Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D33: Quantum Dots, Gates and Single Photon Devices |
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Sponsoring Units: GQI Chair: Anton Zeilinger, University of Vienna Room: LACC 511C |
Monday, March 21, 2005 2:30PM - 3:06PM |
D33.00001: Quantum control of ultracold atomic collisions for quantum logic gates Invited Speaker: Ultracold trapped neutral atoms are a natural system for quantum information processors given the atoms' weak coupling to the environment and the ability to coherently control their dynamics including, electronic, spin, and motional degrees of freedom [1]. By encoding quantum information in the hyperfine magnetic sublevels of an alkali atom, two-qubit quantum logic can be implement through spin-dependent ultracold elastic collisions in optical lattices [2]. I will present a new method for robustly controlling collisions based on the ``trap-induced shape resonance'' (TISR) [3]. Like the magnetic Feschbach resonance, in the TISR a weakly-bound molecular state is made resonant with a trap vibrational state through the trapping potential energy. The TISR allows for strong interaction between trapped but separated atoms, providing new avenues for robust encodings of quantum information, protected from fluctuations in control parameters. A particularly promising candidate species is Cs-133, whose dimer potential posses an extremely weakly bound state near dissociation. Scattering lengths on the order of 100nm are possible for appropriate choices of encodings, larger that the typical trapped wavepacket, and thus leading to very strong interaction. To deal with the complexity of the multichannel scattering problem at short range, and the trapping potential at long range, we have developed a generalized multichannel energy-dependent Fermi pseudo-potential, including higher partial waves, and second order spin-orbit coupling. This provides a powerful method for importing precision molecular data, obtained through free-atom scattering studies, into the trapped-atom protocol. [1] I. H. Deutsch et al., Fort. der Phys. 48, 925 (2000). [2] D. Jaksch et al., Phys. Rev. Lett. 82,1975 (1999); O. Mandel et al., Nature 425, 937 (2003). [3] R. Stock et al., Phys. Rev. Lett. 91, 183201 (2003). [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:18PM |
D33.00002: New Tools for Photonic Entanglement Gregor Weihs Optical technologies are employed in many areas of quantum information, particularly for quantum communication protocols. Apart from detectors and memories, sources of entangled photon pairs are the most important building blocks. After reviewing some recent work on semiconductor microcavities, I will present ideas for novel sources of entangled photon pairs based on semiconductor nanostructures. Parametric down-conversion in photonic crystals can achieve phase and group matching enabling highly efficient devices. Another approach will use semiconductor quantum dots to create time-bin entangled photon pairs. In the latter case only one photon pair will be produced at a time. Single pairs are important for the interferometric construction of multipartite entangled states and linear optic quantum computation. [Preview Abstract] |
Monday, March 21, 2005 3:18PM - 3:30PM |
D33.00003: Collective Storage and Retrieval of Photons James K. Thompson, Adam T. Black, Vladan Vuletic We present results on the development of a triggerable single- photon source. Our hybrid experiment takes advantage of collective coupling between a cold ($T\sim10~\mu$K) atomic ensemble of $N=10^4$ Cs atoms and a low finesse optical cavity ($F=1000$.) The combined cavity-ensemble provides a large optical depth $N\eta\sim100$ potentially allowing near unity success rates ($R\sim99\%$) for converting a collective excitation into a photon in a single output mode. In the initial work presented here, we demonstrate excitation storage times of $\sim1~\mu$s, and peak conversion efficiencies of $R\sim30\%$ (and $R\sim70\%$ in more recent work) in the single photon limit. We currently believe that both quantities are limited by doppler decoherence that can be reduced by storing the atoms in a far detuned optical lattice. The quantum-collective nature of the storage and retrieval process is clearly established by observing several phase matching conditions. This work was supported in parts by the NSF, the ARO, and the Sloan Foundation. [Preview Abstract] |
Monday, March 21, 2005 3:30PM - 3:42PM |
D33.00004: Photo-electron storage in a one-electron quantum dot Deepak Rao, Thomas Szkopek, Hans Robinson, Eli Yablonovitch, Hong-Wen Jiang We report on the trapping, storage and detection of a single photo-electron in an electrostatic quantum dot defined by surface metallic gates on a GaAs/AlGaAs modulation doped heterostructure. The dot can be emptied and reset in a controlled fashion before the arrival of each photon. The trapped photo-electron is detected by the photoconductive gain mechanism of a point contact transistor integrated adjacent to the electron trap. Each photo-electron causes a persistent negative step in the transistor channel current. Such a controllable, non-invasive, single photo- electron detector could allow for quantum information transfer between flying photon qubits and stored electron qubits. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 3:54PM |
D33.00005: Entanglement and dissipation in a 2x2 quantum-dot cell Lesbia Debora Contreras, Fernando Rojas Quantum dot arrays or quantum-dot cellular automata (QCA) have been proposed as elements capable to encode, process and transmit logical information based on quantum effects in terms of charge distributions in specific geometries. and the basis for the charge qubits. Quantum Entanglement is a resource to encode information in a completely new way making possible quantum teleportation, quantum error correction, quantum dense coding. In this work, we explore the dynamical formation of entangled states including dissipative effects, of two parallel double dots (four dots, 2x2 cell), with one extra electron each, coupled by the Coulomb interaction and controlled by a time dependent potential difference applied to one of the double dots, causing the electron to switch. We include dissipative effects via electron-phonon interaction in the Markovian approximation for the reduced density matrix. Dynamical properties of the cell such as charge polarization, measure the entanglement (Wootters concurrence) and the probabilities for each Bell state, are discussed as a function of relevant parameters (tunneling, potential difference, temperature). We find that it is possible to obtain entangled states in the cell based on the electronic charge distribution and produce a specific Bell state from an initially non entangled state through the control of the time dependent potential. The work is supported by DGAPA project IN114403 and CONACyT project 43673-F [Preview Abstract] |
