Site

class tenpy.networks.site.Site(leg, state_labels=None, **site_ops)[source]

Bases: object

Collects necessary information about a single local site of a lattice.

This class defines what the local basis states are: it provides the leg defining the charges of the physical leg for this site. Moreover, it stores (local) on-site operators, which are directly available as attribute, e.g., self.Sz is the Sz operator for the SpinSite. Alternatively, operators can be obained with get_op(). The operator names Id and JW are reserved for the identy and Jordan-Wigner strings.

Warning

The order of the local basis can change depending on the charge conservation! This is a necessary feature since we need to sort the basis by charges for efficiency. We use the state_labels and perm to keep track of these permutations.

Parameters
legLegCharge

Charges of the physical states, to be used for the physical leg of MPS.

state_labelsNone | list of str

Optionally a label for each local basis states. None entries are ignored / not set.

**site_ops :

Additional keyword arguments of the form name=op given to add_op(). The identity operator 'Id' is automatically included. If no 'JW' for the Jordan-Wigner string is given, 'JW' is set as an alias to 'Id'.

Examples

The following generates a site for spin-1/2 with Sz conservation. Note that Sx = (Sp + Sm)/2 violates Sz conservation and is thus not a valid on-site operator.

>>> chinfo = npc.ChargeInfo([1], ['Sz'])
>>> ch = npc.LegCharge.from_qflat(chinfo, [1, -1])
>>> Sp = [[0, 1.], [0, 0]]
>>> Sm = [[0, 0], [1., 0]]
>>> Sz = [[0.5, 0], [0, -0.5]]
>>> site = Site(ch, ['up', 'down'], Splus=Sp, Sminus=Sm, Sz=Sz)
>>> print(site.Splus.to_ndarray())
array([[ 0.,  1.],
       [ 0.,  0.]])
>>> print(site.get_op('Sminus').to_ndarray())
array([[ 0.,  0.],
       [ 1.,  0.]])
>>> print(site.get_op('Splus Sminus').to_ndarray())
array([[ 1.,  0.],
       [ 0.,  0.]])
Attributes
dim

Dimension of the local Hilbert space.

onsite_ops

Dictionary of on-site operators for iteration.

legLegCharge

Charges of the local basis states.

state_labels{str: int}

(Optional) labels for the local basis states.

opnamesset

Labels of all onsite operators (i.e. self.op exists if 'op' in self.opnames). Note that get_op() allows arbitrary concatenations of them.

need_JW_stringset

Labels of all onsite operators that need a Jordan-Wigner string. Used in op_needs_JW() to determine whether an operator anticommutes or commutes with operators on other sites.

opsArray

Onsite operators are added directly as attributes to self. For example after self.add_op('Sz', Sz) you can use self.Sz for the Sz operator. All onsite operators have labels 'p', 'p*'.

perm1D array

Index permutation of the physical leg compared to conserve=None, i.e. OP_conserved = OP_nonconserved[np.ix_(perm,perm)] and perm[state_labels_conserved["some_state"]] == state_labels_nonconserved["some_state"].

JW_exponent1D array

Exponents of the 'JW' operator, such that self.JW.to_ndarray() = np.diag(np.exp(1.j*np.pi* JW_exponent))

Methods

add_op(self, name, op[, need_JW])

Add one on-site operators.

change_charge(self[, new_leg_charge, permute])

Change the charges of the site (in place).

get_op(self, name)

Return operator of given name.

multiply_op_names(self, names)

Multiply operator names together.

op_needs_JW(self, name)

Whether an (composite) onsite operator is fermionic and needs a Jordan-Wigner string.

remove_op(self, name)

Remove an added operator.

rename_op(self, old_name, new_name)

Rename an added operator.

state_index(self, label)

Return index of a basis state from its label.

state_indices(self, labels)

Same as state_index(), but for multiple labels.

test_sanity(self)

Sanity check, raises ValueErrors, if something is wrong.

valid_opname(self, name)

Check whether ‘name’ labels a valid onsite-operator.
change_charge(self, new_leg_charge=None, permute=None)[source]

Change the charges of the site (in place).

Parameters
new_leg_chargeLegCharge | None

The new charges to be used. If None, use trivial charges.

permutendarray | None

The permuation applied to the physical leg, which gets used to adjust state_labels and perm. If you sorted the previous leg with perm_qind, new_leg_charge = leg.sort(), use leg.perm_flat_from_perm_qind(perm_qind). Ignored if None.

test_sanity(self)[source]

Sanity check, raises ValueErrors, if something is wrong.

property dim

Dimension of the local Hilbert space.

property onsite_ops

Dictionary of on-site operators for iteration.

Single operators are accessible as attributes.

add_op(self, name, op, need_JW=False)[source]

Add one on-site operators.

Parameters
namestr

A valid python variable name, used to label the operator. The name under which op is added as attribute to self.

opnp.ndarray | Array

A matrix acting on the local hilbert space representing the local operator. Dense numpy arrays are automatically converted to Array. LegCharges have to be [leg, leg.conj()]. We set labels 'p', 'p*'.

need_JWbool

Whether the operator needs a Jordan-Wigner string. If True, the function adds name to need_JW_string.

rename_op(self, old_name, new_name)[source]

Rename an added operator.

Parameters
old_namestr

The old name of the operator.

new_namestr

The new name of the operator.

remove_op(self, name)[source]

Remove an added operator.

Parameters
namestr

The name of the operator to be removed.

state_index(self, label)[source]

Return index of a basis state from its label.

Parameters
labelint | string

eather the index directly or a label (string) set before.

Returns
state_indexint

the index of the basis state associated with the label.

state_indices(self, labels)[source]

Same as state_index(), but for multiple labels.

get_op(self, name)[source]

Return operator of given name.

Parameters
namestr

The name of the operator to be returned. In case of multiple operator names separated by whitespace, we multiply them together to a single on-site operator (with the one on the right acting first).

Returns
opnp_conserved

The operator given by name, with labels 'p', 'p*'. If name already was an npc Array, it’s directly returned.

op_needs_JW(self, name)[source]

Whether an (composite) onsite operator is fermionic and needs a Jordan-Wigner string.

Parameters
namestr

The name of the operator, as in get_op().

Returns
needs_JWbool

Whether the operator needs a Jordan-Wigner string, judging from need_JW_string.

valid_opname(self, name)[source]

Check whether ‘name’ labels a valid onsite-operator.

Parameters
namestr

Label for the operator. Can be multiple operator(labels) separated by whitespace, indicating that they should be multiplied together.

Returns
validbool

True if name is a valid argument to get_op().

multiply_op_names(self, names)[source]

Multiply operator names together.

Join the operator names in names such that get_op returns the product of the corresponding operators.

Parameters
nameslist of str

List of valid operator labels.

Returns
combined_opnamestr

A valid operator name Operatorname representing the product of operators in names.