## Shaking the lattice uncovers universality

Most of our knowledge of quantum phase transitions (QPTs)—which occur as a result of quantum, rather than thermal, fluctuations—comes from experiments performed in equilibrium conditions. Less is known about the dynamics of a system going through a QPT, which have been hypothesized to depend on a single time and length scale. Clark *et al.* confirmed this hypothesis in a gas of cesium atoms in an optical lattice, which was shaken progressively faster to drive the gas through a QPT.

*Science*, this issue p. 606

## Abstract

The dynamics of many-body systems spanning condensed matter, cosmology, and beyond are hypothesized to be universal when the systems cross continuous phase transitions. The universal dynamics are expected to satisfy a scaling symmetry of space and time with the crossing rate, inspired by the Kibble-Zurek mechanism. We test this symmetry based on Bose condensates in a shaken optical lattice. Shaking the lattice drives condensates across an effectively ferromagnetic quantum phase transition. After crossing the critical point, the condensates manifest delayed growth of spin fluctuations and develop antiferromagnetic spatial correlations resulting from the sub-Poisson distribution of the spacing between topological defects. The fluctuations and correlations are invariant in scaled space-time coordinates, in support of the scaling symmetry of quantum critical dynamics.

Critical phenomena near a continuous phase transition reveal fascinating connections between seemingly disparate systems that can be described via the same universal principles. Such systems can be found in the contexts of superfluid helium (*1*), liquid crystals (*2*), biological cell membranes (*3*), the early universe (*4*), and cold atoms (*5*, *6*). An important universal prediction is the power-law scaling of the topological defect density with the rate of crossing a critical point, as first discussed by T. Kibble in cosmology (*4*) and extended by W. Zurek in the context of condensed matter (*1*). Their theory, known as the Kibble-Zurek mechanism, has been the subject of intense experimental study that has largely supported the scaling laws (*7*). Recent theoretical works further propose the so-called universality hypothesis, according to which the collective dynamics across a critical point should be invariant in the space and time coordinates that scale with the Kibble-Zurek power law (*8*–*10*).

Atomic quantum gases provide a clean, well-characterized, and controlled platform for studying critical dynamics (*6*, *11*, *12*). They have enabled experiments on the formation of topological defects across the Bose-Einstein condensation transition (*13*–*16*) as well as critical dynamics across quantum phase transitions (*17*–*23*). Recent experiments using cold atoms in shaken optical lattices (*24*–*26*) have provided a vehicle for exploring phase transitions in spin models (*27*–*29*).

Here we study the critical dynamics of Bose condensates in a shaken optical lattice near an effectively ferromagnetic quantum phase transition. The transition occurs when we ramp the shaking amplitude across a critical value, causing the atomic population to bifurcate into two pseudo-spinor ground states (*28*). We measure the growth of spin fluctuations and the spatial spin correlations for ramping rates varied over two orders of magnitude. Beyond the critical point, we observe delayed development of ferromagnetic spin domains with long-range antiferromagnetic correlations due to the bunching of the domain sizes, which is not expected in a thermal distribution of ferromagnets. The times and lengths characterizing the critical dynamics agree with the scaling predicted by the Kibble-Zurek mechanism. The measured fluctuations and correlations collapse onto single curves in scaled space and time coordinates, supporting the universality hypothesis.

Our experiments use elongated three-dimensional (3D) Bose-Einstein condensates (BECs) of cesium atoms. We optically confine the condensates with trap frequencies of Hz, where the long () and short () axes are oriented at with respect to the and coordinates (Fig. 1A). The tight confinement along the vertical axis suppresses nontrivial dynamics in that direction (see the discussion on the dynamics in the direction below), which is also the optical axis of our imaging system. We adiabatically load the condensates into a 1D optical lattice (*11*) along the axis with a lattice spacing of _{}nm and a depth of 8.86 , where kHz is the recoil energy and is Planck’s constant.

