Quantum Computing Are We There Yet? Emil Khabiboulline ekhabibo@caltech.edu Ph 070: Popular Presentation 05/10/2016 @ Caltech

Speedup of 100,000,000! Announced by Google in December:

A Working Quantum Computer? There certainly is hype… And all the key players are starting to notice

Outline The Power of Quantum Great. But Can We Do It? Ultracold (Atoms) Ultracold (Wires) What’s Next?

The Power of Quantum Moore’s Law Not bits, qubits An example: Deutsch’s Problem How to break into your bank Other applications (and limitations) The Power of Quantum

Moore’s Law Doubling in computational power every 1.5 years, over the past 50 years

Not bits, qubits Quantum computers also scale exponentially, but in number of qubits, not years What is a quantum bit? =

An example: Deutsch’s Problem You have some function that takes in 0 or 1 and spits out 0 or 1 You want to know 𝑓 0 ≟𝑓(1)

An example: Deutsch’s Problem Classically: Compute 𝑓 0 Compute 𝑓 1 Compare 𝑓 0 with 𝑓 1 Requires 2 computations

An example: Deutsch’s Problem Quantumly: Compute directly 𝑓 0 ≟𝑓(1) Requires 1 computation However, we don’t get to know 𝑓 0 or 𝑓 1 Not magic, just clever use of superposition

An example: Deutsch’s Problem

How to break into your bank RSA encryption works because factoring large numbers is really hard: exponential scaling For quantum computers, the scaling is polynomial An example: 300 digit number Classical: >100 years Quantum: 1 minute

Other applications (and limitations) Two major applications: Generic search / optimization Quantum simulation There are tasks where quantum computers are slower Will not replace your laptop for word processing, browsing the web, etc.

Cats: Alive and dead What do we need? Great. But Can We Do It?

Cats: Alive and dead Superpositions are used all the time in quantum computation: Schrodinger cat state |0>+|1>, True and false, However, we never see Schrodinger cats in our lives

What do we need? Interaction with environment leads to “decoherence” We need exotic conditions: Very clean for isolation Very cold for quantum-ness

Ultracold (Atoms) Translating math to physics “Atom-like” systems An example: Trapped ions Ultracold (Atoms)

Translating math to physics Qubit = vector → physical observable (e.g., energy) that can be in two states Quantum gate = matrix → physical process that acts in different ways depending on the qubit’s state (e.g., electric field) Measurement = dot product → physical measurement (e.g., photon detection)

“Atom-like” systems Broad category: Atoms Ions Quantum dots Defects in diamond

An example: Trapped ions Harty et al., 2014

Superconducting qubits Google’s quantum computer Ultracold (Wires)

Superconducting qubits Superconducting circuit consisting of inductor and capacitor Advantages: easy fabrication and handling Ladd et al., 2010

Google’s quantum computer Google is now using the D-Wave 2X quantum annealer: 1000+ qubits but not a real quantum computer Already has the Quantum AI Lab Recently hired John Martinis, a leading expert

Topological quantum computing Are we there yet? What’s Next?

Topological quantum computing “Topological protection” from noise and decoherence Quantum computing by “braiding” particles No physical examples yet… but wait for superconducting wires

Maybe But the question is no longer if?, but when? Are we there yet? Maybe But the question is no longer if?, but when?