I just released a new paper onto the arXiv entitled “Improving the fidelity of single photon preparation from conditional down-conversion via asymmetric multiport detection”. Quite a mouthful you say. Indeed. In this paper I look at the conditional preparation of single photons via parametric down-conversion. This is a very common technique for preparing single photons for use in quantum optics experiments. The idea is that the down-converter probabilistically spits out pairs of photons, where each photon in the pair goes a different direction. Then, if we measure one of the outputs and detect a photon, we know with highly probability that there was a photon in the other output, due to the fact that they always pair up.

To operate well this requires number resolving photo-detectors. That is, detectors that can tell you exactly how many photons were detected. The reason for this is that the down-converter can spit out multiple pairs simultaneously and we want to filter out the cases where there was exactly one pair. Unfortunately present day detectors can’t do this. They can only tell you if you have no photons or some photons, but they can’t discriminate between different numbers of photons. To remedy this several authors have suggested approaches for improvement. The most elementary is the 2-port device. This consists of a simple beamsplitter, a device that sends half your photons in one direction, and the other half in the other. Both of the two outputs are then measured using normal non-number resolving detectors. Now, suppose two photons were incident. Then there is a 50% probability that the photons will go to different outputs. Then the sum of the two measurement results (i.e. a total of two detection events) gives you the correct result. Note that without the 2-port, 100% of the time you’ll get the wrong answer. So we’ve reduced the error probability. This idea generalizes to an N-port. Then the probability of getting a wrong answer goes as 1/N, which approaches zero for large N. So this trick effectively allows us to approximate a number resolving detector using a sequence of non-number resolving detectors and an N-port.

In this paper I describe a slight variation on the simple 2-port idea and show that by using an asymmetric beamsplitter instead of a half-half beamsplitter, you can significantly improve the operation of this scheme.

A new quantum blog, Room 408, from the boys at the University of Melbourne, has recently started. Their most recent post in particular is very humbling, and at the same time puts a hell of a lot of pressure on me to write more papers. Damn, I was hoping to relex this weekend.

Following on from my previous post on D-Wave’s “quantum computer”, it appears that the company is finally admitting that it isn’t really a quantum computer after all (read the article here). The current claim by CEO Herb Martin is

the machine is not a true quantum computer and is instead a kind of special-purpose machine that uses some quantum mechanics to solve problems.

Furthermore, he says

Users don’t care about quantum computing – users care about application acceleration. That’s our thrust. A general purpose quantum computer is a waste of time. You could spend hundreds of billions of dollars on it and not create a working computer.

Well that’s slightly different to the claims that were originally made before the demonstration.

It’s only two days until D-Wave Systems demonstrates their 16-bit ‘quantum computer’. This has stirred up quite a bit of talk recently. Although in all fairness I ought to give them the benefit of the doubt and wait until the demonstration before passing judgement, I’m not going to. In my mind, without knowing any technical details, I think it’s almost a certainty that this demonstration will be discredited. There are three main reasons for this:

1. If they do successfully demonstrate a 16-bit quantum computer, it means this small upstart company with a hand full of people will have successfully developed technology decades ahead of the entire academic community combined. I think this is quite unlikely.

2. The researchers at D-Wave, to my knowledge, have failed to demonstrate quantum coherence in their system, which rules out quantum computation.

3. D-Wave has provided no details into the working of their system. While you could argue that, being a company, this is to protect their ideas, there are plenty of details that could easily be provided that would confirm functionality without compromising their ideas. For example, quantum state or process reconstructions could easily demonstrate the existence of entanglement and quantum coherence without compromising intellectual property.

4. D-Wave repeatedly makes dubious claims about the type of problems their computers can solve, claiming they are suitable for solving NP-complete problems, which contradicts what is commonly believed by the rest of the scientific community.

In summary, I think it’s extremely unlikely that what D-Wave will demonstrate will be a quantum computer. What I think is likely is that their machine is, in effect, an analogue computer, the happens to produce the desired results in certain situations.

I’ll comment again after the demo, and once I have read more about it and actually know what I’m talking about.

I just released two new papers onto the arXiv, Noise thresholds for entanglement purification and Practical effects in cat state breeding.

Noise thresholds for entanglement purification

In this paper we consider the effects of noisy quantum gates on the operation of the entanglement purification protocol proposed by Bennett et al. We consider two figures of merit, the maximum achievable fidelity of an output state, and the minimum fidelity of an input state for purification to be possible. Both of these measures are a function of gate noise rates. When plotted against gate noise rate, these two measures converge for a particular value of gate noise. At this point purification is no longer possible, defining a noise threshold on gates. For the protocol in question this noise threshold is roughly 0.092, which is achievable in many present-day experimental architectures.

Practical effects in cat state breeding

In this paper (joint work with Austin Lund) we discuss two significant practical effects in the preparation of so-called cat states, which are defined as superpositions of distinct coherent states. Specifically, we consider the effects of loss and mode-mismatch. We find that the size of the cat states that may be prepared is upper bounded as a function of these two parameters. For experimentally realistic parameters we anticipate that cat state magnitudes will be limited to an average photon number of less than 5.

Photos from the QIP conference have been made available online here. Feel free to send in any others you have.

The videos of the infamous QIP’07 panel have been YouTubed. Watch part 1 here, and part 2 here.

Before I say anything else, let me emphasize that this party has NOTHING to do with the QIP conference. It’s purely coincidental that QIP is being held smack-bang on the same day as this wretched party we’re planning. But, since all you world class physicists are here anyway, I might as well invite you. Here are the details:

This Friday Night @ Aggie & Peter’s place.

That’s it.

E-mail for more details.

This post is technical

Today I released “Error propagation in loss- and failure-tolerant quantum computation schemes” with my co-authors Tim Ralph and Bill Munro. This a supplementary paper to our earlier paper “Error tolerance and tradeoffs in loss- and failure-tolerant quantum computing schemes”. We describe in detail how error propagation and accumulation occurs in various loss and failure tolerant quantum computing protocols and derive analytic expressions showing their error scaling behavior.

You can download the paper here.