Post on 11-Mar-2021
Fundamental Physics with Atomic Interferometry
Peter GrahamStanford
Savas DimopoulosJason Hogan
Mark KasevichSurjeet Rajendran
with
PRL 98 (2007)PRD 78 (2008)PRD 78 (2008)PLB 678 (2009)arXiv:1009.2702arXiv:1101.2691
Outline
1. Atomic Interferometry
2. Gravitational Wave Detection
3. Axion Dark Matter Detection
Atomic Interferometry
Atom Interferometry
• established technology
• rapidly advancingdriven by progress in atom cooling (BEC), atomic clocks, ...
1991 - Kasevich & Chu - 3 x 10-6 1998 - A. Peters - 10-10 2011? - 10-15
• high precision already reached 10-11 gearth, 17 digit clock synchronization
path to future improvements (increased atom flux, entangled atoms...)
can test e.g. time variation of fundamental constants
There must be more fundamental physics to be done with it
Atomic Clock|atom〉 = |1〉
1
2∆E
Atomic Clock|atom〉 = |1〉
beamsplitter1√2
(|1〉+ |2〉) 1
2∆E
Atomic Clock|atom〉 = |1〉
beamsplitter1√2
(|1〉+ |2〉) 1
2∆E
1√2
(|1〉+ ei(∆E)t|2〉
)wait time t
Atomic Clock|atom〉 = |1〉
beamsplitter1√2
(|1〉+ |2〉) 1
2∆E
1√2
(|1〉+ ei(∆E)t|2〉
)wait time t
beamsplitter
12
[(1− ei(∆E)t
)|1〉+
(1 + ei(∆E)t
)|2〉
]
Atomic Clock|atom〉 = |1〉
beamsplitter1√2
(|1〉+ |2〉) 1
2∆E
1√2
(|1〉+ ei(∆E)t|2〉
)wait time t
beamsplitter
12
[(1− ei(∆E)t
)|1〉+
(1 + ei(∆E)t
)|2〉
]
N1 N2
output ports
can measure times t ∼ 1∆E ∼ 10−10 s
pulse is a beamsplitterpulse is a mirrorπ
π/2
!1,p" !2,p!k"
Ω1 Ω2
Π""""2
Π 3 Π""""""""2
2 Πt !#Rabi$1"
1""""2
1
,#c1#2 #c2#2
#1,p$#2,p%k$
ψ = c1|1, p〉+ c2|2, p+ k〉
Raman Transition
!1,p" !2,p!k"
Ω1 Ω2
ke f f = ω1 +ω2 ∼ 1 eV
ωe f f = ω1−ω2 ∼ 10−5 eV
ke f f
ω1 ω2
ω2
Raman Transition
Light Pulse Atom Interferometry
r
0
T
2T
t
π
π
2
π
2
beamsplitter
beamsplitter
mirror
Light Pulse Atom Interferometry
a constant gravitational field produces a phase shift:
the interferometer can be as long as T ~ 1 sec ~ earth-moon distance!
sensitivity ~ 10-7 rad∆φ ∼ mg(∆h)T ∼ mg
(k
mT
)T = kgT 2 ∼ 108 rad
∆φ =∫
Ldt =∫
mdτ =∫
pµdxµ
r
0
T
2T
t
π
π
2
π
2
beamsplitter
beamsplitter
mirror
Stanford 10m Interferometer
colocated 85Rb and 87Rb clouds test Principle of
Equivalence initially to 10-15
in controlled (lab) conditions
10 m atom drop tower.
10 m
Atomic Gravitational Wave
Interferometric Sensor (AGIS)
PRD 78 (2008)PLB 678 (2009)arXiv:1009.2702
• provide unique astrophysical informationcompact object binariesblack holes, strong field GR tests
• Every new band opened has revealed unexpected discoveries
Gravitational waves open a new window to the universe
sourced by mass, not chargeuniverse is transparent to gravity waves
Motivation
• rare opportunity to study cosmology before last scattering
inflation and reheatingearly universe phase transitionscosmic strings ...
