Quantum Architecture more unknowns than knowns

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Quantum Architecturemore unknowns than knowns

• Mark OskinUniversity of Washington

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Outline

• What / Why / How
• Design Rules and Technology Abstraction
• Quantum Architecture
• Simulation Infrastructure
• Programming languages

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What is it?
Quantum Architecture

• (1) The organization and optimization of quantum
  and classical structures (i.e. the
  micro-architecture) and the interface (i.e. the
  ISA) for the efficient execution of
  quantum-enabled software.
• (2) A dark vast babble-space

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Why?
- Now?

• Quantum architecture research can
• Identify the weak spots in technology
• Point the way to solutions for some of them
• Push the rest back to the physicists
• Discover what we dont know
• A surprisingly useful thing to know
• Bring a reality check to this process
• Identify physical bounds that alter theoretical
  ones
• Quantify the known aspects gt quite large
• Maybe find the right abstraction?

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How

• Need expertise in both disciplines
• Quantum theorist and physicist
• Architecture Engineers
• Funding is the easiest part
• NSF Nanoscale initiative
• DARPA QuIST
• Students are available
• Lots of interest
• Need only simple background in
• Architecture
• basic QC theory
• Can stay away from the dicey parts at first

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How

• Its not exactly SimpleQubit but
• Currently mathematical models
• Working on an architecture simulator
• Physicists working on component simulator
• Applications are well known
• Its 99 error correction
• They have all the things we like
• Locality
• Parallelism

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Quantum Architecture

1. Abstracting technologies
2. Formulate design constraints
3. Mold into building blocks
4. Form into architectures
5. Simulate application performance

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Technology abstraction

• First order assumptions
• Classical control of quantum gates
• Silicon to interface and control
• Individual control of quantum bits

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Second order assumptions
1.5

• Choose a likely technology Kane
• Spin of 31P holds quantum state
• ? 20nm apart for quantum effect to occur
• ? 1.5Kelvin for reasonable coherence time
• Local magnetic field arbitrates gates
• Controlled by classical pins
• ? 5nm classical pitch
• Driven by high frequency (10-100Mhz) clock
• Gated by lower frequency (0.01 10) Mhz
• Similar to CMOS vs. TTL

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Develop design rules

• 20nm spacing of qubits
• 5nm spacing of control lines
• _at_ 1.5 Kelvin cannot drive AC current
• 2 dimensions must be ? 100nm
• pitch matching issue
• Implies sparseness of quantum state

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Quantum architecture

• Abstractions
• Interconnect
• Memory
• Processor
• Interfacing
• Quantum ISA
• Classical-Quantum interface

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Specialization?
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A Quantum Wire

• Short swapping-channel
• structural implications (sparseness)
• Limited length
• Long teleportation-channel
• Arbitrary length
• Architectural implications
• Overhead
• Latency / bandwidth

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A short quantum wire

• Constructed from swap gates

Unless the particle that holds the quantum state
physically moves, the information flows in
discrete steps from particle to particle.
Each step requires 3 quantum controlled-not
operations, effectively performing a swap of
the quantum states.
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Straightforward approach
5nm access points contain only a handful of
quantum statesfor their electrons at
temperatures less than 1K, compromising
correctoperation.
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One solution
As two physical dimensions ofthe access point
exceed 100nmthousands of electron states are
held.
Classically, thesestates are restrictedto the
access point,however, quantummechanically
theytunnel downward,guided by the via,thus
enabling control.
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Classical access points
100nm
100nm
100nm
100nm
5nm
Narrow tipped control
20nm
20nm
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Incompleteness of lines
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Top-down view
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QCAD Cell Implications

• Minimum wire length
• ? 200nm (10 qubits)
• Excepting custom components
• Minimum junction point size
• ? 44 qubits square
• Realistic sizes will be larger
• Assumes deep 5nm vias

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Why short wires are short

• Limited by decoherence
• Threshold theorem gt distance
• 10-8 ? 1.8mm
• Key difference from classical
• quantum information must be protected,not just
  restored!!
• Can make longer with repeater
• Essentially this is multiple short
  wiresseparated by error correction blocks

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Architecting long wires

• Key insight
• EPR pairs are known states
• No need to protect them
• Purify the good ones
• Discard the bad

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Architecture of a long wire
Quantum EPR channel
EPR Generator
Teleporation Unit
Teleporation Unit
Classical control channel
EPR channel
Purification
Coded Tele- Portation
Entropy Exchange
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Long wires

• Can be of arbitrary length
• A 10mm wire sustains nearly peak bandwidth
• Low latency
• Pre-communicate EPR pairs
• Latency is constant teleportation operation
• Code-conversation for free
• Facilitates Processor lt-gt Memory communication

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Long wires

• Several architectural implications
• EPR generation
• Distributed entropy exchange (zeros)
• Purification
• Teleportation

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QCAD Cells

• Fundamental
• Qubit
• Zero
• Measurement
• Basic
• Line
• Intersection
• Composite / Custom
• Purify (custom error correct)
• Error correct
• Add? Multiply? Memory?

