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Fixes #570.
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@msoeken
msoeken committed Apr 21, 2022
1 parent bd99e82 commit 3596b5d
Showing 1 changed file with 21 additions and 39 deletions.
60 changes: 21 additions & 39 deletions Standard/src/Preparation/QuantumROM.qs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,12 +2,13 @@
// Licensed under the MIT License.

namespace Microsoft.Quantum.Preparation {
open Microsoft.Quantum.Intrinsic;
open Microsoft.Quantum.Canon;
open Microsoft.Quantum.Arithmetic;
open Microsoft.Quantum.Arrays;
open Microsoft.Quantum.Canon;
open Microsoft.Quantum.Convert;
open Microsoft.Quantum.Diagnostics;
open Microsoft.Quantum.Intrinsic;
open Microsoft.Quantum.Math;
open Microsoft.Quantum.Arrays;

/// # Summary
/// Returns an operation that prepares a a purification of a given mixed state.
Expand Down Expand Up @@ -107,10 +108,6 @@ namespace Microsoft.Quantum.Preparation {
return MixedStatePreparation(qubitCounts, oneNorm, op);
}

internal function SplitSign(coefficient : Double) : (Double, Int) {
return (AbsD(coefficient), coefficient < 0.0 ? 1 | 0);
}

/// # Summary
/// Returns an operation that prepares a a purification of a given mixed
/// state, entangled with a register representing a given collection of data.
Expand Down Expand Up @@ -221,6 +218,9 @@ namespace Microsoft.Quantum.Preparation {
/// - Microsoft.Quantum.Preparation.PurifiedMixedState
function PurifiedMixedStateRequirements(targetError : Double, nCoefficients : Int)
: MixedStatePreparationRequirements {
Fact(targetError > 0.0, "targetError must be positive");
Fact(nCoefficients > 0, "nCoefficients must be positive");

let nBitsPrecision = -Ceiling(Lg(0.5*targetError)) + 1;
let nIndexQubits = Ceiling(Lg(IntAsDouble(nCoefficients)));
let nGarbageQubits = nIndexQubits + 2 * nBitsPrecision + 1;
Expand All @@ -235,21 +235,15 @@ namespace Microsoft.Quantum.Preparation {
: (Double, Int[], Int[]) {
let oneNorm = PNorm(1.0, coefficients);
let nCoefficients = Length(coefficients);
if (bitsPrecision > 31) {
fail $"Bits of precision {bitsPrecision} unsupported. Max is 31.";
}
if (nCoefficients <= 1) {
fail $"Cannot prepare state with less than 2 coefficients.";
}
if (oneNorm == 0.0) {
fail $"State must have at least one coefficient > 0";
}
Fact(bitsPrecision <= 31, $"Bits of precision {bitsPrecision} unsupported. Max is 31.");
Fact(nCoefficients > 1, "Cannot prepare state with less than 2 coefficients.");
Fact(oneNorm != 0.0, "State must have at least one coefficient > 0");

let barHeight = 2 ^ bitsPrecision - 1;

mutable altIndex = RangeAsIntArray(0..nCoefficients - 1);
mutable keepCoeff = Mapped(
RoundedDiscretizationCoefficients(_, oneNorm, nCoefficients, barHeight),
coefficient -> Round((AbsD(coefficient) / oneNorm) * IntAsDouble(nCoefficients) * IntAsDouble(barHeight)),
coefficients
);

Expand All @@ -268,15 +262,15 @@ namespace Microsoft.Quantum.Preparation {
mutable barSource = [];

for idxCoeff in IndexRange(keepCoeff) {
if (keepCoeff[idxCoeff] > barHeight) {
if keepCoeff[idxCoeff] > barHeight {
set barSource += [idxCoeff];
} elif (keepCoeff[idxCoeff] < barHeight) {
} elif keepCoeff[idxCoeff] < barHeight {
set barSink += [idxCoeff];
}
}