CosmologyGravitational waves open a new window to the universe
sourced by mass, not chargeuniverse is transparent to gravity waves
Gravitational Wave Signal
laser ranging an atom (or mirror) from a starting distance L sees a position:
and an acceleration
ds2 = dt2 − (1 + h sin(ω(t− z)))dx2 − (1− h sin(ω(t− z)))dy2 − dz2
x ∼ L(1 + h sin(ωt))
a ∼ hLω2 sin(ωt)
For GR calculation see PRD 78 (2008)
Gravitational Wave Signal
laser ranging an atom (or mirror) from a starting distance L sees a position:
and an acceleration
ds2 = dt2 − (1 + h sin(ω(t− z)))dx2 − (1− h sin(ω(t− z)))dy2 − dz2
x ∼ L(1 + h sin(ωt))
a ∼ hLω2 sin(ωt)
gives a phase shift
actual answer
∆φ = kaT 2 ∼ khLω2 sin(ωt)T 2
∆φtot = 4hk
ωsin2
(ωT
2
)sin (ωL) sin (ωt)
For GR calculation see PRD 78 (2008)
0 LHeight
0
T
2T
Time
L~1 km
10 m
Experiment Design
L~1 km
10 m
0 LHeight
0
T
2T
Time
Differential Measurement
similar to comparing two clocks separated by L
• A differential measurement cancels vibrations and laser phase noise (at least to leading order)
allows GW detection down to ω ~ 0.3 Hz (Hughes and Thorne ‘98)
earth vibrations naturally at 1 Hz ! 10−7 m√Hz
• Time-varying gravity gradient noise due to nearby motions of earth
Terrestrial Backgrounds
possible site: DUSEL, Homestake mine (~ 2 km shaft)
• A differential measurement cancels vibrations and laser phase noise (at least to leading order)
allows GW detection down to ω ~ 0.3 Hz (Hughes and Thorne ‘98)
earth vibrations naturally at 1 Hz ! 10−7 m√Hz
• Time-varying gravity gradient noise due to nearby motions of earth
Terrestrial Backgrounds
• All other backgrounds including timing errors, launch position uncertainty, Coriolis effects, and magnetic fields seem controllable to below shot noise
laser vibration control: 10−7 m√Hz
(1 Hz
f
) 32
(1 km
L
)
−140dBcHz
@ 3× 105Hzlaser phase noise control:
δf
f∼ 10−15 over time scales of 1 slaser frequency stability:
already demonstrated by high finesse cavity-locked lasers
possible site: DUSEL, Homestake mine (~ 2 km shaft)
!"#"$%&"'(
$%%"(&%"(
)$
$%%"( &%"(
)*
Possible Satellite Configuration
interferometer regionsatellite
space allows a longer interferometer time and sensitivity to lower ω
• ambient B-field (~ nT), vacuum, etc. in space sufficient for interferometer region
• laser power sufficient for atomic transitions
• the time-varying gravitational fields, vibrations, etc. naturally much smaller than for LISA
Satellite Experiment
Atom Interferometry (AGIS)
baseline L ~ 103 km
requires satellite control (at 10-2 Hz) to
atoms provide inertial proof mass, neutral
gas collisions remove atoms, not a noise source
10µm√Hz
LISA
baseline L = 5×106 km
satellite control to
large EM force on charged proof mass
collisions with background gas are noise
1nm√Hz
motivates more careful consideration of engineering details
several backgrounds and technical requirements may be easier with AI
!"#"$%&"'(
$%%"(&%"(
)$
$%%"( &%"(
)*
interferometer regionsatellite
Projected Terrestrial Sensitivity
L= 1 km and 4 km
10!5 Hz 10!4 Hz 10!3 Hz 10!2 Hz 10!1 Hz 1 Hz 10 Hz 102 Hz 103 Hz 104 Hz
f
10!14
10!15
