I’m happy to use this bully pulpit to advertise that the following paper has been deemed “probably not terrible”, i.e., published.

Here’s the figure^{ a } and caption:

It is my highly unusual opinion that identifying a definition for the branches in the wavefunction is the most conceptually important problem is physics. The reasoning is straightforward: (1) quantum mechanics is the most profound thing we know about the universe, (2) the measurement process is at the heart of the weirdness, and (3) the critical roadblock to analysis is a definition of what we’re talking about. (Each step is of course highly disputed, and I won’t defend the reasoning here.) In my biased opinion, the paper represents the closest yet anyone has gotten to giving a mathematically precise definition.

On the last page of the paper, I speculate on the possibility that branch finding may have practical (!) applications for speeding up numerical simulations of quantum many-body systems using matrix-product states (MPS), or tensor networks in general. The rough idea is this: Generic quantum systems are exponentially hard to simulate, but classical systems (even stochastic ones) are not. A definition of branches would identify *which degrees of freedom* of a quantum system could be accurately simulated classically, and when. Although classical computational transitions are understood in many certain special cases, our macroscopic observations of the real world strongly suggest that *all* systems we study admit classical descriptions on large enough scales.^{ b } There is reason to think a general (abstract) description of the quantum-classical transition is possible, and would allow us to go beyond special cases.

In the rest this blog post I’m going to construct a simple MPS state featuring branches with records that are arbitrarily redundant. The state’s key features will be that it is translationally invariant and has finite correlation length. Translational invariance makes the state much easier to study and compare with the literature, and is a property shared with the simple inflationary model I’m studying with Elliot Nelson. Finite correlation length eliminates the trivial solution of a generalized GHZ state, guarantees that there are an infinite number of branches (if there are any at all), and is in some sense more natural.

#### An MPS with branches

Our strategy will be to build the state up from a *classical* probability distribution that already has the key features. Let , be a classical state of a finite 1D lattice of bits (), and consider the canonical ensemble starting with a uniform distribution and adding an energetic penalty of for misaligned nearest-neighbors^{ c }:

(1)

with . It’s not hard to check that the classical expectation value obeys and

(2)

If we define our correlation length by the asymptotic behavior , then for large (large ) we have

(3)

The idea is to define our quantum state as a superposition of configurations of a spin chain weighted by this distribution: . Contiguous regions of size will be highly correlated. The coarse-grained variables that are being recorded are something like “whether this local region is mostly or mostly ”, but we don’t necessarily want each qubit to have a perfect record of this information. Rather, we choose

(4)

for some fixed (typically small) angle . For one can’t reliably infer the classical variable from a measurement on a single spin, but one can make an arbitrarily reliable inference by measuring lots of them: . If we define our “recording length” by the asymptotic behavior , then for small we have

(5)

Thus our final (unnormalized) state is

(6)

for arbitrary positive parameters and . The state inherits from the classical probability distribution the properties of finite correlation length and translational invariance. Any contiguous set of spins is very likely to all be in the same state, and by measuring a subset of spins one can distinguish between the cases and with high reliability. Therefore in the limit it makes sense to define the redundancy ; this is the approximate number of disjoint records that are available about each branch, where there is a (binary) branch density of .

Now lets turn this into a matrix-product state.^{ d } That is, it should take the form^{ e }^{ f }

(7)

where the range over the local Hilbert spaces of the qubits and the are the “bonds” representing contraction of matrices of as-yet unspecified dimension (depicted in the figure as lines connecting the ‘s horizontally). The trick is to pretend that our classical probability distribution was achieved by starting with a quantum state of appropriately weighted bras,

(8)

and contracting (i.e., multiplying from the right to inner-product over the fictional bras) with a specially designed state that picks out the correct conditional states of the original Hilbert space:

(9)

The object

(10)

then takes the form of (7) and is equal to so long as we choose to satisfy

(11)

or, more cleanly,

(12)

*[I think Martin Ganahl and Guifre Vidal for discussion.]*

### Footnotes

(↵ returns to text)

- The editor tried to convince me that this figure appeared on the cover for purely aesthetic reasons and this does not mean my letter is the best thing in the issue…but I know better!↵
- Note that whether certain degrees of freedom admit a classical effective description is a
*computational*question. The existence of such a description is incompatible with them exhibiting quantum supremacy, but it is compatible with them encoding circumstantial evidence (at the macro scale) for quantum mechanics (at the micro scale).↵ - We assume periodic boundary conditions: ↵
- This will not be a general algorithm for building MPSs since it exploits the special form of the state (6) we’re trying to build, which allows us to identify the ‘s with the ‘s.↵
- Again, periodic boundary conditions↵
- The tikzpicture code was obtained from Piotr Migdał.↵

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