Mark S. Sherwin

Coherent manipulation of semiconductor quantum bits with terahertz radiation.

Quantum bits (qubits) are the fundamental building blocks of quantum information processors, such as quantum computers. A qubit comprises a pair of well characterized quantum states that can in principle be manipulated quickly compared to the time it takes them to decohere by coupling to their environment. Much remains to be understood about the manipulation and decoherence of semiconductor qubits. Here we show that hydrogen-atom-like motional states of electrons bound to donor impurities in currently available semiconductors can serve as model qubits. We use intense pulses of terahertz radiation to induce coherent, damped Rabi oscillations in the population of two low-lying states of donor impurities in GaAs. Our observations demonstrate that a quantum-confined extrinsic electron in a semiconductor can be coherently manipulated like an atomic electron, even while sharing space with ∼105 atoms in its semiconductor host. We anticipate that this model system will be useful for measuring intrinsic decoherence processes, and for testing both simple and complex manipulations of semiconductor qubits.

Quantum computation with quantum dots and terahertz cavity quantum electrodynamics

A quantum computer processes quantum information which is stored in “quantum bits” (qubits). [1] If a small set of fundamental operations, or “universal quantum logic gates,” can be performed on the qubits, then a quantum computer can be programmed to solve an arbitrary problem [2]. The explosion of interest in quantum computation can be traced to Shor’s demonstration in 1994 that a quantum computer could efficiently factorize large integers [3]. Further boosts came in 1996, with the proof that quantum error correcting codes exist [4,5]. It has since been shown that if the quantum error rate is below an accuracy threshold, quantum information can be stored indefinitely [6]. The implementation of a large-scale quantum computer is recognized to be a technological challenge of unprecendented proportions. The qubits must be wellisolated from the decohering influence of the environment, but must also be manipulated individually to initialize the computer, perform quantum logic operations, and measure the result of the computation [7]. Implementations of universal quantum logic gates and quantum computers have been proposed using atomic beams [8], trapped atoms [9] and ions [10], bulk nuclear magnetic resonance [11], nanostructured semiconductors [12–15] and Josephson junctions [16,17]. In schemes based on trapped atoms and ions, qubits couple with collective excitations or cavity photons. Such long-range coupling enables two-bit gates involving an arbitrary pair of qubits, which makes programming straightforward. However, in the atomic and ionic schemes [9,10], the gates must be performed serially, whereas existing error correcting schemes require some degree of parallelism. In semiconductor and superconductor schemes which have been proposed [12–17], only nearest-neighbor qubits can be coupled, and significant overhead is involved in coupling distant qubits. However, some of these schemes have the important advantage that gate operations can be performed in parallel. It is widely agreed that a solid-state quantum computer, if it can be realized, will be the only way to produce a quantum computer containing, for example, 10 qubits. The remainder of this paper describes what is, to our knowledge, the first proposal for a semiconductorbased quantum computer in which quantum gates can be effected between an arbitrary pair of qubits. The qubits consist of the lowest electronic states of speciallyengineered quantum dots (QDs) and are coupled by Terahertz cavity photons. The proposal combines ideas from the atomic and ionic implementations described above with recent developments in the spectroscopy of doped semiconductor nanostructures at Terahertz frequencies [18–20].

Experimental observation of electron-hole recollisions.

An intense laser field can remove an electron from an atom or molecule and pull the electron into a large-amplitude oscillation in which it repeatedly collides with the charged core it left behind. Such recollisions result in the emission of very energetic photons by means of high-order-harmonic generation, which has been observed in atomic and molecular gases as well as in a bulk crystal. An exciton is an atom-like excitation of a solid in which an electron that is excited from the valence band is bound by the Coulomb interaction to the hole it left behind. It has been predicted that recollisions between electrons and holes in excitons will result in a new phenomenon: high-order-sideband generation. In this process, excitons are created by a weak near-infrared laser of frequency fNIR. An intense laser field at a much lower frequency, fTHz, then removes the electron from the exciton and causes it to recollide with the resulting hole. New emission is predicted to occur as sidebands of frequency fNIR + 2nfTHz, where n is an integer that can be much greater than one. Here we report the observation of high-order-sideband generation in semiconductor quantum wells. Sidebands are observed up to eighteenth order (+18fTHz, or n = 9). The intensity of the high-order sidebands decays only weakly with increasing sideband order, confirming the non-perturbative nature of the effect. Sidebands are strongest for linearly polarized terahertz radiation and vanish when the terahertz radiation is circularly polarized. Beyond their fundamental scientific significance, our results suggest a new mechanism for the ultrafast modulation of light, which has potential applications in terabit-rate optical communications.

Two-dimensional terahertz photonic crystals fabricated by deep reactive ion etching in Si

Two-dimensional terahertz photonic crystals were manufactured from Si using deep reactive ion etching. Arrays of square holes with widths of 80 (100) μm and lattice constants of 100 (125) μm were etched through 500-μm-thick wafers with high resistivity. Stop bands with transmittance 200 GHz were observed near 1 THz for light with an electric field vector in the plane of the wafers (TE polarization). The observed stop bands are close to TE photonic band gaps predicted by a two-dimensional calculation.