10!16
10!17
10!18
10!19
10!20
10!21
10!22
10!23
h ! Hz10!5 Hz 10!4 Hz 10!3 Hz 10!2 Hz 10!1 Hz 1 Hz 10 Hz 102 Hz 103 Hz 104 Hz
10!14
10!15
10!16
10!17
10!18
10!19
10!20
10!21
10!22
10!23
LIGO
LISA
WhiteDwarf
Binaryat
10kpc
10 3
Ms,1Ms
BHbinary
at10
kpc
10 3
Ms ,
1Ms BH
binaryat
10Mpc
WhiteDwarf
Binaryat
10Mpc
Projected Satellite Sensitivity
L= 100 km, 103 km, and 104 km
10!5 Hz 10!4 Hz 10!3 Hz 10!2 Hz 10!1 Hz 1 Hz 10 Hz 102 Hz 103 Hz 104 Hz
f
10!14
10!15
10!16
10!17
10!18
10!19
10!20
10!21
10!22
10!23
h ! Hz10!5 Hz 10!4 Hz 10!3 Hz 10!2 Hz 10!1 Hz 1 Hz 10 Hz 102 Hz 103 Hz 104 Hz
10!14
10!15
10!16
10!17
10!18
10!19
10!20
10!21
10!22
10!23
LIGO
LISA
WhiteDwarf
Binaryat
10kpc10 3
Ms ,
1Ms BH
binaryat
10Mpc
10 5
Ms,1Ms
BHbinary
at10
Gpc
WhiteDwarf
Binaryat
10Mpc
Stochastic GW’s - Phase Transitions
RS1
also get observable gravitational waves from some SUSY models (NMSSM)
10!3Hz 10!2Hz 10!1Hz 1 Hz 10 Hz 102 Hz 103 Hz
f
10!17
10!16
10!15
10!14
10!13
10!12
10!11
10!10
10!9
10!8
10!7
10!6
10!5
10!4
10!3
10!2
10!1
1
"GW
10!3Hz 10!2Hz 10!1Hz 1 Hz 10 Hz 102 Hz 103 Hz
10!17
10!16
10!15
10!14
10!13
10!12
10!11
10!10
10!9
10!8
10!7
10!6
10!5
10!4
10!3
10!2
10!1
1
LIGO S4
LISA
inflation
Initial LIGO
Advanced LIGO
BBN # CMB
10!3Hz 10!2Hz 10!1Hz 1 Hz 10 Hz 102 Hz 103 Hz
f
10!17
10!16
10!15
10!14
10!13
10!12
10!11
10!10
10!9
10!8
10!7
10!6
10!5
10!4
10!3
10!2
10!1
1
"GW
10!3Hz 10!2Hz 10!1Hz 1 Hz 10 Hz 102 Hz 103 Hz
10!17
10!16
10!15
10!14
10!13
10!12
10!11
10!10
10!9
10!8
10!7
10!6
10!5
10!4
10!3
10!2
10!1
1
10!3Hz 10!2Hz 10!1Hz 1 Hz 10 Hz 102 Hz 103 Hz
f
10!17
10!16
10!15
10!14
10!13
10!12
10!11
10!10
10!9
10!8
10!7
10!6
10!5
10!4
10!3
10!2
10!1
1
"GW
10!3Hz 10!2Hz 10!1Hz 1 Hz 10 Hz 102 Hz 103 Hz
10!17
10!16
10!15
10!14
10!13
10!12
10!11
10!10
10!9
10!8
10!7
10!6
10!5
10!4
10!3
10!2
10!1
1
LIGO S4
LISA
inflation
Initial LIGO
Advanced LIGO
BBN # CMB
10!3Hz 10!2Hz 10!1Hz 1 Hz 10 Hz 102 Hz 103 Hz
f
10!17
10!16
10!15
10!14
10!13
10!12
10!11
10!10
10!9
10!8
10!7
10!6
10!5
10!4
10!3
10!2
10!1
1
"GW
10!3Hz 10!2Hz 10!1Hz 1 Hz 10 Hz 102 Hz 103 Hz
10!17
10!16
10!15
10!14
10!13
10!12
10!11
10!10
10!9
10!8
10!7
10!6
10!5
10!4
10!3
10!2
10!1
1
Cosmic Strings
Gμ=10-10
DePies & Hogan PRD 75 125006
Gμ=10-16
7
LIGO
LISA
Adv LIGO
c
a
d
b AGIS!LEO
10!5Hz 10
!4Hz 10
!3Hz 10
!2Hz 10
!1Hz 1 Hz 10 Hz 10
2Hz 10
3Hz 10
4Hz
10!14
10!15
10!16
10!17
10!18
10!19
10!20
10!21
10!22
10!23
10!24
f
h Hz
FIG. 4: The thick (red) curve shows the enveloped strain sensitivity versus gravitational wave frequency of a five-pulse inter-ferometer sequence for a single arm of the proposed AGIS-LEO instrument. Analogous plots for LIGO [31], Advanced LIGO[32], and LISA [33] are included for comparison. Curves a-d show gravitational wave source strengths after integrating over thelifetime of the source or one year, whichever is shorter: (a) represents inspirals of 103M!, 1M! intermediate mass black holebinaries at 10 kpc, (b) inspirals of 105M!, 1M! massive black hole binaries at 10 Mpc, (c) white dwarf binaries at 10 kpc, and(d) inspirals of 103M!, 1M! intermediate mass black hole binaries at 10 Mpc.
