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ABSTRACT: The interaction of an electronic spin with its nuclear environment, an issue known as the central spin problem, has been the subject of considerable attention due to its relevance for spin-based quantum computation using semiconductor quantum dots. Independent control of the nuclear spin bath using nuclear magnetic resonance techniques and dynamic nuclear polarization using the central spin itself offer unique possibilities for manipulating the nuclear bath with significant consequences for the coherence and controlled manipulation of the central spin. Here we review some of the recent optical and transport experiments that have explored this central spin problem using semiconductor quantum dots. We focus on the interaction between 10(4)-10(6) nuclear spins and a spin of a single electron or valence-band hole. We also review the experimental techniques as well as the key theoretical ideas and the implications for quantum information science.
Nature Material 06/2013; 12(6):494-504. · 32.84 Impact Factor
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ABSTRACT: We investigate phonon-induced spin and charge relaxation mediated by spin-orbit and hyperfine interactions for a single electron confined within a double quantum dot. A simple toy model incorporating both direct decay to the ground state of the double dot and indirect decay via an intermediate excited state yields an electron spin relaxation rate that varies nonmonotonically with the detuning between the dots. We confirm this model with experiments performed on a GaAs double dot, demonstrating that the relaxation rate exhibits the expected detuning dependence and can be electrically tuned over several orders of magnitude. Our analysis suggests that spin-orbit mediated relaxation via phonons serves as the dominant mechanism through which the double-dot electron spin-flip rate varies with detuning.
Physical Review Letters 05/2013; 110(19):196803. · 7.37 Impact Factor
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ABSTRACT: Controlling long-distance quantum correlations is central to quantum computation and simulation. In quantum dot arrays, experiments so far rely on nearest-neighbour couplings only, and inducing long-distance correlations requires sequential local operations. Here, we show that two distant sites can be tunnel-coupled directly. The coupling is mediated by virtual occupation of an intermediate site, with a strength that is controlled via the energy detuning of this site. It permits a single charge to oscillate coherently between the outer sites of a triple dot array without passing through the middle, as demonstrated through the observation of Landau-Zener-Stückelberg interference. The long-distance coupling significantly improves the prospects of fault-tolerant quantum computation using quantum dot arrays, and opens up new avenues for performing quantum simulations in nanoscale devices.
Nature Nanotechnology 04/2013; · 27.27 Impact Factor
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ABSTRACT: We investigate the electric manipulation of a single-electron spin in a single gate-defined quantum dot. We observe that so-far neglected differences between the hyperfine- and spin-orbit-mediated electric dipole spin resonance conditions have important consequences at high magnetic fields. In experiments using adiabatic rapid passage to invert the electron spin, we observe an unusually wide and asymmetric response as a function of the magnetic field. Simulations support the interpretation of the line shape in terms of four different resonance conditions. These findings may lead to isotope-selective control of dynamic nuclear polarization in quantum dots.
Physical Review Letters 03/2013; 110(10):107601. · 7.37 Impact Factor
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ABSTRACT: We study the formation of wrinkles in graphene upon wet transfer onto a
target substrate, whereby draining of water appears to play an important role.
We are able to control the orientation of the wrinkles by tuning the surface
morphology. Wrinkles are absent in flakes transferred to strongly hydrophobic
substrates, a further indication of the role of the interaction of water with
the substrate in wrinkle formation. The electrical and structural integrity of
the graphene is not affected by the wrinkles, as inferred from Raman
measurements and electrical conductivity measurements.
07/2012;
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ABSTRACT: Contamination of graphene due to residues from nanofabrication often
introduces background doping and reduces charge carrier mobility. For samples
of high electronic quality, post-lithography cleaning treatments are therefore
needed. We report that mechanical cleaning based on contact mode AFM removes
residues and significantly improves the electronic properties. A mechanically
cleaned dual-gated bilayer graphene transistor with hBN dielectrics exhibited a
mobility of ~36,000 cm2/Vs at low temperature.
12/2011;
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ABSTRACT: We present a scheme for achieving coherent spin squeezing of nuclear spin states in semiconductor quantum dots. The nuclear polarization dependence of the electron spin resonance generates a unitary evolution that drives nuclear spins into a collective entangled state. The polarization dependence of the resonance generates an area-preserving, twisting dynamics that squeezes and stretches the nuclear spin Wigner distribution without the need for nuclear spin flips. Our estimates of squeezing times indicate that the entanglement threshold can be reached in current experiments.
