Frequent Asked Questions about x2 technology
 | What is x2 technology? |
Conventional cross correlator measures cross correlation function, <A(t)B(t+T)>, where T >0, A(t), B(t) are two fluctuating input signal. A correlator with x2 technology measures <A(t)B(t+T)> and <B(t)A(t+T)> simultaneously. This is equivalent to measuring <A(t)B(t+T)> for T in both positive and negative domain.
x2 technology is relevant only to cross correlation function measurement. It is especially beneficial to those who measure short time autocorrelation function using pseudo-cross correlation function techniques to overcome detector imperfections.
| When and why are <A(t)B(t+T)> and <B(t)A(t+T)> statistically independent? |
When the signal is shot noise dominated, <A(t)B(t+T)> and <B(t)A(t+T)> are statistically independent. For example, consider a photon pair where a photon arrive at channel B before another photon arrive at channel A. This pair contributes to <A(t)B(t+T)> but not <B(t)A(t+T)>. On the other hand, if the photon arrives at channel B before another photon arrive at channel A, the pair will contribute to <B(t)A(t+T)> but not <A(t)B(t+T)>.
| Under what circumstances x2 technology improves measurement efficiency? |
When the signal is shot noise dominated, x2 technology improves the measurement efficiency by a factor of two.
| Why is x2 technology especially beneficial for short time correlation function measurements? |
Short time correlation function measurements are always limited by the photon shot noise. For example, at 100KHz average intensity in each channel (channel A and B) and a 12.5ns sample time, the average number of photons per sample time is 0.00125. Because only a photon pair (one photon from channel A and one photon from channel B) contributes to 2nd order correlation function, this means, on average, one contribution per 8ms. To average over 10^6 events, the measurement time for a conventional correlator will be 8000 seconds, or 2.2 hours.
With x2 technology, the measurement time is cut by half to achieve the same statistics. This is because <A(t)B(t+T)> is statistically independent from <B(t)A(t+T)>, and yet both represent the same physics.
| What about the prices and availability? |
We announce products when they are actually available. There are three models with x2 technology. Flex2K-12x2 is available now. It's a fast (12.5ns) multiple tau digital correlator with x2 technology. The price is $5,500. The lead time is about two weeks. Flex2K-09x2 and Flex2K-07x2 are available for orders. The prices are $6,500 and $8,500. The delivery time is approximately 8 weeks.