Control of quantum two-level systemsΒ Β· changes π on Bloch sphere Oscillating fields...
Transcript of Control of quantum two-level systemsΒ Β· changes π on Bloch sphere Oscillating fields...
Control of quantum two-level systems
AS-Chap. 10 - 2
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How to control a qubit?
10.3 Control of quantum two-level systems β¦.. General concept
Does the answer depend on specific realization? No, only its implementation
Qubit is pseudo spin General concept exists Independent of qubit realization Methods from nuclear magnetic resonance (NMR)
Nuclear magnetic resonance Method to explore the magnetism of nuclear spins Important application Magnetic resonance imaging (MRI) in medicine MRI exploits the different nuclear magnetic signatures of different tissues
AS-Chap. 10 - 3
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10.3 Control of quantum two-level systems β¦.. General concept
early suggestions H. Carr (1950) and V. Ivanov (1960)
Brief MRI history
1972 MRI imaging machine proposed by R. Damadian (SUNY)
1973 1st MRI image by P. Lauterbur (Urbana-Champaign)
Late 1970ies Fast scanning technique proposed by P. Mansfield (Nottingham)
2003 Nobel Prize in Medicine for P. Lauterbur and P. Mansfield
AS-Chap. 10 - 4
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10.3 Control of quantum two-level systems β¦.. NMR techniques
Overview: Important NMR techniques
Basic idea Rotate spins by static or oscillating magnetic fields
Static fields parallel to quantization axis free precession changes π on Bloch sphere Oscillating fields perpendicular to quantization axis change population changes π on Bloch sphere
Important protocols Rabi population oscillations Relaxation measurement π1 Ramsey fringes π2
β Spin echo (Hanh echo) π2
β corrected for reversible dephasing
AS-Chap. 10 - 5
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10.3. Control of quantum two-level systems β¦.. Quantum two-level system
Arbitrary TLS π» =1
2
π»11 π»12π»21 π»22
is Hermitian matrix
π»11, π»22 β β and π»21 = π»12β
we choose π»11 > π»22
Hamiltonian of a quantum two-level system (TLS)
Symmetrize Subtract global energy offset π»11+π»22
4Γ 1
π» =1
2
π Ξ β πΞ
Ξ + πΞ βπ=
π
2π π§ +
Ξ
2π π¦ +
Ξ
2π π₯
π β‘π»11 β π»22
2> 0 Ξ, Ξ β β
Natural or physical basis π+ , |πββ©
π» 0 =1
2
π 00 βπ
=π
2π π§
π» πΒ± = Β±π
2
AS-Chap. 10 - 6
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10.3 Control of quantum two-level systems β¦.. Quantum two-level system
Diagonalization of π― π» =
1
2
π Ξ β πΞ
Ξ + πΞ βπ
tan π β‘Ξ2+Ξ 2
π with 0 β€ π β€ π
tanπ β‘Ξ
Ξ with 0 β€ π β€ 2π
Eigenvalues detπ β 2πΈΒ± Ξ β πΞ
Ξ + πΞ βπ β 2πΈΒ±= 0
π β 2πΈΒ± βπ β 2πΈΒ± β Ξ β πΞ Ξ + πΞ = 0
πΈΒ± = Β±1
2π2 + Ξ2 + Ξ 2 β‘ Β±
1
2βπq
Eigenvectors |Ξ¨+β© = +πβππ 2 cosπ
2π+ + π+ππ 2 sin
π
2|πββ©
|Ξ¨ββ© = βπβππ 2 sinπ
2π+ + π+ππ 2 cos
π
2|πββ©
π» Ψ± = EΒ±|Ψ±β©
C. Cohen-Tannoudji, B. Diu, and F. LaloΓ«, Quantum Mechanics Volume One, Wiley-VCH
Basis Ξ¨+ , |Ξ¨ββ© energy eigenbasis (because π» has energy units)
transition energy
conveniently expressed in
frequency units
AS-Chap. 10 - 7
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10.3 Control of quantum two-level systems β¦.. Quantum two-level system
Visualization on Bloch sphere
tan π β‘Ξ2+Ξ 2
π with 0 β€ π β€ π
tanπ β‘Ξ
Ξ with 0 β€ π β€ 2π
|πΉ+β© = +πβππ 2 cosπ
2π+ + πππ 2 sin
π
2|πββ©
Basis rotation on Bloch sphere!
