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AS-Chap. 3.6 - 1
Introduction to Chapter 6(from Chapter 3 of the lecture notes)
Quantum treatment of JJ
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AS-Chap. 3.6 - 2
Classical treatment of Josephson junctions (so far)
Phase ๐ and charge ๐ = ๐ถ๐ โ๐๐
๐๐ก Purely classical variables
(๐, ๐) are assumed to be measurable simultaneously
Dynamics Tilted washboard potential, rotating pendulum
Classical energies:
Potential energy ๐(๐)(Josephson coupling energy / Josephson inductance)
Kinetic energy ๐พ แถ๐
(Charging energy via1
2๐ถ๐2 =
๐2
2๐ถโ
๐๐
๐๐ก
2/ junction capacitance)
Current-phase & voltage-phase relation from macroscopic quantum model
Quantum origin
Primary macroscopic quantum effects
Second quantization
Treat (๐, ๐) as quantum variables (commutation relations, uncertainty)
Secondary macroscopic quantum effects
3.6 Full Quantum Treatment of Josephson JunctionsSecondary Quantum Macroscopic Effects
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AS-Chap. 3.6 - 3
E
j
2๐ธJ0
3.6.1 Quantum Consequences of the small Junction Capacitance
Validity of classical treatment
Classical treatment valid for ๐ธ๐ฝ0
โ๐๐โ
๐ธ๐ฝ0
๐ธ๐ถ
1/2โซ 1 (Level spacing โช Potential depth)
Enter quantum regime by decreasing junction area ๐ด
Consider an isolated, low-damping junction, ๐ผ = 0 Cosine potential, depth 2๐ธ๐ฝ0 Close to potential minimum
Harmonic oscillatorFrequency ๐p, level spacing โ๐p
Vacuum energyโ๐p
2
โโ๐p
2
โ โ๐p
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AS-Chap. 3.6 - 4
Example 1Area ๐ด = 10 ฮผm2, Tunnel barrier ๐ = 1 nm, ๐ = 10,
๐ฝc = 100A
cm2
๐ธJ0 = 3 ร 10โ21 J
๐ธJ0/โ = 4500 GHz
๐ถ =๐๐0๐ด
๐= 0.9 pF
๐ธ๐ถ = 2 ร 10โ26 J
๐ธ๐ถ
โ= 30 MHz
Classical junction
We also need ๐ โช 500 mK for ๐B๐ โช ๐ธJ0, ๐ธ๐ถ!
3.6.1 Quantum Consequences of the small Junction Capacitance
Parameters for the quantum regime
Example 2Area ๐ด = 0.02 ฮผm2
๐ถ โ 1 fF ๐ธ๐ถ โ ๐ธJ0 Quantum junction
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AS-Chap. 3.6 - 5
Kinetic energy:
Total energy:
๐ ๐ โ 1 โ cos๐ Potential energy๐พ แถ๐ โ แถ๐2
Kinetic energy
Energy due to extra charge ๐ on one junction electrode due to ๐
Consider ๐ธ ๐, แถ๐ as junction Hamiltonian, rewrite kinetic energy
๐พ = ๐2/2๐
๐ =โ
2๐๐
3.6.1 Quantum Consequences of the small Junction Capacitance
Position coordinate associated to phase ๐, momentum associated to charge ๐
Hamiltonian of a strongly underdamped junction (with ๐๐
๐๐กโ 0)
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AS-Chap. 3.6 - 6
Canonical quantization (operator replacement)
with ๐ =๐
2๐ # of Cooper pairs
we get the Hamiltonian
Describes only Cooper pairs
๐ธ๐ถ =๐2
2๐ถnevertheless defined as
charging energy for a single electron charge
๐ โก๐
2๐ Deviation of # of CP in electrodes from equilibrium
3.6.1 Quantum Consequences of the small Junction Capacitance
Commutation rules for the operators
Heisenberg uncertainty relation
ฮ๐ดฮ๐ต โฅ1
2แ๐ด, ๐ต
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AS-Chap. 3.6 - 7
Hamiltonian in the flux basis (๐ =โ
2๐๐ =
ฮฆ0
2๐๐)
3.6.1 Quantum Consequences of the small Junction Capacitance
Commutator ๐, ๐ = ๐โ
๐ and Q are canonically conjugate (analogous to x and p)
Circuit variables are now quantized
Superconducting quantum circuits
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AS-Chap. 3.6 - 8
Lowest energy levels localized near bottom of potential wells at ๐๐ = 2๐ ๐
Taylor series for ๐ ๐ Harmonic oscillator,
Frequency ๐p, eigenenergies ๐ธ๐ = โ๐p ๐ +1
2
Ground state: narrowly peaked wave function at ๐ = ๐๐
Large fluctuations of ๐ on electrodes since ฮ๐ โ ฮ๐ โฅ 2๐(small EC pairs can easily fluctuate, large ฮ๐)
Small phase fluctuations ฮ๐Negligible ฮ๐ โ classical treatment of phase dynamics is good approximation
3.6.2 Limiting Cases: The Phase and Charge Regime
The phase regime
โ๐p โช ๐ธJ0 , ๐ธ๐ถ โช ๐ธJ0 Phase ๐ is a good quantum number!
