Quantum Quantum Architectures Architectures June 1, 2005 June 1, 2005
Computing? Computing? e of computing devices that exhibit quantu e of computing devices that exhibit quantu chanical behavior chanical behavior ehavior of isolated ions ehavior of isolated ions Bose- -Einstein condensate in a magnetic well Einstein condensate in a magnetic well Bose hoton interactions hoton interactions ire and transistor interactions in the near future ire and transistor interactions in the near future computing model is largely unexplored e computing model is largely unexplored ⊆ ? QP ⊆ ? NP ⊆ ? QP ⊆ ? NP usage model is still being debated e usage model is still being debated
hy are Architects Involved hy are Architects Involved e know what the “killer app” will be e know what the “killer app” will be Error correction will be >99% of the work Error correction will be >99% of the work e physicists don’t know computation e physicists don’t know computation “Don’t worry, it’s polynomial...” “Don’t worry, it’s polynomial...” e theorists don’t know physics e theorists don’t know physics “Simplify the problem by removing communication...” “Simplify the problem by removing communication...” short, architects can be the reality check short, architects can be the reality check Identify physical bounds that supersede theoretical on Identify physical bounds that supersede theoretical on Determine what aspects of computation will be the mo Determine what aspects of computation will be the m challenging challenging
Outline Outline hat makes quantum different? hat makes quantum different? Quantum bits Quantum bits Operations and measurement Operations and measurement Decoherence Decoherence hat will a quantum computer look like hat will a quantum computer look like uantum architecture research at UW uantum architecture research at UW Current research: Simulation Current research: Simulation Building quantum wires Building quantum wires
Classical vs. Quantum Classical vs. Quantum sic element: 0 or 1 n Bits are continuous va Bits are continuous va sic element: 0 or 1 n ts are independent n Bits may be entangled Bits may be entangled ts are independent n interfere interfere n Data may not be copie Data may not be copie ata may be copied ata may be copied n destroyed at will n Operations must be Operations must be destroyed at will n reversible reversible n Data Data decoheres decoheres with t with t ata is static ata is static n
Qubits: Quantum Bits : Quantum Bits Qubits and qubits qubits both have two states: 0 an both have two states: 0 an and perposition: perposition: ubit may be in both states simultaneously may be in both states simultaneously ubit ase: ase: ubit may have a negative quantity of a st may have a negative quantity of a st ubit
uantum States and Measuremen uantum States and Measuremen e qubit qubit probabilistically represents two states probabilistically represents two states e a |0> + b |1> a |0> + b |1> ch additional quibit quibit doubles the number of sta doubles the number of sta ch additional a |00> + b |01> +c |10> + d |11> a |00> + b |01> +c |10> + d |11> easurement sends a qubit qubit into a classical stat into a classical stat easurement sends a is may alter the states of other qubits qubits is may alter the states of other a |00> + b |11> (EPR state) a |00> + b |11> (EPR state)
easurement and Copy Protectio easurement and Copy Protectio uantum data cannot be copied uantum data cannot be copied Copying involves a read and a write Copying involves a read and a write “Reading” destroys the state “Reading” destroys the state uantum data can be transferred uantum data can be transferred One qubit qubit can swap its state with another can swap its state with another One Quantum state can be “teleported” over infinite Quantum state can be “teleported” over infinite distance (but the sender loses the data) distance (but the sender loses the data) any quantum algorithms are probabilistic any quantum algorithms are probabilistic d involve iteration d involve iteration
Other Operations Other Operations easurement: the only irreversible operati easurement: the only irreversible operati l other operations are reversible l other operations are reversible 2nd Law: Reversible operations conserve ener 2nd Law: Reversible operations conserve ener “Not” is reversible “Not” is reversible “And” and “Or” are not reversible “And” and “Or” are not reversible ost “traditional” operations must produce ost “traditional” operations must produce ditional output ditional output How do you make “+” reversible? How do you make “+” reversible? Scratch bits must be protected Scratch bits must be protected
Noise: Decoherence Decoherence Noise: eoretical systems are “closed” eoretical systems are “closed” No energy may enter the system unless explicitly No energy may enter the system unless explicitly introduced introduced al systems can’t be completely isolated al systems can’t be completely isolated Performing an op adds “noise” to a qubit qubit Performing an op adds “noise” to a Over time, qubits qubits will simply “ will simply “decohere decohere” ” Over time, At higher temperatures (higher energy), decoherence decoherence At higher temperatures (higher energy), occurs more quickly occurs more quickly e will need massive cooling systems e will need massive cooling systems y usable quantum system will require error y usable quantum system will require error
rror Correction is Crucial rror Correction is Crucial ror correcting codes are available ror correcting codes are available Operations exist for computing on encoded data Operations exist for computing on encoded dat Since qubits qubits cannot be read, an error correction cannot be read, an error correction Since routine manipulates the coded bits to fix errors routine manipulates the coded bits to fix errors ror correction must often be applied ror correction must often be applied cursively to error correction routines cursively to error correction routines e Threshold Theorem e Threshold Theorem 4 per op can be tolerated -4 An error rate of 10 - per op can be tolerated An error rate of 10 This error rate requires nearly continuous error This error rate requires nearly continuous error correction correction
To Summarize... To Summarize... uantum bits are awesome uantum bits are awesome ...except that no one has really done anything ...except that no one has really done anything amazing with them (yet) amazing with them (yet) …because measurement really hurts …because measurement really hurts oring and moving data will be difficult oring and moving data will be difficult Decoherence limits storage and transit time limits storage and transit time Decoherence e know what quantum computers will do e know what quantum computers will do Error correcting...all the time Error correcting...all the time rforming computation in the presence of rforming computation in the presence of
Outline Outline hat makes quantum different? hat makes quantum different? Quantum bits Quantum bits Operations and measurement Operations and measurement Decoherence Decoherence hat will a quantum computer look like hat will a quantum computer look like uantum architecture research at UW uantum architecture research at UW Current research: Simulation Current research: Simulation Building quantum wires Building quantum wires
Pick a Technology Pick a Technology hat device technology will be used? hat device technology will be used? Who knows... Who knows... evelop first order assumptions evelop first order assumptions Classical control of quantum gates Classical control of quantum gates Silicon to interface and control Silicon to interface and control n Provides rough size constraints Provides rough size constraints n Individual control of quantum bits Individual control of quantum bits ck a likely technology that fits ck a likely technology that fits For example, ion traps For example, ion traps
onsider Building Blocks Consider Building Blocks Processor: Computation Processor: Computation Several sets of universal gates exist Several sets of universal gates exist Different device technologies can more easily implem Different device technologies can more easily implem some gates than others some gates than others Measurement and “zero” creation is also important Measurement and “zero” creation is also important emory: Storage emory: Storage Storage is difficult because of decoherence decoherence Storage is difficult because of Constant error correction may be performed Constant error correction may be performed n Decoherence Decoherence- -free subspaces are being researched free subspaces are being researched n Hence, memory looks a lot like the processor Hence, memory looks a lot like the processor Interconnect: Communication Interconnect: Communication
reger- -Stickles and Stickles and Balensiefer Balensiefer reger e computer is a grid of traps and wires e computer is a grid of traps and wires ch trap has a flexible gate that can perform ch trap has a flexible gate that can perform easurement or a quantum op easurement or a quantum op mmunication is performed by moving ions mmunication is performed by moving ions
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