Programming a quantum computer with Cirq (QuantumCasts)

Key Takeaways

a quantum computer is a new type of computer that stores an axon information in its quantum form today we are entering into an exciting era where quantum computers are beginning to be large enough and performant enough to execute tasks in less than a second that would take years to execute on a normal computer here is google's own Bristlecone chip this is a quantum computer made of superconducting circuits with 72 quantum bits google's researchers are using this chip to attempt to achieve a task that cannot be solved in years on a supercomputer hi I'm Dave bacon from Google I run the team that builds software for Google's kuan computers and today I'd like to tell you a little bit about how we program Google's quantum computers we do this using an open source framework called circ a quantum computer stores its information using quantum bits or qubits the information in these qubits is then maneuvered around using the laws of quantum physics describe an algorithm on a quantum computer you use what is called the quantum circuit model the quantum circuit model is essentially a diagram describing how to perform a quantum computation here is an example of a quantum circuit you read this diagram like a sheet of music from left to right each of the qubits in a quantum computer corresponds to a single horizontal wire in the quantum circuit here we see this quantum circuit operates on four cubits the boxes in the diagram correspond to quantum gates that are applied to one or more qubits depending on how many wires the box is connected to here's a single qubit gate and here is a two qubit game quantum gates are instructions to send control signals to the quantum computer to perform a certain quantum action on the qubits finally one has instructions for reading out the quantum information this corresponds to quantum gates that perform measurements and turn quantum bits into classical bit if we have a quantum circuit diagram we can use this diagram to send microwave pulses and instructions to our quantum computing hardware to execute the quantum gates and then read out the result of the circuit like reading a sheet of music as we sweep from left to right we play the given gates as they appear the quantum circuit diagram is cool-looking but of course if we had to draw a diagram to do this and we are writing really long quantum programs this would quickly become very challenging to do to solve this problem researchers developed frameworks or programming languages to write more traditional looking programs that represent the quantum circuit an open source framework that we use that Google for this effort is called Cirque welcome to the surface let's write a simple circuit Cirque is a Python framework this means you can use all the goodness of Python in helping to write your quantum program the central object in Cirque is a circuit object here we show creating a circuit object another key set of objects are qubits let's define two qubits with simple names now we can perform some quantum gates on these qubits let's apply a single qubit Hadamard gate denoted by h2 one of the qubits followed by a two qubit controlled-not or c not gate between the qubits finally let's measure the quantum bits what circuit have we produced simply use print to see the circuit no it's not 1993 but ASCII diagrams are useful for seeing the quantum circuit that you've built here we see the H gate for a Hadamard followed by the controlled night date with the ampersand symbol and the X symbol and finally the measurements represented by M once we've constructed a quantum circuit what do we do with it insert right now you can perform a simulation of the circuit here we run the circuit ten times and see the measurement results measurement results in quantum computers don't always give the same values of bits one run of the circuit may result in the output being zero and another and it being one here we see that the measurement results differ for each run of the simulation circ also contains an interface for running the quantum circuit against actual quantum hardware now that you've seen a simple quantum circuit in circ you might think great now I can write really large quantum programs for example it is known that quantum computers can efficiently factor numbers something that breaks modern public key cryptography that's pretty scary today's quantum computers however are very far from being able to perform this task this is because essentially quantum computers could only perform so much quantum computation before the quantum computation falls apart consider again a quantum circuit every date that you apply in this circuit corresponds to some pretty complicated electronics shaping and setting of electromagnetic fields to the quantum computer these pulses are not always perfect and so every single one of the gates you perform has some effective chance of failing boom one of our single qubit gates has failed in addition to not being able to execute gates exactly quantum computers also have a problem just doing nothing that is if you leave quantum bits around over time the quantum information stored in them will decay away we call this process decoherence boom while waiting to execute the next gate one of our qubits has failed due to the coherence today's quantum computers don't perform exactly as we specify them in the quantum circuit model we call this the problem of noise and quantum computers because of noise the size of our quantum computation is limited for today's quantum computers it is limited both in the number of qubits and the number of operations we can perform on these qubits if quantum computers are noisy can we ever build a really large quantum computer the answer to this is yes using a magic protocol called quantum air correction we won't focus on air correction here but it is a procedure for turning a bunch of noisy qubits into a fewer number of much less noisy cubits since today's quantum computers cannot perform arbitrary large or long quantum computation an important question is what can they do this is the main question of what people call the nist era this stands for noisy intermediate scale quantum and it is used to distinguish today's quantum computers from future err corrected quantum computers are their algorithms a practical value in the nist era we do not know the answer to this question on the other hand we also know that we are starting to build quantum computers which exceed the capabilities of classical computers the so called supremacy frontier because of this there's tremendous excitement in quantum computing we are entering into the unknown an era where there is potential for important discovery we built circ for NIST computers and to aid in this discovery because Sirk is focused on nist computers and not on quantum air corrector computers we made some choices that we believe are important for these near-term devices as an example one choice we made is that we believe that the programmer who is coding Furness calgary limbs needs to be very aware of the idiosyncrasies of the Hardware upon which the quantum computation is run hardware is not abstracted away insert insert this is captured by device objects here is the device object for our Bristlecone device printed out gives a representation of the layout of the qubits on the device we see that it is a strange grid of qubits the qubits are represented by pluses the lines between the qubits represents the fact that only adjacent qubits can be subjected to a two qubit game for example we can perform a two qubit eight between these qubits but not these which are too far apart to directly interact another subtlety of the Bissel cone device is that there are important constraints on when you can simultaneously perform two qubit gates if you apply a two qubit gate to two adjacent qubits then you cannot simultaneously apply a two qubit gate to any of the adjacent qubits of this two qubit gate because the hardware isn't abstracted away in circ we can use the device objects directly when building our quantum program to enforce these constraints for example here we try to perform to two qubit Z gates at the same time on adjacent qubits but because we had passed in the device object to the circuit it is aware of the Bristlecone constraint and when we print out the circuit we see that the circuit has correctly moved one of the CZs to a later time slice in order to avoid violating the constraint Cirque is an open source project licensed under the Apache 2 license if you want to install the latest release of Cirque you can simply run pip install circ in most properly configured environments for running Python circ is an alpha release that is it is under constant and active change we welcome contributions to do this you can go to Turks github repo and follow the instructions for contributing the github repo it also contains links to further documentation for Cirque I'm excited by the next few years of quantum computers are there Ness algorithms that can perform problems of practical importance circ is a tool we are using to help explore this exciting question we hope that you've enjoyed this brief introduction to programming a NIST computer for a more information about Google's efforts in quantum computing I encourage you to check out Google's quantum page [Music]