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-- BOB Quantum Kernel β Single-Qubit Gate Application
-- Loop: do i = 0, state%dim - 1
-- Phase 2: Formal Structure (no proofs yet)
-- WORM-sealed observable bookkeeping
module Invariants.GateApplicationLoop where
open import Data.Nat using (β; _+_; _β€_; _<_; _β₯_; _β‘_; zero; suc)
open import Data.Real using (β; _+_; _*_; _-_; _<_; _β€_; _β₯_)
open import Data.Bool using (Bool; true; false)
open import Relation.Binary.PropositionalEquality using (_β‘_; refl; cong)
open import Core.ErrorCode using (ErrorCode; BOB_SUCCESS)
open import Core.QuantumState using (QuantumState; Dimension; isValidDim; canApplyGate)
open import Core.Predicates using (basisStateInRange; qubitIndexValid; dimensionPreserved)
open import Core.BitCounting using (gate_exit_pairs_count; pairs_updated_j_equals_i)
-- ============================================================================
-- Loop Context: Gate Application Setup
-- ============================================================================
-- Gate matrix (2Γ2)
record GateMatrix : Set where
field
entry_1_1 : β -- top-left
entry_1_2 : β -- top-right
entry_2_1 : β -- bottom-left
entry_2_2 : β -- bottom-right
is_unitary : Bool -- verified in apply_single_qubit_gate precondition
-- Immutable context throughout loop
record GateContext : Set where
field
state : QuantumState -- being mutated
gate : GateMatrix -- immutable operator
qubit_index : β -- target qubit (immutable)
num_qubits : β -- from state dimension
dim : β -- state%dim (2^num_qubits)
bit_mask : β -- precomputed: 1 << qubit_index
-- ============================================================================
-- Loop State at iteration i
-- ============================================================================
record GateLoopState : Set where
field
ctx : GateContext
i : β -- basis state iterator [0, dim)
-- Tracking updates
num_amplitudes_processed : β -- how many basis states examined
num_pairs_updated : β -- number of (state_0, state_1) pairs updated
-- Gate application bookkeeping
last_state_0_qubit_0 : Bool -- was last examined state's qubit bit 0?
error_status : β -- 0 = BOB_SUCCESS
-- ============================================================================
-- Loop Invariant: What holds at each iteration i?
-- ============================================================================
record GateInvariant (s : GateLoopState) (i : β) : Set where
field
-- 1. Loop iterator in valid range: 0 β€ i β€ dim
h_i_in_range : i β€ GateContext.dim (GateLoopState.ctx s)
-- 2. State remains valid
h_state_valid : isValidDim (GateContext.state (GateLoopState.ctx s))
-- 3. State can have gates applied
h_state_can_apply : canApplyGate (GateContext.state (GateLoopState.ctx s))
-- 4. Gate matrix is unitary (verified precondition)
h_gate_unitary : GateMatrix.is_unitary (GateContext.gate (GateLoopState.ctx s)) β‘ true
-- 5. Qubit index is valid
h_qubit_valid : qubitIndexValid (GateContext.qubit_index (GateLoopState.ctx s)) (GateContext.num_qubits (GateLoopState.ctx s))
-- 6. Number of basis states examined = i
h_states_examined : GateLoopState.num_amplitudes_processed s β‘ i
-- 7. For each processed basis state i where qubit bit is 0,
-- the (state_0, state_1) pair has been updated
h_pairs_updated :
β (j : β) β
j < i β
let qubit_bit = (j mod (2 * GateContext.bit_mask (GateLoopState.ctx s))) / GateContext.bit_mask (GateLoopState.ctx s)
in qubit_bit β‘ 0 β
-- Then: this pair was processed
(GateLoopState.num_pairs_updated s β₯ (j / (2 * GateContext.bit_mask (GateLoopState.ctx s))) + 1)
-- 8. Dimension preserved
h_dim_preserved : dimensionPreserved (GateContext.dim (GateLoopState.ctx s)) (β.logβ (GateContext.dim (GateLoopState.ctx s)))
-- 9. Gate context unchanged
h_ctx_immutable : β j β j < i β GateContext.gate (GateLoopState.ctx s) β‘ GateContext.gate (GateLoopState.ctx s)
-- 10. No errors
h_error_clear : GateLoopState.error_status s β‘ 0
-- ============================================================================
-- Base Case: i = 0 (before loop starts)
-- ============================================================================
gate_base :
(s : GateLoopState) β
GateLoopState.i s β‘ 0 β
isValidDim (GateContext.state (GateLoopState.ctx s)) β
canApplyGate (GateContext.state (GateLoopState.ctx s)) β
GateMatrix.is_unitary (GateContext.gate (GateLoopState.ctx s)) β‘ true β
qubitIndexValid (GateContext.qubit_index (GateLoopState.ctx s)) (GateContext.num_qubits (GateLoopState.ctx s)) β
GateLoopState.num_amplitudes_processed s β‘ 0 β
GateLoopState.num_pairs_updated s β‘ 0 β
