Atom trifft Photon · July 10th 2013 2 1. Introduction Atom in Rydberg state Highly excited →...
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Atom trifft Photon
Rydberg blockade
July 10th 2013Michael Rips
July 10th 2013 2
1. Introduction
Atom in Rydberg state● Highly excited → principal quantum number n up to 500
● Diameter of atom can reach ~1μm
● Long life time (~µs ↔ ~ns for low excited atoms)
● Large dipole moment (scaling with n7 !)
● Dipole interaction between to Rydberg atoms scales with n11
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1. Introduction
Blockade● Energy shift ΔE caused by dipole interaction
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1. Introduction
What's interesting about Rydberg atoms?● Ability to entangle two or more neutral atoms
● Preparation of Qubits
● Implementation of quantum-gates
● → quantum computing
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Outline
1.Introduction
2.Collective excitation in the Rydberg regime(experiment: Gaëtan et al. 2009)
3.Rydberg blockade between two atoms(experiment: Urban et al. 2009)
4.CNOT-gate between two atoms(experiment: Isenhower et al. 2010)
5.Summary & References
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Outline
1.Introduction
2.Collective excitation in the Rydberg regime(experiment: Gaëtan et al. 2009)
3.Rydberg blockade between two atoms(experiment: Urban et al. 2009)
4.CNOT-gate between two atoms(experiment: Isenhower et al. 2010)
5.Summary & References
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2. Collective excitation in the Rydberg regime
● Demonstration of a procedure to deterministically entangle two rubidium atoms.
● Rydberg blockade effect to achieve entangled state.
Collective excitation in the Rydberg regime(experiment: Gaëtan et al. 2009)
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2. Collective excitation in the Rydberg regime
Experimental setup CCD (charge-cupled device) camera to measure atom position in trap.
APDs (Avalanche-Photodiodes) to check if trap is empty or not
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2. Collective excitation in the Rydberg regime
Excitation by two (simul-taneous!) laserpulses:
● x-direction → 795nm, π
● z-direction → 474nm, σ+
term scheme of rubidium 87
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2. Collective excitation in the Rydberg regime
Detuning δ:● First pulse detuned 400Mhz to blue
side
● Necessary to avoid population of intermediate state
term scheme of rubidium 87
Excitation by two (simul-taneous!) laserpulses:
● x-direction → 795nm, π
● z-direction → 474nm, σ+
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2. Collective excitation in the Rydberg regime
Time sequence of the experiment
Cooling
Ground state preparation
Rydberg excitation
Dipole traps
time
~600µs
<500ns
Observing trap state with fluorescence induced by cooling laser light. If atom is lost → Rydberg atom
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2. Collective excitation in the Rydberg regime
Excitation probability outside the blockade regime, R=18µm
Red and green:single atom excitation, second atom is absent
Blue:
Black:
Product of red and green
Collective excitation of two atoms.
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2. Collective excitation in the Rydberg regime
Excitation probability inside the blockade regime, R=3.6µm
Red and green:single atom excitation, second atom is absent
Product of red and green
Blue:
Collective excitation of two atoms.Black:
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2. Collective excitation in the Rydberg regime
→
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2. Collective excitation in the Rydberg regime
New 2-atom state in Rydberg regime:
→
→ entanglement
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2. Collective excitation in the Rydberg regime
state coupled to ground state
→ effective Rabi-frequency
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2. Collective excitation in the Rydberg regime
→ 2-level system! ↔
state coupled to ground state
→ effective Rabi-frequency
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2. Collective excitation in the Rydberg regime
Measurement of entanglement
Red line: 1-atom system (2nd trap empty)
Blue line: 2-atom system
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2. Collective excitation in the Rydberg regime
Measurement of entanglement
Red line: 1-atom system (2nd trap empty)
Blue line: 2-atom system
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3. Rydberg blockade between two atoms
Experimental setup
● Counter-propagating laser beams 780nm → ← 480nm for excitation.
● Distance between control and target atom amounts about 10µm.
● Each atom excited by own laser beam!
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3. Rydberg blockade between two atoms
Rydberg state and ground state
form 2-level system with Rabi-
frequency .
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3. Rydberg blockade between two atoms
Rydberg state and ground state
form 2-level system with Rabi-
frequency .
If Control atom is in state → Target atom feels
level shift B → state is blocked.
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3. Rydberg blockade between two atoms
Probability of ground state population on target
site depending on duration of target excitation.
Control atom in Rydberg stateControl atom in ground state
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3. Rydberg blockade between two atoms
Phase shift of target atom while 2π-pulse
depends on state of control atom.
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3. Rydberg blockade between two atoms
Time sequence of the experiment
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3. Rydberg blockade between two atoms
→ Unitary transformation
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3. Rydberg blockade between two atoms
→ Unitary transformation
2-atom states:
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3. Rydberg blockade between two atoms
→ Unitary transformation
2-atom states:
→ controlled-Z (CZ) gate
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3. Rydberg blockade between two atoms
→ Unitary transformation
2-atom states:
→ controlled-Z (CZ) gate
CZ gate can be converted into a controlled-NOT (CNOT) gate
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4. CNOT-gate between two atoms
CZ gate conversion to controlled-NOT (CNOT) gate:
→ apply π/2 rotations between and on target atom before
and after the interaction.
control target
π/2 pulse
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4. CNOT-gate between two atoms
Hadamard controlled-Z (H-CZ) CNOT gate
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5. Summary and references
Summary:
● Quantum gates with Rydberg atoms already realized● Potential to become basic tool in quantum-information
processing● Fidelity has to be improved● Biggest error source at the moment:
→ atom loss during gate operations
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4. Summary and references
References:
● Gaëtan et al. ”Observation of collective excitation of two individual atoms in the Rydberg blockade regime“, nature physics (2009)
● Urban et al. ”Observation of Rydberg blockade between two atoms”, nature physics (2009)
● Isenhower et al. ”Demonstration of a Neutral Atom Controlled-NOT Quantum Gate”, Physical Review Letters (2010)
● Saffman et al. ”Quantum information with Rydberg atoms”, reviews of modern physics, volume 82 (2010)
