LadderOperator.h

/* Copyright 2016 Kristofer Björnson
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
 
 
#ifndef COM_DAFER45_TBTK_LADDER_OPERATOR
#define COM_DAFER45_TBTK_LADDER_OPERATOR
 
#include "TBTK/FockState.h"
#include "TBTK/HoppingAmplitudeSet.h"
#include "TBTK/Statistics.h"
 
namespace TBTK{
 
template<typename BIT_REGISTER>
class LadderOperator{
public:
    enum class Type {Creation, Annihilation};
 
    LadderOperator();
 
    LadderOperator(
        Type type,
        Statistics statistics,
        const HoppingAmplitudeSet *hoppingAmplitudeSet,
        unsigned int state,
        unsigned int numBitsPerState,
        unsigned int maxOccupation,
        const FockState<BIT_REGISTER> &templateState,
        const BIT_REGISTER &fermionMask
    );
 
    ~LadderOperator();
 
    Type getType() const;
 
    const Index getPhysicalIndex() const;
 
    unsigned int getState() const;
 
    unsigned int getNumParticles(
        const FockState<BIT_REGISTER> &fockState
    ) const;
 
    FockState<BIT_REGISTER>& operator*(FockState<BIT_REGISTER> &rhs) const;
private:
    Type type;
 
    Statistics statistics;
 
    const HoppingAmplitudeSet *hoppingAmplitudeSet;
 
    unsigned int state;
 
    BIT_REGISTER stateMask;
 
    BIT_REGISTER leastSignificantBit;
 
    unsigned int leastSignificantBitIndex;
 
    BIT_REGISTER maxOccupation;
 
    BIT_REGISTER moreSignificantFermionMask;
};
 
template<typename BIT_REGISTER>
LadderOperator<BIT_REGISTER>::LadderOperator(){
}
 
template<typename BIT_REGISTER>
LadderOperator<BIT_REGISTER>::LadderOperator(
    Type type,
    Statistics statistics,
    const HoppingAmplitudeSet *hoppingAmplitudeSet,
    unsigned int state,
    unsigned int numBitsPerState,
    unsigned int maxOccupation,
    const FockState<BIT_REGISTER> &templateState,
    const BIT_REGISTER &fermionMask
) :
    stateMask(templateState.bitRegister),
    leastSignificantBit(templateState.bitRegister),
    maxOccupation(templateState.bitRegister),
    moreSignificantFermionMask(fermionMask)
{
    this->type = type;
    this->statistics = statistics;
    this->hoppingAmplitudeSet = hoppingAmplitudeSet;
    this->state = state;
 
    leastSignificantBitIndex = state*numBitsPerState;
 
    for(unsigned int n = 0; n < stateMask.getNumBits(); n++){
        if(n >= leastSignificantBitIndex && n < leastSignificantBitIndex + numBitsPerState)
            stateMask.setBit(n, 1);
        else
            stateMask.setBit(n, 0);
 
        if(n == leastSignificantBitIndex)
            leastSignificantBit.setBit(n, 1);
        else
            leastSignificantBit.setBit(n, 0);
    }
 
    this->maxOccupation = maxOccupation;
    this->maxOccupation = (this->maxOccupation << leastSignificantBitIndex);
 
    for(unsigned int n = 0; n < moreSignificantFermionMask.getNumBits(); n++){
        this->moreSignificantFermionMask.setBit(n, false);
        if(leastSignificantBit.getBit(n))
            break;
    }
}
 
template<typename BIT_REGISTER>
LadderOperator<BIT_REGISTER>::~LadderOperator(){
}
 
template<typename BIT_REGISTER>
typename LadderOperator<BIT_REGISTER>::Type LadderOperator<BIT_REGISTER>::getType() const{
    return type;
}
 
template<typename BIT_REGISTER>
const Index LadderOperator<BIT_REGISTER>::getPhysicalIndex() const{
    return hoppingAmplitudeSet->getPhysicalIndex(state);
}
 
template<typename BIT_REGISTER>
unsigned int LadderOperator<BIT_REGISTER>::getState() const{
    return state;
}
 
template<typename BIT_REGISTER>
unsigned int LadderOperator<BIT_REGISTER>::getNumParticles(
    const FockState<BIT_REGISTER> &fockState
) const{
    return ((fockState.getBitRegister() & stateMask) >> leastSignificantBitIndex).toUnsignedInt();
}
 
template<typename BIT_REGISTER>
FockState<BIT_REGISTER>& LadderOperator<BIT_REGISTER>::operator*(
    FockState<BIT_REGISTER> &rhs
) const{
    switch(type){
    case Type::Creation:
        if((rhs.bitRegister & stateMask) == maxOccupation){
            rhs.bitRegister.setMostSignificantBit();
            break;
        }
        rhs.bitRegister += leastSignificantBit;
        break;
    case Type::Annihilation:
        if(!(rhs.bitRegister & stateMask).toBool()){
            rhs.bitRegister.setMostSignificantBit();
            break;
        }
        rhs.bitRegister -= leastSignificantBit;
        break;
    default:
        TBTKExit(
            "LadderOperator<BIT_REGISTER>::operator*()",
            "This should never happen.",
            "Contact the developer."
        );
    }
 
    switch(statistics){
    case Statistics::FermiDirac:
        rhs.prefactor *= pow(-1, (rhs.bitRegister & moreSignificantFermionMask).getNumOneBits());
        break;
    case Statistics::BoseEinstein:
        break;
    default:
        TBTKExit(
            "LadderOperator<BIT_REGISTER>::operator*()",
            "This should never happen.",
            "Contact the developer."
        );
    }
 
    return rhs;
}
 
};  //End of namespace TBTK
 
#endif