Physical Review Letters 11/2011; 107(20):206806. · 7.37 Impact Factor
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ABSTRACT: Measurement of coupled quantum systems plays a central role in quantum information processing. We have realized independent single-shot read-out of two electron spins in a double quantum dot. The read-out method is all-electrical, cross-talk between the two measurements is negligible, and read-out fidelities are ~86% on average. This allows us to directly probe the anticorrelations between two spins prepared in a singlet state and to demonstrate the operation of the two-qubit exchange gate on a complete set of basis states. The results provide a possible route to the realization and efficient characterization of multiqubit quantum circuits based on single quantum dot spins.
Science 08/2011; 333(6047):1269-72. · 31.20 Impact Factor
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ABSTRACT: Entanglement generation and detection are two of the most sought-after goals
in the field of quantum control. Besides offering a means to probe some of the
most peculiar and fundamental aspects of quantum mechanics, entanglement in
many-body systems can be used as a tool to reduce fluctuations below the
standard quantum limit. For spins, or spin-like systems, such a reduction of
fluctuations can be realized with so-called squeezed states. Here we present a
scheme for achieving coherent spin squeezing of nuclear spin states in
few-electron quantum dots. This work represents a major shift from earlier
studies in quantum dots, which have explored classical "narrowing" of the
nuclear polarization distribution through feedback involving stochastic spin
flips. In contrast, we use the nuclear-polarization-dependence of the electron
spin resonance (ESR) to provide a non-linearity which generates a non-trivial,
area-preserving, "twisting" dynamics that squeezes and stretches the nuclear
spin Wigner distribution without the need for nuclear spin flips.
01/2011;
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ABSTRACT: Artificial molecules containing just one or two electrons provide a powerful platform for studies of orbital and spin quantum dynamics in nanoscale devices. A well-known example of these dynamics is tunnelling of electrons between two coupled quantum dots triggered by microwave irradiation. So far, these tunnelling processes have been treated as electric-dipole-allowed spin-conserving events. Here we report that microwaves can also excite tunnelling transitions between states with different spin. We show that the dominant mechanism responsible for violation of spin conservation is the spin-orbit interaction. These transitions make it possible to perform detailed microwave spectroscopy of the molecular spin states of an artificial hydrogen molecule and open up the possibility of realizing full quantum control of a two-spin system through microwave excitation.
Nature Communications 01/2011; 2:556. · 7.40 Impact Factor
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ABSTRACT: Artificial molecules containing just one or two electrons provide a powerful
platform for studies of orbital and spin quantum dynamics in nanoscale devices.
A well-known example of these dynamics is tunneling of electrons between two
coupled quantum dots triggered by microwave irradiation. So far, these
tunneling processes have been treated as electric dipole-allowed
spin-conserving events. Here we report that microwaves can also excite
tunneling transitions between states with different spin. In this work, the
dominant mechanism responsible for violation of spin conservation is the
spin-orbit interaction. These transitions make it possible to perform detailed
microwave spectroscopy of the molecular spin states of an artificial hydrogen
molecule and open up the possibility of realizing full quantum control of a two
spin system via microwave excitation.
10/2010;
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ABSTRACT: Two-qubit interactions are at the heart of quantum information processing.
For single-spin qubits in semiconductor quantum dots, the exchange gate has
always been considered the natural two-qubit gate. The recent integration of
magnetic field or g-factor gradients in coupled quantum dot systems allows for
a one-step, robust realization of the controlled phase (C-Phase) gate instead.
We analyze the C-Phase gate durations and fidelities that can be obtained under
realistic conditions, including the effects of charge and nuclear field
fluctuations, and find gate error probabilities of below 10-4, possibly
allowing fault-tolerant quantum computation.
10/2010;
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ABSTRACT: Transport through spin-blockaded quantum dots provides a means for electrical
control and detection of nuclear spin dynamics in the host material. Although
such experiments have become increasingly popular in recent years,
interpretation of their results in terms of the underlying nuclear spin
dynamics remains challenging. Here we point out a fundamental process in which
nuclear spin dynamics can be driven by electron shot noise; fast electric
current fluctuations generate much slower nuclear polarization dynamics, which
in turn affect electron dynamics via the Overhauser field. The resulting
extremely slow intermittent current fluctuations account for a variety of
observed phenomena that were not previously understood.
