Mesoscopic Physics, Quantum Optics, and Quantum Information

May 10-12, 2004

Organizers: Mikhail Lukin, Charles Marcus, Anders Sørensen

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Online Talks

Beenakker

Buttiker

Das Sarma 

Girvin

Glazman

Hirayama 

Kastner

Kouwenhoven

Levitov

Loss

Marcus

 Monroe

 Schoelkopf

Sørensen 

 Steel

Schwab

 Tarucha

 Taylor

 Tian

Vuckovic 

 Vuletic

 Westervelt

 Zoller

one 

Workshop Participants

Prof. Boris Altshuler
Princeton University
305 Jadwin Hall
Princeton, NJ 08544
bla@feynman.Princeton.EDU
 
Prof. Carlo Beenakker
P.O. Box 9506
Instituut-Lorentz
Leiden University
Leiden, NL 2300 RA, The Netherlands
Univ. of Leiden, Netherlands
beenakker@lorentz.leidenuniv.nl
 
Prof. Markus Buttiker
University of Geneva
Dept. Phys. Theor.
24 Quai E. Ansermet
Geneva,Ch-1211 Switzerland
buttiker@serifos.unige.ch
 
Prof. Sankar Das Sarma
Physics Department
University of Maryland
College Park, MD 20672
cmtc@physics.umd.edu
 
Prof. Joseph Eberly
Department of Physics
University of Rochester
Rochester, NY 14627
eberly@pas.rochester.edu
 
Prof. Steven M. Girvin
Yale University
Physics Department
Sloane Physics Lab
PO Box 208120
New Haven, CT 06520-8120
steven.girvin@yale.edu
 
Prof. Leonid Glazman
Theoretical Physics Institute
University of Minnesota
116 Church St SE
Minneapolis, MN 55455
glazman@umn.edu
 
Prof. Bertrand I. Halperin
Physics Department
Harvard University
17 Oxford Street
Cambridge, MA 02138
halperin@hall.harvard.edu
 
Dr. Yoshiro Hirayama
NTT Basic Research Laboratories
3-1, Morinosato Wakamiya
Atsugi, Kanagawa 243-0198, JAPAN
hirayama@nttbrl.jp

Prof. Marc Kastner
MIT
77 Massachusetts Ave.
Cambridge, MA 02139
mkastner@MIT.EDU
 
Prof. Leo Kouwenhoven
Univ. of Delft - Netherlands
Lorentzweg 1
Delft,
CT
2628CJ The Netherlands
leo@qt.tn.tudelft.nl
 
Dr. Leonid Levitov
Physics Department
Massachusetts Institute of Technology
12-112, 77 Massachusetts Ave.
Cambridge, MA 02139
levitov@mit.edu
 
Prof. Daniel Loss
Department of Physics
University of Basel
Klingelbergstrasse 82
Basel, Switzerland 4056
daniel.loss@unibas.ch
 
Prof. Mikhail Lukin
Physics Department
Harvard University
Cambridge, MA 02138
lukin@physics.harvard.edu
 
Prof. Charles Marcus
Physics Department
Harvard University
Cambridge, MA 02138
marcus@harvard.edu
 
Prof. Christopher R. Monroe
2477 Randall Laboratory
University of Michigan
Ann Arbor, MI 48109-1120
crmonroe@umich.edu
 
 
Prof. Emmanuel I. Rashba
Department of Physics
MIT
Cambridge, MA 02139
erashba@mailaps.org
 
Dr. Hossein Sadeghpour
ITAMP
60 Garden Street, MS 14
Camridge, MA 02138
hsadeghpour@cfa.harvard.edu
 
Dr. Keith C. Schwab
Univ. of Maryland
8050 Greenmead Drive
College Park, MD 20740
schwab@lps.umd.edu
 
Prof. Robert J Schoelkopf
Department of Applied Physics
Yale University
PO Box 208284;
401 Becton Center
New Haven, CT 06520-8284
robert.schoelkopf@yale.edu

Dr. Anders Sørensen
ITAMP
60 Garden Street, MS 14
Cambridge, MA 02138
asorensen@cfa.harvard.edu
 
Prof. Duncan G. Steel
Randall Physics Laboratory
University of Michigan
Ann Arbor, MI 48109
dst@umich.edu
 
Prof. Seigo Tarucha
Department of Physics
University of Tokyo
7-3-1, Hongo
Bunkyo-ku
Tokyo, Japan 113-0033
tarucha@phys.s.u-tokyo.ac.jp
 
Mr. Jacob Taylor
Harvard University
Jefferson Lab
17 Oxford St.
Cambridge, MA 02138
jmtaylor@fas.harvard.edu
 
Dr. Lin Tian
Technikerstrasse 25
Institute for Theoretical Physics
University of Innsbruck
Innsbruck 6020 Austria
lin.tian@uibk.ac.at
 
Prof. Jelena Vuckovic
Ginzton Laboratory
450 Via Palou
Stanford University
Stanford, CA 94305-4088
jela@stanford.edu
 
Prof. Vladan Vuletic
MIT
Center for Cold Atoms
77 Massachusetts Avenue, 26-231
Cambridge, MA 02139
vuletic@mit.edu
 
Prof. Robert M. Westervelt
Division of Engineering and Applied Sciences
Harvard University
9 Oxford Street
Cambridge, MA 02138
westervelt@deas.harvard.edu
 
Prof. Peter Zoller
Institute for Theoretical Physics
Univ. of Innsbruck
6020 Innsbruck, Austria
peter.zoller@uibk.ac.at

ATTENDEES

Mr. Dmitry A. Abanin
Masshachusetts Institute of Technology
77 Massachusetts Ave, 12-113
Cambridge, MA 02139
abanin@mit.edu
Axel Andre
Physics Department
Harvard University
Cambridge, MA 02138
andre@physics.harvard.edu
 