π
π
|πββ©
|π+β©
|π³+β©
|π³ββ©
π
π½
Vectors |πΉββ© and |πΉ+β© define new quantization axis
π» =π2+Ξ2+Ξ 2
2π π§ in basis πΉβ , |πΉ+β©
|πΉββ© and |πΉ+β© become new poles
|πΉββ© = βπβππ 2 sinπ
2π+ + πππ 2 cos
π
2|πββ©
π
AS-Chap. 10 - 8
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10.3 Control of quantum two-level systems β¦.. Qubit Control: Fictitious spin Β½
Fictitious spin Β½ in fictitious magnetic field π©
π» β = βπΎβ π© β πΊ = βπΎβ
2
π΅π§ π΅π₯ β ππ΅π¦π΅π₯ + ππ΅π¦ βπ΅π§
πΎ is the gyromagnetic ratio
π΅ = π΅π₯ , π΅π¦ , π΅π§π
is the magnetic field vector
Analogy to spin Β½ in static magnetic field
Fictitous spin in fictitious π©-field quantum TLS β π+ β πβ β π Ξ¨+ β π Ξ¨β (π denotes the quantization axis along which π» β is diagonal) β πΎ π© E+ β Eβ = βππ
Polar angles of π© π, π βπΎβπ΅π§ π βπΎβπ΅π₯ Ξ βπΎβπ΅π¦ Ξ
Any quantum TLS has a βbuilt-
inβ static magnetic field
β NMR situation!
Orientation with respect to the quantization axis
depends on π, Ξ, Ξ
AS-Chap. 10 - 9
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Free precession
Free precession
free evolution corresponds to free
precession about the π-axis
Also called Larmor precession
In absence of decoherence, only π π evolves linearly with time π‘
In the energy eigenbasis g , |eβ© , the qubit state vector |πΉ0β© is parallel to built-in field for πΉ0 = |gβ© antiparallel to the built-in field for πΉ0 = |eβ© Built-in field points along π§-axis on Bloch sphere
π
π
|π β©
|πβ©
ππ
|π³πβ©
When qubit state vector |πΉ0β© is not parallel or antiparallel to built-in field
|π³(π)β©
π(π)
πΉ = cosπ
2e + πππ sin
π
2g
π½π
π
AS-Chap. 10 - 10
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Free precession
Larmor precession β formal calcualtion
Energy eigenbasis π» =βππ
2π π§ fictitious field aligned along quantization axis
π
π
|π β©
|πβ©
ππ
π½π
|π³πβ©
|π³(π)β©
π(π)
Time-independent problem Develop into stationary states
πΉ π‘ = e πΉ0 πβππqπ‘/2|eβ© + g πΉ0 πππqπ‘/2|gβ©
πΉ0 = cosπ02
e + πππ0 sinπ02
g
πΉ π‘ = πβππqπ‘/2 cosπ02|eβ© + πππ0πππqπ‘/2 sin
π02|gβ©
= cosπ02|eβ© + ππ(π0+πqπ‘) sin
π02|gβ©
π(π)
πΉ = cosπ
2e + πππ sin
π
2g
π
AS-Chap. 10 - 11
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10.3 Control of quantum two-level systems β¦.. Qubit Control: Rabi Oscillations
Rotating drive field
Consider the qubit state vector |πΉβ© expressed in energy eigenbasis g , |eβ©
Apply a drive field with amplitude βπd rotating about the π-axis at frequency π
π» π‘ =β
2
πq πdπβπππ‘
πdππππ‘ βπq
π
π
π» β = βπΎβ
2
π΅π§ π΅π₯ β ππ΅π¦π΅π₯ + ππ΅π¦ βπ΅π§
π©π = βππͺ
ππ π©π π©π
0 πd 0
π 4 πd 2 πd 2
π 2 0 πd
3π 4 βπd 2 πd 2
π βπd 0
5π 4 βπd 2 βπd 2
3π 2 0 βπd
7π 4 πd 2 βπd 2
π
AS-Chap. 10 - 12
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10.3 Control of quantum two-level systems β¦.. Qubit Control: Rabi Oscillations
Driven quantum TLS
π» π‘ =β
2
πq πdπβπππ‘
πdππππ‘ βπq
=βπq
2π π§ +
βπd
2π +π
+πππ‘ + π βπβπππ‘
Drive rotating about arbitrary equatorial axis on the Bloch sphere
π» d =βπd
2π +π
+π ππ‘+π + π βπβπ ππ‘+π
without loss of generality we choose π = 0
β‘ π» 0 β‘ π» d
Operators π β β‘ e g and π + β‘ e g create or annihilate an excitation in the TLS
Here, our physics convention is not very intuitive, but consistent!