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Building Block (I)

• Measurement unit computational Bell basis

Classical control
Measure
Qubit to measure
Classical 0,1 output with probability determined
by ?
Zero qubit
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Building Block
EX

• Entropy exchange unit

Polarized Light
P

Ground
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Macro Block
EPR

• EPR generation unit

Classical control
Quantum output of an EPR state
EPR Generator
Zero qubits
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Macro Block
Pur

• Purification unit error correction

Classical control
Purification Unit
Zero bits
Purified EPR states
EPR states to purify
Garbage state (to Entropy Exch)
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Quantum Memory
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Quantum memory?

• Is dedicated memory viable?
• Yes
• DRAM like (needs refreshing)
• Hierarchical error codes?
• Quantum caches
• DFS (Decoherence Free Subspace)?
• Really phase coherent subspace
• Need less error correction/qubit
• No
• Qubit Refresh almost as complex as computation!
• Big Almost gt No T gate / all transversal

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Quantum ALU / ISA
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Quantum Functional Unit

• Complex, have to tightly integrate
• Measurement
• Zeros
• Quantum I/O
• Irregular classical logic
• Maybe custom data-paths for
• H/X/Z
• CNot
• T
• Complicated by hierarchical error coding

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Processing

• Likely to use just-in-time compilation
• Huge O(nck) savings with error correction
• Optimize overhead to data size
• Clustering
• Smaller O(nc) savings
• Packing / unpacking
• Application specific error processing
• Phase error independence
• Bit-flip error independence

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Flexible execution units
Classic analogy MMX (except more complicated to
combine)
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Interfacing and Control

• Quantum operations occur at different speeds
• 10-100Mhz for single qubit rotations
• 10-100Khz for two-qubit operations
• 1Mhz on average (50/50 split)
• Even at 1Mhz operation
• Ample opportunity for interesting classical work
• Error correction creates even more time for
  top-level control (5k)
• Low-level must simultaneously decide on the
  control of millions of qubits/Mhz

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Controlling the classical control

• Highly parallel
• O(n) operations per-cycle!
• Required for fault-tolerant operation
• Specialized classical processors?
• Certainly ASIC logic for drive/control
• Quantum co-processor ISA interface?

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Quantum ISA

• Single qubit rotations
• rotate(qubit, axis, angle)
• Controlled operations
• rotate(qubit, axis, angle, on list)
• Just-Enough-Compilation
• Control error correction overhead
• Invoke(program, input, input complexity)

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Simulation

• Architecture Simulation
• Abstraction layer
• QCAD Cells
• Macro blocks (memory, etc)
• Classical interfacing
• Bolt onto SimpleScalar??
• Design path
• QVHDL -gt Cell Layout

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How?

• Quantum simulation is O(2n) hard
• Obtaining the right algorithmic answer is not
  going to happen
• Symbolic simulation is only O(nt)
• Classic n-body simulation
• Eminently Parallelizable
• Look for this in the Fall

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Programming Abstractions

• Quantum computing lacks a clear abstraction for
  computer scientists
• Matrix algebra just isnt intuitive enough
• Difficult to abstract
• 2n states for n bits
• entanglement

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A Classical Representation of Quantum Circuits
Example Quantum Teleportation
H
X
Z
H
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Critic

• Concise
• Familiar
• Classical decisions are explicit
• - Super-position is hidden
• - Entanglement is hidden

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Alternative Representation
H
H
X
C
C
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Critic

• - Not very concise (exponential!)
• - Not very familiar (where are the qubits?)
• - Classical decisions are implicit
• Super-position is exposed
• Entanglement is exposed

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Ideal Programming Abstraction

• Concise
• Familiar within reason
• Integrates Classical/Quantum
• Exposes super-position and entanglement

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Conclude

• Choose your area of interest and there is work to
  do
• Design rules / cell development
• Architecture abstractions
• Classical-Quantum interfacing
• Programming languages

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Notes / Graduate course

• http//www.cs.washington.edu homes/oskin/quantum-
  tutorial
• Notes based on book by Michael Nielsen and Isaac
  Chuang (with some info from John Preskill)
• Graduate course w/UGs on request
• Geared for computer scientists
• Begins with linear algebra review
• Ends with error correction
• Sequence of programming assignments in QCL

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QARC Project

• Quantum Architecture project
• Isaac Chuang, MIT
• Fred Chong, UC Davis
• John Kubiatowicz, UC Berkeley
• Mark Oskin, UW