Transcript Slide - TerpConnect - University of Maryland

Atoms Coupled to SQUIDs
Saurabh Paul
C. P. Vlahacos, Jonathan Hoffman, Jeffrey Grover, Daniel
Hemmer, Z.Kim, B.Palmer, A. Dragt, J. Taylor, C.J. Lobb, J.R.
Anderson, L. Orozco, S. Rolston, F.C. Wellstood
Physics Frontier Center -NSF
Joint Quantum Institute
Department of Physics
University of Maryland, College Park, Maryland
Funding by NSF-PFC, JQI, and CNAM
Outline
• Motivation
• Basic idea
• Main challenge- Trapping atoms next to cold
circuits
• Basics of atom trapping
• Designing the Proof-of-Principle Experiment
• Future work
• Conclusion
Motivation
• Construct a hybrid qubit system by coupling neutral atoms to
superconducting qubits
• In this case, we will be using the hyperfine splitting
of the Rb(87) atoms (6.83 GHz), coupled to flux qubit at same
frequency.
• The long coherence time of atomic qubits combined with the
scalability in superconducting qubits opens the possibility
to create devices that can perform at levels unachievable by
either technology alone.
• Also, learn how to manipulate individual atoms at cryogenic
temperatures and couple them to extremely sensitive solid
state devices.
Basic Idea
Coupling Neutral Atoms to Superconducting qubits
• Cool flux qubit (SQUID) to 20 mK,
manipulate and read out electrically
• Trap and manipulate atoms with light
• Move atoms close to SQUID loop to
couple to magnetic moment of the
atoms
cloud
of Rb87
atoms
2 mm
flux qubit
• Demonstrate proof of principle
N
H  H fluxqubit  H atoms   mi  Bloop
i 1
expect typically about
100 Hz to 1 kHz for a Rb87 atom
• Improve SQUID coherence and
coupling to atoms
• Push to few atom limit
Main Challenge - trapping atoms
next to cold circuits
• We need to trap atoms just a few microns above
superconducting circuits.
• Trap atoms just a few microns above superconducting
circuits…… without significantly heating the
superconducting device.
• Trapping method may involve large magnetic fields,
or time varying fields, which may interfere with the
superconducting devices.
Outline
• Motivation
• Basic idea
• Main challenge- Trapping atoms next to cold
circuits
• Basics of atom trapping
• Designing the Proof-of-Principle Experiment
• Conclusion
• Future work
Optical Dipole Trap
Trapping atoms in dipole potentials resulting from interaction with
red-detuned light
atoms
Laser
Dipole moment induced by electric field,
Where
 is the polarizability
The interaction potential is given by
p E
U dip  
1
p.E
2
 E2 I
If worked out fully, we will get
U dip
3 c 2

3 c 2 

I (r ) 
I (r )
3
3
20   0
20 
where     0 is called the detuning
So, when   0 (i.e., red-detuned) we have an attractive trapping potential
Focused-beam traps
Use a red-detuned focused Gaussian laser beam to create dipole traps

2P
r2 
I
exp  2 2

2
 w ( z)
w
(
z
)


U dip   I
0
lens
U
r
Large Power incident on edge of chip
Disadvantages:
• Large optical power (100mW)
required.
• Difficult to get focal point close to
surface (few mm) without
introducing significant heating
• Atoms not very tightly confined.
zo
zo
zo
Superconducting
device
Magneto-Optic Trap (MOT)
1 Dimensional MOT
E
J=1


0

-1
Atoms are in
state
0 The ground
Moving
  randomly
In +z and –z directions
0  
0
J=0
B(z)=A.z
1
0
z
Disadvantages of using MOT
3D view of a MOT
• Requires application of relatively
large and time-varying magnetic fields
which may couple to
superconducting devices
• The problem of large amount of Laser
power and heating effect is also
significant.
• Atoms not tightly confined (big trap)
Two coils in anti-helmholtz
configuration
Trapping atoms with a tapered optical fiber
• What is a tapered optical fiber?
• Why is it useful?.....Evanescent Wave
Normal optical fiber
500 nm
125
microns
Evanescent wave

