# Batching simulations

Batching allows **running many independent simulations concurrently**. It can dramatically speedup simulations, especially on GPUs. In this tutorial, we explain how to batch quantum simulations in dynamiqs.

```
import dynamiqs as dq
import jax.numpy as jnp
import timeit
```

## Batching in short

Batching in dynamiqs is achieved by **passing a list of Hamiltonians, initial states, or jump operators** to the simulation functions. The result of a batched simulation is a single array that contains all the individual simulations results. For example, let's simulate the Schrödinger equation for all combinations of the three Hamiltonians \(\{\sigma_x, \sigma_y, \sigma_z\}\) and the four initial states \(\{\ket{g}, \ket{e}, \ket{+}, \ket{-}\}\):

```
# define three Hamiltonians
H = [dq.sigmax(), dq.sigmay(), dq.sigmaz()] # (3, 2, 2)
# define four initial states
g = dq.basis(2, 0)
e = dq.basis(2, 1)
plus = dq.unit(g + e)
minus = dq.unit(g - e)
psi = [g, e, plus, minus] # (4, 2, 1)
# run the simulation
tsave = jnp.linspace(0.0, 1.0, 11) # (11,)
result = dq.sesolve(H, psi, tsave)
print(f'Shape of result.states: {result.states.shape}')
```

```
Shape of result.states: (3, 4, 11, 2, 1)
```

The returned states is an array with shape *(3, 4, 11, 2, 1)*, where *3* is the number of Hamiltonians, *4* is the number of initial states, *11* is the number of saved states, and *(2, 1)* is the shape of a single state.

Note

All relevant `result`

attributes are batched. For example if you specified `exp_ops`

, the resulting expectation values `result.expects`

will be an array with shape *(3, 4, len(exp_ops), 11)*.

Importantly, **batched simulations are not run sequentially in a for loop**. What is meant by

*batching*is that instead of evolving from initial to final time a single state with shape

*(2, 1)*for each combination of argument, the whole batched state

*(3, 4, 2, 1)*is evolved

**once**from initial to final time, which is much more efficient.

## Batching modes

There are two ways to batch simulations in dynamiqs: **cartesian batching** and **flat batching**.

### Cartesian batching

The simulation runs for all possible combinations of Hamiltonians, jump operators and initial states. This is the default mode.

For `dq.sesolve`

, the returned array has shape:

```
result.states.shape = (...H, ...psi0, ntsave, n, 1)
```

`...x`

indicates the batching shape of `x`

, i.e. its shape without the last two dimensions.
Example: Cartesian batching with `dq.sesolve`

`H`

has shape*(2, 3, n, n)*,`psi0`

has shape*(4, n, 1)*,

then `result.states`

has shape *(2, 3, 4, ntsave, n, 1)*.

For `dq.mesolve`

, the returned array has shape:

```
result.states.shape = (...H, ...L0, ...L1, (...), ...rho0, ntsave, n, n)
```

`...x`

indicates the batching shape of `x`

, i.e. its shape without the last two dimensions.
Example: Cartesian batching with `dq.mesolve`

`H`

has shape*(2, 3, n, n)*,`jump_ops = [L0, L1]`

has shape*[(4, 5, n, n), (6, n, n)]*,`rho0`

has shape*(7, n, n)*,

then `result.states`

has shape *(2, 3, 4, 5, 6, 7, ntsave, n, n)*.

### Flat batching

The simulation runs for each set of Hamiltonians, jump operators and initial states using broadcasting. This mode can be activated by setting `cartesian_batching=False`

in `dq.Options`

. In particular for `dq.mesolve()`

, each jump operator can be batched independently from the others.

## What is broadcasting?

JAX and NumPy broadcasting semantics are very powerful and allow you to write concise and efficient code. For more information, see the NumPy documentation on broadcasting.

For `dq.sesolve`

, the returned array has shape:

```
result.states.shape = (..., ntsave, n, 1)
```

`... = jnp.broadcast_shapes(H, psi0)`

is the broadcasted shape of all arguments.
Example: Flat batching with `dq.sesolve`

`H`

has shape*(2, 3, n, n)*,`psi0`

has shape*(3, n, 1)*,

then `result.states`

has shape *(2, 3, ntsave, n, 1)*.

For `dq.mesolve`

, the returned array has shape:

```
result.states.shape = (..., ntsave, n, n)
```

`... = jnp.broadcast_shapes(H, L0, L1, ..., rho0)`

is the broadcasted shape of all arguments.
Example: Flat batching with `dq.mesolve`

`H`

has shape*(2, 3, n, n)*,`jump_ops = [L0, L1]`

has shape*[(3, n, n), (2, 1, n, n)]*,`rho0`

has shape*(3, n, n)*,

then `result.states`

has shape *(2, 3, ntsave, n, n)*.

Note

Any batch shape is valid as input as long as it is broadcastable with other arguments.

For example for `dq.sesolve()`

with `H`

of shape *(2, 3, n, n)*, `psi0`

can be of shape: *(n, 1)*, *(3, n, 1)*, *(2, 1, n, 1)*, *(2, 3, n, 1)*, *(..., 2, 3, n, 1)*, etc. By playing with the arguments shape, you have complete freedom over the simulation you want to run.

