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Experimental work on entangled photon holes
T.B. Pittman, S.M. Hendrickson, J. Liang, and J.D. Franson
UMBC
ICSSUR Olomouc, June 2009
Experimental work on entangled photon holes
T.B. Pittman, S.M. Hendrickson, J. Liang, and J.D. Franson
UMBC
ICSSUR Olomouc, June 2009
Linear Optics Quantum Computing,
Zeno Gates
Entangled-Photon Holes
Outline

Entangled Photon holes?

Generation of these states by
 two-photon absorption
 quantum interference

Experimental observation of photon holes using
quantum interference

Towards Bell’s inequality tests
Optical Entanglement

Entanglement of photon pairs:




polarization
momentum
….
….combinations of properties
Polarization entanglement
from Type II PDC (Kwiat ‘95)

We are investigating a new form of entanglement


arises from the absence of photon pairs themselves
correlated absences…. “Entangled photon holes”

Creation of entangled photon holes can have macroscopic
effects on two-photon absorption
 effects of entanglement can be observed with “classical detector”

This talk will focus instead on the basic concept and recent
experimental work
What are entangled photon holes?
parametric
down-conversion

First, consider photon pairs from typical PDC scenario:

Photons generated at same time, but that time is uncertain
 superposition of these times  entanglement


background in each beam is empty
but uniform probability amplitude to find photon pair anywhere
What are entangled photon holes?
Two-photon
absorption medium
3
2
1
weak coherent
state inputs

(3-level atoms)
Now consider ideal two-photon absorption

Photons annihilated at same time, but that time is uncertain
 superposition of these times  entanglement


Background in each beam is constant
But uniform probability amplitude to find hole pair anywhere
Consider two single-photon inputs
“holes” correlated in time, but
could be generated at any time:
photon 1
amplitude
coherent superposition
photon 2
coinc. rate
time
-
0
+
(t1-t2)
PDC with narrowband pump
photon pair could be
produced at any time
amplitude
photon 1
coherent superposition
of these times
photon 2
coinc. rate
time
-
0
+
(t1-t2)
Photon pairs vs. Photon holes


empty background
photon pair anywhere



constant background
hole pair anywhere
Entangled photon holes: “negative image” of PDC
Ideal two-photon absorption?
3


Generation of entangled photon holes in this
way requires strong two-photon absorption
at the single-photon level

Very difficult to achieve (works in progress)

example system: tapered optical fiber in atomic vapor
Can entangled photon holes be generated
through quantum interference instead?

Yes
2
1
(3-level atoms)
TPA in tapered optical fibers
Rb atoms
optical fiber
evanescent field
outside fiber


“heat and pull”: sub-wavelength diameter wires
evanescent field interacts with Rubidium vapor
Reduced mode volume beats optimal free-space focusing (for TPA)
Recent experiments with tapered optical fibers in Rb
d ~ 125 mm


taper: d ~ 450 nm
(over L ~ 5 mm)
gives ~106 improvement in TPA rate
over focused beam
even this is way too small for observing
TPA at single-photon levels!
H.You et.al. PRA 78, 053803 (2008)
Side note: nonlinear transmission through TOF
Nonlinear transmission

Rb atoms tend to accumulate on TOF


can be removed using optical beam
propagating through the TOF


Reduces transmission (scattering)
probably LIAD & thermal effects
results in nonlinear transmission %
S.M. Hendrickson et.al. JOSA B 26, 267 (2009)
S. Spillane et.al PRL 100, 233602 (2008)
saturation spectroscopy
Photon holes via quantum interference
?
weak coherent state
Interference effect to suppress the probability P11 of finding
one photon in each output mode?
Photon holes via quantum interference
PDC source
50/50 beam splitter
phase locked, 
weak coherent state
Interference effect to suppress the probability P11 of finding
one photon in each output mode?
mix with phase-locked PDC source at 50/50 BS
Note: TPA case: classical in  nonlinearity  quantum out
this case: classical in + quantum in  interference  quantum out
Photon holes via quantum interference
PDC source
50/50 beam splitter
what is P11 ?
phase locked, 
 1,1 ~  2 1,1  ei  2 1,1
weak coherent state
due to 2-photon term
of weak coherent state

due to PDC pair
If indistinguishable amps and  = p, destructive interference (P11 = 0)
 suppress any pairs from “splitting” at 50/50
 leaves photon hole pairs in constant laser background

experimental challenge: how to phase-lock PDC & weak laser?
 answer: Koashi et.al. phase-coherence experiment (1994)
 frequency-doubled laser (2w) for PDC pump  PDC pairs at w
 fundamental (w) as weak coherent state
 MZ-like interferometer  phase 
Versatile method: many implementations possible…
Koashi et.al. PRA (1994)
Resch et.al. two-photon switch
PRL 87, 123603 (2001)
Kuzmich et.al. homodyned Bell-test
PRL 85, 1349 (2000)
Lu and Ou, cw experiment
PRL 88, 023601 (2002)
Photon holes experiment
laser
pick-off
PDC
crystal
SHG
“HOM” beam splitter
delay
filter
mode-locked
laser
primary
beam splitter
PDC
APD-2

delay
APD-1
ND
l-plate
PBS
laser
stop
filters
start
data aq.
TAC
Photon holes experiment
“HOM dip”
V~99%
laser
pick-off
PDC
crystal
SHG
“HOM” beam splitter
delay
filter
mode-locked
laser
primary
beam splitter
PDC
APD-2