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AS-Chap. 3.6 - 9
Hamiltonian
define ๐ = ๐ธ โ ๐ธ๐ฝ0 /๐ธ๐ถ, ๐ = ๐ธ๐ฝ0/2๐ธ๐ถ and ๐ง = ๐/2
known from periodic potential problem in solid state physics Energy bandsGeneral solution
Bloch waves
Charge/pair number variable ๐ is continuous (charge on capacitor!) ๐น ๐ is not 2๐-periodic
3.6.2 Limiting Cases: The Phase and Charge Regime
Mathieu equation
The phase regime
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AS-Chap. 3.6 - 10
1D problem Numerical solution straightforwardVariational approach for approximate ground state
Trial function for ๐ธ๐ถ โช ๐ธJ0
Choose ๐ to find minimum energy:
๐ฌ๐ช
๐ฌ๐๐= ๐. ๐
๐ฌ๐ฆ๐ข๐ง = ๐.๐ ๐ฌ๐๐
first order in EJ0
Emin โ 0 for EC โช EJ0
3.6.2 Limiting Cases: The Phase and Charge Regime
The phase regime
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AS-Chap. 3.6 - 11
Tunneling coupling โ exp โ2๐ธJ0โ๐ธ
โ๐p Very small since โ๐p โช ๐ธJ0
Tunneling splitting of low lying states is exponentially small
3.6.2 Limiting Cases: The Phase and Charge Regime
The phase regime
๐ฌ๐ช
๐ฌ๐๐= ๐. ๐
๐ฌ๐ฆ๐ข๐ง = ๐.๐ ๐ฌ๐๐
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AS-Chap. 3.6 - 12
Kinetic energy โ ๐ธ๐๐๐
๐๐ก
2dominates
Complete delocalization of phase Wave function should approach constant value, ฮจ ๐ โ const. Large phase fluctuations, small charge fluctuations (๐ฅ๐ โ ๐ฅ๐ โฅ 2๐)
3.6.2 Limiting Cases: The Phase and Charge Regime
โ๐p โซ ๐ธJ0 , ๐ธ๐ถ โซ ๐ธJ0 Charge ๐ (momentum) is good quantum number
Appropriate trial function:
Hamiltonian
Approximate ground state energy
second order in EJ0
๐ผ โช 1
The charge regime
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AS-Chap. 3.6 - 13
๐ฌ๐ช๐ฌ๐ฑ๐
= ๐. ๐
๐ฌ๐๐๐ = ๐. ๐๐ ๐ฌ๐ฑ๐
Periodic potential is weak Strong coupling between neighboring phase states Broad bands Compare to electrons moving in strong (phase regime) or weak (charge regime) periodic
potential of a crystal
3.6.2 Limiting Cases: The Phase and Charge Regime
The charge regime
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AS-Chap. 3.6 - 14
Voltage ๐ Charge ๐ = ๐ถ๐, energy ๐ธ =๐2
2๐ถ
Observation of CB requires small thermal fluctuations
๐ธ๐ถ =๐2
2๐ถ> ๐๐ต๐ โ ๐ถ <
๐2
2๐๐ต๐
๐ถ โ 1 fF at T = 1 K, ๐ = 1 nm, and ๐ = 5 ๐ด โฒ 0.02 ฮผm2 Small junctions!
