### Friday, October 18, 2002

**Quantum Parody of Shakespeare's Hamlet III.i**

To end the week I give you the following presented by Ken Regan at the Dagstuhl meeting.
To evolve, or not to evolve: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of coherent waves,
Or to take arms against superpositions,
And by observing end them? To observe; to evolve---
No more; and by a measurement to say we end
The mixed states and the thousand natural shocks
NMR is heir to, 'tis a computation
Devoutly to be wish'd. To evolve, to run;
Perchance to decohere---ay, there's the rub;
For in too long a run what waves may come
When they have shuffled off magnetic coil,
Must give us pause: there's the respect
That makes calamity of so long runs;
For who would bear the whips and scorns of time,
Qubits flipped wrong, the proud man's "quantumly",
But that the dread of something after halting,
The undiscover'd light cone from whose bourn
No classical info returns, puzzles the will
And makes us rather study those models we know
Than fly to others that we know not of?
Thus deadlines do make cowards of us all;
And thus the native hue of evolution
Is sicklied o'er with the pale cast of thought,
And enterprises of great pith and moment
With this regard their funding turns awry,
To lose the name of action.---Soft you now!
The fair Ophelia! Nymph, in thy bra and kets
Be all my states remember'd.

1:29 PM
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### Thursday, October 17, 2002

**Scooping the Loop Snooper**

Thanks to Jose Balcazar for showing me this poem by Geoffrey K. Pullum giving a proof, in verse, that the halting problem is undecidable.
10:45 AM
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### Wednesday, October 16, 2002

**More from Dagstuhl**

The highlight of Tuesday was seeing Manindra Agrawal present the new primality algorithm. History in the making.
Graph Isomorphism is a common topic in the conference. Next to factoring, graph isomorphism is the most well-studied problem in NP not known to be in P or NP-complete. Graph isomorphism sits in co-AM, i.e. there is a two-round interactive proof system for showing that two graphs are not isomorphic. The best deterministic algorithm uses time exponential in (n log n)^{0.5}.

Jacobo Toran today gave a talk on the hardness of graph isomorphism. He showed that given a black box for GI, one can compute the number of accepting paths in a directed graph, a class known as #L functions or equivalently the determinant of an integer matrix. He also showed that matching reduces to graph isomorphism. Whether every language in P is log-space reducible to graph isomorphism remains an interesting open question.

Later in the conference V. Arvind will present his result that GI is in SPP, or more specifically that one can determine graph isomorphism in P^{SAT} where every query made to the SAT oracle has at most one satisfying assigment.

Graph Isomorphism is the current algorithmic challenge for quantum computers. It is an example of the hidden subgroup problem, a special case of which was used for Shor's factoring algorithm. Perhaps the interaction between the classical and quantum theorists at this conference may help find a efficient quantum algorithm for GI.

8:21 AM
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### Monday, October 14, 2002

**Howdy from Dagstuhl**

Howdy from Schloss Dagstuhl, a conference center in Germany that hosts weekly computer science workshops. This week I´m here for the Seminar on Algebraic Methods in Quantum and Classical Models of Computation.
The interesting open problem of the day comes from Pierre McKenzie. Consider a circuit that works on sets of nonnegative integers. Inputs are the sets {0} and {1}. The gates consist of union, intersection, complement, addition and multiplication. Addition of two sets A and B is the set consisting of x+y for x in A and y in B. Multiplication is similar.

Given such a circuit with specified input sets and an integer w, is it decidable whether w is in the set generated by the output gate?

A decision algorithm for this problem yields a way to settle Goldbach´s conjecture that every even number greater than 2 is the sum of two primes. I´ll leave this implication as an excercise.

1:50 PM
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