Optical Control in Semiconductors Optical Control in Semiconductors for Spintronics for Spintronics and and Quantum Information Processing Quantum Information Processing Jun Kono Department of Electrical & Computer Engineering, Rice University October 8, 2003 Supported by NSF DMR-0325474 (ITR), NSF DMR- 0134058 (CAREER), NSF INT-0221704, and DARPA MDA972-00-1-0034 (SpinS) ECE Affiliates Meeting
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Optical Control in SemiconductorsOptical Control in Semiconductorsfor Spintronicsfor Spintronics
andandQuantum Information ProcessingQuantum Information Processing
Jun Kono
Department of Electrical & Computer Engineering,Rice University
October 8, 2003
Supported by NSF DMR-0325474 (ITR), NSF DMR-0134058 (CAREER), NSF INT-0221704, and DARPAMDA972-00-1-0034 (SpinS)
ECE Affiliates Meeting
October 8, 2003 J. Kono
Our Spin/Q.I.P. TeamOur Spin/Q.I.P. Team
Jigang Wang Ajit Srivastava Xiangfeng Wang Rahul Srivastava Giti Khodaparast
Dave ReitzeChris Stanton Young-Dahl Jho
Lu Sham Hiro Munekata
Univ. of Florida
UCSD Tokyo Inst.of Technology
October 8, 2003 J. Kono
Need for Need for QuantumQuantum Technologies Technologies
ß Novel quantum technologies being sought ‡ better performanceand new functionality and multi-functionality
ß Spin-based electronics, quantum electronics based on quantumcoherence, interference, and entanglement, … etc.
“Quantum physics holds the keyto the further advance ofcomputing in the postsilicon era.”
- J. Birnbaum and R. S. Williams
“Coherent spin packets may offergenuine quantum devicesthrough their wave-like properties.”
- D. D. Awschalom
October 8, 2003 J. Kono
OutlineOutline
n Semiconductor ‘Spintronics’n Towards Solid-State Realization of Quantum
Information ProcessingOur Approach: Ultrafast Optical ControlOur Recent Discoveries
Ultrafast Photoinduced Softening (UPS) in aFerromagnetic SemiconductorUltrafast Photoinduced Transparency (UPT) using theDynamic Franz-Keldysh Effect (DFKE)
Summary
October 8, 2003 J. Kono
Emerging Technologies forEmerging Technologies forSolid-State Information ProcessingSolid-State Information Processing
Spintronics
ß Use spinsspins in solid statedevices
ß Improve informationprocessing
ß Add new functionalities
Quantum InformationProcessing
ß Devise and implementquantum-coherent
strategies for computationcomputationand communicationcommunication
Use discrete, quantized degrees of freedom in a physicaldevice to perform information processing functions.
October 8, 2003 J. Kono
Magneto-ElectronicsMagneto-Electronics
Magnetic tunneling junction(MTJ) or “spin valve” ‡Nonvolatile MRAM:“Microchips that neverforget ”
S. Parkin (1990)Compatibility with Si andGaAs ‡ next phase:semiconductor spintronics
GMR ‡ read-out heads in hard drives
1st generation spintronic devices based onferromagnetic metals – already in commercial use
October 8, 2003 J. Kono
Recent Discoveries in SemiconductorsRecent Discoveries in Semiconductors
n A room temperature, optically induced, verylong lived quantum coherent spin state insemiconductors that responds at Terahertzwith no dissipation and can be transported bysmall electric fields (UCSB).
n Ferromagnetism in semiconducting GaMnAsat 120K (Japan, Europe, U.S.A.).
DARPA ‘Spins in Semiconductors’Program (2000 – present)
October 8, 2003 J. Kono
Spin-Enhanced and Spin-Spin-Enhanced and Spin-Enabled ElectronicsEnabled Electronics
• Quantum Spin Electronics– Tunneling/transport of quantum confined spin states
– Spin dependent resonant tunneling devices and spin filtering
– Spin FETs (“spin gating”)
– Spin LEDs, electroluminescent devices, and spin lasers
• Coherent Spin Electronics– Optically generated coherent spin states and coherent control of
propagating spin information - optical encoders and decoders
• Quantum Information Processing– Qubits using coherent spin states a|0> + b|1>, a2 + b2 = 1
– Spin based quantum computing, teleportation, code breaking andcryptography
October 8, 2003 J. Kono
Quantum Information ProcessingQuantum Information Processing
- Coherent superposition: 10 ba +=y
‡ Inherent parallelism ‡ can solve problems that arecomputationally too intensive for classical computers
- 2 examples of ‘quantum algorithms’:Shor’s factorization (1994)Grover’s search (1997)
- Physical Implementations:Trapped ions (1995)Cavity QED (1995)Bulk NMR (1997), …
Proof-of-principledemonstrations, butnot scalable
October 8, 2003 J. Kono
Toward Toward Solid-StateSolid-State Realization of QIP Realization of QIPThe Race is on!The Race is on!