Prof. Raymond Ashoori
MIT
Room 13-2053
77 Massachusetts Ave.
Cambridge, MA 02139
ashoori@mit.edu
 
Dr. Chagaan Baatar
Office of Naval Research
800 N. Quincy Street
Arlington, VA 22217
BaatarC@onr.navy.mil
 
Ms. Ania Bleszynski
Division of Engineering and Applied Sciences
Harvard University
9 Oxford Street
Cambridge, MA 02138
ania@physics.harvard.edu
 
Dr. Erli Chen
Center for Imaging and Mesoscale Structures
Harvard University
9 Oxford St.
Cambridge, MA 02138
erlichen@deas.harvard.edu
 
Ms. Lilian Childress
Harvard University
Jefferson 430
Department of Physics
17 Oxford St.
Cambridge, MA 02138
childres@fas.harvard.edu
 
Mr. Sang Chu
Georgia Institute of Technology
837 State, NW
Atlanta, GA 30332
gte813m@prism.gatech.edu
 
Prof. Eugene Demler
Physics Department
Harvard University
Cambridge, MA 02138
demler@cmt.harvard.edu
 
Dr. Mandar Deshmukh
Harvard University
12 Oxford St
Dept of CCB
Cambridge, MA 02138
deshmukh@fas.harvard.edu
 
Dr. Henry Everitt
4300 South Miami Blvd.
US Army Research Office
Durham, NC 27703
henry.o.everitt@us.army.mil
 
Ms. Parisa Fallahi
Division of Engineering and Applied Sciences
Harvard University
9 Oxford Street
Cambridge, MA 02138
fallahi@fas.harvard.edu
 
Dr. Fawwaz Habbal
Associate Dean for Research and Planning
Harvard University
Pierce 219
Cambridge, MA 02138
habbalf@deas.harvard.edu
 
Mohammad Hafezi
Harvard University
Physics Department
17 Oxford St.
Cambridge, MA 02138
hafezi@physics.harvard
 
Jack Harris
17 Oxford Street
Harvard/MIT CUA
Cambridge, MA 02138
jack@cua.harvard.edu
 
Dr. Mark Heiligman
ARDA
National Security Agency
Fort Meade, MD 20755
miheili@nsa.gov
 
Prof. Eric Heller
Physics Department
Harvard University
17 Oxford Street
Cambridge, MA 02138
heller@physics.harvard.edu
 
Dr. Walter Hofstetter
Lyman Physical Laboratory
Harvard University
17 Oxford Street
Cambridge, MA 02138
hofstett@cmt.harvard.edu
 
Mr. Michael Hohensee
Physics Department
Harvard University
Cambridge, MA 02138
hohensee@fas.harvard.edu
 
Prof. Alex Kamenev
Physics Department
University of Minnesota
Minneapolis, MN 55455
kamenev@physics.umn.edu
 
Dr. Austen Lamacraft
Princeton University
313 Jadwin Hall
Princeton, NJ 08544
alamacra@Princeton.EDU
 
Mr. Benjamin G. Lee
Gordon McKay Lab
Harvard University
9 Oxford Street
Cambridge, MA 02138
bglee@fas.harvard.edu
 
Ramesh Mani
9 Oxford Street
Gordon McKay Lab.
Harvard U.
Cambridge, MA 02138
mani@deas.harvard.edu
 
Mr. Vladimir Manucharyan
Yale University
Physics Department
217 Prospect Street
New Haven, CT 06511
vladimir.manucharyan@yale.edu
 
Ms. Florent Massou
Physics Department
Harvard University
Cambridge, MA 02138
massou@fas.harvard.edu
 
Mr. Aryesh Mukherjee
Harvard University
35 Oxford Street,
Perkins Hall, RM 134
Cambridge, MA 02138
mukherj@fas.harvard.edu
 
Mr. Mukund Vengalattore
150 Jefferson Labs
17 Oxford St.
Cambridge, MA 02138
mukund@atom.harvard.edu
 
Mr. Andy Vidan
Harvard University
207 McKay Lab
9 Oxford St.
Cambridge, MA 02138
vidan@fas.harvard.edu
 
Mr. Andreas Wallraff
Department of Applied Physics
Yale University
PO Box 208284;
401 Becton Center
New Haven, CT 06520-8284
wallraff@yale.edu
 
Dr. Daw-Wei Wang
Physics Department
Harvard University
Cambridge, MA 02138
dwwang@cmt.harvard.edu
 
Dr. Stuart Wolf
DARPA-DSO
3701 N. Fairfax Drive
Arlington, VA 22203
swolf@darpa.mil
 
Dr. Junqiao Wu
Harvard Univ, Dept of CCB,
12 Oxford St.
Cambridge, MA 02138
wu2@fas.harvard.edu
 
Mr. Saijun Wu
Harvard University
Jefferson Lab
17 Oxford Street
Cambridge, MA, 02138
saijun@atom.harvard.edu
Ms. Xiao Yanhong
Jefferson Labs Room 150a
17a Oxford Street
Cambridge, MA 02138
yxiao@fas.harvard.edu
 
Prof. Amir Yacoby
Department of Condensed Matter Physics
Weizmann Institute of Science
Rehovot 76100, Israel
amir.yacoby@weizmann.ac.il

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Workshop Schedule

 

May 10, 2004, Monday

Phillips Auditorium (all day)
8:45-9:00 Welcome and Coffee

Session 1: Nuclear Spins and Quantum Dots

Chair: Boris Altshuler

9:00-9:40 S. Tarucha: Tunable Coupling of Electron Spin and Nuclear Spin in Quantum Dots
9:40-10:20 Y. Hirayama: Semiconductor Charge Qubit
10:20-11:00 R.M. Westervelt:  Imaging Electrons in a Single-Electron Quantum Dot
11:00-11:30 Coffee
11:30-12:10 J. Taylor/M. Lukin: Controlling the Nuclear Spin Environment of Quantum Dots: From Dephasing to Robust Qubit Storage and Manipulation
12:10-12:50 S. Das Sarma: Coherence and Entanglement in Semiconductor Quantum Bits
12:50-1:50 Lunch