To induce the ferromagnetic quantum phase transition, we modulate the phase of the lattice beam to periodically translate the lattice potential by , where is the shaking amplitude and the modulation frequency is tuned to mix the ground and first excited lattice bands (fig. S1) (*28*, *30*). The hybridized single-particle ground band energy can be modeled for small quasimomentum by(1)where is the atomic mass, and the coefficients of its quadratic () and quartic () terms depend on the shaking amplitude (Fig. 1B). For shaking amplitudes below the critical value, the coefficient is positive and the BEC occupies the lone ground state at momentum . The quantum phase transition occurs when the quadratic term crosses zero at , where and . At this point, the speed of sound for superfluid excitations, formally studied in (*31*), drops to zero along but remains nearly constant along and . Even stronger shaking converts the ground band into a double well with , yielding two degenerate ground states with . Repulsively interacting bosons with this double-well ground band are effectively ferromagnetic, having two degenerate many-body ground states with all atoms either pseudo-spin up () or down () (*28*). Notably, transitioning to one of these two ground states requires the system to spontaneously break the symmetry of its Hamiltonian. Describing the dynamics across the critical point presents a major challenge because of the divergence of the correlation length of quasimomentum and the relaxation time (critical slowing).

The Kibble-Zurek mechanism provides a powerful insight into quantum critical dynamics. According to this theory, when the time remaining to reach the critical point inevitably becomes shorter than the relaxation time, the system becomes effectively frozen (Fig. 1C). The system only unfreezes at a delay time after passing the critical point, when relaxation becomes faster than the ramp. At this time, topological defects become visible, and the typical distance between neighboring defects is proportional to the equilibrium correlation length. The Kibble-Zurek mechanism predicts that and depend on the quench rate as(2)(3)where and are the equilibrium dynamical and correlation length exponents given by the universality class of the phase transition. Although the details of this picture may not apply to every phase transition, the general scaling arguments are very robust, and similar predictions hold for a variety of quench types across the transition (*12*) and for phase transitions that break either continuous or discrete symmetries (*7*).

For slow ramps, and diverge and become separated from other scales in the system, making them the dominant scales for characterizing the collective critical dynamics (*8*–*10*). This idea motivates the universality hypothesis, which can be expressed as(4)indicating that the critical dynamics of any collective observable obeys the scaling symmetry and can be described by a universal function of the scaled coordinates and . The only effect of the quench rate is to modify the length and time scales.

We test the scaling symmetry of time by monitoring the emergence of quasimomentum fluctuations at different quench rates. Here, fluctuations refer to deviations of quasimomentum from zero, which vary across space and between individual samples; fluctuations should saturate to a large value when domains having are fully formed. After loading the condensates into the lattice, we ramp the shaking amplitude linearly from to values well above the critical amplitude nm (*32*) and interrupt the ramps at various times to perform a brief time-of-flight (TOF) before detection. After TOF, we measure the density deviation (*32*), which is nearly proportional to the quasimomentum distribution (fig. S2), where is the density profile of the th Bragg peak and the angle brackets denote averaging over multiple images. This detection method is particularly sensitive near the critical point when the quasimomentum just starts deviating from zero, indicating the emergence of fluctuations in the ferromagnetic phase where the ground states have nonzero quasimomentum. The spin density measurement used later to study spatial correlations is viable only when atoms settle to .

Over a wide range of quench rates, the evolution of quasimomentum fluctuation can be described in three phases (Fig. 2A). First, below the critical point, quasimomentum fluctuation does not exceed its baseline level. Second, just after passing the critical point, critical slowing keeps the system “frozen,” and fluctuation remains low. Finally, the system unfreezes and quasimomentum fluctuation quickly increases and saturates, indicating the emergence of ferromagnetic domains. We quantify this progression by investigating the fluctuation of contrast (Fig. 2B) that tracks quasimomentum fluctuation in our condensates via the fluctuation of (fig. S2), where is the total density and the angle brackets denote averaging over space and over multiple images. For comparison between different quench rates, we calculate the normalized fluctuation , where subscripts i and f indicate the fluctuation at early and late times, respectively (*32*). We find empirically that the growth of normalized fluctuations is well fit by the function
(5)where the time is defined relative to when the system crosses the critical point at , characterizes the delay time when the system unfreezes, and is the formation time over which the fluctuation grows. The measurement of fluctuation over time provides a critical test for both the Kibble-Zurek scaling and the universality hypothesis. First, both and exhibit clear power-law scaling with the quench rate varied over more than two orders of magnitude (Fig. 2C). Power-law fits yield the exponents of and , respectively. The nearly equal exponents are suggestive of the universality hypothesis, which requires all times to scale identically. Indeed, the growth of contrast fluctuation follows a universal curve when time is scaled by (Fig. 2D), strongly supporting the universality hypothesis (Eq. 4). Note that any observable time characterizing the collective dynamics can be chosen as in Eq. 4, including and .

