Brian Swingle

Any quantum state is fully specified by the expectation values of a complete set of Hermitian operators. For a system of Majorana fermions, such as the Sachdev-Ye-Kitaev (SYK) model, this set of observables can be taken to be all possible strings of Majorana fermion operators. The expectation values of these fermion strings in a thermal state depend erratically on the microscopic couplings that specify the SYK Hamiltonian, and we study their statistical properties directly in the thermodynamic limit using path integral techniques. When the underlying SYK Hamiltonian is chaotic, we find that these expectation values are well-modeled as real Gaussian random variables with zero mean and a variance that we compute. In contrast, for the integrable variant of SYK, we find that the expectation values are actually non-Gaussian. We then use these results to study measures of magic in the SYK thermal state, including the robustness of magic and the stabilizer Rényi entropy. We also show that our results can be quantitatively reproduced with a dual gravity calculation in the chaotic case at sufficiently low temperature. In this dual gravity model the variance of a given microscopic operator string is related to a wormhole geometry stabilized by a massive particle which is dual to the operator string. Our results thus provide a concrete and quantitative setting in which to study the relationship between randomness, wormholes, and closed universes as well as a holographic dual of quantum magic.

Big bang/big crunch closed universes can be realized in AdS/CFT, even though they lack asymptotically AdS boundaries. With enough bulk entanglement, the bulk Hilbert space of a closed universe can be holographically encoded in the CFT. We clarify the relation of this encoding to observer-clone proposals and refute recent arguments about the breakdown of semiclassical physics in such spaces. In the limit of no bulk entanglement, the holographic encoding breaks down. The oft-cited one-dimensional nature of the closed universe Hilbert space represents the limitation of the external (CFT) Hilbert space to access the quantum information in the closed universe, similar to the limitations imposed on observers outside a perfectly isolated quantum lab. We advocate that the CFT nevertheless continues to determine the physical properties of the closed universe in this regime, showing how to interpret this relationship in terms of a final state projection in the closed universe. We provide a dictionary between the final state wavefunction and CFT data. We propose a model of the emergence of an arrow of time in the universe with a given initial or final state projection. Finally, we show that the conventional EFT in the closed universe, without any projection, can be recovered as a maximally ignorant description of the final state. This conventional EFT is encoded in CFT data, and it can be probed by computing coarse-grained observables. We provide an example of one such observable. Taken together, these results amount to a clean bill of health for baby universes born of AdS/CFT.

What do the typical entangled states of two black holes look like? Do they
contain firewalls? We approach these questions constructively, providing
ensembles of states which densely explore the black hole Hilbert space. None of
the states contain firewalls. On the contrary, they contain very long
Einstein-Rosen (ER) caterpillars: wormholes with large numbers of matter
inhomogeneities. Distinguishing these ensembles from the typical entangled
states of the black holes is hard. We quantify this by deriving the
correspondence between a microscopic notion of quantum randomness and the
geometric length of the wormhole. This formalizes a "complexity = geometry''
relation.

We study the equilibrium thermodynamics of quantum hard spheres in the infinite-dimensional limit, determining the boundary between liquid and glass phases in the temperature-density plane by means of the Franz-Parisi potential. We find that as the temperature decreases from high values, the effective radius of the spheres is enhanced by a multiple of the thermal de Broglie wavelength, thus increasing the effective filling fraction and decreasing the critical density for the glass phase. Numerical calculations show that the critical density continues to decrease monotonically as the temperature decreases further, suggesting that the system will form a glass at sufficiently low temperatures for any density.

The spectral form factor (SFF) is an important diagnostic of energy level repulsion in random matrix theory (RMT) and quantum chaos. The short-time behavior of the SFF as it approaches the RMT result acts as a diagnostic of the ergodicity of the system as it approaches the thermal state. In this work we observe that for systems without time-reversal symmetry, there is a second break from the RMT result at late time around the Heisenberg time. Long after thermalization has taken hold, and after the SFF has agreed with the RMT result to high precision for a time of order the Heisenberg time, the SFF of a large system will briefly deviate from the RMT behavior in a way exactly determined by its early time thermalization properties. The conceptual reason for this second deviation is the Riemann-Siegel lookalike formula, a resummed expression for the spectral determinant relating late time behavior to early time spectral statistics. We use the lookalike formula to derive a precise expression for the late time SFF for semi-classical quantum chaotic systems, and then confirm our results numerically for more general systems.