Session 2: Mesoscopic AMO

1:50-2:30 J. Vuckovic: Photonic Crystals Devices for Quantum Information Processing
2:30-3:10 C. Monroe:  Scaling the Ion Trap: Microtraps and the Probabalistic Photon Bus
3:10-3:40 Coffee
3:40-4:20 V. Vuletic:  Stability Of Bose-Einstein Condensates Near Room-Temperature Surfaces
4:20-5:00 P. Zoller: Single Atom Switches and Mirrors by Quantum Interference: "Single Atom Transitors"
5:15-6:15 Reception in Perkin Lobby
Tuesday, May 11, 2004

Session 3: Electron Spin and Quantum Dots
9:00-9:40 M.A. Kastner:  Effect of Microwave Excitation on the Kondo Conductance of a Single-Electron Transistor
9:40-10:20 C.M. Marcus: Measuring the Entanglement of Two Electrons
10:20-11:00 L. Kouwenhoven: Spin States in Few-Electron Quantum Dots
11:00-11:30 Coffee
11:30-12:10 D. Loss: Relaxation and Decoherence of Electron Spins in Quantum Dots
12:10-12:50 L. I. Glazman: Transport Spectroscopy of Coupled Quantum Dots in Conditions of the Kondo Effect
12:50-2:00 Lunch

Session 4: Mesoscopic Cavity QED
2:00-2:40 A.S. Sorensen: Capacitive Coupling of Atomic Systems to Mesoscopic Conductors
2:40-3:20 L. Tian: Nanoelectromechanics: From Quantum Computing to Motional State Engineering
3:20-3:50 Coffee
3:50-4:30 R. Schoelkopf: Circuit Quantum Electrodynamics: Observing Strong Coupling between a Superconducting Qubit and a Photon
4:30-5:10 S.M. Girvin: Atomic Physics with Electrical Circuits:
Cavities as Amplifiers, Detectors and Buses

Wednesday, May 12, 2004

Session 5: New Directions in Mesoscopic Electronics and Optics
9:00-9:40 M. Buttiker: Two-Particle Aharonov-Bohm Effect and Entanglement in the Electronic Hanbury Brown-Twiss Setup
9:40-10:20 C. Beenakker: Free-Electron Quantum Computation
10:20-11:00 Schwab: TBA
11:00-11:30 Coffee
11:30-12:10 Steel: Optical Control of Spin in Semiconductor Dots for Quantum Operations
12:10-12:50 L. Levitov: Pattern Formation as a Signature of Quantum Degeneracy in a Cold Exciton System
12:50 Adjourn

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Abstracts

Beenakker

Buttiker

Das Sarma 

Girvin

Glazman

Hirayama 

Kastner

Kouwenhoven

Levitov

Loss

Marcus

 Monroe

 Schoelkopf

Sørensen 

 Steel

Tarucha

Taylor 

 Tian

Vuckovic 

 Vuletic

 Westervelt

 Zoller

Free-Electron Quantum Computation

Carlo Beenakker

Instituut-Lorentz
Leiden University

It is known that a quantum computer operating on electron-spin qubits with single-electron Hamiltonians and assisted by single-spin measurements is not more powerful than a classical computer. This no-go theorem of Terhal and DiVincenzo applies only to fermions --- not to bosons. Indeed, Knill, Laflamme, and Milburn showed that the exponential speed-up over a classical algorithm afforded by quantum mechanics can be reached using only linear optics with single-photon detectors. The detectors interact with the qubits, providing the nonlinearity needed for the computation, but qubit-qubit interactions (e.g. nonlinear optical elements) are not required in the bosonic case.

We have found a way to remove the constraint on the efficiency of quantum algorithms for free fermions, by using the fact that the electron carrying the qubit in its spin degree of freedom has also a charge degree of freedom. Spin and charge commute, so a measurement of the charge leaves the spin qubit unaffected. To measure the charge the qubit should interact with a detector, but no qubit-qubit interactions are needed. Charge detectors enable the
construction of a CNOT gate for free fermions, using only beam splitters and spin rotations. The gate is nearly deterministic if the charge detector counts the number of electrons in a mode, and fully deterministic if it only measures the parity of that number.

This work was done with David DiVincenzo, Clive Emary, and Markus Kindermann.

 Two-Particle Aharonov-Bohm Effect and Entanglement in the Electronic Hanbury Brown­Twiss Setup

Markus Buttiker

Department of Theoretical Physics
University of Geneva

We are interested in the generation and detection of entanglement in mesoscopic conductors. Most proposals consider spin entanglement which for detection requires a spin to charge conversion in the detection process. In our proposals we instead focus on orbital entanglement: the role of the spin degree of freedom is taken by two contacts which inject particles into the conductor which fly off to measurement contacts.

A particularly interesting and realistic geometry consists of an electronic analog of the optical Hanbury Brown Twiss geometry. In the electronic analog particles are guided along edge states in a geometry in which scattering is controlled by four adiabatic quantum point contacts playing the role of (half-) silvered mirrors. The geometry has the property that all partial waves end in different contacts without generating any single particle (second order) interference. In particular there is no Aharonov-Bohm effect. However, exchange effects lead to two-particle Aharonov-Bohm oscillations in the zero-frequency current cross correlations (a forth order interference effect).

We demonstrate that the two-particle Aharonov-Bohm effect is related to two-particle orbital entanglement by demonstrating that a Bell inequality is violated. Importantly the Bell inequality can be tested via a measurement of zero-frequency current-noise correlations. In the highly asymmetric set-up the entangled states are orbitally entangled electron-hole states. In the nearly symmetric set-up entanglement is due to postselection of two electron states generated by the measurement only.

