michael a. nielsen university of queensland quantum shwantum goal: to explain all the things that...
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Michael A. Nielsen
University of Queensland
Quantum Shwantum
Goal: To explain all the things that got left out of my earlier lectures because I tried to stuff too much in.
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The measurement postulate formulatedin terms of “observables”
A measurement is described by a complete set of projectors onto orthogonal subspaces. Outcome occurs with probability Pr( ) .The corresponding post-measure
Our
ment state is
f orm
:j
j
P j
j P
P
.j
jP
A measurement is described by an ,
a Hermitian operator , with spectral decomposiOld f orm: o
tion
bservable
.j jj
MM P
The possible measurement outcomes correspond to theeigenvalues , and the outcome occurs with probability Pr( ) .
j j
j jP
The corresponding post-measurement state is
.j
j
P
P
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An example of observables in action
Suppose we "measE uxample: re ".Z has spectral decomposition 0 0 - 1 1, so
this is just like measuring in the computational basis,and calling the outcomes "1" and "-1", respectively, f or0 and 1.
Z Z
Find the spectral decomposition of .Show that measuring corresponds to measuringthe parity of two qubits, with the result +1 correspondingto even parity, and the result
Exercis
-1 correspon
:
i
e
d
Z ZZ Z
ng to oddparity.
Suppose we measure the observable f or astate which is an eigenstate of that observable. Showthat, with certainty, the outcome of the measurement isthe corresponding eigenvalue
Exerci
of the ob
se: M
servable.
00 00 11Hint: 11 10 10 01 01Z Z
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What can be measured in quantum mechanics?
Computer science can inspire fundamental questions about physics.
We may take an “informatic” approach to physics.(Compare the physical approach to information.)
Problem: What measurements can be performed in quantum mechanics?
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“Traditional” approach to quantum measurements: A quantum measurement is described by an observable,M, that is, a Hermitian operator acting on the state spaceof the system.
Measuring a system prepared in an eigenstate ofM gives the corresponding eigenvalue of M as themeasurement outcome.
“The question now presents itself – Can every observablebe measured? The answer theoretically is yes. In practiceit may be very awkward, or perhaps even beyond the ingenuityof the experimenter, to devise an apparatus which could measure some particular observable, but the theory always allows one to imagine that the measurement could be made.” - Paul A. M. Dirac
What can be measured in quantum mechanics?
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Does program number x halt on input of x?
0 if program halts on input ( )
1 otherwise
x xh x
Is there an algorithm to solve the halting problem, thatis, to compute h(x)?
Suppose such an algorithm exists.PROGRAM: TURING(x)
IF h(x) = 1 THEN HALTELSE loop forever
Let T be the program number forTURING.
TURING(T) halts h(T) = 1 h(T) = 0
Contradiction!
The halting problem
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The halting observable
0
( )x
M h x x x
Can we build a measuring device capable of measuringthe halting observable?
Yes: Would give us a procedure to solve the halting problem.
No: There is an interesting class of “superselection” rules controlling what observables may, in fact, be measured.
Consider a quantum system with an infi nite-dimensionalstate space with orthonormal basis 0 , 1 , 2 ,
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Research problem: Is the halting observablereally measurable? If so, how? If not,why not?
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Quantum computation via measurement alone
A quantum computation can be done simply by a sequence of two-qubit measurements. (No unitarydynamics required, except quantum memory!)
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Can we build a programmable quantum computer?
U Ufixedarray
U
,UP
1
1
Unitary operators ,...,which are distinct, even up to global phase f actors, require ortho
No-programming theor
gonal programs ,..
e
. .
:
,
m n
n
U U
U U
Challenge exercise: Prove the no-programming theorem.
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A stochastic programmable quantum computer
Bell
Measurement
{U 00 11
2U I U
jU
0,1,2,3j
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Why it works
Bell
Measurement
{00 11
2
jU
0,1,2,3j
UU
00 11
2U I U
j
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How to do single-qubit gates using measurements alone
Bell
Measurement
{00 11
2
jkU
0,1,2,3j
kUkU
00 11
2k kU I U
kU
14With probability , ,
and the gate succeeds.j k
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Coping with failure
Action was , - known unitary erra .orjkU j k †Now attempt to apply the gate ( ) to the
qubit, using a similar procedure based on measurements alone.
jkU U
14Successf ul with probability , otherwise repeat.
1Failure probability can be ac loghievr
ed wepet
itit
h .ions
O
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How to do the controlled-not
Bell
Measurement
{ m j klU
, 0,1,2,3j k
2Bellmlm lU I U
lmU
116With probability , , , and the gate succeeds.j l k m
2
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Simplification: reducing to two-qubit measurements
The only measurement of more than two qubitspresently required is for preparing the input to thecontrolled-not.
2Bellmlm lU I U
Simplif y by taking 0.l m
00 0000 0101 1011 1110U
The fi rst and third bits always have even parity, so it suffi ces to create:( 0 1)( 000 101 011 110 ), and then measure the parity of bits 1 and 3.
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Simplification: reducing to two-qubit measurements
Reduces to the problem of creating:
( 000 101 011 110 ).
Recall the identity underlying teleportation:
00 11 00 11 00 11
01 10 01 10
Z
X XZ
0 00 11 00 11 0 00 11 0
01 10 1 01 10 1
Project onto the subspace of the fi rst two qubitsspanned by 00 11 and 01 10 .
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DiscussionMeasurement is now recognized as a powerful tool in manyschemes for the implementation of quantum computation.
Research problem: Is there a practical variant of this scheme?
Research problem: What sets of measurement are sufficient to do universal quantum computation?
Research problem: We have talked about attempts to quantify the “power” of different entangled states. Can a similar quantitative theory of the power of quantum measurements be developed?
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Thankyou to:Organizing Committee
Michael BremnerJennifer DoddAnna Rogers
Tutors
Michael BremnerPaul CochraneChris DawsonJennifer Dodd
Alexei GilchristSarah Morrison
Duncan MortimerTobias Osborne
Rodney PolkinghorneDamian Pope
Speakers
Dave BaconSteve Bartlett
Bob ClarkAlex HamiltonPing Koy LamKeith SchwabMatt Sellars
Ben TravaglioneAndrew White
Howard Wiseman
The Godparents
Gerard MilburnHalina Rubinsztein-Dunlop
Andrew White
VolunteersTamyka Bell, Andrew Hines Mark Dowling, Michael Gagen, Joel Gilmore, Nathan Langford,Austin Lund, Jeremy O’Brien, Geoff Pryde, Doug Turk