Quantum Information Processing

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Quantum Information Processing
Neutron Interferometry
Dmitry Pushin Physics, MIT
as told by David G. Cory Department of Nuclear
Science Engineering Massachusetts Institute of
Technology
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Dr. Sekhar Ramanathan Dr. Timothy
Havel Professor Seth Lloyd Dr. Sergio
Valenzuela Dr. Will Oliver Dr. John Bernard Dr.
M. Arif, NIST University of Waterloo Professor
Joseph Emerson Professor Raymond Laflamme Dr.
Jonanthan Baugh
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Dr. Timothy Havel Professor Seth Lloyd Dr. Sekhar
Ramanathan Dr. Joseph Emerson Paola Cappellaro
Michael Henry Jonathan Hodges Suddhasattwa
Sinha Jamie Yang
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Heisenberg - new QT
Planck - photons
1900
1910
1920
1930
Einstein
Schrödinger - wave equation
Bohr - old QT, interpretation
Dirac - relativistic wave-equation
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1900
1920
1940
1980
2000
1960
2020
Haroche
Zeilinger
Aspect
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Macroscopic Quantum Coherence
Quantum mechanics permits information processing
beyond the classical limit These new
possibilities are
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Neutron interferometry an example of macroscopic
quantum coherence
3-blade, interferometer
Size 10 cm
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Neutron interferometry an example of macroscopic
quantum coherence
Bragg scattering Each neutron is coherently
spread over two paths
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Neutron interferometry an example of macroscopic
quantum coherence
Ignore the beam that is scattered out of the
interferometer
No information lost. The transmitted and
reflected beams carry the same information,
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Neutron interferometry an example of macroscopic
quantum coherence
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Neutron interferometry an example of macroscopic
quantum coherence
t - transmitted r - reflected
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Neutron interferometry an example of macroscopic
quantum coherence
Measure the neutron Intensity. In this case that
is the number of neutrons per unit time.
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Interference
A
B
C
D
path II
A
H-beam
path Iñ
path IIñ
B
3He detectors
C
O-beam
path I
O-beam
D
H-beam
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Neutron interferometry an example of macroscopic
quantum coherence
Neutons/ 3 min
phase
Clothier et al, (1991) PRA 44, 5357
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Neutron interferometry an example of macroscopic
quantum coherence
A simple example of probability amplitudes. Set ?
so that IH0.
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Coherent Neutron Imaging
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Spatially encoding of the neutron beam

• By spatially encoding beam we are introducing
• a new degree of freedom. By tracing this degree
• we can
• measure spatial properties of materials
  (softmatter)
• use it as controlled decoherence in QIP

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Coherent Neutron Imaging
Vary k to collect a complete set of Fourier
components. The resolution depends on S/N not
the detector.
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Spatial encoding
The fit is to the known sample geometry,
parameters are step location and size. Notice
that each point is 50 minutes of averaging.
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Spin Polarized Neutrons
Analyzer
p/2
Detector
p
not


Polarizer
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Interference and spin
A
B
C
D
path II
Polarizer
H-beam
A
Analyzer
?
?/2
path Iñ
path IIñ
path I
B
O-beam
C
O-beam
not


D
H-beam
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Spin based phase grating
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Coherence Measurements
Radius of neutron 0.7 fm
A neutron interferometer is a macroscopic quantum
coherence device, we will measure the coherence
length of the neutrons wave-function.
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Coherent neutron scattering
First example of coherent neutron wave-funtion
over two interferometers
3-blade interferometer with prisms to vertically
shift the beam.
Add second interferometer.
Adjust phase for only O-beam.
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When will we have a neutron Interferometer at MIT?
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Each crystal blade acts as a beam splitter.
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Neutron interferometry with vibrations
Vibrations change the momentum of the n and thus
the Bragg angle. Note, the two paths change in
opposite directions.
Even low frequency vibrations are deadly.
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Neutron interferometry with vibrations
Decoherence Free Subspace
No interference
Low frequency vibrations are OK.
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When will we have a neutron Interferometer at MIT?
Multiple interferometers for controlling
neutron information. Multiple paths to code
for errors. Spin dependent measurements to
correct for momentum spread.
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Stern-Gerlach (details)
Gradient magnets
Sample
p
Analyzer
Polarizer
p/2
p/2
V
VI
I
II
Detector
III
IV
I
V
II
VI
III
IV
where
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Center for Materials Science and Engineering
Summer Students (NSF)
2000
1998
1999
2001
2002
2003
2004
N. Seiberlich
C. Breen S. Kumaresan
J. S. Hodges
J. C. Gore
K. Edmonds J. Yang
A. Gorshkov M. Henry
D. Khanal
Journal of Magnetic Resonance
New Journal of Physics
Physical Reviews A
Concepts in Magnetic Resonance
Physical Reviews A
Decoherence Free Subspace
Quantum and Classical Channel
Construction and Implementation of Logic Gates on
two Spins
EIT
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Dr. Timothy Havel Professor Seth Lloyd Dr. Sekhar
Ramanathan Dr. Joseph Emerson Dr. Grum
Teklemariam Dr. Greg Boutis Nicolas
Boulant Paola Cappellaro Zhiying (Debra)
Chen Hyung Joon Cho Daniel Greenbaum Michael
Henry Jonathan Hodges Suddhasattwa Sinha Jamie
Yang