We briefly discuss the effect of dephasing and find that the visibility of the two-particle Aharonov-Bohm effect is directly related to the degree with which the Bell inequality can by violated.


Work in collaboration with Peter Samuelsson and Eugene V. Sukhorukov.

 

Coherence and Entanglement in Semiconductor Quantum Bits

Sankar Das Sarma

Condensed Matter Theory Center
Department of Physics
University of Maryland
College Park; MD 20742-4111

I will discuss in this talk the related issues ofspin entanglement and spin coherence in semiconductor nanostructure-based electron spin qubits. In particular, I will describe the theory of spin spectral diffusion in localized spins in semiconductors through the nuclear spin flip-flop mechanism, which is the main spin decoherence mechanism at low temperatures. In this context, I will discuss the exchange coupling induced entanglement between neighboring electron spins in Si and GaAs nanostructures, emphasizing the various physical mechanisms which affect this entanglement. Finally, I will discuss the prospects for the direct observation of spin entanglement in transport measurements using the double quantum dot turnstile structure as a spin entangler.

This work is done in collaboration with Xuedong Hu, Rogerio De Sousa, and Belita Koiller, and is supported by the ARDA, the LPS, the ARO, and the NSF. The relevant references are Phys. Rev. B 69, 115312 (2004); B 68, 115322 (2003); B 68, 115330 (2003); B 67, 033301 (2003); A 68, 052310 (2003); A 66, 012312 (2002); Phys. Rev. Lett. 88, 027903 (2002); 90, 067401 (2003), and other related and unpublished work.

 

Atomic Physics with Electrical Circuits:
Cavities as Amplifiers, Detectors and Buses

Steven M. Girvin

Departments of Physics and Applied Physics
Yale University

Recent experimental progress in the Schoelkopf group at Yale has opened up a new frontier of strong coupling cavity QED using superconducting electrical circuits. I will discuss the theory of quantum noise and back action for our dispersive QND readout scheme, quantum control issues for qubits in cavities, quantum Zeno effects, the potential use of cavities as an information buses for entangling superconducting qubits, and finally prospects for the construction of single microwave photon emitters and detectors.

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Transport Spectroscopy of Coupled Quantum Dots in Conditions of the Kondo Effect

L.I. Glazman

Theoretical Physics Institute
University of Minnesota
116 Church St SE
Minneapolis, MN 55455

We develop electron transport theory for novel devices [1,2], which are interesting in the context of correlated electrons physics. The device proposed in Ref. [1] is designed for an observation of a non-Fermi-liquid behavior of itinerant electrons. The device measured in Ref. [2] may serve a similar purpose, and also may become important for quantum computing.

In the case of Ref. [1], our theory provides a strategy for tuning to the non-Fermi-liquid fixed point -- a quantum critical point in the space of device parameters. We explore the corresponding quantum phase transition, and make explicit predictions for the behavior of differential conductance in the vicinity of the quantum critical point.

Motivated by the measurements [2], we developed a theory of conductance of Kondo quantum dots coupled by the RKKY interaction. Investigation of the differential conductance at fixed interaction strength may allow one to distinguish between the possible ground states of the system. Transition between the ground states is achieved by tuning the interaction strength; the nature of the transition (which includes a possibility of a non-Fermi-liquid point) can be extracted from the temperature dependence of the linear conductance.

1. Y. Oreg and D. Goldhaber-Gordon, Phys. Rev. Lett. v. 90, p. 136602, 2003.
2. N. J. Craig J. M. Taylor, E. A. Lester, C. M. Marcus, M. P. Hanson, A. C. Gossard, to appear in Science, cond-mat/0404213.

 

Semiconductor Charge Qubit

Y. Hirayama[1,2], T. Hayashi[1] and T. Fujisawa[1]

[1]NTT Basic Research Laboratories, NTT Corporation

[3-1] Monrinosato-Wakamiya, Atsugi-Shi, 243-0198, Japan

[2]SORST-JST, 4-1-8 Honmachi, Kawaguchi-Shi, 331-0012, Japan

E-mail: hirayama@nttbrl.jp

A small-scale test-bed for a quantum computer has been demonstrated by using solution NMR. However, from the viewpoint of scalability, a solid-state quantum computer is desirable. Among the many candidates for solid-state qubits, semiconductor systems have advantages in that they use existing cutting-edge IC technologies. However, introducing coherent control in the systems is challenging. For this purpose, the coherent control of charge, spin, nuclear spin, and exciton has been studied in semiconductor systems.
In this presentation, we will discuss a semiconductor charge qubit embedded in a quantum dot system. We used an electrical pump-and-probe technique to measure the coherent oscillation of a single electron in a coupled-quantum-dot system, where electron occupation in the right or left dot operates as a quantum two-level system. The first pulse, i.e. the initialization stage, puts the electron in the left dot. Then, the system is set to the resonant condition during the second pulse. We successfully observed a current oscillation depending on whether an electron locates in the left or right dot as a function of the duration of the second pulse. [1] We have demonstrated a modulation of the oscillation frequency by electrical control of the coupling between two dots. Arbitrary control of the pseudospin rotation on the Bloch sphere has also been realized by changing the electrical pulse applied to the system. [2] It is noteworthy that this charge qubit can be controlled all-electrically in semiconductor systems.

A single shot measurement is also important for reading calculated results in a quantum computer. We have applied a radio-frequency (RF) single-electron transistor (SET) or quantum point contact (QPC) technique to a single-shot measurement of the charge. The time-domain detection of each single-electron tunneling event has been successfully demonstrated for a quantum dot electrostatically coupled to the RF-SET. [3]

[1] T. Hayashi, T. Fujisawa, H. D. Cheong, Y. H. Jeong and Y. Hirayama, Phys. Rev. Lett. 91, 226804 (2003).
[2] T. Fujisawa, T. Hayashi, H. D. Cheong, Y. H. Jeong and Y. Hirayama, Physica E21, 1053 (2004).
[3] T. Fujisawa, T. Hayashi, Y. Hirayama, H. D. Cheong and Y. H. Jeong, Appl. Phys. Lett. 84, 2343 (2004).