(1), the interferometer is maximally sensitive to GW frequencies at and above the corner frequency ωc =2T cos−1
√38 ≈
2π · 0.29/T at which point ∆φGW = (125/16)keffhL sin θGW. For lower frequencies ω < ωc, the 3 dB point occurs atω3dB = 2
T csc−1[4/√5] ≈ 2π · 0.19/T and sensitivity falls off as ω4. At higher frequencies ω > ωc, the envelope of
the sensitivity curve is constant and the periodic anti-resonant frequencies that are present in (Eq. 1) can easily beavoided by varying T [26].In order to observe a gravitational wave, the phase shift∆φGW must be sampled repeatedly as it oscillates in time due
to the evolution of θGW. To avoid aliasing, the sampling rate fr must be at least twice the frequency of the gravitationalwave. The sampling rate can be increased (without decreasing T ) by operating multiple concurrent interferometersusing the same hardware [26]. In these multiplexed interferometers, atom clouds with different velocities would beaddressed via Doppler shifts of the laser light.
C. Gravitational Wave Sensitivity
The phase sensitivity of an atom interferometer is limited by quantum projection noise δφ = 1/SNR = 1/√Na,
where Na is the number of detected atoms per second. For shot-noise limited atom detection, the resulting strain
sensitivity to gravitational waves becomes h = δφ8keffL
csc4(ωT/2)(
27+8 cosωT
). Figure 4 shows the AGIS-LEO mission
strain sensitivity (enveloped) for a single pair of satellites using the five-pulse interferometer sequence from Fig. 3with !keff = 200!k beamsplitters, an L = 30 km baseline, a T = 4 s interrogation time, and a phase sensitivity of
AGIS-LEOlow Earth orbit may be simpler
White paper with detailed proposal, backgrounds, etc: arXiv:1009.2702
a: IMBH@ 10 kpc
103M!
b: BH@ 10 Mpc105M!
c: WD binary@ 10 kpc
d: IMBH@ 10 Mpc
103M!
Axion Dark Matter Detection
arXiv:1101.2691
Strong CP problem:
Cosmic Axions
measurements ⇒ θ ! 3× 10−10
creates a nucleon EDM d ∼ 3× 10−16 θ e cmL ⊃ θ GG
Strong CP problem:
Cosmic Axions
measurements ⇒ θ ! 3× 10−10
creates a nucleon EDM d ∼ 3× 10−16 θ e cmL ⊃ θ GG
the axion is a simple solution:
withL ⊃ a
faGG + m2
aa2 ma ∼(200 MeV)2
fa∼ MHz
(1016 GeV
fa
)
Strong CP problem:
Cosmic Axions
measurements ⇒ θ ! 3× 10−10
creates a nucleon EDM d ∼ 3× 10−16 θ e cmL ⊃ θ GG
the axion is a simple solution:
withL ⊃ a
faGG + m2
aa2 ma ∼(200 MeV)2
fa∼ MHz
(1016 GeV
fa
)
a
Va(t) ∼ a0 cos (mat)
cosmic expansion reduces amplitude a0
Strong CP problem:
Cosmic Axions
measurements ⇒ θ ! 3× 10−10
creates a nucleon EDM d ∼ 3× 10−16 θ e cmL ⊃ θ GG
the axion is a simple solution:
withL ⊃ a
faGG + m2
aa2 ma ∼(200 MeV)2
fa∼ MHz
(1016 GeV
fa
)
the axion is a good cold dark matter candidate
this field has momentum = 0 ⇒ it is non-relativistic matter
a
Va(t) ∼ a0 cos (mat)
cosmic expansion reduces amplitude a0
Axions From High Energy Physics
Easy to generate axions from high energy theories
have a global symmetry broken at a high scale fa
string theory or extra dimensions naturally have axions from non-trivial topology
naturally gives large fa ~ GUT (1016 GeV) or Planck (1019 GeV) scales
Axions and WIMPs are the best motivated cold dark matter candidates
Constraints and Searches
1014
1018
1016
fa (GeV)
1012
1010
108
Constraints and Searches
1014
1018
1016
fa (GeV)
1012
1010
108
Axion dark matter
Constraints and Searches
a γ
B
L ⊃ a
faFF =
a
fa
!E · !B
in most models also have:
microwave cavity (ADMX)
1014
1018
1016
fa (GeV)
1012
1010
108
Axion dark matter
Constraints and Searches
a γ
B
L ⊃ a
faFF =
a