The recently introduced Quantum Lego framework provides a powerful method for generating complex quantum error correcting codes (QECCs) out of simple ones. We gamify this process and unlock a new avenue for code design and discovery using reinforcement learning (RL). One benefit of RL is that we can specify \textit{arbitrary} properties of the code to be optimized. We train on two such properties, maximizing the code distance, and minimizing the probability of logical error under biased Pauli noise. For the first, we show that the trained agent identifies ways to increase code distance beyond naive concatenation, saturating the linear programming bound for CSS codes on 13 qubits. With a learning objective to minimize the logical error probability under biased Pauli noise, we find the best known CSS code at this task for $\lesssim 20$ qubits. Compared to other (locally deformed) CSS codes, including Surface, XZZX, and 2D Color codes, our $[[17,1,3]]$ code construction actually has \textit{lower} adversarial distance, yet better protects the logical information, highlighting the importance of QECC desiderata. Lastly, we comment on how this RL framework can be used in conjunction with physical quantum devices to tailor a code without explicit characterization of the noise model.

We study the thermalization properties of one-dimensional open quantum systems coupled to baths at their boundary. The baths are driven to their thermal states via Lindblad operators, while the system undergoes Hamiltonian dynamics. We specifically consider multi-site baths and investigate the extent to which the late-time steady state resembles a Gibbs state at some controllable temperature set by the baths. We study three models: a non-interacting fermion model accessible via free-fermion technology, and two interacting models, the XZ model and the chiral clock model, which are accessible via tensor network methods. We show that, by tuning towards the weak coupling and slow driving limits, one can engineer low temperatures in the bulk of the system provided the bath size is big enough. We use this capability to study energy transport in the XZ model at lower temperatures than previously reported. Our work paves the way for future studies of interacting open quantum systems at low temperatures.

In this paper, we explore the possibility of building a quantum memory that
is robust to thermal noise using large $N$ matrix quantum mechanics models.
First, we investigate the gauged $SU(N)$ matrix harmonic oscillator and
different ways to encode quantum information in it. By calculating the mutual
information between the system and a reference which purifies the encoded
information, we identify a transition temperature, $T_c$, below which the
encoded quantum information is protected from thermal noise for a memory time
scaling as $N^2$. Conversely, for temperatures higher than $T_c$, the
information is quickly destroyed by thermal noise. Second, we relax the
requirement of gauge invariance and study a matrix harmonic oscillator model
with only global symmetry. Finally, we further relax even the symmetry
requirement and propose a model that consists of a large number $N^2$ of
qubits, with interactions derived from an approximate $SU(N)$ symmetry. In both
ungauged models, we find that the effects of gauging can be mimicked using an
energy penalty to give a similar result for the memory time. The final qubit
model also has the potential to be realized in the laboratory.

Many simulation tasks require that one first prepare a system's Gibbs state.
We present a family of quantum circuits for variational preparation of thermal
Gibbs states on a quantum computer; we call them the thermal multi-scale
entanglement renormalization ansatz (TMERA). TMERA circuits transform input
qubits to wavepacket modes localized to varying length scales and approximate a
systems Gibbs state as a mixed state of these modes. The TMERA is a based on
the deep multi-scale entanglement renormalization ansatz (DMERA); a TMERA
modifies a ground-state DMERA circuit by preparing each input qubit as a mixed
state. The excitation probabilities for input qubits serve as variational
parameters used to target particular temperature Gibbs states. Since a TMERA is
a special case of the product spectrum ansatz for thermal states, it is simple
to prepare, analyze, and optimize. We benchmark the TMERA on the transverse
field Ising model in one dimension and find that for $D=6$ it produces global
fidelities $\mathcal F > 0.4$ for 512-site systems across all temperatures.