In quantum dots made from materials with nonzero nuclear spins, hyperfine coupling creates a fluc... more In quantum dots made from materials with nonzero nuclear spins, hyperfine coupling creates a fluctuating effective Zeeman field (Overhauser field) felt by electrons, which can be a dominant source of spin qubit decoherence. We characterize the spectral properties of the fluctuating Overhauser field in a GaAs double quantum dot by measuring correlation functions and power spectra of the rate of singlet-triplet mixing of two separated electrons. Away from zero field, spectral weight is concentrated below 10 Hz, with ∼ 1/f 2 dependence on frequency, f . This is consistent with a model of nuclear spin diffusion, and indicates that decoherence can be largely suppressed by echo techniques. arXiv:0712.4033v3 [cond-mat.mes-hall]
Circuit quantum electrodynamics allows spatially separated superconducting qubits to interact via... more Circuit quantum electrodynamics allows spatially separated superconducting qubits to interact via a "quantum bus", enabling two-qubit entanglement and the implementation of simple quantum algorithms. We combine the circuit quantum electrodynamics architecture with spin qubits by coupling an InAs nanowire double quantum dot to a superconducting cavity. We drive single spin rotations using electric dipole spin resonance and demonstrate that photons trapped in the cavity are sensitive to single spin dynamics. The hybrid quantum system allows measurements of the spin lifetime and the observation of coherent spin rotations. Our results demonstrate that a spin-cavity coupling strength of 1 MHz is feasible. PACS numbers: 03.67.Lx, 42.50.Pq, 73.63.Kv, 85.35.Be 1 arXiv:1205.6767v1 [cond-mat.mes-hall] 30 May 2012
We demonstrate high speed manipulation of a few-electron double quantum dot. In the one-electron ... more We demonstrate high speed manipulation of a few-electron double quantum dot. In the one-electron regime, the double dot forms a charge qubit. Microwaves are used drive transitions between the (1,0) and (0,1) charge states of the double dot. A local quantum point contact charge detector measures the photon-induced change in occupancy of the charge states. Charge detection is used to measure T 1 ∼16 ns and also provides a lower bound estimate for T * 2 of 400 ps for the charge qubit. In the two-electron regime we use pulsed-gate techniques to measure the singlet-triplet relaxation time for nearly-degenerate spin states. These experiments demonstrate that the hyperfine interaction leads to fast spin relaxation at low magnetic fields. Finally, we discuss how two-electron spin states can be used to form a logical spin qubit.
We describe a technique for quantum information processing based on localized ensembles of nuclea... more We describe a technique for quantum information processing based on localized ensembles of nuclear spins. A qubit is identified as the presence or absence of a collective excitation of a mesoscopic ensemble of nuclear spins surrounding a single quantum dot. All single and two-qubit operations can be effected using hyperfine interactions and single-electron spin rotations, hence the proposed scheme avoids gate errors arising from entanglement between spin and orbital degrees of freedom. Ultra-long coherence times of nuclear spins suggest that this scheme could be particularly well suited for applications where long lived memory is essential.
Techniques for coherent control of electron spin-nuclear spin interactions in quantum dots can be... more Techniques for coherent control of electron spin-nuclear spin interactions in quantum dots can be directly applied in spintronics and in quantum information processing. In this work we study numerically the interaction of electron and nuclear spins in the context of storing the spin-state of an electron in a collective state of nuclear spins. We take into account the errors inherent in a realistic system: the incomplete polarization of the bath of nuclear spins and the different hyperfine interactions between the electron and individual nuclei in the quantum dot. Although these imperfections deteriorate the fidelity of the quantum information retrieval, we find reasonable fidelities are achievable for modest bath polarizations.
We show how realistic charge manipulation and measurement techniques, combined with the exchange ... more We show how realistic charge manipulation and measurement techniques, combined with the exchange interaction, allow for the robust generation and purification of four-particle spin entangled states in electrically controlled semiconductor quantum dots. The generated states are immunized to the dominant sources of noise via a dynamical decoherence-free subspace; all additional errors are corrected by a purification protocol. This approach may find application in quantum computation, communication, and metrology.