Monday, March 21, 2005 3:54PM - 4:06PM |
D33.00006: Tunnel coupling of charge qubits in semiconductor quantum computer architectures Belita Koiller, Xuedong Hu, S. Das Sarma Charge-based qubits in solid state quantum computer proposals have the attractive advantage of being relatively easy to manipulate and measure. We study the feasibility of P$_2^+$ charge qubits in Si, focusing on single qubit properties in terms of tunnel coupling between the two phosphorus donors. We take into consideration the multi-valley structure of the Si conduction band and show that valley interference could have important effects on the operations of P$_2^+$ charge qubits by producing a tunnel-coupling distribution centered at zero value. We conclude that the Si bandstructure significantly (and adversely) influences the tunnel coupling between the two phosphorous donors in terms of defining elementary charge qubits in the $P_2^+$ system in Si, since the energy splitting in these efective two-level systems is essential for quantum computation. We also critically compare charge qubits properties for Si:P$_2^+$ and GaAs double quantum dots and discuss effects of dot size variations. [Preview Abstract] |
Monday, March 21, 2005 4:06PM - 4:18PM |
D33.00007: Double Occupancy Errors in Quantum Computing Operations: Corrections to Adiabaticity Ryan Requist, John Schliemann, Alexander Abanov, Daniel Loss We study the corrections to adiabatic dynamics of two coupled quantum dot spin-qubits, each dot singly occupied with an electron, in the context of a quantum computing operation. Tunneling causes double occupancy at the conclusion of an operation and constitutes a processing error. We model the gate operation with an effective two- level system, where non-adiabatic transitions correspond to double occupancy. The model is integrable and possesses three independent parameters. We confirm the accuracy of Dykhne's formula, a nonperturbative estimate of transitions, and discuss physically intuitive conditions for its validity. Our semiclassical results are in excellent agreement with numerical simulations of the exact time evolution. A similar approach applies to two-level systems in different contexts. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:30PM |
D33.00008: Quantum gates using a pulsed bias scheme Preethika Kumar, Steven Skinner, Elizabeth Behrman, James Steck We present here a novel scheme to realize quantum gates by means of a pulsed bias. We show how a NOT gate (one qubit), a C-NOT gate (two qubits) and a Toffoli gate (three qubits) can be realized by clocking the bias on one of the qubits in the system. When the bias is kept high, a qubit will remain in the state it has been initialized to, according to the governing two-level Hamiltonian. We call this state a memory state. When the bias is clocked low, the two-level system oscillates between its two basis states. We call this state a transitional state. In all cases, we force one qubit in the system into a transitional state for a certain transitional time. With proper choice of parameters, bias, coupling and tunneling, we show in this talk how these quantum gates can be realized. The key to the C-NOT gate is to maintain the control qubit in a memory state while forcing the target qubit into a transitional state. This reduces the Hamiltonian to a two level system, where the frequency, amplitude, and offset of oscillation of the target qubit are functions of the state of the control qubit. The reduced Hamiltonian approach is then extended further to a Toffoli gate which has two control qubits and one target qubit. [Preview Abstract] |
Monday, March 21, 2005 4:30PM - 4:42PM |
D33.00009: Measuring a qubit using stimulated wave of polarization in a 1D spin chain Jae-Seung Lee, Anatoly Khitrin Measurement of a single qubit state is one of requirements for quantum information processors. A measuring device should show macroscopically distinguishable read out depending on the state of a single qubit. One logically simple way is applying a sequence of quantum controlled-NOT gates, globally conditioned on the state of the measured qubit, when a measuring device is initialized in a specific state. Recent NMR experiments have shown that an algorithm of quantum measurement can be realized using multiple-quantum dynamics of nuclear spins. However, this dynamics is slow and not very efficient. In this work, we propose a simple and realistic model of amplified quantum measurement which uses a stimulated wave of polarization in a 1D chain. An exact solution for the dynamics of this system is presented. It describes a process when a flip of the first qubit in the chain triggers a wave of flipped qubits, eventually covering the entire system. [Preview Abstract] |
Monday, March 21, 2005 4:42PM - 4:54PM |
D33.00010: Decoherence Free States for Two and Four charge qubits under static local fluctuations Tetsufumi Tanamoto, Shinobu Fujita, Xuedong Hu We analyze the effects of static fluctuations of qubit parameters on decoherence free subspace (DFS)[1] in charge qubits that are two-level atoms based on coupled quantum dots. We solve the master equations of four and two charge qubits and the detector as two serially coupled quantum point contacts (QPCs)[2,3]. We show that robustness of DFS depends on the magnitude of the fluctuations. We also show exact solutions for two-qubit detection under collective decoherence measurement environment. [1]D. A. Lidar and K. B. Whaley, quant-ph/0301032 [2]T. Tanamoto and X. Hu, Phys. Rev. B69 115301 (2004). [3]T. Tanamoto and X. Hu, cond-mat/0310293. [Preview Abstract] |
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D33.00011: Signatures of quantum effects in weak measurements of qubits Rusko Ruskov, Alexander N. Korotkov, Ari Mizel In many proposed qubit designs, especially solid state qubit systems, it is experimentally difficult to perform ideal projective von Neumann measurements, and one must resort to weak measurements. In this work, we describe signatures of quantum coherence and entanglement in the output of weak measurements. Both one and two qubit cases are addressed. We consider the ability of a measuring device to generate, as well as detect, entanglement. The interpretation of the power spectrum of weak measurements is discussed, especially its ``quantum'' and ``classical'' parts. [Preview Abstract] |
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