π+ =0 01 0
πβ =0 10 0
Matrix representation and
AS-Chap. 10 - 13
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10.3 Control of quantum two-level systems β¦.. Qubit Control: Rabi Oscillations
Time evolution of driven quantum TLS π» π‘ =
β
2
πq πdπβπππ‘
πdππππ‘ βπq
Qubit state πΉ π‘ = πe π‘ e + πg π‘ g
obeys SchrΓΆdinger equation
πd
dπ‘πe π‘ =
πq
2πe(π‘) +
πd
2πβπππ‘πg(π‘)
πd
dπ‘πg π‘ =
πd
2ππππ‘πe(π‘) β
πq
2πg(π‘)
πβd
dπ‘πΉ π‘ = π» πΉ π‘
Time-dependent Difficult to solve Move to rotating frame!
SchrΓΆdinger equation looses explicit time dependence
πd
dπ‘πe π‘ =
πq β π
2πe(π‘) +
πd
2πg(π‘)
πd
dπ‘πg π‘ =
πd
2πe(π‘) β
πq β π
2πg(π‘)
πe π‘ β‘ ππππ‘ 2 πe π‘
πg π‘ β‘ πβπππ‘ 2 πg π‘
AS-Chap. 10 - 14
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Rabi Oscillations
Formal treatment Effective Hamiltonian π» describing the same dynamics as π» (π‘)
πβd
dπ‘πΉ π‘ = π» πΉ π‘
πΉ π‘ β‘ πe π‘ e + πg π‘ g
πd
dπ‘πe π‘ =
πq β π
2πe(π‘) +
πd
2πg(π‘)
πd
dπ‘πg π‘ =
πd
2πe(π‘) β
πq β π
2πg(π‘)
Interpretation of the rotating frame π» π‘ =
β
2
πq πdπβπππ‘
πdππππ‘ βπq
The frame rotates at the angular speed π of the drive Driving field appears at rest Drive can be in resonance with Larmor precession frequency πq
Away from resonance |π β πq| β« 0
red terms dominate no π β |πβ© transitions induced by drive Near resonance π β πq
blue terms dominate π β |πβ© transitions induced by drive
H =β
2
βΞπ πd
πd Ξπ
with Ξπ β‘ π β πq
AS-Chap. 10 - 15
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Rabi Oscillations
Expand into stationary states
Dynamics of the effective Hamiltonian
H =β
2
βΞπ πd
πd Ξπ
Ξπ β‘ π β πq
Diagonalize H =ββ Ξπ2 + πd
2
2π π§
Initial state π³π = π (energy ground state)
πΉ π‘ = πΉ β πΉ 0 π+ππ‘2 Ξπ2+πd
2
πΉ β + πΉ + πΉ 0 πβππ‘2 Ξπ2+πd
2
πΉ +
tan π = βπd
Ξπ
tanπ = 0
|πΉ +β© = + cosπ
2e + sin
π
2|gβ©
|πΉ ββ© = β sinπ
2e + cos
π
2|gβ©
|Ξ¨+β© = +πβππ 2 cosπ
2π+ + π+ππ 2 sin
π
2|πββ©
|Ξ¨ββ© = βπβππ 2 sinπ
2π+ + π+ππ 2 cos
π
2|πββ©
New eigenstates
πΉ π‘ = cosπ
2π+ππ‘2 Ξπ2+πd
2
πΉ β + sinπ
2πβππ‘2 Ξπ2+πd
2
πΉ +
AS-Chap. 10 - 16
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Rabi Oscillations
Probability π·π to find TLS in |πβ©
Ξπ β‘ π β πq
πe β‘ e πΉ π‘ 2 = e πΉ π‘2=
|πΉ +β© = + cosπ
2e + sin
π
2|gβ©
|πΉ ββ© = β sinπ
2e + cos
π
2|gβ©
= sinπ
2cos
π
2πβππ‘2 Ξπ2+πd
2
β π+ππ‘2 Ξπ2+πd
22
πΉ π‘ = cosπ
2π+ππ‘2
Ξπ2+πd2
πΉ β + sinπ
2πβππ‘2
Ξπ2+πd2
πΉ +
= sin π sinπ‘ Ξπ2 +πd
2
2
2
πe =πd2
Ξπ2 + πd2 sin
2π‘ Ξπ2 + πd
2
2
tan π = βπd
Ξπ
Driven Rabi oscillations TLS population oscillates under transversal drive
AS-Chap. 10 - 17
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Rabi Oscillations
Rabi Oscillations on the Bloch sphere
Ξπ β‘ π β πq
πΉ π‘ = cosπ
2π+ππ‘2
Ξπ2+πd2
πΉ β + sinπ
2πβππ‘2
Ξπ2+πd2
πΉ +
πe =πd2
Ξπ2 + πd2 sin
2π‘ Ξπ2 + πd
2
2
tan π = βπd
Ξπ
On resonance π = ππͺ
Rotating frame cancels Larmor precession
