dots coupled in series. A single electron is delocalized between the left and
right quantum dots while the centre one remains always empty. Superpositions
are formed and both charge and spin are exchanged between the outermost dots.
Detection of the process is achieved via the observation of narrow resonances,
insensitive to the transport Pauli spin blockade.
[Show abstract][Hide abstract] ABSTRACT: Spin qubits involving one or two spins have emerged as potential building blocks for quantum information processing applications, resulting in many double quantum dot (DQD) studies. Coherent control of a two-electron spin qubit close to the singlet/triplet (S/T+) anticrossing through Landau-Zener-St\"uckelberg (LZS) oscillations has been studied theoretically and demonstrated experimentally in DQDs. Recent advances with triple quantum dot (TQD) technology have suggested additional advantages, such as their potential for encoding quantum information, that may soon be possible. Towards these goals we demonstrate, for for first time, the coherent manipulation of three-particle spin states in a TQD where all three spins play a role.
[Show abstract][Hide abstract] ABSTRACT: Spin qubits based on interacting spins in double quantum dots have been demonstrated successfully. Readout of the qubit state involves a conversion of spin to charge information, which is universally achieved by taking advantage of a spin blockade phenomenon resulting from Pauli's exclusion principle. The archetypal spin blockade transport signature in double quantum dots takes the form of a rectified current. At present, more complex spin qubit circuits including triple quantum dots are being developed. Here we show, both experimentally and theoretically, that in a linear triple quantum dot circuit the spin blockade becomes bipolar with current strongly suppressed in both bias directions and also that a new quantum coherent mechanism becomes relevant. In this mechanism, charge is transferred non-intuitively via coherent states from one end of the linear triple dot circuit to the other, without involving the centre site. Our results have implications for future complex nanospintronic circuits.
[Show abstract][Hide abstract] ABSTRACT: We report on the Overhauser shift (OHS) of electric dipole spin resonance peaks in the reverse direction to that previously reported. Measuring electric dipole spin resonance in a double quantum dot, we observe two resonance peaks reflecting a Zeeman energy difference between the two quantum dots, and find that the dependence of the peak position on microwave power is different for each dot. The perturbation theory for hybridizing the flip-flop and photon-assisted tunneling mechanisms is discussed and is found to successfully explain the power dependence. This theory can also consistently explain the OHS in both directions, explaining the experimental results previously reported. Our results reveal the bidirectionality of OHSs and this is expected to play an important part in the development of effective nuclear spin squeezing techniques.
New Journal of Physics 12/2012; 14(12):123013. DOI:10.1088/1367-2630/14/12/123013 · 3.67 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The original spin qubit proposal  suggested a linear array of spins
for quantum computations and the exchange interaction for 2 qubit
operations. An essential component of the proposal was the ability to
control pairwise the exchange interaction between neighbouring pairs of
spins. In this work we experimentally demonstrate such a pairwise
control of the exchange interaction between three spins localized in a
triple quantum dot (TQD) device. The TQD potential was formed using
electrostatic lateral split-gate technology on a GaAs/GaAlAs
heterostructure with a high-mobility two-dimensional electron gas .
We employ fast pulsing technique based on the Landau-Zener-Stuckelberg
(LZS) approach for creating and manipulating coherent superpositions of
three spin quantum states . We show that we are able to maintain
coherence when increasing the exchange coupling of one spin with another
while simultaneously decreasing its coupling with the third.[4pt]  D.
Loss, and D.P. DiVincenzo, Phys. Rev. A57, 120-126 (1998).[0pt]  L.
Gaudreau , et al., Appl. Phys. Lett. v.95, 193101 (2009). [0pt]  J.R.
Petta, H. Lu, and A.C. Gossard, Science v.327, 669-672 (2010).
[Show abstract][Hide abstract] ABSTRACT: Electric dipole spin resonance of two individual electrons and the influence of hyperfine coupling on the spin resonance are studied for a double quantum dot equipped with a micro-magnet. The spin resonance occurs by oscillating the electron in each dot at microwave (MW) frequencies in the presence of a micro-magnet induced stray field. The observed continuous wave (CW) and time-resolved spin resonances are consistent with calculations in which the MW induced AC electric field and micro-magnet induced stray field are taken into account. The influence of hyperfine coupling causes an increase and broadening of the respective CW spin resonance peaks through dynamical nuclear polarization when sweeping up the magnetic field. This behaviour appears stronger for the larger of the two spin resonance peaks and in general becomes more pronounced as the MW power increases, both reflecting that the electron-nuclei interaction is more efficient for the stronger spin resonance. In addition the hyperfine coupling effect only becomes pronounced when the MW induced AC magnetic field exceeds the fluctuating nuclear field.