An improved model for non-resonant terahertz detection in field-effect transistors

Transistors operating well above the frequencies at which they have gain can still rectify terahertz currents and voltages, and have attracted interest as room-temperature terahertz detectors. We show that such rectifying field-effect transistors may still be treated as a lumped element device in the limit where plasma resonances of the electron gas do not occur. We derive analytic formulas for important transistor parameters, such as effective rectification length and device impedance using a transmission-line model. We draw conclusions for plasma-resonant detection where possible. We derive the THz response of a field-effect transistor with a two-dimensional electron-gas channel by a Taylor expansion of the drain–source bias. We connect circuit theory to the existing theories that describe the bias in the gated region by differential equations. Parasitic effects, such as the access resistance, are included. With the approach presented in this paper, we derive the responsivity for a novel field detector ...

Determining the Oligomeric Structure of Proteorhodopsin by Gd3+-Based Pulsed Dipolar Spectroscopy of Multiple Distances

The structural organization of the functionally relevant, hexameric oligomer of green-absorbing proteorhodopsin (G-PR) was obtained from double electron-electron resonance (DEER) spectroscopy utilizing conventional nitroxide spin labels and recently developed Gd3+ -based spin labels. G-PR with nitroxide or Gd3+ labels was prepared using cysteine mutations at residues Trp58 and Thr177. By combining reliable measurements of multiple interprotein distances in the G-PR hexamer with computer modeling, we obtained a structural model that agrees with the recent crystal structure of the homologous blue-absorbing PR (B-PR) hexamer. These DEER results provide specific distance information in a membrane-mimetic environment and across loop regions that are unresolved in the crystal structure. In addition, the X-band DEER measurements using nitroxide spin labels suffered from multispin effects that, at times, compromised the detection of next-nearest neighbor distances. Performing measurements at high magnetic fields with Gd3+ spin labels increased the sensitivity considerably and alleviated the difficulties caused by multispin interactions.

Near-infrared sideband generation induced by intense far-infrared radiation in GaAs quantum wells

GaAs quantum wells are simultaneously illuminated with near-infrared (NIR) radiation at frequency ωnir and intense far-infrared (FIR) radiation from a free-electron laser at ωfir. Magnetic fields up to 9 T are applied. Strong and narrow sidebands are observed at ωsideband=ωnir±2ωfir. The intensity of the sidebands is enhanced when either ωsideband or ωnir is near the onset of NIR absorption in the quantum well, or when ωfir is near the free-electron cyclotron frequency. We attribute these sidebands to four-wave mixing of NIR and FIR photons whose energies differ by more than a factor of 100.

Coherent Manipulation and Decoherence of S = 10 Single-Molecule Magnets

We report coherent manipulation of S=10 Fe8 single-molecule magnets. The temperature dependence of the spin decoherence time T2 measured by high-frequency pulsed electron paramagnetic resonance indicates that strong spin decoherence is dominated by Fe8 spin bath fluctuations. By polarizing the spin bath in Fe8 single-molecule magnets at magnetic field B=4.6 T and temperature T=1.3 K, spin decoherence is significantly suppressed and extends the spin decoherence time T2 to as long as 712 ns. A second decoherence source is likely due to fluctuations of the nuclear spin bath. This hints that the spin decoherence time can be further extended via isotopic substitution to smaller nuclear magnetic moments.

High-Q terahertz microcavities in silicon photonic crystal slabs

Photonic crystal cavities consisting of three holes missing along a principal axis in a triangular lattice of holes in a silicon slab were fabricated. Each cavity was built into a waveguide to form a Lorentzian filter. Two samples were constructed with lattice constants a=80 and 76 μm, with a radius r=0.30a in a 44 μm thick high-resistivity silicon wafer. Transmission experiment show a single sharp resonance near 1 THz in each sample with quality factor Q as high as 1020. Contributions to the measured Q from cavity-waveguide coupling, radiative loss, and material loss are determined.

Lightwave-driven quasiparticle collisions on a subcycle timescale

Ever since Ernest Rutherford scattered α-particles from gold foils, collision experiments have revealed insights into atoms, nuclei and elementary particles. In solids, many-body correlations lead to characteristic resonances—called quasiparticles—such as excitons, dropletons, polarons and Cooper pairs. The structure and dynamics of quasiparticles are important because they define macroscopic phenomena such as Mott insulating states, spontaneous spin- and charge-order, and high-temperature superconductivity. However, the extremely short lifetimes of these entities make practical implementations of a suitable collider challenging. Here we exploit lightwave-driven charge transport, the foundation of attosecond science, to explore ultrafast quasiparticle collisions directly in the time domain: a femtosecond optical pulse creates excitonic electron–hole pairs in the layered dichalcogenide tungsten diselenide while a strong terahertz field accelerates and collides the electrons with the holes. The underlying dynamics of the wave packets, including collision, pair annihilation, quantum interference and dephasing, are detected as light emission in high-order spectral sidebands of the optical excitation. A full quantum theory explains our observations microscopically. This approach enables collision experiments with various complex quasiparticles and suggests a promising new way of generating sub-femtosecond pulses.