decoupling with random quantum circuits
DESCRIPTION
Decoupling with random quantum circuits. S. Omar Fawzi (ETH Zürich) Joint work with Winton Brown (University College London). Random unitaries. Encoding for almost any quantum information transmission problem Entanglement generation Thermalization Scrambling (black hole dynamics) - PowerPoint PPT PresentationTRANSCRIPT
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Decoupling with random quantum
circuits
Omar Fawzi (ETH Zürich)
Joint work with Winton Brown (University College London)
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Random unitaries• Encoding for almost any quantum information
transmission problem • Entanglement generation• Thermalization• Scrambling (black hole dynamics) • Uncertainty relations / information locking• Data hiding• …
Decoupling
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Decoupling
S cannot see correlations between A and E
Decoupling theorem: how large can s be?
U S: s qubits
E
A: n qubits Sc: n-s qubits
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Decoupling theorem
[Schumacher, Westmoreland, 2001],…, [Abeyesinghe, Devetak, Hayden, Winter, 2009],…, [Dupuis, Berta, Wullschleger, Renner, 2012]
U S: s qubits
E
A: n qubits Sc: n-s qubits
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Decoupling theorem: examples
• A Pure:• Max. entanglement: • k EPR pairs:
In this talk
U S: s qubits
E
A: n qubits Sc: n-s qubits
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Computational efficiency
• A typical unitary needs exponential time!• Two-design is sufficient: O(n2) gates• O(n) gates possible?
• Physics motivation:o Time scale for thermalizationo Fast scramblers (black hole information)
• How fast can typical “local” dynamics decouple?
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Random quantum circuits
• Random gate on random pair of qubits • Complexity measures:
o Number of gateso Depth
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Random quantum circuits
• RQCs of size O(n2) are approximate two-designs [Harrow, Low, 2009]
• Approx two-designs decouple [Szehr, Dupuis, Tomamichel, Renner, 2013]
=> RQCs of size O(n2) decouple
Objective: Improve to O(n)
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Decoupling vs. approx. two-designs
• Approx. two design ≠ decoupling•[Szehr, Dupuis, Tomamichel, Renner, 2013]
• [Dankert, Cleve, Emerson, Livine, 2006]
o Random circuit model: e-approx two-design with O(n log(1/e)) gates
o Does NOT decouple unless Ω(n2)Cannot use route
More details [Brown, Poulin,
soon]
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Main result
RQC’s with O(n log2n) gates decouple
Depth: O(log3n)
U s
E
nn-s
Compare to Ω(log n)
Compare to Ω(n)
Almost tight
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Proof stepsRecall:
o Pure input ρ, no E systemo Study decoupling directly
U s
E
nn-s
S
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Proof setup
Total mass on strings with support on S
Fourier coefficient
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Evolution of mass dist.
IX IZ YI XX XZ YY ZX ZZ
Distribution of masses
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The Markov chain
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Putting things together
Main technical contribution
Initial mass at level l
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Conclusion• Summary
o Random quantum circuits with O(n log2 n) gates and depth O(log3 n) decouple
• Open questionso Depth improved to O(log n)?o Quantum analogue of randomness extractors• Explicit constructions of efficient unitaries?• Number of unitaries?
o Geometric locality, d-dimensional lattice?o Hamiltonian evolutions?