# [−][src]Crate qip

Quantum Computing library leveraging graph building to build efficient quantum circuit simulations. Rust is a great language for quantum computing with gate models because the borrow checker is very similar to the No-cloning theorem.

See all the examples in the examples directory of the Github repository.

# Example (CSWAP)

Here's an example of a small circuit where two groups of Registers are swapped conditioned on a third. This circuit is very small, only three operations plus a measurement, so the boilerplate can look quite large in comparison, but that setup provides the ability to construct circuits easily and safely when they do get larger.

```use qip::*;

// Make a new circuit builder.
let mut b = OpBuilder::new();

// Make three registers of sizes 1, 3, 3 (7 qubits total).
let q = b.qubit();  // Same as b.register(1)?;
let ra = b.register(3)?;
let rb = b.register(3)?;

// We will want to feed in some inputs later, hang on to the handles
// so we don't need to actually remember any indices.
let a_handle = ra.handle();
let b_handle = rb.handle();

// Define circuit
// First apply an H to q
// Then swap ra and rb, conditioned on q.
let (q, _, _) = b.cswap(q, ra, rb)?;
// Finally apply H to q again.

// Add a measurement to the first qubit, save a reference so we can get the result later.
let (q, m_handle) = b.measure(q);

// Now q is the end result of the above circuit, and we can run the circuit by referencing it.

// Make an initial state: |0,000,001> (default value for registers not mentioned is 0).
let initial_state = [a_handle.make_init_from_index(0b000)?,
b_handle.make_init_from_index(0b001)?];
// Run circuit with a given precision.
let (_, measured) = run_local_with_init::<f64>(&q, &initial_state)?;

// Lookup the result of the measurement we performed using the handle, and the probability
// of getting that measurement.
let (result, p) = measured.get_measurement(&m_handle).unwrap();

// Print the measured result
println!("Measured: {:?} (with chance {:?})", result, p);```

# The Program Macro

While the borrow checker included in rust is a wonderful tool for checking that our registers are behaving, it can be cumbersome. For that reason qip also includes a macro which provides an API similar to that which you would see in quantum computing textbooks

```use qip::*;

let n = 3;
let mut b = OpBuilder::new();
let ra = b.register(n)?;
let rb = b.register(n)?;

fn gamma(b: &mut dyn UnitaryBuilder, mut rs: Vec<Register>) -> Result<Vec<Register>, CircuitError> {
let rb = rs.pop().unwrap();
let ra = rs.pop().unwrap();
let (ra, rb) = b.cnot(ra, rb);
let (rb, ra) = b.cnot(rb, ra);
Ok(vec![ra, rb])
}

let (ra, rb) = program!(&mut b, ra, rb;
// Applies gamma to |ra[0] ra[1]>|ra[2]>
gamma ra[0..2], ra[2];
// Applies gamma to |ra[0] rb[0]>|ra[2]>
gamma |ra[0], rb[0],| ra[2];
// Applies gamma to |ra[0]>|rb[0] ra[2]>
gamma ra[0], |rb[0], ra[2],|;
// Applies gamma to |ra[0] ra[1]>|ra[2]> if rb == |111>
control gamma rb, ra[0..2], ra[2];
// Applies gamma to |ra[0] ra[1]>|ra[2]> if rb == |110> (rb[0] == |0>, rb[1] == 1, ...)
control(0b110) gamma rb, ra[0..2], ra[2];
)?;
let r = b.merge(vec![ra, rb])?;
```

To clean up gamma we can use the `wrap_fn` macro:

```use qip::*;

let n = 3;
let mut b = OpBuilder::new();
let ra = b.register(n)?;
let rb = b.register(n)?;

fn gamma(b: &mut dyn UnitaryBuilder, ra: Register, rb: Register) -> (Register, Register) {
let (ra, rb) = b.cnot(ra, rb);
let (rb, ra) = b.cnot(rb, ra);
(ra, rb)
}
// Make a function gamma_op from gamma which matches the spec required by program!(...).
// Here we tell wrap_fn! that gamma takes two registers, which we will internally call ra, rb.
wrap_fn!(gamma_op, gamma, ra, rb);
// if gamma returns a Result<(Register, Register), CircuitError>, write (gamma) instead.
// wrap_fn!(gamma_op, (gamma), ra, rb)

let (ra, rb) = program!(&mut b, ra, rb;
gamma_op ra[0..2], ra[2];
)?;
let r = b.merge(vec![ra, rb])?;
```

And with these wrapped functions, automatically produce their conjugates / inverses:

```use qip::*;

let n = 3;
let mut b = OpBuilder::new();
let ra = b.register(n)?;
let rb = b.register(n)?;

fn gamma(b: &mut dyn UnitaryBuilder, ra: Register, rb: Register) -> (Register, Register) {
let (ra, rb) = b.cnot(ra, rb);
let (rb, ra) = b.cnot(rb, ra);
(ra, rb)
}
wrap_fn!(gamma_op, gamma, ra, rb);
invert_fn!(inv_gamma_op, gamma_op);

// This program is equivalent to the identity (U^-1 U = I).
let (ra, rb) = program!(&mut b, ra, rb;
gamma_op ra, rb[2];
inv_gamma_op ra, rb[2];
)?;
```

## Re-exports

 `pub use self::builders::*;` `pub use self::common_circuits::*;` `pub use self::errors::*;` `pub use self::macros::*;` `pub use self::pipeline::run_local;` `pub use self::pipeline::run_local_with_init;` `pub use self::pipeline::run_with_state;` `pub use self::pipeline::QuantumState;` `pub use self::pipeline_debug::run_debug;` `pub use self::qubits::Register;` `pub use self::types::Precision;`

## Modules

 boolean_circuits Quantum analogues of boolean circuits builders Opbuilder and such common_circuits Common circuits for general usage. errors Error values for the library. feynman_state A state which favors memory in exchange for computation time. iterators Efficient iterators for sparse kronprod matrices. macros Macros for general ease of use. measurement_ops Functions for measuring states. pipeline Code for building pipelines. pipeline_debug Tools for displaying pipelines. qfft Quantum fourier transform support. qubits Basic classes for defining circuits/pipelines. sparse_state Sparse quantum states state_ops Functions for running ops on states. trace_state Tracing state types Commonly used types. unitary_decomposition Break unitary matrices into circuits. utils Commonly used short functions.

## Macros

 bridge Convert the iterator to par_iter if allowed into_iter Choose between into_iter and into_par_iter invert_fn Wrap a function to create a version compatible with `program!` as well as an inverse which is also compatible. iter Choose between iter and par_iter iter_mut Choose between iter_mut and par_iter_mut program A helper macro for applying functions to specific qubits in registers. sort_by_key Choose between sort_by_key and par_sort_by_key sort_unstable_by Choose between sort_by_key and par_sort_by_key wrap_and_invert Wrap a function to create a version compatible with `program!` as well as an inverse which is also compatible. wrap_fn Allows the wrapping of a function with signature: `Fn(&mut dyn UnitaryBuilder, Register, Register, ...) -> (Register, ...)` or `Fn(&mut dyn UnitaryBuilder, Register, Register, ...) -> Result<(Register, ...), CircuitError>` to make a new function with signature: `Fn(&mut dyn UnitaryBuilder, Vec) -> Result, CircuitError>` and is therefore compatible with `program!`.

## Structs

 Complex A complex number in Cartesian form.