Classical Control in Quantum Programs

1. Classical Control inQuantum Programs Dominique Unruh IAKS, Universität Karlsruhe Founded by the European Project ProSecCo IST-2001-39227

2. Design Goals • Quantum programming language We present a method of modelling a quantum programming language in a way that should allow to write quantum programs in a formal way, without loosing the similarity to pseudo-code which is essential for an intuitive approach to algorithms. • Terminating and non-terminating Our modelling shall support the design of terminating as well of non-terminating programs within the same mathematical framework. • Mixed classical and quantum operations We want to be able to represent both pure quantum (unitary) as well as classical operations (measurements, erasure) using the same framework. That is, we do not impose a strict separation between a classical controlling computer and the controlled quantum registers. • Output during the program’s run We show how to model programs which give classical (i.e. measured) output during the program’s run. This is especially useful, if the programs in consideration do not terminate but give a continuous stream of results, since the usual run-and-then-measure-after-termination approach is not sensible in that case. • Physical interpretation A programs behaviour can be modelled as a measurement process (with defined resulting state in case of termination). In order to avoid artificial constructions we adhere to these operational semantics and show how to transform any program into the mathematical description of a measurement process.

3. Operational Semantics • A program is a physical experiment The process of running a program is very much comparable to performing a physical experiment: At the beginning, the machine is in a given initial state. Then this state is modified by performing various operations. These may include measurements during the experiment (outputs of the program). The experiment possibly terminates (real life experiments always do, due to limited resources, programs not necessarily). If the program terminates, the machine afterwards is in a well-defined post-measurement state. • A physical experiment is a measurement Any physical experiment can be seen as a measurement operation. We distinguish two kinds: a measurement operation defining the post-measurement state (called general measurement or PMVM), and a measurement not defining the post-measurement state (a POVM). To describe measurements which may or may not have a post-measurement state, we need to use the convex combination of a general measurement and a PMVM. This combination we will call a mixed measurement. • A program is a measurement Since we can consider a program as an experiment, and an experiment as a mixed measurement, it is natural to model a program as a mixed measurement. The outputs of the program are measurement results and are finite or infinite sequences over an alphabet Σ. This gives rise to operational semantics of programs, which are simply mathematical descriptions of measurements. • Classical, measurement, unitary operations The mixed measurement formalism is strong enough to have as special cases classical (probabilistic) programs, orthogonal measurements, and unitary operations, thus capturing all kinds of operations in a single mathematical framework.

4. Composing programs • Sequentially execute several programs If two programs P and Q are defined, the composed program P;Q can be interpreted as first executing P (yielding some output), and then—if P terminated—executing Q on the post-execution state of P. The output of the composed program is then given by the concatenation of the respective inputs.

5. Print-Command • Any outputs can be build up from constant outputs By defining an elementary program print x, one can build up any output by using switch (see below). E.g. switch (M as m) print m outputs the result of measurement M. • Constant outputs are constant measurements The program print x outputting x is simply a measurement with constant outcome x and which does not modify the programs state.

6. Branching (if/switch) • Branching as classical control In our modelling we consider branching which is conditional upon the outcomes of measurements. This is opposed to controlled unitary transformation (e.g. CNOT), where the controlling qubit is not measured. More complex quantum algorithms usually have such classical control, be it an overall loop repeating the experiment until a useful result is obtained (e.g. a non-trivial factor in Shor’s factoring algorithm). • Branching programs are composed measurements A program if (M) P can be interpreted as the experiment measuring M and then executing the program P if M yielded the outcome true. This experiment can be expressed as a measurement in a straightforward manner: if Mi denotes the superoperator describing the post-measurement state on outcome i, then if (M) P is simply defined as PMtrue+Mfalse. Analogously we define if (M) P else Q. • More powerful: the switch-statement An if-statement can only distinguish between two cases: true and false. In many cases a more powerful statement, the switch statement. switch (M as m) P(m) measures M and then executes the program P(m) where m is the outcome of the measurement.

7. Loops • Informal description simple The program while (M) Pdenotes the experiment of repeatedly measuring M and if the outcome is true then executing P. If the outcome is false, abort. • Mathematical description turns out to be difficult Two approaches immediately come to mind for defining while. The first would be to define while as a least fixed point. However due to the possibility of outputs there is no natural lattice structure. The second would be to define while as the limit of if(M){P;if(M){P;...}}. However, there is no natural topology giving the desired results. • Axiomatic approach Since a direct limit/fixed point approach fails, we define while (M) Pby stating some properties the experiment above should fulfill. These axioms can informally be stated as follows: • The loop runs n times and terminates, iff M yields n times true and then false, and if P terminates n times. • The loop runs n times and does not terminate, iff M yields n times true, and if P terminates n–1 times and does not terminate the n-th time. • The loop runs an infinite number of times, iff always M yields true, and if P always terminates. • The probability that one of these events occurs is 1. It can be shown that there is exactly one program (i.e. mixed measurement) exists fulfilling these axioms.

8. Reasoning about programs • Conditions are sets of states Modelling a pre-/postcondition as a set of states (density operators) gives as the ability to express even complex conditions like “variable x contains an uniform random value” or “variables x and y are unentangled” or “variable x is in the computational basis”. • Pre-/postconditions If for any state ρ in M, the program P takes ρ to a state in N, we write {M} P {N} • Conditional equivalence of programs If two programs P, Q behave identically upon a given set M of initial states, we say they are equivalent on M, written {M} P = Q. • Programs can be conditioned on outputs If P is a program possibly having output a, we can define P|a as the program P restricted to having output a. Note that P|a is not probability preserving.