## Creating batched arguments

### Single-dimensional batching

There are multiple ways to create a batched argument.

The most straightforward way is to pass a list of values:

```
# define several Hamiltonians
Hx, Hy, Hz = dq.sigmax(), dq.sigmay(), dq.sigmaz()
H = [Hx, Hy, Hz] # (3, 2, 2)
```

It is often useful to sweep a parameter:

```
# define several Hamiltonians
omega = jnp.linspace(0.0, 1.0, 21)
H = omega[:, None, None] * dq.sigmaz() # (21, 2, 2)
```

Or you can use dynamiqs utility functions directly:

```
# define several initial states
alpha = [1.0, 2.0, 3.0]
psis = dq.coherent(16, alpha) # (3, 16, 1)
```

### Multi-dimensional batching

The previous examples illustrate batching over one dimension, but you can batch over as many dimensions as you want:

```
# define several Hamiltonians
H = [
[Hx, 2 * Hx, 3 * Hx, 4 * Hx],
[Hy, 2 * Hy, 3 * Hy, 4 * Hy],
[Hz, 2 * Hz, 3 * Hz, 4 * Hz]
] # (3, 4, 2, 2)
```

```
# define several Hamiltonians
omega = jnp.linspace(0.0, 1.0, 21)[:, None, None, None]
eps = jnp.linspace(0.0, 10.0, 11)[:, None, None]
H = omega * dq.sigmaz() + eps * dq.sigmaz() # (21, 11, 2, 2)
```

```
# define several initial states
alpha_real = jnp.linspace(0, 1.0, 5)
alpha_imag = jnp.linspace(0, 1.0, 6)
alpha = alpha_real[:, None] + 1j * alpha_imag # (5, 6)
psis = dq.coherent(16, alpha) # (5, 6, 16, 1)
```

### Batching over a TimeArray

We have seen how to batch over time-independent objects, but how about time-dependent ones? It's essentialy the same, you have to pass a batched `TimeArray`

, in short:

The batching of the returned time-array is specified by `values`

. For example, to define a PWC operator batched over a parameter \(\theta\):

```
>>> thetas = jnp.linspace(0.0, 1.0, 11) # (11,)
>>> times = [0.0, 1.0, 2.0]
>>> values = thetas[:, None] * jnp.array([3.0, -2.0]) # (11, 2)
>>> array = dq.sigmaz()
>>> H = dq.pwc(times, values, array)
>>> H.shape
(11, 2, 2)
```

The batching of the returned time-array is specified by the array returned by `f`

. For example, to define a modulated Hamiltonian \(H(t)=\cos(\omega t)\sigma_x\) batched over the parameter \(\omega\):

```
>>> omegas = jnp.linspace(0.0, 1.0, 11) # (11,)
>>> f = lambda t: jnp.cos(omegas * t)
>>> H = dq.modulated(f, dq.sigmax())
>>> H.shape
(11, 2, 2)
```

The batching of the returned time-array is specified by the array returned by `f`

. For example, to define an arbitrary time-dependent operator batched over a parameter \(\theta\):

```
>>> thetas = jnp.linspace(0.0, 1.0, 11) # (11,)
>>> f = lambda t: thetas[:, None, None] * jnp.array([[t, 0], [0, 1 - t]])
>>> H = dq.timecallable(f)
>>> H.shape
(11, 2, 2)
```

## Why batching?

When batching multiple simulations, the state is not a 2-D array that evolves in time but a N-D array which holds all independent simulations. This allows running **multiple simulations simultaneously** with great efficiency, especially on GPUs. Moreover, it usually simplifies the simulation code and also the subsequent analysis of the results, because they are all gathered in a single large array.

Common use cases for batching include:

- simulating a system with different values of a parameter (e.g. a drive amplitude),
- simulating a system with different initial states (e.g. for gate tomography),
- performing an optimisation using multiple starting points with random initial guesses (for parameters fitting or quantum optimal control).

### Quick benchmark

To illustrate the performance gain of batching, let us compare the total run time between using a for loop vs using a batched simulation. We will simulate a set of 3,000 Hamiltonians on a two-level system:

```
# define 3000 Hamiltonians
omega = jnp.linspace(0.0, 10.0, 100)[:, None, None]
epsilon = jnp.linspace(0.0, 1.0, 30)[:, None, None, None]
H = omega * dq.sigmaz() + epsilon * dq.sigmax() # (100, 30, 2, 2)
# other simulation parameters
psi0 = dq.basis(2, 0)
tsave = jnp.linspace(0.0, 1.0, 50)
options = dq.Options(progress_meter=None)
# running the simulations successively
def run_unbatched():
results = []
for i in range(len(omega)):
for j in range(len(epsilon)):
result = dq.sesolve(H[i, j], psi0, tsave, options=options)
results.append(result)
return results
# running the simulations simultaneously
def run_batched():
return dq.sesolve(H, psi0, tsave, options=options)
# time functions
%timeit run_unbatched()
%timeit run_batched()
```

```
2.59 s ± 52.8 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
44.1 ms ± 2.66 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)
```

As we can see, the batched simulation is **much faster** than the unbatched one. In this simple example, it is about 60 times faster. The gain in performance will be even more significant for larger simulations, or when using a GPU.