delay
APD-1
ND
l-plate
PBS
laser
stop
filters
start
data aq.
TAC
Photon holes experiment
“HOM dip”
V~99%
laser
pick-off
PDC
crystal
SHG
“HOM” beam splitter
delay
filter
mode-locked
laser
primary
beam splitter
PDC
APD-2

delay
APD-1
ND
l-plate
PBS
laser
stop
filters
start
giant MZ interferometer
(fiber and free-space)
data aq.
key point: phase 
TAC
step 1: calibration

coinc.
counts
coincidence counts
2000
1500
matched two-photon
amplitudes
2000
weak laser only
(76 MHz pulse train)
PDC only
1500
1000
1000
500
500
0
0
-20
0
20
40
-20
relative delay (ns)
0
20
40
step 2: phase control

coinc.
counts
 = 0o
 = 180o
Visibility ~90%
step 3: observation of photon holes

Probability of finding one photon
in each beam is suppressed
coincidence counts
coinc.
counts
2000
1500
1000
500
0
Note: not completely eliminated.
due to imperfect mode-matching
-20
0
20
40
relative delay (ns)
Pittman et.al. PRA 74, 041801R (2006)
Data summary
laser only
main result
PDC only
Data summary
laser only
PDC only
Important: data collected shows existence of photon
holes, but does not demonstrate entangled nature of state
-- analogous to just measuring “photon pairs” in, say,
Kwiat ’95 polarization experiments
additional measurements are required:
-- Bell test with entangled photon holes
main result
Bell’s inequality tests
basic idea: use “Franson interferometer”
1
L
S
coinc.
counts
S
L
2
PDC source
only S1S2 and L1 L2 amplitudes
  
Rc ~ cos 2  1 2 
 2 
can be used to violate Bell’s ineq.
Bell’s inequality tests
basic idea: use “Franson interferometer”
1
L
S
coinc.
counts

S
L
2
PDC source
photon holes source
only S1S2 and L1 L2 amplitudes
Photons never emitted at same time
only S1L2 and L1S2 amplitudes
  
Rc ~ cos 2  1 2 
 2 
can be used to violate Bell’s ineq.
   
Rc ~ cos 2  1 2 
 2 
Bell’s inequality tests
Interpretation is difficult: detectors
only
register
background
photons
basic idea:
use
“Franson
interferometer”
-- photon holes suppress detection process in a nonlocal way
1
L
S
coinc.
counts

S
L
2
PDC source
photon holes source
only S1S2 and L1 L2 amplitudes
Photons never emitted at same time
only S1L2 and L1S2 amplitudes
  
Rc ~ cos 2  1 2 
 2 
can be used to violate Bell’s ineq.
   
Rc ~ cos 2  1 2 
 2 
Time-bin entangled photon holes

Photon hole generation: relies on interference of independent sources




short-pulsed lasers/narrowband filters for indistinguishability
no cw “energy-time” type entanglement
this puts our Bell test exp’s into the “time-bin” regime (Gisin’s group)
Experiments currently underway (4 stabilizations req’d)
Time-bin entangled photon holes

photon hole source

Photon hole generation: relies on interference of independent sources




short-pulsed lasers/narrowband filters for indistinguishability
no cw “energy-time” type entanglement
this puts our Bell test exp’s into the “time-bin” regime (Gisin’s group)
Experiments currently underway (4 stabilizations req’d)
Summary and outlook

New form of entanglement
 entangled photon holes
 “negative image” of PDC

Generation via ideal TPA
or quantum interference effects
 recent experiments

Many open questions:
…
 quantum communications
 …

Some comments on photon hole data

Data looks similar to that typically obtained by splitting a
conventional anti-bunched state

PDC
But that kind of (two-beam) state is very different than photon hole
states of interest here
50/50 beam splitter
-lock
1
laser
excitation pulse train
statistics of either beam
resemble a coherent state
splitting an antibunched beam
gives two antibunched states
>> also different than the (single-mode) states produced by “hole-burning” in
Fock space: B. Basiea et.al. Phys. Lett A 240, 277 (1998)
>> and not the same as the two-mode single-photon states
of the form |0,1> + | 1,0>
(HISTORICAL SIDE NOTE)
Bouwmeester et.al. Teleporation
Nature 390, 575 (1997)
Koashi et.al. PDC phase coherence
PRA 50, R3605 (1994)

1st demo that required “Multi-photon”
experimental conditions
 Ultra-fast pulsed-PDC and narrowband filters for indistinguishability
 now used for many experiments
Rarity et.al. PDC & |>
Philos. Trans. 355, 2567 (1997)
Fiber-based interferometer
primary
beam splitter
HOM & primary
beam splitters
PDC photons
weak
laser pulse
HOM beam splitter
Rb TPA frequency-locking system
5 2D5/2
6 2P3/2
778 nm
420 nm
optimal PDC
bandwidth ~ 3 nm
Doppler-broadened peaks ~ 1 GHz
~ 2 nm
PDC
lock
5 2P3/2
fluor. counts (arb)
778 nm
780 nm
5 2S1/2
Doppler-free peaks
0.0
0.5
1.0
1.5
2.0
2.5
3.0
laser frequency scan (GHz)
spectral analysis
wavelength meter
SM fiber
MM fiber
778 nm input
fluorescence
collection
PBS
aux. output beam
l/4
Rb vapor cell
in TC’d oven
narrowband
filter
detector
(SPCM or PIN)
f = 80 mm lenses
3.5
4.0