Semiconductors vs. Superconductors
Spin Qubits vs. Charge Qubits
Electrical Manipulation vs. Optical Manipulation
• Intensive search for realistic approaches to building aquantum computer
• Solid-state systems offer a much greater degree ofcontrol over design and fabrication, necessary forconstructing large-scale devices
Cooper Pairs vs. Flux QubitsElectron Spin vs. Nuclear Spins
October 8, 2003 J. Kono
Decoherence Decoherence ProblemProblem
T : decoherence timet : operation timeR = T/t : figure of merit
How can we increase T and/or decrease t?
• Coherent states are very easily damaged by uncontrolledinteractions with the environment – decoherence
• Unavoidable decoherence will cause the quantuminformation to decay ‡ main obstacle
• Decoherence causes a collapse of the superposition stateinto a single eigenstate ‡ loss of parallelism
October 8, 2003 J. Kono
To develop novel ultrafast optical methodsin semiconductors that may find application inspintronics or quantum information sciencethrough coherent light-matter interactionsinvolving ferromagnetism, band structures,lattice vibrations, and excitons
Our GoalOur Goal
e-
October 8, 2003 J. Kono
Ultrafast Ultrafast Photoinduced SofteningPhotoinduced Softening(UPS) in InMnAs(UPS) in InMnAs
DFKE Simulation for a GaAs FilmDFKE Simulation for a GaAs Film1.0
0.8
0.6
0.4
0.2
0.0
Tra
nsm
ittan
ce
no pump pumped
-0.6
-0.4
-0.2
0.0
0.2
T ?
T0
1.551.501.451.401.351.30
Energy (eV)
• Undoped GaAsfilm (2.7 mm thick)
• Multiple-reflectionincluded
• lpump = 9 mm (138meV)
• Ipump = 1010 W/cm2
ll InducedInducedtransparencytransparency
October 8, 2003 J. Kono
DFKE: DFKE: ConclusionsConclusions
n Intense MIR laser fields can coherentlymodify electronic states in solids throughnon-resonant pumping
n No sample damage, no real carriers
n Ultrafast transmission quenching belowband edge observed
n First observation of induced transparencyabove the band edge
n Main features of observations qualitativelyagree with theory
October 8, 2003 J. Kono
Accomplished:
• Photogenerated transient carriers ‡ modify magneticproperties in InMnAs ‡ first demonstration of ultrafastsoftening: Hc (coercivity) decreases (‘hard’ ‡ ‘soft’)
• Intense and coherent midinfrared radiation modified bandstructure through DFKE ‡ first observation of ultrafastphotoinduced transparency
SummarySummaryUltrafast Optical Control in SemiconductorsUltrafast Optical Control in Semiconductors
In Progress:
• Demonstration of ultrafast photoinduced magnetizationreversal
• Transient modifications of Tc and photoinduced transientpara- to ferromagnetic transition
October 8, 2003 J. Kono
Realization of Spin-Based DevicesRealization of Spin-Based Devices
Technical issuesTechnical issues
• How strongly can one create carriers of agiven spin?
• How long can one sustain the spinpolarization?
• How can one modulate or control the spin?
• How sensitively can one detect the spin?