 Effect of Microwave Excitation on the Kondo Conductance
of a Single-Electron Transistor

M. A. Kastner

Department of Physics
Massachusetts Institute of Technology, Room 6-113
77 Massachusetts Avenue
Cambridge, MA 02139-4307

The discovery of the Kondo effect in nanometer-size semiconductor structures has caused a renaissance in the study of this quantum mechanical many-body phenomenon. We study a quantum dot coupled to two leads, called the drain and the source, with a gate electrode nearby. Because of localization of electronic wavefunctions on the dot, the charge is quantized. As a result, this transistor turns on and off again for every electron added by the gate, and it is therefore called a single-electron transistor (SET). When the coupling of electrons on the dot to the source and drain is very weak, the conductance of an SET at zero drain-source bias is very small, except for values of the gate voltage at which two charge states of the quantum dot are degenerate, giving a series of peaks in the SET's conductance. However, if the dot contains an unpaired electron spin, the residual coupling between the source or drain and the dot leads to screening of the localized electron's spin by the delocalized electrons with opposite spin in the leads, so that at zero temperature a spin-zero singlet is formed. The temperature scale for the formation of this entangled many-body state is the Kondo temperature TK. The Kondo effect enhances the conductance between the charging peaks when the number of electrons is odd but not when it is even, because the screening of an unpaired electron creates a new quantum mechanical entangled many-body ground state that extends from one lead through the dot into the other lead.

Whereas the Kondo effect was first observed in metals doped with magnetic impurities, SETs provide new ways of studying the Kondo effect not possible in conventional systems. In particular, the capability of applying voltages larger than kTK/e between the two leads of an SET makes it possible to study the Kondo effect out of equilibrium.

In the mid 1990's one particularly interesting non-equilibrium phenomenon was predicted-photon restoration of the Kondo singlet. With only dc electric fields applied, the Kondo coupling enhances the conductance only when the source-drain voltage is less than kTK/e because the applied voltage destroys the coherence between the two clouds of screening electrons in the source and drain. Thus, the Kondo effect is evinced by a sharp peak in the differential conductance at dc voltage Vds=0. The theorists predicted, as early as 1995, that were a microwave voltage at frequency w applied, in addition to Vds, the Kondo peak in differential conductance would be restored at the voltage Vds=hw/e. This corresponds to a new state, which involves not only the spatial and spin variables of the electrons but also the electromagnetic field. Despite significant effort, this photon-restoration of the Kondo effect has not been previously observed.

I will describe how we have recently succeeded in observing photon restoration of the Kondo state and what the results tell us about the Kondo effect. Previous attempts to observe this ac Kondo effect provided evidence that the microwave excitation causes decoherence of the Kondo singlet. We confirm this and find that the Kondo features in differential conductance all disappear when the microwave voltage Vac is larger than ~kTK/e. We use a microwave cavity to excite the SET, which limits the excitation to a very narrow band of frequencies and therefore minimizes the decoherence. The sidebands are expected to be most prominent when eVac = hw, but because of the decoherence, this means that hw cannot be much larger than kTK. Careful tuning of TK is therefore necessary to observe the photon restoration of the Kondo singlet. The results of this work are soon to appear in Science Magazine.

Andrei Kogan and Sami Amasha are the leaders of this experiment. The devices we use were fabricated by David Goldhaber-Gordon, Hadas Shtrickman and Diana Mahalu. We are grateful for research support from the US Army Research Office under Contract DAAD19-01-1-0637, the National Science Foundation under Grant No. DMR-0102153, and the NSEC Program of the National Science Foundation under Award Number DMR-0117795.

 Spin States in Few-Electron Quantum Dots

Leo Kouwenhoven

Kavli Institute of NanoScience, Delft University of Technology
POB 5046, 2600GA Delft, The Netherlands

We discuss measurements of spin states in semiconductor quantum dots in the regime of few electrons or few holes. In split-gate GaAs/AlGaAs quantum dots we have measured the spin of individual electrons using electrical pulses for a pump-probe type technique. From repeating these measurements on many electrons we find that at 8 Tesla the T1 spin relaxation time is about 1 ms. This spin life time is expected to be significantly longer in carbon nanotubes where the spin-orbit interaction is much weaker. We have realized disorder-free semiconducting nanotubes which can be filled by a small number of either electron or holes. In addition we present an experimental scheme for coherently transferring the electron spin into photon polarization.

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Pattern Formation as a Signature of Quantum Degeneracy in a Cold Exciton System

Leonid Levitov

Physics Department
Massachusetts Institute of Technology
12-112, 77 Massachusetts Ave.
Cambridge, MA 02139

The development of a Turing instability to a spatially modulated state in a photoexcited electron-hole system is proposed as a novel signature of exciton Bose statistics. We show that such an instability, which is driven by kinetics of exciton formation, can result from stimulated processes that build up near quantum degeneracy. In the spatially uniform 2d
electron-hole system, the instability leads to a triangular lattice pattern while, at an electron-hole interface, a periodic 1d pattern develops. We analyze the mechanism of wavelength selection at instability, and show that the transition is abrupt (type I) for the uniform 2d system, and continuous (type II) for the electron-hole interface.

We discuss the extent to which this mechanism may be responsible for the patterns of photoluminescence (PL) recently recorded in photoexcited quantum well devices. The large scale ring-like patterns are explained by classical mechanism involving carrier imbalance, transport and recombination. The rings originate from the spatial separation of p and n
carriers, and occur at the interface of the p and n domains, where excitons are generated. Excitons in the rings are expected to be cold with a temperature close to that of the lattice. Interestingly, at Kelvin temperatures, the exciton rings undergo a transition via a modulational instability. The low-temperature ordered exciton state represents a periodic array of aggregates.

