Two Physics 471 students want to have some fun. They set up an experiment similar to the one in the previous problem: as before, the first Stern-Gerlach analyzer allows only particles with spin up along the z-axis to pass through, but this time the second Stern-Gerlach analyzer allow only particles with spin up along the x-axis to pass through. The analyzers are separated by a region of space where there is a uniform magnetic field Bo. The students are free to choose the direction of Bo, but they want to arrange it so that 100% of the silver atoms that were transmitte by the first analyzer are also transmitted by the second analyzer. Student A finished the two previous problems and says that they should orient the magnetic fiel Bo along the +ŷ direction so she can use her results from those problems. Student B looked at Figure 3.9(b) in McIntyre and thought it would be cool to orient the field B

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I need some help with this quantum mechanics question. Figure 3.9(b) is in the second image. It might also be helpful for you to draw the spatial coordinate system.

Two Physics 471 students want to have some fun. They set up an experiment similar to the one
in the previous problem: as before, the first Stern-Gerlach analyzer allows only particles with
spin up along the z-axis to pass through, but this time the second Stern-Gerlach analyzer allows
only particles with spin up along the x-axis to pass through. The analyzers are separated by a
region of space where there is a uniform magnetic field Bo. The students are free to choose the
direction of Bo, but they want to arrange it so that 100% of the silver atoms that were transmitted
by the first analyzer are also transmitted by the second analyzer.
Student A finished the two previous problems and says that they should orient the magnetic field
Bo along the +ŷ direction so she can use her results from those problems.
Student B looked at Figure 3.9(b) in McIntyre and thought it would be cool to orient the field Bo
in the x-z plane at an angle of 45° with respect to the z-axis. (In spherical coordinates, Bo would
be pointing in the n direction given by 0 = π/4 and =0.)
a) If they follow student A's suggestion, what is the minimum time the particles must spend
in the magnetic field so that 100% of the particles transmitted by the initial z-analyzer are
transmitted by the final x-analyzer? Define wo = Bo and express your answer in terms of 000.
m
Hint: Although the field is oriented along +y axis rather than the +x axis as in problems 1 and 2,
the physics is the same, so the results should be the same.
b) If they follow student B's suggestion, what is the minimum time the particles must spend
in the magnetic field so that 100% of the particles transmitted by the initial z-analyzer are
transmitted by the final x-analyzer? Hint: You may be tempted to use Equation (3.48), but
that's not necessary. Take seriously the claim that "expectation values in quantum mechanics
obey the laws of classical mechanics" and think about how the direction of the spin vector varies
in time. McIntyre's Figure 3.9(b) should be helpful.
c) If they follow student A's suggestion and do the calculations correctly, but they
accidentally orient the B-field along the -ŷ direction instead of the + direction, what fraction of
the particles transmitted by the initial z-analyzer will be transmitted by the final x-analyzer?
Could they get that fraction back to 100% by changing the time that the particles spend in the
magnetic field?
d) If they follow student B's suggestion but they accidentally orient the B-field in the
opposite direction, what fraction of the particles transmitted by the initial z-analyzer will be
transmitted by the final x-analyzer?
Transcribed Image Text:Two Physics 471 students want to have some fun. They set up an experiment similar to the one in the previous problem: as before, the first Stern-Gerlach analyzer allows only particles with spin up along the z-axis to pass through, but this time the second Stern-Gerlach analyzer allows only particles with spin up along the x-axis to pass through. The analyzers are separated by a region of space where there is a uniform magnetic field Bo. The students are free to choose the direction of Bo, but they want to arrange it so that 100% of the silver atoms that were transmitted by the first analyzer are also transmitted by the second analyzer. Student A finished the two previous problems and says that they should orient the magnetic field Bo along the +ŷ direction so she can use her results from those problems. Student B looked at Figure 3.9(b) in McIntyre and thought it would be cool to orient the field Bo in the x-z plane at an angle of 45° with respect to the z-axis. (In spherical coordinates, Bo would be pointing in the n direction given by 0 = π/4 and =0.) a) If they follow student A's suggestion, what is the minimum time the particles must spend in the magnetic field so that 100% of the particles transmitted by the initial z-analyzer are transmitted by the final x-analyzer? Define wo = Bo and express your answer in terms of 000. m Hint: Although the field is oriented along +y axis rather than the +x axis as in problems 1 and 2, the physics is the same, so the results should be the same. b) If they follow student B's suggestion, what is the minimum time the particles must spend in the magnetic field so that 100% of the particles transmitted by the initial z-analyzer are transmitted by the final x-analyzer? Hint: You may be tempted to use Equation (3.48), but that's not necessary. Take seriously the claim that "expectation values in quantum mechanics obey the laws of classical mechanics" and think about how the direction of the spin vector varies in time. McIntyre's Figure 3.9(b) should be helpful. c) If they follow student A's suggestion and do the calculations correctly, but they accidentally orient the B-field along the -ŷ direction instead of the + direction, what fraction of the particles transmitted by the initial z-analyzer will be transmitted by the final x-analyzer? Could they get that fraction back to 100% by changing the time that the particles spend in the magnetic field? d) If they follow student B's suggestion but they accidentally orient the B-field in the opposite direction, what fraction of the particles transmitted by the initial z-analyzer will be transmitted by the final x-analyzer?
(a)
P
+→→
1.0
0.8
0.6
0.4
0.2
0
2T
wo
4T
@o
6T
wo
(b)
ZA
(S(0))
B
(S(t))
y
X
FIGURE 3.9 (a) Spin-flip probability for a uniform magnetic field with x- and z-components and
(b) the corresponding precession of the expectation value of the spin.
Transcribed Image Text:(a) P +→→ 1.0 0.8 0.6 0.4 0.2 0 2T wo 4T @o 6T wo (b) ZA (S(0)) B (S(t)) y X FIGURE 3.9 (a) Spin-flip probability for a uniform magnetic field with x- and z-components and (b) the corresponding precession of the expectation value of the spin.
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