fa
!E · !B
in most models also have:
microwave cavity (ADMX)
1014
1018
1016
fa (GeV)
1012
1010
108
Axion dark matter
axion-photon conversion suppressed by fa
size of cavity increases with fa
signal ∝1f4
a
Constraints and Searches
a γ
B
L ⊃ a
faFF =
a
fa
!E · !B
in most models also have:
microwave cavity (ADMX)
1014
1018
1016
fa (GeV)
1012
1010
108
Axion dark matter
axion emission affects SN1987A, White Dwarfs, other astrophysical objects
axion-photon conversion suppressed by fa
size of cavity increases with fa
signal ∝1f4
a
Constraints and Searches
a γ
B
L ⊃ a
faFF =
a
fa
!E · !B
in most models also have:
microwave cavity (ADMX)
1014
1018
1016
fa (GeV)
1012
1010
108
How search for high fa axions?
Axion dark matter
axion emission affects SN1987A, White Dwarfs, other astrophysical objects
axion-photon conversion suppressed by fa
size of cavity increases with fa
signal ∝1f4
a
Strong CP problem:
Axion Dark Mattercreates a nucleon EDM d ∼ 3× 10−16 θ e cmL ⊃ θ GG
witha(t) ∼ a0 cos (mat) ma ∼(200 MeV)2
fa∼ MHz
(1016 GeV
fa
)
the axion: L ⊃ a
faGG + m2
aa2 creates a nucleon EDM d ∼ 3× 10−16 a
fae cm
Strong CP problem:
Axion Dark Mattercreates a nucleon EDM d ∼ 3× 10−16 θ e cmL ⊃ θ GG
witha(t) ∼ a0 cos (mat) ma ∼(200 MeV)2
fa∼ MHz
(1016 GeV
fa
)
the axion: L ⊃ a
faGG + m2
aa2 creates a nucleon EDM d ∼ 3× 10−16 a
fae cm
the axion gives all nucleons a rapidly oscillating EDM
axion dark matter is the biggest BEC ρDM ∼ m2aa2
0 ∼ 0.3GeVcm3
Strong CP problem:
Axion Dark Mattercreates a nucleon EDM d ∼ 3× 10−16 θ e cmL ⊃ θ GG
witha(t) ∼ a0 cos (mat) ma ∼(200 MeV)2
fa∼ MHz
(1016 GeV
fa
)
the axion: L ⊃ a
faGG + m2
aa2 creates a nucleon EDM d ∼ 3× 10−16 a
fae cm
the axion gives all nucleons a rapidly oscillating EDM
axion dark matter is the biggest BEC ρDM ∼ m2aa2
0 ∼ 0.3GeVcm3
thus all (free) nucleons radiate
Axion is more observable through its effects on atomic energy levels(significantly different from standard EDM searches)
Atomic Axion SearchesUnlike a normal EDM search, can look directly for the axion,
without external EM fields to modulate the signal
Eint ∼eA
2 ∼ 1012 Vm
instead use internal fields, much larger:
induces a shift to the energy levels: δω ∼ #Eint · #dn ∼ 10−24 eV
Atomic Axion SearchesUnlike a normal EDM search, can look directly for the axion,
without external EM fields to modulate the signal
Eint ∼eA
2 ∼ 1012 Vm
instead use internal fields, much larger:
induces a shift to the energy levels: δω ∼ #Eint · #dn ∼ 10−24 eV
if cool 108 atoms per sec, interferometer lasts ~ 1 sec, and take 106 trials
then shot noise limit is δω ∼(1 s ·
√1014 atoms
)−1∼ 7× 10−23 eV
There are two important caveats: Parity and Schiff’s Theorem
Parity Breaking
Atomic states have very little parity breaking
thus !Eint · !dn ≈ 0
Axion breaks parity (CP)
Parity Breaking
Atomic states have very little parity breaking
thus !Eint · !dn ≈ 0
Axion breaks parity (CP)
!dnmolecules can naturally break parity at O(1), though more difficult to work with due to low-lying modes
One possible solution:
Must control molecular rotation with applied E field (~ 106 V/m)
Use applied B field (< 0.1 T) to rotate nuclear spin with axion’s frequency (easily scanned)
Schiff’s TheoremSchiff’s theorem: in electrostatic equilibrium the E field on any point charge is zero
thus !Eint · !dn ≈ 0
Schiff’s TheoremSchiff’s theorem: in electrostatic equilibrium the E field on any point charge is zero
thus !Eint · !dn ≈ 0
higher moments take into account corrections to this (e.g. finite size of nucleus...)