for rep in 0..nCoefficients * 10 {
if (Length(barSink) > 0 and Length(barSource) > 0) {
if Length(barSink) > 0 and Length(barSource) > 0 {
let idxSink = Tail(barSink);
let idxSource = Tail(barSource);
set barSink = Most(barSink);
Expand All @@ -285,12 +279,12 @@ namespace Microsoft.Quantum.Preparation {
set keepCoeff w/= idxSource <- keepCoeff[idxSource] - barHeight + keepCoeff[idxSink];
set altIndex w/= idxSink <- idxSource;

if (keepCoeff[idxSource] < barHeight) {
if keepCoeff[idxSource] < barHeight {
set barSink += [idxSource];
} elif (keepCoeff[idxSource] > barHeight) {
} elif keepCoeff[idxSource] > barHeight {
set barSource += [idxSource];
}
} elif (Length(barSource) > 0) {
} elif Length(barSource) > 0 {
let idxSource = Tail(barSource);
set barSource = Most(barSource);
set keepCoeff w/= idxSource <- barHeight;
Expand All @@ -302,12 +296,6 @@ namespace Microsoft.Quantum.Preparation {
return (oneNorm, keepCoeff, altIndex);
}

// Used in QuantumROM implementation.
internal function RoundedDiscretizationCoefficients(coefficient: Double, oneNorm: Double, nCoefficients: Int, barHeight: Int)
: Int {
return Round((AbsD(coefficient) / oneNorm) * IntAsDouble(nCoefficients) * IntAsDouble(barHeight));
}

// Used in QuantumROM implementation.
internal operation PrepareQuantumROMState(
nBitsPrecision: Int, nCoeffs: Int, nBitsIndices: Int,
Expand All @@ -332,7 +320,7 @@ namespace Microsoft.Quantum.Preparation {
ApplyToEachCA(H, uniformKeepCoeffRegister!);

// Write bitstrings to altIndex and keepCoeff register.
let unitaryGenerator = (nCoeffs, QuantumROMBitStringWriterByIndex(_, keepCoeff, altIndex, data));
let unitaryGenerator = (nCoeffs, idx -> WriteQuantumROMBitString(idx, keepCoeff, altIndex, data, _, _, _, _));
MultiplexOperationsFromGenerator(unitaryGenerator, indexRegister, (keepCoeffRegister, altIndexRegister, dataRegister, altDataRegister));

// Perform comparison
Expand All @@ -341,23 +329,17 @@ namespace Microsoft.Quantum.Preparation {
let indexRegisterSize = Length(indexRegister!);

// Swap in register based on comparison
ApplyToEachCA((Controlled SWAP)([flagQubit], _), Zipped(indexRegister! + dataRegister, altIndexRegister! + altDataRegister));
}

// Used in QuantumROM implementation.
internal function QuantumROMBitStringWriterByIndex(idx : Int, keepCoeff : Int[], altIndex : Int[], data : Bool[][])
: ((LittleEndian, LittleEndian, Qubit[], Qubit[]) => Unit is Adj + Ctl) {
return WriteQuantumROMBitString(idx, keepCoeff, altIndex, data, _, _, _, _);
ApplyToEachCA(Controlled SWAP([flagQubit], _), Zipped(indexRegister! + dataRegister, altIndexRegister! + altDataRegister));
}

// Used in QuantumROM implementation.
internal operation WriteQuantumROMBitString(idx: Int, keepCoeff: Int[], altIndex: Int[], data : Bool[][], keepCoeffRegister: LittleEndian, altIndexRegister: LittleEndian, dataRegister : Qubit[], altDataRegister : Qubit[])
: Unit is Adj + Ctl {
if (keepCoeff[idx] >= 0) {
if keepCoeff[idx] >= 0 {
ApplyXorInPlace(keepCoeff[idx], keepCoeffRegister);
}
ApplyXorInPlace(altIndex[idx], altIndexRegister);
if (Length(dataRegister) > 0) {
if Length(dataRegister) > 0 {
ApplyToEachCA(CControlledCA(X), Zipped(data[idx], dataRegister));
ApplyToEachCA(CControlledCA(X), Zipped(data[altIndex[idx]], altDataRegister));
}
Expand Down

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