01/2010;
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ABSTRACT: We observe multiple stable states of nuclear polarization and nuclear self-tuning over a large range of fields in a double quantum dot under conditions of electron spin resonance. The observations can be understood within an elaborated theoretical rate equation model for the polarization in each of the dots, in the limit of strong driving. This model also captures unusual features of the data, such as fast switching and a "wrong" sign of polarization. The results reported enable applications of this polarization effect, including accurate manipulation and control of nuclear fields.
Physical Review Letters 08/2009; 103(4):046601. · 7.37 Impact Factor
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ABSTRACT: The main obstacle to coherent control of two-level quantum systems is their coupling to an uncontrolled environment. For electron spins in III-V quantum dots, the random environment is mostly given by the nuclear spins in the quantum dot host material; they collectively act on the electron spin through the hyperfine interaction, much like a random magnetic field. Here we show that the same hyperfine interaction can be harnessed such that partial control of the normally uncontrolled environment becomes possible. In particular, we observe that the electron spin resonance frequency remains locked to the frequency of an applied microwave magnetic field, even when the external magnetic field or the excitation frequency are changed. The nuclear field thereby adjusts itself such that the electron spin resonance condition remains satisfied. General theoretical arguments indicate that this spin resonance locking is accompanied by a significant reduction of the randomness in the nuclear field. Comment: 6 pages, 5 figures, 4 pages supplementary material
02/2009;
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ABSTRACT: We report a measurement of the spin-echo decay of a single electron spin confined in a semiconductor quantum dot. When we tip the spin in the transverse plane via a magnetic field burst, it dephases in 37 ns due to the Larmor precession around a random effective field from the nuclear spins in the host material. We reverse this dephasing to a large extent via a spin-echo pulse, and find a spin-echo decay time of about 0.5 micros at 70 mT. These results are in the range of theoretical predictions of the electron spin coherence time governed by the electron-nuclear dynamics.
Physical Review Letters 06/2008; 100(23):236802. · 7.37 Impact Factor
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ABSTRACT: Manipulation of single spins is essential for spin-based quantum information processing. Electrical control instead of magnetic control is particularly appealing for this purpose, because electric fields are easy to generate locally on-chip. We experimentally realized coherent control of a single-electron spin in a quantum dot using an oscillating electric field generated by a local gate. The electric field induced coherent transitions (Rabi oscillations) between spin-up and spin-down with 90 degrees rotations as fast as approximately 55 nanoseconds. Our analysis indicated that the electrically induced spin transitions were mediated by the spin-orbit interaction. Taken together with the recently demonstrated coherent exchange of two neighboring spins, our results establish the feasibility of fully electrical manipulation of spin qubits.
Science 12/2007; 318(5855):1430-3. · 31.20 Impact Factor
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ABSTRACT: We study, both theoretically and experimentally, driven Rabi oscillations of a single electron spin coupled to a nuclear-spin bath. Because of the long correlation time of the bath, two unusual features are observed in the oscillations. The decay follows a power law, and the oscillations are shifted in phase by a universal value of approximately pi/4. These properties are well understood from a theoretical expression that we derive here in the static limit for the nuclear bath. This improved understanding of the coupled electron-nuclear system is important for future experiments using the electron spin as a qubit.
Physical Review Letters 10/2007; 99(10):106803. · 7.37 Impact Factor
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ABSTRACT: The authors employ a cryogenic high electron mobility transistor (HEMT) amplifier to increase the bandwidth of a charge detection setup with a quantum point contact (QPC) charge sensor. The HEMT is operating at 1 K and the circuit has a bandwidth of 1 MHz. The noise contribution of the HEMT at high frequencies is only a few times higher than that of the QPC shot noise. The authors use this setup to monitor single-electron tunneling to and from an adjacent quantum dot. The authors measure fluctuations in the dot occupation as short as 400 ns, 20 times faster than in previous work.
Applied Physics Letters 09/2007; 91(12):123512-123512-3. · 3.84 Impact Factor
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ABSTRACT: We study, both theoretically and experimentally, driven Rabi oscillations of a single electron spin coupled to a nuclear spin bath. Due to the long correlation time of the bath, two unusual features are observed in the oscillations. The decay follows a power law, and the oscillations are shifted in phase by a universal value of ~pi/4. These properties are well understood from a theoretical expression that we derive here in the static limit for the nuclear bath. This improved understanding of the coupled electron-nuclear system is important for future experiments using the electron spin as a qubit.
04/2007;