GateLoopState.error_status s β‘ 0 β
GateInvariant s 0
gate_base s h_i h_state_valid h_can_apply h_unitary h_qubit h_examined h_pairs h_error =
record
{ h_i_in_range = zero
; h_state_valid = h_state_valid
; h_state_can_apply = h_can_apply
; h_gate_unitary = h_unitary
; h_qubit_valid = h_qubit
; h_states_examined = h_examined
; h_pairs_updated = Ξ» j h_j_lt_zero _ β absurd (Β¬(<-zero j) h_j_lt_zero)
; h_dim_preserved = refl
; h_ctx_immutable = Ξ» j h_j_lt_zero β absurd (Β¬(<-zero j) h_j_lt_zero)
; h_error_clear = h_error
}
where
Β¬(<-zero : β n β Β¬(n < 0)
Β¬(<-zero _ ()
-- ============================================================================
-- Inductive Step: i β i+1
-- ============================================================================
-- One iteration of basis state processing
record GateIterationStep (s s' : GateLoopState) : Set where
fields
-- Context unchanged
ctx_same : GateLoopState.ctx s β‘ GateLoopState.ctx s'
-- Basis state iterator increments
i_increments : GateLoopState.i s' β‘ GateLoopState.i s + 1
-- Amplitudes processed count incremented
states_processed_incremented : GateLoopState.num_amplitudes_processed s' β‘ GateLoopState.num_amplitudes_processed s + 1
-- Pairs updated: either incremented or unchanged
-- (incremented if this basis state's qubit bit was 0)
pairs_updated_invariant :
let i = GateLoopState.i s
qubit_bit = (i mod (2 * GateContext.bit_mask (GateLoopState.ctx s))) / GateContext.bit_mask (GateLoopState.ctx s)
in (qubit_bit β‘ 0 β
GateLoopState.num_pairs_updated s' β‘ GateLoopState.num_pairs_updated s + 1) β§
(qubit_bit β 0 β
GateLoopState.num_pairs_updated s' β‘ GateLoopState.num_pairs_updated s)
-- new_amplitudes written for this iteration
new_amplitudes_written : Bool
-- No errors during iteration
error_unchanged : GateLoopState.error_status s' β‘ 0
-- Inductive step
gate_step :
(s s' : GateLoopState) (i : β) β
GateInvariant s i β
GateIterationStep s s' β
GateInvariant s' (i + 1)
gate_step s s' i inv_i step =
record
{ h_i_in_range = Nat.succ_le_of_lt (by-i-lt-dim-from-invariant inv_i)
; h_state_valid = GateInvariant.h_state_valid inv_i
; h_state_can_apply = GateInvariant.h_state_can_apply inv_i
; h_gate_unitary = GateInvariant.h_gate_unitary inv_i
; h_qubit_valid = GateInvariant.h_qubit_valid inv_i
; h_states_examined = cong suc (GateInvariant.h_states_examined inv_i)
; h_pairs_updated = Ξ» j h_j_lt_suc_i h_qubit_zero β
let h_j_le_i = <-to-β€ h_j_lt_suc_i
in case (decide (j β‘ i)) of Ξ» where
(yes p) β
-- j = i, and qubit_bit = 0, so this pair just got updated
let (h_pairs_inc, _) = GateIterationStep.pairs_updated_invariant step
h_pairs_pred = h_pairs_inc h_qubit_zero
in pairs_updated_j_equals_i i (GateContext.bit_mask (GateLoopState.ctx s)) h_qubit_zero
(Ξ» pairs β h_pairs_pred)
(no Β¬p) β
-- j < i, use previous invariant
let h_j_lt_i = β€-to-< h_j_le_i Β¬p
in GateInvariant.h_pairs_updated inv_i j h_j_lt_i h_qubit_zero
; h_dim_preserved = GateInvariant.h_dim_preserved inv_i
; h_ctx_immutable = Ξ» j h_j_lt β GateInvariant.h_ctx_immutable inv_i j (β€-to-< (<-to-β€ h_j_lt) (by-step-i-increments))
; h_error_clear = GateIterationStep.error_unchanged step
}
-- ============================================================================
-- Exit Condition: Loop termination (i = dim)
-- ============================================================================
-- All basis states processed
gate_exit :
(s : GateLoopState) (i : β) β
GateInvariant s i β
i β‘ GateContext.dim (GateLoopState.ctx s) β
-- Then:
-- 1. All basis states examined
(GateLoopState.num_amplitudes_processed s β‘ GateContext.dim (GateLoopState.ctx s)) β§
-- 2. All valid (state_0, state_1) pairs updated
(GateLoopState.num_pairs_updated s β‘ GateContext.dim (GateLoopState.ctx s) / 2) β§
-- 3. new_amplitudes array complete and ready to swap
(β (i : β) β i < GateContext.dim (GateLoopState.ctx s) β basisStateInRange i (GateContext.dim (GateLoopState.ctx s))) β§
-- 4. No errors
(GateLoopState.error_status s β‘ 0) β§
-- 5. State dimension preserved
(dimensionPreserved (GateContext.dim (GateLoopState.ctx s)) (GateContext.dim (GateLoopState.ctx s)))
gate_exit s i inv_i h_done =
β¨ trans (GateInvariant.h_states_examined inv_i) h_done
, gate_exit_pairs_count i (GateContext.dim (GateLoopState.ctx s))
(GateContext.bit_mask (GateLoopState.ctx s)) h_done
(by-dim-is-power-of-2)
(GateLoopState.num_pairs_updated s)
, Ξ» j h_j_lt β
basisStateInRange j (GateContext.dim (GateLoopState.ctx s))
, GateInvariant.h_error_clear inv_i
, refl
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