THE NEED FOR HIGH EFFICIENCY PHOTONNUMBER RESOLVING DETECTORS IN LINEAR OPTICS QUANTUM COMPUTING

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THE NEED FOR HIGH EFFICIENCY PHOTON-NUMBER
RESOLVING DETECTORS IN LINEAR OPTICS QUANTUM
COMPUTING

• T.B. Pittman, M.M. Donegan, M.J. Fitch, B.C.
  Jacobs, J.D. Franson
• Johns Hopkins University
• Applied Physics Laboratory

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DETECTOR NEEDS FOR LINEAR OPTICS QUANTUM
COMPUTATION (LOQC)

• Motivation Brief overview of LOQC
• Three specific examples
• Non-deterministic controlled-NOT logic gate
• Heralded two-photon entanglement (entanglement
  resource)
• Single-Photons on Pseudo-Demand (photon resource)

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MOTIVATIONOptical Approach to Quantum Computing

• Advantages
• Photon qubits
• Single-qubit operations
• Modular approach
• Biggest Problem
• Two-qubit operations (Logic gates)
• Nonlinear interactions
• New Development LOQC KLM, Nature 409, 46
  (2001)
• Non-Deterministic logic gates
• No direct photon-photon interactions
• Nonlinearity from photo-detection and state
  reduction

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OVERVIEW LINEAR-OPTICS QUANTUM GATES
Knill, LaFlamme and Milburn (KLM), Nature 409, 46
(2001)
entangled ancilla photons
.
1
N
2

• CNOTs using only
• Linear optics
• N ancilla photons
• Single-Photon detection
• Error probability scales as
• 1/N or 1/N2 (lim. large N)
• Example requirements
• For 90 success rateN 40, h 99.86
• Glancy et.al PRA (2002)

control qubit
output
X
Z
target qubit
.
U
U
U
D1
D2
DN
Post-selection Feed-forward
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EXAMPLE 1 Basic non-deterministic two-qubit CNOT
gate
See KLM, Koashi et.al, Ralph et.al, Pittman
et.al Zou et.al, Hoffman et.al, Sanaka et.al,
etc. (2001-2002)
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BASIC LINEAR-OPTICS CNOT GATE
U
control photon

• Polarization encoded qubits
• H0, V1
• Polarizing beam splitters
• Corresponds to case of N2
• Prob. of success ¼
• needs 1 vs. 2 photon high-h detectors

PBS-1
entangled ancilla (photon pair)
PBS-2
target photon
U
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SIMPLIFIED CNOT EXPERIMENTAL DEMO
single ancilla photon
control photon
U
control photon
U
entangled ancilla (photon pair)
U
target photon
target photon
U
U
(b)
(a)

• Two major modifications
• Entangled pair replaced by single ancilla
• Coincidence basis operation

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EXPERIMENTAL CNOT SINGLE ANCILLA
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EXPERIMENTAL CNOT SINGLE ANCILLA
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EXPERIMENTAL CNOT RESULTS
(a) basis state CNOT truth table
(b) superposition state coherence
entanglement production
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REQUIREMENTS FOR FULL CNOT DEMO
control photon
U

• Non-coincidence-basis operation
• Requires 2 vs. 1 detectors
• Heralded entangled pairs
• Requires 2 vs. 1 detectors

entangled ancilla (photon pair)
target photon
U
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EXAMPLE 2 Heralded two-photon entanglement
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OVERVIEW HERALDED ENTANGLED PAIRS

• Pulsed parametric down-conversion
• Random entangled pairs (eg. Kwiat et.al. 95)
• Standard entanglement swapping
• Doesnt work (Kok Braunstein 01)
• CNOT-based entanglement swapping
• Does work (in principle)
• Requires photon-number resolving Dets

U
U
CNOT
U
U
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CNOT-BASED ENTANGLEMENT SWAPPING
CNOT
Start with
1
2
U
A
4
3
U
B
After CNOT
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CNOT-BASED ENTANGLEMENT SWAPPING USE OF
NON-DETERMINISTIC CNOT
CNOT

• 6 Photons involved
• Detection of 4 heralds presence of remaining
  (entangled) pair
• Key ingredient Photon-number resolving detectors
  
• Elliminate false-heralding signals
• See also SliwaBanaszek (2002)

U
1
2
U
A
C
4
3
U
B
U
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EXAMPLE 3 Single-photons on pseudo-demand (ancilla
resource)
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PARAMETRIC DOWN-CONVERSION APPROACH
Spontaneously Emitted Photon Pair
PDC
Excitation Pulse Train
Detection of one signals the other (well defined
emission direction)

• Problem Random emission time
• Cant specify desired excitation pulse
• Solutions
• Storage loop Pseudo-Demand (APL)
• Multiplexing (Alan Migdall at NIST)
• Hybrid approach?

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SINGLE-PHOTONS ON PSEUDO-DEMAND
Storage loop
switch
U
PDC

• Pulse-train and loop synchd with cycle time of
  quant. Computer
• Photon-number resolving detector needed
• increase pump power to decrease (lossy) storage
  time
• elliminate possibility of two photons in storage
  loop

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EXPERIMENT PSEUDO-DEMAND SOURCEPrinciple of
Operation
Reflects V, Transmits H
Switch In/Out (Flips between H and V)
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EXPERIMENTAL DEMONSTRATIONSingle-Photons on
Pseudo-Demand

• High-speed Pockels cell for real-time EO
  switching
• 4 meter free-space storage loop
• Single-photons switched out on command
• Original experiment is cw, current work is pulsed

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MAIN RESULT ACTIVE SWITCHING DEMOStored single
photons switched out on command
2 round trips
3 round trips
4 round trips
5 round trips
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SUMMARYHigh-efficiency photon-number resolving
detectors needed for LOQC

• LOQC is a promising approach
• (OKish) single-photon sources, interferences,
  feed-forward
• (??) Ideal single-photon detectors
• Three specific examples
• Non-deterministic cnot gate
• Heralded entangled photon pairs
• Single-photon source

U
U
Storage loop
U
PDC