Observation of CB requires small quantum fluctuations
Quantum fluctuations due to Heisenberg principle ฮ๐ธ โ ฮ๐ก โฅ โ Finite tunnel resistance ๐๐ ๐ถ = ๐ ๐ถ (decay of charge fluctuations)
ฮ๐ก = 2๐๐ ๐ถ, ฮ๐ธ =๐2
2๐ถ ๐ โฅ
โ
๐2= ๐ ๐พ = 24.6 kฮฉ Typically satisfied
3.6.3 Coulomb and Flux Blockade
Coulomb blockade in normal metal tunnel junctions
Single electron tunneling
Charge on one electrode changes to ๐ โ ๐
Electrostatic energy ๐ธโฒ =๐โ๐ 2
2๐ถ
Tunneling only allowed for ๐ธโฒ โค ๐ธ Coulomb blockade: Need ๐ โฅ ๐/2 or ๐ โฅ ๐CB = ๐c = ๐/2๐ถ
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AS-Chap. 3.6 - 15
Coulomb blockade in superconducting tunnel junctions
For ๐2
2๐ถ> ๐๐ต๐, ๐๐ (๐ = 2๐) No flow of Cooper pairs
Threshold voltage ๐ฝ โฅ ๐ฝ๐ช๐ฉ = ๐ฝ๐ =๐๐
๐๐ช=
๐
๐ช
Coulomb blockade Charge is fixed, phase is completely delocalized
3.6.3 Coulomb and Flux Blockade
S S2e-
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AS-Chap. 3.6 - 16
Current ๐ผ Flux ๐ท = ๐ฟ ๐ผ, energy ๐ธ = ๐ท2/2๐ฟ
In presence of fluctuations we need
๐ธJ0 โซ ๐B๐ (large junction area)
And ๐ฅ๐ธ โ ๐ฅ๐ก โฅ โ with ๐ฅt = 2๐๐ฟ
๐ and ๐ฅ๐ธ = 2๐ธ๐ฝ0 ๐ โค
โ
(2๐)2=
1
4๐ ๐พ
3.6.3 Coulomb and Flux Blockade
Phase or flux blockade in a Josephson junction
Phase is blocked due to large ๐ธJ0 = ๐ท0๐ผ๐/2๐
๐ผc takes the role of ๐CB Phase change of 2๐ equivalent to flux change of ๐ท0
Flux blockade ๐ผ โฅ ๐ผFB = ๐ผc =ฮคฮฆ0 2๐
๐ฟc
Analogy to CB ๐ผ โ ๐, 2๐ โ๐ท0
2๐, ๐ถ โ ๐ฟ
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AS-Chap. 3.6 - 17
Coherent charge statesIsland charge continuously changed by gate
Independent charge states (๐ธJ0 = 0)
Parabola ๐ธ ๐ = ๐ โ ๐ โ 2๐ 2/2๐ถฮฃ
Cooper pair box
3.6.4 Coherent Charge and Phase States
๐ธJ0 > 0
Interaction of |๐โฉ and |๐ + 1โฉ at the level crossing points ๐ = ๐ +1
2โ 2๐
Avoided level crossing (anti-crossing)
Coherent superposition states ๐นยฑ = ๐ผ ๐ ยฑ ๐ฝ ๐ + 1
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AS-Chap. 3.6 - 18
Average charge on the island as a function of the applied gate voltage Quantized in units of 2๐ (no coherence yet)
3.6.4 Coherent Charge and Phase States
First experimental demonstration of coherent superposition charge states by Nakamura, Pashkin, Tsai (1999, not this picture)
Coherent charge states
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AS-Chap. 3.6 - 19
Coherent phase states Interaction of two adjacent phase states Example is rf SQUID
magnetic energy
of flux ๐ =ฮฆ0
2๐๐ in the ring ๐ฝ๐๐๐ = ๐ฝ๐/๐
Tunnel coupling Experimental evidence for quantum coherent superposition (Mooij et al., 1999)
3.6.4 Coherent Charge and Phase States
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AS-Chap. 3.6 - 20
Violation of conservation of energy on small time scales, obey ฮ๐ธ โ ฮ๐ก โฅ โ
Creation of virtual excitations Include Langevin force ๐ผ๐น with adequate statistical properties Fluctuation-dissipation theorem
๐ธ ๐, ๐ = energy of a quantum oscillator