October 8, 2003 J. Kono
Decoherence Decoherence ProblemProblem
T : coherence timet : operation timeR = T/t : figure of merit
Factoring a 4-bit number using Shor’s algorithmRequires ~2 x 104 gate operations on 20 qubits ‡R has to be > 4 x 105
• Coherent states are very easily damaged by uncontrolledinteractions with the environment – decoherence
• Unavoidable decoherence will cause the quantuminformation to decay, thus inducing errors in thecomputation
• Decoherence occurs rapidly in complex big systems, whichis why we never observe macroscopic superpositions
October 8, 2003 J. Kono
Ultrafast Optics in Ferromagnetic Ultrafast Optics in Ferromagnetic MetalsMetals
Ni and Co: E. Beaurepaire et al., Phys. Rev. Lett. 76, 4250 (1996). M. Aeschlimann et al., Phys. Rev. Lett. 79, 5158 (1997). A. Scholl et al., Phys. Rev. Lett. 79, 5146 (1997). J. Hohlfeld et al., Phys. Rev. Lett. 78, 4861 (1997). J. Güdde et al., Phys. Rev. B 59, R6608 (1999). B. Koopmans et al., Phys. Rev. Lett. 85, 844 (2000).
CoPt3: G. Ju et al., Phys. Rev. B 57, R700 (1998). E. Beaurepaire et al., Phys. Rev. B 58, 12134 (1998). L. Guidoni et al., Phys. Rev. Lett. 89, 017401 (2002).
n Curie temperature (Tc) of InMnAs below roomtemperature ‡ explore new materials (e.g.,GaMnN)
n Small band gap of InMnAs requires powerfulmid-infrared (MIR) laser pulses ‡ requiresdevelopment of compact MIR source or largeband gap materials (e.g., GaMnN)
n Kerr rotation angle of InMnAs small ‡ newmaterials or sophisticated opticalarrangements necessary
LimitationsLimitations
October 8, 2003 J. Kono
E-field-induced changes in optical absorption near the E-field-induced changes in optical absorption near the bandedgebandedge
rEeÇrr
⋅-='Additional term in the Hamiltonian
Exponential for positive arguments
Oscillations fornegativearguments
Exponential tail below bandgap
Oscillationsabovebandgap
Franz-Keldysh Franz-Keldysh EEffectffect
absorption
wavefunction
dc fielddc field
October 8, 2003 J. Kono
Unusual Power DependenceUnusual Power Dependence
Dec
reas
ing
inte
nsi
ty
Incr
easi
ng
eff
ect!
!
Below band gap absorption in bulk GaAs at 300 KBelow band gap absorption in bulk GaAs at 300 K
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
Tra
nsm
issi
on
1.3901.3801.3701.360
Photon Energy (eV)
4.7 mm 6 mm 8 mm 9 mm
October 8, 2003 J. Kono
Extent of Induced AbsorptionExtent of Induced AbsorptionPolycrystalline ZnSe, 3.5 mm driving field, g ~ 1
0.5
0.4
0.3
0.2
0.1
0.0
tran
smis
sion
2.62.42.22.01.8photon energy (eV)
Induced absorption extends almost 1 eV below Eg!
October 8, 2003 J. Kono
DFKE is adramatic effect
Other Strong-Field-Induced EffectsOther Strong-Field-Induced Effectsin Semiconductorsin Semiconductors
DC Franz-Keldysh effect AC Stark effect
shift ~ only a few meV at 100 kV/cm !
B.G. Yacobi et al.A. Mysyrowicz et al.
October 8, 2003 J. Kono
Based on: Y. Yacoby, Phys. Rev. 169, 610 (1967).
DFKE Simulation for GaAsDFKE Simulation for GaAs
2.5
2.0
1.5
1.0
0.5
0.0
?
(106
cm-1
)
1.601.551.501.451.401.351.30
Energy (eV)
no pump pumped
GaAs? pump = 9 ?m (h? = 138 meV)
Ipump = 1010 W/cm2 (Up~ h?)
Blue shift of band edge ‡Inducedtransparencytransparency
• A. H. Chin, J. M. Bakker, and J. Kono, Phys. Rev. Lett. 85, 3293 (2000).• A. H. Chin, O. G. Calderón, and J. Kono, Phys. Rev. Lett. 86, 3292 (2001).• O. G. Calderón, A. H. Chin, and J. Kono, Phys. Rev. A 63, 053807 (2001).• M. A. Zudov, J. Kono, A. P. Mitchell, and A. H. Chin, Phys. Rev. B 64,
121204(R), (2001).• A. H. Chin, J. Kono, and G. S. Solomon, Phys. Rev. B 65, 121307(R),
(2002).• A. Srivastava and J. Kono, in: Quantum Electronics and Laser Science
Conference, OSA Technical Digest (Optical Society of America,Washington DC, 2003), QFD2.