While the scenario in which the observed patterns are related to kinetic effects due to exciton Bose statistics is very appealing, one also needs to assess the possibility that the aggregation is caused by inter-exciton interaction. At short distances, the interaction is repulsive, of an
electric dipole form, making the excitons stable with respect to formation of multi-exciton complexes. At long distances, however, a comparatively weak, attractive interaction may appear as a result of coupling by plasmon wind generated at exciton formation and recombination. We estimate this effect and find the condition under which it can drive an instability in a uniform exciton system.

Another setting in which exciton degeneracy effects can be revealed is provided by recent experiments suggesting that the 2d excitons can collect in shallow in-plane traps. We find that Bose condensation in a trap results in a dramatic change of the exciton PL angular distribution. The long-range coherence of the condensed state gives rise to a sharply
focussed peak of radiation in the direction normal to the plane. By comparing the PL profile with and without Bose Condensation we provide a simple diagnostic for the existence of a Bose condensate. The PL peak has strong temperature dependence due to the thermal order parameter phase fluctuations across the system. The angular PL distribution can also be
used for imaging vortices in the trapped condensate. Vortex phase spatial variation leads to destructive interference of PL radiation in certain directions, creating nodes in the PL distribution that imprint the vortex configuration.

The work reviewed in this talk has been done in collaboration with Leonid Butov, Jonathan Keeling, Peter Littlewood, and Ben Simons.

 

 

Relaxation and Decoherence of Electron Spins in Quantum Dots

Daniel Loss

Department of Physics
University of Basel
Switzerland

I discuss the quantum dynamics and decay of a single electron spin confined to a GaAs quantum dot. The most important sources of spin decay are hyperfine interaction with N nuclear spins [1,2] and the spin-orbit interaction coupling spins to phonons [3]. The hyperfine interaction leads to non-exponential decay laws (due to memory effects in the nuclear spin system) on a scale of microsecs, but the amount of decay can be efficiently suppressed by applying magnetic fields (comparable to the Overhauser field) and/or by dynamically generating a finite polarization p of the nuclear spins, with the mild requirement that p>1/sqrt{N} [2]. The detailed dynamics of the nuclear spins is very rich and can be obtained in a systematic approach in terms of a generalized master equation for a wide physical parameter range [2], thereby generalizing exact results we have obtained for full polarization p=1 [1]. The decay due to phonons [3] is described in terms of an effective Hamiltonian (obtained via Schrieffer-Wolf transformation) which couples the electron spin to phonons or any other fluctuation of the dot potential. It is shown that the spin decoherence time T2 is as large as the spin relaxation time T1, under realistic conditions. For the Dresselhaus and Rashba spin-orbit couplings, we found [3] that, in leading order, the effective magnetic field can have only fluctuations transverse to the applied magnetic field. As a result, T2=2T1 for arbitrarily large Zeeman splittings, in contrast to the naively expected case T2<< T1. The spin decay is drastically suppressed for certain magnetic field directions and values of the Rashba coupling constant. Finally, for the spin coupling to acoustic phonons, we have shown that T2=2T1 for all spin-orbit mechanisms in leading order in the electron-phonon interaction. The theoretical values for T1 are in the range of milliseconds (for B=8T, and at low temperatures), and agree well with recent experiments by the Delft group.

[1] A. Khaetskii, D. Loss, and L. Glazman, PRB 67, 195329 (2003).
[2] B. Coish and D.Loss, preprint.
[3] V. Golovach, A. Khaetskii, and D. Loss, cond-mat/0310655 (to appear in PRL).

 

Measuring the Entanglement of Two Electrons

C. M. Marcus

Department of Physics
Harvard University
17 Oxford St., Cambridge MA 02138

This talk presents recent measurements of tunneling and co-tunneling transport through a two-electron quantum dot (D. M. Zumbuhl, et al.), and how from these measurements one can extract information about the concurrence of the two-electron state. Concurrence provides a measure of entanglement and separability of the two-electron state. Remarkably, the measurement of concurrence in this case does not require a technically difficult noise cross-correlation measurement, but instead can be extracted from dc transport.

 

Scaling the Ion Trap: Microtraps and the Probabalistic Photon Bus

Christopher Monroe

FOCUS Center and University of Michigan
University of Michigan
Ann Arbor, MI 48109-1120


Ideas for scaling the ion trap quantum computer are presented, along with recent progress toward generating remote entanglement between quantum memories and the fabrication of next-generation ion trap structures.

Work supported by ARDA, NSA, ARO, and NSF.

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Circuit quantum electrodynamics: observing strong coupling between a superconducting qubit and a photon

Rob Schoelkopf


Department of Applied Physics
Yale University
New Haven, CT 06520-8284

I will describe recent experiments in which the strong coupling limit of cavity quantum electrodynamics has been realized for the first time using superconducting circuits. In our approach, we use a Cooper pair box acting as an artificial atom coupled to a transmission
line resonator forming a one-dimensional cavity. The strong coupling limit is reached as the vacuum Rabi frequency for the coupling of cavity photons to quantized excitations of the
qubit exceeds the damping rates of both the cavity and the qubit. When the qubit is detuned from the cavity resonance frequency a high-fidelity dispersive quantum non-demolition read-out of the qubit state is achieved. Using this read-out technique we have characterized the qubit properties spectroscopically and attained coherence times greater than 100 ns,
indicating that this architecture is extremely attractive for quantum computing and control. I will also discuss the prospects for single-shot readout and entanglement of multiple qubits, using the cavity as a quantum bus.