Hydrogen flame
I  exp 2 r


• Atoms can be trapped in this evanescent wave, but how?
Trapping atoms with a tapered optical fiber
(continued)
atoms
Red-detuned light
We already know that
U dip  I
E
And for a red-detuned light, it looks like
So, if we only have a red-detuned light
The atoms will stick to the fiber
The blue-detuned light provides the
Repulsive force to trap the atoms a few hundred
nm from the surface
Distance(nm)
Used Experimental Setup
(Arno Rauschenbeutel, Univ. of Mainz)
Beam splitter
Red
light
atoms
Blue light
Tapered
Optical fiber
Potential wells
Once the atoms are trapped in the wells, it is also possible to move
them along the length of the tapered region
Potential well along the tapered fiber
(Arno Rauschenbeutel, Univ. of Mainz)
Trapping atoms using a tapered optical fiber
tapered optical fiber
3 cm
linearly polaized
red+blue light
2 mm
2 mm
Advantages:
• Relatively low power is required
• 99% of the light can be confined to very near
the fiber, strong coupling to atom.
• Buildable with a large tapered part
Disadvantages:
• Still requires mW of power.
• Maybe 1% of light lost as heat or
scattering out of fiber.
• Not used much previously at low
temperature
• Need to figure out how to load atoms
Outline
• Motivation
• Basic idea
• Main challenge- Trapping atoms next to cold
circuits
• Basics of atom trapping
• Designing the Proof-of-Principle Experiment
• Conclusion
• Future work
Proof-of-Principle Experiment
Couple atoms
to superconducting
LC resonator
Couple
atoms to SQUIDs
load
atoms
red/blue light
tapered optical fiber
move to within
1 to 10 mm of chip
pump resonator at 6.83 GHz,
monitor absorbed power
Pin
- High-Q, LC resonator
- Cool to 20 mK
- Needs to be tunable
to 6.83 GHz
Pout
6.83 GHz
Advantages for proof of principle:
relatively simple to build LC resonator, "simple" to measure, robust, very sensitive
How do we know when the atoms couple
to the resonator ?
• Monitor the microwave power absorbed, and look for sharp absorption drop
due to the atoms at 6.83 GHz….i.e. NMR
• Look for any possible shifts in the resonant frequency
• From the optics side, we can detect the state of the atomic spins by fluorescence
detection
• For larger atomic samples, Faraday rotation may also be a possible detection
technique
Superconducting LC-Resonator – thin film Al on sapphire
(Z. Kim and B. Palmer, LPS)
1 mm
Al on Sapphire
Transmission (dB)
0
100 mm
T = 350 mK
-2
-4
fo = 5.5773 GHz
Qtot = 25,000
Qintrinsic ~ 75,000
-6
-8
Qintrinsic up to 106
at 20 mK
-10
5.5765
5.5770
5.5775
Frequency (GHz)
5.5780
Mechanical Tuning
We will use a mechanical
tuning arm, made of Al.
The effective inductance
of the inductor is
Tuning arm
L  L0 (1   2 )
where,  2  M 2 / L0 2
Z= distance between
Tuning arm and inductor/chip
a
Inductor with self
Inductance L0
By varying “z”, we can
vary  and L, and in
turn the resonant
frequency of the
oscillator.
Basic Idea of Tuning assembly
bottom of dilution fridge
Mechanical
z-stage
copper
axial shield
Piezo z-stage
(Attocube)
copper
fabric
frame
sample
metallized
fabric
tuning
arm
SMA connectors
Basic Idea of Tuning assembly
bottom of dilution fridge
Mechanical
z-stage
copper
axial shield
Piezo z-stage
(Attocube)
copper
fabric
frame
sample
metallized
fabric
tuning
arm
SMA connectors
Basic Idea of Tuning assembly
bottom of dilution fridge
Mechanical
z-stage
copper
axial shield
Piezo z-stage
(Attocube)
copper
fabric
frame
sample
metallized
fabric
tuning
arm
SMA connectors
Tuning assembly bolted to bottom of DR
bottom of
dilution fridge
copper base
plate
copper retaining
ring
axial shield
z-axis stage
metalized fabric
z-Attocube
Al tuning arm
sample box
Nb resonator
Fiber-Resonator spacing and alignment adjustment
load atoms
~ 200 mW
Pin~ 20 mW
fiber
fiber support frame
Metalized fabric
ANPz101
ANPx101
ANRv101
ANR101
q
f
Fiber-Resonator spacing and alignment adjustment
load atoms
~ 200 mW
Pin~ 20 mW
fiber
ANPz101
ANPx101
ANPz101
ANRv101
ANR101
q
f
fiber support frame
Metalized fabric
Fiber-Resonator spacing and alignment adjustment
load atoms
~ 200 mW
Pin~ 20 mW
fiber
ANPz101
ANPx101
q
ANR101
f
fiber support frame
Cu diaphragm
Fiber-Resonator spacing and alignment adjustment
load atoms
~ 200 mW
Pin~ 20 mW
fiber
ANPz101
ANPx101
ANRv101
ANR101
q
f
fiber support frame
Cu diaphragm
Fiber-Resonator spacing and alignment adjustment
load atoms
~ 200 mW
Pin~ 20 mW
fiber
fiber support frame
Metalized fabric
ANPz101
ANPx101
ANRv101
ANR101
q
f
The entire assembly for the sample box
Mechanical
z-stage
Base
of DR
Tuning
assembly
c
c
load
atoms
Piezo z-stage
(Attocube)
Pin
fiber
ANPz101
ANPx101
ANRv
101
q
ANR101 f
fiber support
frame
Metalized
fabric
Inside the Fridge
Dilution
Refrigerator
100mK plate
20 cm
Mixing
chamber
plate
Sample box
Future Work
• Demonstrate mechanical tunability and stability of the superconducting
resonator while maintaining very high Q.
• Build and operate a mechanism for moving chip very close to fiber at mK
temperatures.
• Fabricate tapered optical fibers and test their robustness at cryogenic
temperatures.
• Study the light scattered/lost from the fibers, and check the heat load at the
sample.
• Load and trap atoms on the fiber at mK.
• Do the proof-of-principle experiment by trapping atoms a few microns
above the superconducting resonator and measure their magnetic coupling
to the resonator.
Conclusion
• We are trying to make a hybrid quantum system by coupling
neutral atoms to superconducting devices.
• As a proof-of-principle, we are working on an experiment
which involves coupling Rb atoms to a superconducting LC
resonator at mK.
• The resonant frequency of the resonator will be tuned in situ
mechanically using piezo stages (attocubes).
• We propose to trap atoms along a tapered optical fiber, and
bring it to within a few microns of the resonator surface using
piezo stages.
• We will measure the coupling between the atoms and the
resonator by microwave absorption and optically.
Thanks