Introduction to Introduction to Photonic Photonic Quantum Quantum Logic Logic QUAMP Summer School SEPT 2006 QUAMP Summer School SEPT 2006 J. G. Rarity J. G. Rarity University of Bristol University of Bristol [email protected][email protected]EU FET: QAP EU FET: QAP o, J. Fulconis, J. Duligall, C. Hu, R. Gibson, O Alibart, J. O’Brien sworth, lnick, D. Whittaker, M. Fox, J. Timpson , T. Spiller, K. Harrison EPSRC EPSRC 1-phot 1-phot FP6:IP FP6:IP SECOQC SECOQC www.ramboq.org www.ramboq.org
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Introduction to PhotonicIntroduction to PhotonicQuantum Quantum
LogicLogic QUAMP Summer School SEPT 2006QUAMP Summer School SEPT 2006
J. G. RarityJ. G. RarityUniversity of BristolUniversity of Bristol
Bristol: Daniel Ho, J. Fulconis, J. Duligall, C. Hu, R. Gibson, O Alibart, J. O’Brien Bath: William Wadsworth, Sheffield: M. Skolnick, D. Whittaker, M. Fox, J. TimpsonHP Labs: W. Munro, T. Spiller, K. Harrison
EPSRCEPSRC1-phot1-phot
FP6:IP FP6:IP SECOQCSECOQC
www.ramboq.orgwww.ramboq.org
Structure– What is light?– Decoherence of photons– Single photon detection– Encoding bits with single photons and single bit manipulation.– Linear logic– Entangled state sources– Single photon sources– Quantum Cryptography
The electro-magnetic spectrum
Optical Photon energy Eph=hf>>KT
λ=1.5umEph=0.8eV
λ=0.33umEph=4eV
V+
Particlelike Wave-like during propagation
Particlelike
Decoherence of photons: associated with loss
• Storage time in fibre 5μs/km, loss 0.17 dB/km (96%)
• Polarised light from stars==Storage for 6500 years!
Photon is absorbed in the avalanche region to create an electron hole pair
Electron and hole are accelerated in the high electric field
Collide with other electrons and holes to create more pairs
With high enough field the device breaks down when one photon is absorbed
InGaAs avalanche detectors:Gated modules operation at 1550nmLower efficiency ~20-30%Higher dark counts ~1E4/secAfterpulsing (10 us dead time)www.idquantique.com
Other detectors
• The Geiger mode avalanche diodes count one photon then switch off for a dead time before they are ready to detect another-NOT PHOTON NUMBER RESOLVING
• Photon number resolving detectors may become available in the near future:– Cryogenic superconducting to resistive transitions Jaspan et al
Single photon can only be detected in one detectorHowever interference pattern built up from many individual countsP. Grangier et al, Europhysics Letters 1986
L
U
In the interferometer we have superposition state
)11(2
1L
iU
e
2/)sin1()1(
2/)sin1()0(
y probabilitDetection
1 10
generalIn
1)(0)1(21
21,
2
)11(2
1
2
22
P
P
eiie
tir
e
iiout
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After the interferometer:
Phaseplate
Beamsplitter50:50
Mirror
D(1)
D(0)
Mirror
D(0)CountRate
0 Thicknessof phase plate
Encoding one bit per photon and single qubit rotations
Encoding single photons using two polarisation modes Superposition states of ‘1’ and ‘0’
|Ψ>= α|0> +β|1> Probability amplitudes α , β
Detection Probability: |α|2
Single photon encoding showing QBER<5.10-4
(99.95% visibility)
-20 0 20 40 60 80 100 120 140 160 180 200
102
103
104
105
Gat
ed c
ount
rat
e
/2 plate angle
2QUBIT logic: Photonic CNOT Gate.
ccccttin
1010
ctcctcctcctcout10011100
D(1)
PBS D(0)
Targeta bt t| H>+ | >V
Controla bc c| H>+ | >V
QR is a quantum polarisation rotatorRotates polarisation if control is vertically polarisedDoes nothing if control is Horizontally polarised
Requires non-linearity at single photon level: Atoms: Turchette and Kimble PRL1995, Solid state: J. P. Reithmaier/ A. Forchel, NATURE 432, Nov 2004.
Bennett and Brassard 1984Secure key exchange using quantum cryptography
Demonstration of an all-optical quantum controlled-NOT gate
Knill et al Nature 409, 46–52 (2001)J L O’Brien et al, Nature 426, 264 (2003) / quant-ph/0403062
Polarisation KLM gate
Parity Measurement
Parity and conditional CNOTKnill et al Nature 409, 46–52 (2001)
Pittman et al (2002) PRL 88, 257902
Not 100% efficient but Up to 50%
NotesTarget V-->H+V Control V-->H+VParity-->HH+VV -45--> H(H+V)-V(H-V) Confirm click is H-->(H-V) out -45--> |H>Confirm click is V-->(H+V) out -45--> |V>
Target V-->H+V Control H-->H-VParity-->HH-VV -45--> H(H+V)+V(H-V) Confirm click is H-->(H+V) out -45--> |V>
A ‘scalable’ 2-qubit CNOT gate
In the proposal Actual realisationTruth tableFidelity ~0.8
S. Gasparoni, J-W Pan, P. Walther, T. Rudolph, and A. Zeilinger, Phys. Rev. Lett. 93, 020504 (2004)
IST-2001-38864: RAMBOQ
Optical Cluster State ComputingP. Walther et al Nature 434, 169-176 (2005)