Quantum Key Distribution (QKD)

• Read about the three polarizing filter experiment.
• Recall that a photon secretly remembers its polarization state.  It's not possible to copy or nondestructively read this state, but you can pass it through a filter and see if it's absorbed or transmitted.
  

class photon {
private:
	// true polarization state:
	//   0 vertical
	//   1 diagonal up
	//   2 horizontal
	//   3 diagonal down
	unsigned int polarization; 
public:
	photon() {
		polarization=rand()%4;
	}

	/* Return true if this photon is detected at this polarization filter angle.
	   This method is not const, due to observation's effect on the particle. */	
	bool detect(unsigned int angle) {
		while (true) {
			if (polarization==angle)
				return true; // yup
			else if ((polarization+2)%4==angle)	
				return false; // nope--cross polarized	
			
			// else randomly project polarization onto angle
			polarization=(angle+(rand()&2))%4;
		}
	}
};

void run_test(bool with_eve) {
	int agree_angle=0, agree_detect=0;
	for (int test=0;test<1000000;test++) {
		photon p; // generated by alice
		int alice_angle=rand()%4;
		bool alice_detect=p.detect(alice_angle);

		if (with_eve) {
			p.detect(rand()%4);
		}
		
		int bob_angle=rand()%4;
		bool bob_detect=p.detect(bob_angle);
		
		if (alice_angle == bob_angle) {
			agree_angle++;
			if (alice_detect == bob_detect) agree_detect++;
		}
	}
	std::cout<<"Bob and alice's filter angles agreed "<<agree_angle<<" times, and photons agreed "<<agree_detect<<" times\n";
}

void foo() {
	run_test(false);
	run_test(true);
}

(Try this in NetRun now!)

The weird part about this is Alice and Bob can detect Eve's effect on the polarization of the photon no matter *how* Eve measures the polarization. 

Without Eve, Bob and Alice's filter angles agreed 250115 times, and photons agreed 250115 times.
With Eve, Bob and Alice's filter angles agreed 249833 times, and photons agreed 187838 times.

Even if Eve uses a fancy beam splitter to make two copies of the photon, sends one copy through unmodified, and only later measures her local copy, entanglement means we can still detect this operation!

There are some definite drawbacks:

• This only works as long as we can keep a single photon alive and unmodified; modern work can only reach a few hundred kilometers even across dedicated fiber optic cable.
• Generating and detecting single photons is tricky and expensive, meaning it's easy enough to mess up most implementations, for example by overwhelming the detector with bright light, or bouncing your own photons off Bob's detector to read its state.