 Capacitive Coupling of Atomic Systems to Mesoscopic Conductors

Anders S. Sørensen*, Caspar H. van der Wal, Lilian I. Childress, and
Mikhail D. Lukin

*ITAMP
Harvard-Smithsonian Center for Astrophysics
60 Garden Street, MS 14
Cambridge, MA 02138

We describe a technique that enables a strong, coherent coupling between isolated neutral atoms and mesoscopic conductors. The coupling is achieved by exciting atoms trapped above the surface of a superconducting transmission line into Rydberg states with large electric dipole moments that induce voltage fluctuations in the transmission line. Using a mechanism analogous to cavity quantum electrodynamics, an atomic state can be transferred to a long-lived mode of the fluctuating voltage, atoms separated by millimeters can be entangled, or the quantum state of a solid-state device can be mapped onto atomic or photonic states.

Phys. Rev. Lett. 92, 063601 (2004)

 Optical Control of Spin in Semiconductor Dots for Quantum Operations

Duncan G. Steel
The University of Michigan (FOCUS)
Ann Arbor, Michigan

D. Gammon
Naval Research Laboratory
Washington, DC

L.J. Sham
University of California
San Diego, California

Semiconductor quantum dots have optical properties similar to simple atomic systems, unlike higher dimensional semiconductor structures that are dominated by manybody physics associated with the continuum states. They also provide a potentially ideal electronic structure appropriate for quantum computing. Data shows that the electronic and spin state in these structures can be coherently controlled on a time scale short compared to the quantum decoherence time and that entangled states of qubits can be created. The system is remarkably robust against pure dephasing and we have been able to demonstrate a simple conditional quantum logic device involving multiple Rabi flops of the exciton and biexciton where the optical Bloch vector is the qubit. We also show that we can coherently control the state of a single electron spin that, because of its longer coherence time, may be more useful for quantum computing.

This work supported by ARO/ARDA, DARPA, AFOSR, ONR and NSF.

 

Tunable Coupling of Electron Spin and Nuclear Spin in Quantum Dots

S. Tarucha[1,2], S. Yamaguchi[1], and K. Ono[1]

[1]Department of Applied Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan


[2]ERATO-JST, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

Recent experiments of electron spin and nuclear spin coupling in semiconductor nanostructures have lead to a novel way of manipulating nuclear spin in solid state system. We have observed the coupling in a double dot system and found a way of electrical manipulation of nuclear spin. In this talk I will discuss the possible interactions between electron spin and nuclear spin observed in our double dot system and demonstrate the coherent manipulation of nuclear spins. One key ingredient in this work is to prepare an excited but long-lived electron-spin triplet state in a double dot system. Formation of such an electron spin triplet state blocks single electron tunneling current through the double dot system by Pauli exclusion. Current can flow when the spin triplet state undergoes a spin-flip transition, mediated by hyperfine interaction with nuclear spins. Contributions from nuclei of 71Ga and 69Ga to this current are explored using a technique similar to NMR. We show that the hyperfine coupling effect is turned on and off by adjusting degeneracy of electron-spin singlet and triplet states. This degeneracy is tunable with source-drain voltage as well as magnetic field. Using this technique and a standard pulsed NMR technique, we succeed in demonstrating the coherent nuclear-spin manipulation. From experiments of Rabi oscillations and spin echo we are able to derive the decoherence time and dephasing time for nuclear spins in quantum dots.

 Controlling the Nuclear Spin Environment of Quantum Dots: From Dephasing to Robust Qubit Storage and Manipulation

J.M. Taylor and M.D. Lukin

Department of Physics
Harvard University,
Cambridge, Massachusetts 02138

Electrons in semiconductor quantum dots typically interact with a large ensemble of surrounding nuclear spins via hyperfine coupling. When uncontrolled, this coupling can produce rapid dephasing of electron spin degrees of freedom. In this talk we will describe several approaches to control the interaction between electronic and nuclear degrees of freedom, which allow one to eliminate the dephasing associated with nuclei and to use the localized ensembles of nuclear spins as a useful resource. These approaches make use of the long-lived memory associated with nuclear spins.

Specifically we will describe a technique for storing electronic spin qubits in collective states of nuclear ensembles. This can be achieved by controlling hyperfine interaction with external effective magnetic fields, and can result in a robust quantum memory for mesoscopic quantum bits with a coherence times approaching seconds.

We further show that properly prepared collective states of nuclear ensembles may allow for this technique to be effective even in the regime of low nuclear spin polarization. Specifically, such preparation involves cooling of the nuclear bath by coupling to the degrees of freedom of localized electron spin (quantum bit). Although such a cooling procedure rapidly saturates and the resulting saturated states of the spin bath ("dark states'') generally have low degrees of polarization and purity, their symmetry properties make them a valuable resource for the coherent manipulation of quantum bits. We demonstrate that the dark states of nuclear ensembles can be used to coherently control the system-bath interaction and to provide a robust, long-lived memory for quantum information.

We present a unified description of decoherence, cooling and manipulation of a mesoscopic collection of nuclear spins via coupling to a single quantum system of electronic spin. Extensions of these techniques to implement quantum information protocols in quantum computation and quantum communication are discussed.

 Nanoelectromechanics: From Quantum Computing to Motional State Engineering

Lin Tian

Institute for Theoretical Physics
University of Innsbruck, Austria
Technikerstraße 25
6020 Innsbruck

The development of micro-fabrication technology enables the construction of submicron structures with exotic quantum mechanical properties and potential applications. Recent experiments include displacement sensing of a GHz nanoresonator approaching the quantum limit and the observation of the thermal vibration of a single wall Carbon nanotube. Here, we present examples that bridge nanomechanical systems, mesoscopic physics and quantum optics with applications in quantum computing. 1. ground state cooling of the a small resonator by the single Cooper pair box ---the superconducting charge qubit. We show that an analog of the well known ``laser'' cooling of a nanomechanical resonator capacitively coupled to the qubit can be performed by applying an AC driving to the qubit or the resonator. 2. We present a scheme that integrates the superconducting charge qubit and the ion trap qubits and achieves scalable ion trap quantum computing without moving the ions around. 3. Submicron sized traps for charged particles and neutral atoms, e.g. ion traps, can be constructeded by using nanowires or nanotubes as the electrodes. By charging the electrodes with radio frequency voltage, trapping frequency up to $GHz$ can be achieved. Besides up scaling the time scales of ion trap, the nanotrap provides another type of data bus---the flexual mode vibration of the elelctrodes ---