We next test the spatial scaling symmetry based on the structures of pseudo-spin domains that emerge after the system unfreezes. Here, we cross the critical point with two different protocols: The first is a linear ramp starting from , whereas the second begins with a jump to , followed by a linear ramp. We detect domains near the time in the spin density distribution based on the density of atoms with spin up/down (fig. S3). At this time, the spin domains are fully formed and clearly separated by topological defects (domain walls), as shown in Fig. 3A. Furthermore, choosing this time just after domain formation minimizes the time available for nonuniversal relaxation processes. We characterize the domain distribution with the spin correlation function (*17*, *28*)
(6)averaged over multiple images (Fig. 3B). Both ramping protocols lead to similar correlation functions, suggesting that the domain distribution is insensitive to increases in the quench rate below the critical point.

The spin correlations reveal rich domain structure that strongly depends on the quench rate. For slower ramps nm/ms, the structures are predominantly one-dimensional and the density of topological defects increases with the quench rate. The tighter confinement and finite speed of sound near the critical point along the and _{}axes allow spin correlations to span the gas in those directions. The dynamics thus appear one-dimensional. When the quench rate exceeds nm/ms, defects start appearing along the _{}axis, and the domain structures become multidimensional. We attribute this dimensional crossover to the unfreezing time becoming too short to establish correlation along the axis. For the remainder of this work, we focus on the slower quenches and investigate the spin correlations along the axis.

We examine the 1D correlations using line cuts of the density-weighted correlation functions (*17*, *28*). The results exhibit prominent decaying oscillation (Fig. 3C). We extract two essential length scales from the correlation functions: the average domain size , or equivalently, the distance between neighboring topological defects, and the correlation length , indicating the width of the envelope function. These two scales are determined from the position and width of the peak in the Fourier transform of (*32*).

These length scales enable us to test the spatial scaling symmetry. The lengths and both display power-law scaling consistent with the Kibble-Zurek mechanism (Fig. 3D), with fits yielding exponents for the domain size and for the correlation length. Similarly, the correlations, measured at the same scaled time, collapse to a single curve in spatial coordinates scaled by the domain size (Fig. 3E). This result strongly supports the spatiotemporal scaling from the universality hypothesis (Eq. 4). An empirical curve(7)provides a good fit to the universal correlation function, yielding = 1.01(1) and = 1.04(1), indicating that the width of the envelope is close to the typical domain size.

The most striking feature of the universal correlation function is the emergence of oscillatory, antiferromagnetic order in the ferromagnetic phase. In thermal equilibrium, ferromagnets are expected to have a finite correlation length but no anticorrelation. The appearance of strong anticorrelation at suggests that domains of size form preferentially during the quantum critical dynamics. A statistical analysis of the topological defect distribution reveals that the domain sizes are bunched with their standard deviation well below their mean, indicating that the topological defects are created by a sub-Poisson process (fig. S4).

Finally, the combined scaling exponents of space and time allow us to extract the equilibrium critical exponents based on the Kibble-Zurek mechanism (*33*) (Fig. 3F). Solving Eqs. 2 and 3, we obtain the dynamical exponent 1.9(2) and correlation length exponent 0.52(5), which are close to the mean-field values and up to our experimental uncertainty. Note that the dynamical critical exponent results from the unique quartic kinetic energy of our system at the critical point (*32*).

Direct identification of domain walls presents intriguing possibilities for future studies of the topological defects generated during critical dynamics. These opportunities would be particularly interesting if the shaking technique were extended to higher dimensions in such a way that the transition breaks a continuous symmetry. In addition, the scaling of the correlation functions suggests that the antiferromagnetic order may be a shared feature of quantum critical dynamics for phase transitions in the same universality class, meriting future experiments.

## Supplementary Materials

## References and Notes

**Acknowledgments:**We thank L.-C. Ha, C. V. Parker, B. M. Anderson, B. J. DeSalvo, A. Polkovnikov, S. Sachdev, and Q. Zhou for helpful discussions. L.W.C. was supported by a National Defense Science and Engineering Graduate Fellowship and a Nambu Fellowship. This work was supported by NSF Materials Research Science and Engineering Centers (DMR-1420709), NSF grant PHY-1511696, and Army Research Office–Multidisciplinary Research Initiative grant W911NF-14-1-0003. The data presented in this paper are available upon request to C.C. (cchin@uchicago.edu).