Journal of Physics Conference Series 12/2011; 334(1). DOI:10.1088/1742-6596/334/1/012009
[Show abstract][Hide abstract] ABSTRACT: We have established a quantitative method to evaluate the charge sensing
sensitivity using a quantum point contact. From the experimental data of
a lateral triple quantum dot, we have obtained potential modulations at
the saddle point imposed by an electron in each quantum dot of ~1
μeV. The estimated screening lengths for each quantum dot showed a
clear position dependence probably due to the gate structures.
[Show abstract][Hide abstract] ABSTRACT: A crucial requirement for quantum information processing is the realization
of multiple-qubit quantum gates. Here, we demonstrate an electron spin based
all-electrical two-qubit gate consisting of single spin rotations and inter-dot
spin exchange in a double quantum dot. A partially entangled output state is
obtained by the application of the two-qubit gate to an initial, uncorrelated
state. We find that the degree of entanglement is controllable by the exchange
operation time. The approach represents a key step towards the realization of
universal multiple qubit gates.
[Show abstract][Hide abstract] ABSTRACT: We have demonstrated Landau-Zener-Stuckelberg oscillations in a triple quantum dot circuit related to pairs of triple quantum dot states. Different initialization schemes and pulse shapes involving all three dots will be discussed. However, the complexity of a triple quantum dot system suggests that in general coherent behaviour can be expected from interplays between various combinations of states. Here we demonstrate both experimentally and theoretically in a triple quantum circuit containing three spins, a coherent interplay between two coexisting qubits as a function of pulse amplitude and rise time. To further clarify the behaviour within the system we also observe and study coherent oscillations after a fourth spin has been added to the system in one of the relevant dots.
[Show abstract][Hide abstract] ABSTRACT: Recently, Landau-Zener-Stuckelberg (LZS) oscillations have been demonstrated in a double quantum dot device . In this talk we demonstrate LZS oscillations in a triple quantum dot environment. Our triple quantum dot design allows us to tune to either the charge or spin qubit regimes. Using a pulsing technique in the spin qubit regime, we create a superposition of triple quantum dot states, allow for phase accumulation, and interfere. We demonstrate coherent LZS oscillations with three spins across the triple quantum dot structure. We investigate their dependence on pulse rise time, separation time, energy detuning, and magnetic field. [4pt]  J. R. Petta et al., Science 327, 669 (2010).
[Show abstract][Hide abstract] ABSTRACT: Electron spin confined in quantum dots is a promising candidate for experimental qubits. Aiming at realizing a three spin-qubit system, we designed split micromagnets suitable for the lateral triple quantum dots. From numerical simulations of the stray magnetic field distribution, field gradients ∼0.8 T /μ m and differences of in-plane components ∼10 mT can be attained, which enable the electrical and addressable manipulation of three qubits. Furthermore, this technique can be applied for up to 25 qubits in realistic multiple quantum dots. For the first step of implementing such three-qubit systems, a relevant triple quantum dot device has been fabricated and characteristic charge states were observed.
[Show abstract][Hide abstract] ABSTRACT: We measure a triple quantum dot in the regime where three addition lines, corresponding to the addition of an electron to each of three dots, pass through each other. In particular, we probe the interplay between transport and the three-dimensional nature of the stability diagram. We choose the regime most pertinent for spin qubit applications. We find that at low bias transport through the triple quantum dot circuit is only possible at six quadruple point locations. The results are consistent with an equivalent circuit model.
Physical Review B 08/2010; 82(7). DOI:10.1103/PhysRevB.82.075304 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report the coherent manipulation of electron spins in a double quantum dot integrated with a micromagnet. We performed electric dipole spin resonance experiments in the continuous wave (CW) and pump-and-probe modes. We observed two resonant CW peaks and two Rabi oscillations of the quantum dot current by sweeping an external magnetic field at a fixed frequency. Two peaks and oscillations are measured at different resonant magnetic field, which reflects the fact that the local magnetic fields at each quantum dot are modulated by the stray field of a micromagnet. As predicted with a density matrix approach, the CW current is quadratic with respect to microwave (MW) voltage while the Rabi frequency (νRabi) is linear. The difference between the νRabi values of two Rabi oscillations directly reflects the MW electric field across the two dots. These results show that the spins on each dot can be manipulated coherently at will by tuning the micromagnet alignment and MW electric field.