Schiff moment: δω ∼ Eint dS ∼(10−9 Z3
)Eint dn
δω ∼ 10−3 Eint dn ∼ 10−27 eVoften use Hg or Tl
Schiff’s TheoremSchiff’s theorem: in electrostatic equilibrium the E field on any point charge is zero
thus !Eint · !dn ≈ 0
higher moments take into account corrections to this (e.g. finite size of nucleus...)
Schiff moment: δω ∼ Eint dS ∼(10−9 Z3
)Eint dn
δω ∼ 10−3 Eint dn ∼ 10−27 eVoften use Hg or Tl225Ra
223Fr
225Ac
229Pa
0.2
0.4
0.6
9
2× 10−25 eV
4× 10−25 eV
6× 10−25 eV
9× 10−24 eV
239Pu 3× 10−25 eV0.3
Schiff’s TheoremSchiff’s theorem: in electrostatic equilibrium the E field on any point charge is zero
thus !Eint · !dn ≈ 0
higher moments take into account corrections to this (e.g. finite size of nucleus...)
Schiff moment: δω ∼ Eint dS ∼(10−9 Z3
)Eint dn
δω ∼ 10−3 Eint dn ∼ 10−27 eVoften use Hg or Tl225Ra
223Fr
225Ac
229Pa
0.2
0.4
0.6
9
2× 10−25 eV
4× 10−25 eV
6× 10−25 eV
9× 10−24 eV
239Pu 3× 10−25 eV0.3
22 min
10 d
1.4 d
2.4×104 y
15 d
half-life
Schiff’s TheoremSchiff’s theorem: in electrostatic equilibrium the E field on any point charge is zero
thus !Eint · !dn ≈ 0
higher moments take into account corrections to this (e.g. finite size of nucleus...)
Schiff moment: δω ∼ Eint dS ∼(10−9 Z3
)Eint dn
δω ∼ 10−3 Eint dn ∼ 10−27 eVoften use Hg or Tl225Ra
223Fr
225Ac
229Pa
0.2
0.4
0.6
9
2× 10−25 eV
4× 10−25 eV
6× 10−25 eV
9× 10−24 eV
239Pu 3× 10−25 eV0.3
22 min
10 d
1.4 d
2.4×104 y
15 d
half-life
Argonne
Stony Brook
NIST cooled polar 40K87Rb to < µK
microwave cavity (ADMX)
1014
1018
1016
fa (GeV)
1012
1010
108
Axion dark matter
astrophysical constraints
Molecular Axion Searches
GUT
Planck
microwave cavity (ADMX)
1014
1018
1016
fa (GeV)
1012
1010
108
Axion dark matter
astrophysical constraints
Molecular Axion Searches
molecular interferometry
can most easily search in kHz - MHz frequencies high fa
GUT
Planck
microwave cavity (ADMX)
1014
1018
1016
fa (GeV)
1012
1010
108
Axion dark matter
astrophysical constraints
Molecular Axion Searches
molecular interferometry
can most easily search in kHz - MHz frequencies high fa
GUT
Planck
difficult technological challenges, similar to early stages of WIMP detection
axion dark matter is very well-motivated, no other way to search for at high fa
would be both the discovery of dark matter and a glimpse into physics at very high energies
Summary
1. Considered the use of molecular interferometry to detect Axion dark matter
• Axion dark matter is well motivated
• unlike WIMPs, currently no way to detect over most of parameter space
2. Proposed an Atomic Gravitational Wave Interferometric Sensor (AGIS)
• both terrestrial and satellite versions
• AGIS-LEO in low Earth orbit
3. Proposed laboratory tests of General Relativity
• including test of Equivalence Principle to 10-15 under construction @ Stanford
4. New ideas?