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Photonic Crystals Devices for Quantum Information Processing

Jelena Vuckovic

Department of Electrical Engineering and Ginzton Laboratory
Stanford University, Stanford, CA 94305-4088

http://www-ee.stanford.edu/~jela

Photonic crystal structures can be built to operate in two opposite regimes: one is a suppression of photon states inside the photonic band gap, and the other is a large enhancement of the density of photon states. Both regimes are of consequence to a number of applications in nanoscale and nonlinear optics, as well as to quantum information processing in the optical domain. Our work on the employment of photonic crystals to build hardware of solid-state photonic quantum information systems, as well as to construct miniaturized optical devices will be reviewed in this talk.

We have demonstrated sources of single photons based on quantum dots in (one-dimensional photonic crystal) micropost microcavities and have shown that such sources exhibit a large Purcell factor together with a small multi-photon probability. For a quantum dot on resonance with a cavity, the spontaneous emission rate has been increased by a factor of five, while the probability to emit two or more photons in the same pulse has been reduced to 2% compared to a Poisson-distributed source of the same intensity. In addition to the small multi-photon probability, such a strong Purcell effect is important in a single-photon source for improving the photon outcoupling efficiency and the single-photon generation rate, and for making the emitted photon pulses indistinguishable. We have tested the emitted single photons from such a source through a Hong-Ou-Mandel-type two-photon interference experiment, and found that consecutive photons are largely indistinguishable, with a mean wave-packet overlap as large as 0.81, making this source useful in a variety of experiments in quantum optics and quantum information.

We have also developed two-dimensional photonic crystal microcavities of finite depth that exhibit large quality factors together with small mode volumes and with maximum field intensity in the high-index region, which is of importance for enhanced interaction with quantum dot excitons. By employing such cavities instead of microposts, Purcell factor could be dramatically increased and even the strong-coupling regime of the cavity quantum electrodynamics (QED) with a single exciton could be reached. In addition to the fundamental studies of the solid-state cavity QED, this is also of importance for construction of single photon sources with improved efficiency, visibility, and speed, as well as for construction of entangled photon sources on demand. Finally, two-dimensional coupled arrays of such photonic crystal microcavities can be employed in making miniaturized lasers or sensors.

 Stability Of Bose-Einstein Condensates Near Room-Temperature Surfaces

Prof. Vladan Vuletic

MIT
Center for Ultracold Atoms
77 Massachusetts Avenue, 26-231
Cambridge, MA 02139

Using a Bose-Einstein condensate of rubidium atoms prepared near a microfabricated silicon chip, we investigate the impact of the room-temperature surface on the condensate at small distances down to 1 micrometer. We find that for electrically conducting films the condensate lifetime is limited by magnetic field noise generated by Johnson-noise induced currents, as previously predicted, and observed for bulk conductors. A dielectric surface has no adverse effects on condensate stability or temperature until the Casimir-Polder attractive force between surface and atoms overcomes the magnetic confinement. We discuss implications for microchip-based quantum devices, such as interferometers and Josephson-junctions for Bose-Einstein condensates.

 Imaging Electrons in a Single-Electron Quantum Dot

Parisa Fallahi, Ania Bleszynski, R.M. Westervelt, E.J. Heller and A.C. Gossard

Division of Engineering and Applied Sciences
Harvard University
9 Oxford Street
Cambridge, MA 02138

A scanning probe microscope (SPM) operating at liquid He temperatures was used to image electrons inside a single-electron quantum dot. In previous research,1 a charged SPM tip backscattered electron waves arriving from a quantum point contact (QPC) through a two-dimensional electron gas. As the tip was scanned over the sample, changes in QPC conductance imaged the coherent flow of electron waves, showing interference fringes spaced by half the Fermi wavelength. In the current experiments, we use changes in the Coulomb blockade conductance of a quantum dot to image electrons inside. A small quantum dot was formed in a GaAs/AlGaAs heterostructure by electrostatic gates. As the side-gate voltage VG and source to drain voltage VSD were changed, Coulomb blockade diamonds show that it can contain 0, 1 or 2 electrons. When the SPM tip is held at a fixed position, similar Coulomb blockade diamonds are produced by changing the tip voltage Vtip instead of VG. Images of electrons inside the single electron dot were obtained by fixing both VG and Vtip and scanning the tip position over the dot. The images show a ring of Coulomb blockade conductance centered on the dot, where the number of electrons changes from 0 to 1 - the tip pushes an electron off the dot. The radius of the ring reduces as the Vtip is increased. From the shape of these rings, we measure the shift in energy of the electron ground state that is induced by the tip.

1M.A. Topinka, R.M. Westervelt and E.J. Heller, "Imaging Electron Flow", Physics Today 56, 47 (2003).

 

Single Atom Switches and Mirrors by Quantum Interference: "Single Atom Transitors"

Peter Zoller

Institute for Theoretical Physics
University of Innsbruck, and
Institute for Quantum Optics and Quantum Information of the
Austrian Academy of Sciences
Innsbruck, Austria

We consider a single impurity (qubit) in a 1D optical lattice. The qubit acts as a single atom "quantum mirror": depending on the state of the qubit, the single atom mirror can either be transparent or block the transport of atoms. The blocking of transport is based on an EIT type quantum interference.

In particular we will discuss in detail: (i) an exact solution of the scattering problem of a single atom from the impurity, displaying the quantum interference features as Fano-type energy profiles in the Bloch band; (ii) the semi-analytical solution of a many-atom Hubbard model either for hard-core bosons and using a Vidal type DMRG approach to solve the time dependent many particle Schroedinger equation.

The situation studied is in loose analogy to a single electron transitor in mesoscopic physics.

 

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