We show that spin-orbit coupling in a quantum dot molecule allows for coherent manipulation of tw... more We show that spin-orbit coupling in a quantum dot molecule allows for coherent manipulation of two electron spin states using Raman transitions. Such two-electron spin states defined by the singlet and triplet states of two exchange coupled quantum dots can have favorable coherence properties. In addition, two of the four metastable ground states in this system can be used as auxiliary states that could facilitate implementation of tasks such as mapping of spin states to that of a single propagating photon. We find that even weak spin-orbit effects -manifesting themselves as slightly different g-factors for the electron and the hole -would allow for the coherent Raman coupling of the singlet-triplet states. We also discuss the possibilities for implementing quantum optical techniques for spin preparation and manipulation.
We describe a coherent control technique for coupling electron spin states associated with semico... more We describe a coherent control technique for coupling electron spin states associated with semiconductor double-dot molecule to a microwave stripline resonator on a chip. We identify a novel regime of operation in which strong interaction between a molecule and a resonator can be achieved with minimal decoherence, reaching the so-called strong coupling regime of cavity QED. We describe potential applications of such a system, including low-noise coherent electrical control, fast QND measurements of spin states, and long-range spin coupling.
We report initialization, complete electrical control, and single-shot readout of an exchange-onl... more We report initialization, complete electrical control, and single-shot readout of an exchange-only spin qubit. Full control via the exchange interaction is fast, yielding a demonstrated 75 qubit rotations in under 2 ns. Measurement and state tomography are performed using a maximum-likelihood estimator method, allowing decoherence, leakage out of the qubit state space, and measurement fidelity to be quantified. The methods developed here are generally applicable to systems with state leakage, noisy measurements, and non-orthogonal control axes.
We explore a method for laser cooling and optical detection of excitations in a LC electrical cir... more We explore a method for laser cooling and optical detection of excitations in a LC electrical circuit. Our approach uses a nanomechanical oscillator as a transducer between optical and electronic excitations. An experimentally feasible system with the oscillator capacitively coupled to the LC and at the same time interacting with light via an optomechanical force is shown to provide strong electro-mechanical coupling. Conditions for improved sensitivity and quantum limited readout of electrical signals with such an "optical loud speaker" are outlined. PACS numbers: 42.50.Wk, 78.20.Jq, 85.60.Bt Cooling plays an essential roles in most areas of physics, in part because it reduces detrimental thermal fluctuations. For sensing application, where thermal fluctuations may hide the small signals one is trying to measure, strong coupling of mechanical and electrical oscillators to systems in a pure quantum state, such as light or polarized atomic ensembles, opens up new possibilities for quantum sensing of fields and forces [1]. This in principle allows for enhanced sensitivity of the oscillators, where readout of their state is limited only by quantum fluctuations. In recent years, dramatic advances in optomechanical coupling and cooling of high quality-factor (Q) mechanical systems have been made [2].
Artificial molecules containing just one or two electrons provide a powerful platform for studies... more 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.
We show how to bridge the divide between atomic systems and electronic devices by engineering a c... more We show how to bridge the divide between atomic systems and electronic devices by engineering a coupling between the motion of a single ion and the quantized electric field of a resonant circuit. Our method can be used to couple the internal state of an ion to the quantized circuit with the same speed as the internal-state coupling between two ions. All the well-known quantum information protocols linking ion internal and motional states can be converted to protocols between circuit photons and ion internal states. Our results enable quantum interfaces between solid state qubits, atomic qubits, and light, and lay the groundwork for a direct quantum connection between electrical and atomic metrology standards.
We describe a thin-film superconducting Nb microwave resonator, tunable to within 0.3 ppm of the ... more We describe a thin-film superconducting Nb microwave resonator, tunable to within 0.3 ppm of the hyperfine splitting of 87 Rb at f Rb = 6.834683 GHz. We coarsely tuned the resonator using electron-beam lithography, decreasing the resonance frequency from 6.8637 GHz to 6.8278 GHz.