State vector πΉ π‘ has no π-evolution
πΉ π‘ = cosπdπ‘
2g + π sin
πdπ‘
2e
Rotation about π₯-axis
π
π
π³ π = |π β©
|π³ (π)β©
π πππ
Finite detuning π«π > π Additional precession at π₯π
Population oscillates faster Reduced oscillation amplitude
AS-Chap. 10 - 18
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Rabi Oscillations
Rabi Oscillations β Graphical representation πe =
πd2
Ξπ2 + πd2 sin
2π‘ Ξπ2 + πd
2
2
On resonance π = ππͺ
Detuning π«π = πππ
First experimental demonstration with superconducting qubit Y. Nakamura et al., Nature 398, 786 (1999)
π·π
π
π
π·π
π
π
π. π
AS-Chap. 10 - 19
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Rabi Oscillations
Oscillating vs. rotating drive β Microwave pulses πe =
πd2
Ξπ2 + πd2 sin
2π‘ Ξπ2 + πd
2
2
On resonance (π = ππͺ) π·π
π
π
2π/π
2πd
Oscillating drive 2βπd cosππ‘ = βπd π+iππ‘ + πβiππ‘
in frame rotating with +π the πβiππ‘ -component rotates fast with β2π For πd βͺ π this fast contribtion averages out on the timescale of the slowly rotating component Rotating wave approximation
AS-Chap. 10 - 20
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Rabi Oscillations
Important drive pulses on the Bloch sphere
π
π
π
π
π
π
|π β©
π
π -pulse (πdΞπ‘ = π) g β |eβ© flips, refocus phase evolution
π /π-pulse (πdΞπ‘ = π/2) Rotates into equatorial plane and back
π
π β π π
π
2π/π
2βπd
Ξπ‘
π + π π
π
AS-Chap. 10 - 21
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Rabi Oscillations
Rabi Oscillations in presence of decoherence πe =
πd2
Ξπ2 + πd2 sin
2π‘ Ξπ2 + πd
2
2
π«π =0 π·π
π
π
Effect of decoherence (qualitative approach) Loss of coherent properties to environment at a constant rate Ξdec = 2π/πdec βChange of coherenceβ (time derivative) proportional to βamount of coherenceβ
Exponential decay of coherent property with factor πβπͺππππ
ππ = πβ
π
π»πππ Argument holds well for population decay (energy relaxation, π1) Loss of phase coherence more diverse depending on environment (exponential, Gaussian, or power law) Experimental timescales range from few ns to 100 ΞΌs
π. π π. π π + πβπ
π»πππ
Decay envelope
AS-Chap. 10 - 22
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Rabi Oscillations
Rabi decay time πe =
πd2
Ξπ2 + πd2 sin
2π‘ Ξπ2 + πd
2
2
π«π =0 π·π
π
π
π. π π. π π + πβπ
π»πππ
Complicated interplay between π1, π2β, and the drive
At long times, small oscillations persist Nevertheless useful order-of-magnitude check for πdec Important tool for single-qubit gates To determine π1, π2
β, π2 correctly, more sophisticated protocols are required energy relaxation measurements, Ramsey fringes, spin echo
AS-Chap. 10 - 23
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Rabi Oscillations
Energy relaxation on the Bloch sphere
π
π
|π β©
π
π½
π³ π½,π
π
Environment induces energy loss
State vector collapses to g Implies also loss of phase information Intrinsically irreversible