Transition from โthermalโ Johnson-Nyquist noise to quantum noise:
classical limit (โ๐, ๐๐ โช ๐๐ต๐):
quantum limit (โ๐, ๐๐ โซ ๐๐ต๐):
3.6.5 Quantum Fluctuations
vacuumfluctuations
occupation probabilityof oscillator (Planck distribution)
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AS-Chap. 3.6 - 21
Escape of the โphase particleโ from minimum of washboard potential by tunneling
Macroscopic, i.e., phase difference is tunneling (collective state) States easily distinguishable
Competing process
Thermal activation Low temperatures
Neglect damping
Dc-bias Term โโ๐ผ๐
2๐in Hamiltonian
Curvature at potential minimum:
(Classical) small oscillation frequency:
3.6.6 Macroscopic Quantum Tunneling
๐ = ๐ผ/๐ผ๐
(attempt frequency)
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AS-Chap. 3.6 - 22
Quantum mechanical treatment
Tunnel coupling of bound states to outgoing waves Continuum of states But only states corresponding to quasi-bound states have high amplitude In-well states of width ฮ = โ/๐ (๐ = lifetime for escape)
Determination of wave functions
Wave matching method Exponential prefactor within WKB approximation Decay in barrier:
decay of wave function of particle with mass M and energy E
3.6.6 Macroscopic Quantum Tunneling
for ๐ ๐ โซ ๐ธ0 = โ๐A/2
mass effective barrier height
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AS-Chap. 3.6 - 23
Constant barrier height Escape rate
Increasing bias current
๐0 decreases with ๐ ๐ค becomes measurable
Temperature ๐โ where ๐คtunnel = ๐คTA โ exp โ๐0
๐๐ต๐
for ๐ผ > 0: For ๐p โ 1011 ๐ โ1
T* 100 mK
very small for ๐ โ 0
3.6.6 Macroscopic Quantum Tunneling
for ๐ผ โ 0:
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AS-Chap. 3.6 - 24
Junction couples to the environment (e.g., Caldeira-Leggett-type heat bath)
Crossover temperature ๐๐ต๐โ โ
โ๐R
2๐with ๐R = ๐๐ด 1 + ๐ผ2 โ ๐ผ and ๐ผ โก
1
2๐ N๐ถ๐A
Strong damping ๐ผ โซ 1 ๐R โช ๐A Lower ๐โ
Damping suppresses MQT
3.6.6 Macroscopic Quantum Tunneling
Additional topic: Effect of damping
lightly damped(small capacitance)
moderately damped(larger capacitance)
After: Martinis et al., Phys. Rev. B. 35, 4682 (1987).
๐R/๐
A
๐ผ10310โ1 10 105
1.0
010โ3
0.8
0.6
0.4
0.2
Phase diffusion by MQTSee lecture notes
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AS-Chap. 3.6 - 25
Classical description only in the phase regime (large junctions): ๐ธ๐ถ โช ๐ธ๐ฝ0
For ๐ธ๐ โซ ๐ธ๐ฝ0: quantum description (negligible damping):
Phase difference ๐ and Cooper pair number ๐ =๐
2๐are canonically conjugate variables
phase regime: ฮ๐ โ 0 and ฮ๐ โ โcharge regime: ฮ๐ โ 0 and ฮ๐ โ โ
Charge regime at ๐ = 0
Coulomb blockade Tunneling only for ๐CB โฅ๐
๐ถ
Flux regime at ๐ = 0
Flux blockade Flux motion only for ๐ผFB โฅฮฆ0
2๐๐ฟc
At ๐ผ < ๐ผc Escape out of the washboard by thermal activation or macroscopic quantum tunneling
TA-MQT crossover temperature ๐โ
Summary (secondary quantum macroscopic effects)