The reflection spectrum of an optical cavity is exquisitely sensitive to length variations, enabl... more The reflection spectrum of an optical cavity is exquisitely sensitive to length variations, enabling precise and accurate displacement measurements . When combined with mechanical oscillators, such cavities can yield accelerometers of unprecedented resolution. Previously, accelerometer sensitivity enhancements were achieved by lowering the sensor's natural frequency and bandwidth. Detection near the thermal limit was achieved, but at high acceleration levels due to low oscillator mass[4]. We present a novel self-calibrating accelerometer, capable of reaching nano-g n / √ Hz sensitivities (µGal/ √ Hz -1 g n = 9.81 m/s 2 -equivalent displacement of 10 −18 m/ √ Hz) over a bandwidth of several kHz, and compare its accuracy to a calibrated commercial system. It consists of a compact (10.6×15 mm), high-mQ (5 kg) fused-silica oscillator that utilizes fiber-optic micro-mirror cavities, for self-calibrated detection of the motions of its test-mass . This device provides a substantial improvement over conventional systems in accelerometry, standards and calibrations, optomechanics, seismology and gravimetry.
We investigate phonon-induced spin and charge relaxation mediated by spin-orbit and hyperfine int... more 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 non-monotonically 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.
δ Δ ^F IG. 1: (a) Schematics of the interface: Atoms (sphere), in a 1D lattice of the length L, a... more δ Δ ^F IG. 1: (a) Schematics of the interface: Atoms (sphere), in a 1D lattice of the length L, are electrically (magnetically) coupled to light in the nanofiber (the superconducting waveguide), respectively. The quantum microwave (optical) field with Rabi frequencyÊM (Êo) is manipulated using a classical control radio-frequency (optical) field with Rabi frequency ΩM (Ωo), respectively. Trapping lights are not shown in the (b) The quantum (ÊM) and control (ΩM) electromagnetic fields arrive while the atomic system is in the ground state. (c) The quantum field is stored as an atomic spin excitation (S(z)). (d) Internal level structure of a 87 Rb atom and transitions induced by the four electromagnetic fields. δ, ∆2 are two-photon and one-photon detunings, respectively. (e) Dimensions of an example LC resonator. (f) Magnetic field profile viewed from the top, lighter colors show higher fields.
Self-assembled semiconductor quantum dots show remarkable optical and spin coherence properties, ... more Self-assembled semiconductor quantum dots show remarkable optical and spin coherence properties, which have lead to a concerted research effort examining their potential as a quantum bit for quantum information science 1-6 .
We propose a method to achieve coherent coupling between Nitrogen Vacancy (NV) centers in diamond... more We propose a method to achieve coherent coupling between Nitrogen Vacancy (NV) centers in diamond and superconducting flux qubits. The resulting coupling can be used to create a coherent interaction between the spin states of distant NV centers mediated by the flux qubit. Furthermore, we show that the coupling can be used to achieve a coherent transfer of quantum information between the flux qubit and an ensemble of NV centers. This opens the possibility of using nuclear spin ensembles as a long-term memory for a superconducting quantum information processor and may also enable interfacing superconducting qubits with optical photons.
We describe an opto-electronic structure in which charge and spin degrees of freedom in electrica... more We describe an opto-electronic structure in which charge and spin degrees of freedom in electrical gate-defined quantum dots can be coherently coupled to light. This is achieved via electron-electron interaction or via electron tunneling into a proximal self-assembled quantum dot. We illustrate potential applications of this approach by considering several quantum control techniques, including optical read-out of gate-controlled semiconductor quantum bits and controlled generation of entangled photon-spin pairs.
We investigate the quantum dynamics of systems involving small numbers of strongly interacting ph... more We investigate the quantum dynamics of systems involving small numbers of strongly interacting photons. Specifically, we develop an efficient method to investigate such systems when they are externally driven with a coherent field. Furthermore, we show how to quantify the many-body quantum state of light via correlation functions. Finally, we apply this method to two strongly interacting cases: the Bose-Hubbard and fractional quantum Hall models, and discuss an implementation of these ideas in atom-photon system.
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Papers by Jacob Taylor