π1-time rate Ξ1 =2π
π1
Golden Rule argument
Ξ1 β π πq
π π is noise spectral density High frequency noise Intuition: Noise induces transitions
Quantum jumps
Single-shot, quantum nondemotiton measurement yields a discrete jump to g at a random time
Probability is equal for each point of time
Exponential decay with πβ
π‘
π1
AS-Chap. 10 - 24
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Rabi Oscillations
Dephasing on the Bloch sphere Environment induces random phase changes
Ξ2 β π π β 0 , π2 =2π
Ξ2
Low-frequency noise is detuned No energy transfer
1/π-noise π π β1
π
Example: Two-level fluctuator bath To some extent reversible
Decay laws πβ
π‘
π2, πβ
π‘
π2
2 ,
π‘
π2
π½
π
π
|π β©
|πβ©
ππ π
π³ π½π, ππ
|π³(π)β©
π
π
π½π
Visualization Phase π becomes
more and more unknown with time
Classical probility, no superposition!
Phase coherence lost when arrows are distributed over whole equatorial plane
AS-Chap. 10 - 25
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Rotating frame & no detuning (Ξπ = π β πq = 0) β no π₯π¦-evolution
x
y 1
0
x
y 1
0
x
y 1
0
x
y 1
0
x
y 1
0
Qubit dynamics β Relaxation
10.3 Control of quantum two-level systems
β¦.. Qubit Control: Energy Relaxation
π«π
π Measurement Init
π»π
π·π
π«π
π
πβπ
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x
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free evolution time
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x
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x
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Ramsey fringes
Qubit dynamics β Ramsey fringes (π»πβ )
x
y
π«πππ
π /π Measurement Init
π /π
π«πππ π»πβ
π«π = π
π·π
π
π. π π. π π + πβπ
π«π > π Beating Ξπ = 2π/Ξπ π»π
β βπ = ππ»πβπ + π»π
βπ Energy decay always present!
AS-Chap. 10 - 27
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free evolution time
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Spin echo
Qubit dynamics β Spin echo (π»ππ¬β )
π«πππ/π
π /π Measurement Init
π /π
π«πππ π»ππ¬β
π«π = π π·π
π. π π. π π β πβπ
π
Refocussing time
π«πππ/π
x
y
x x x x
y
x x x x
y x
y 1
0
Refocussing pulse reverses low-frequency phase dynamics π»ππ¬
β > π»πβ
AS-Chap. 10 - 28
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10.3 Control of quantum two-level systems
β¦.. Qubit Control: Ramsey vs. Spin echo sequence
Ramsey vs. spin echo sequence
0
0.2
0.4
0.6
0.8
1
0 2 4 6 8 10
F(Ο
t/2)
Οt/2
Ramsey filter FR
spin echo filter FE
Spin echo cancels the effect of low-frequency noise in the environment
Pulse sequences act as filters! Environment described by noise spectral density π(π)
ππ ππ‘ =sin2
ππ‘2
ππ‘2
2
ππ ππ‘ =sin4
ππ‘4
ππ‘4
2
Decay envelope β πβπ‘2 S π πΉR,E
ππ‘
2ππ
+βββ
Spin echo sequence
Filters low-frequency noise for ππ‘ β 0 ππ‘ β 2 Noise field fluctuates synchronously with π-pulse No effect
Sequence length π‘ is important!