Entanglement in quantum critical phenomena

Quantum phase transitions occur at zero temperature and involve the appearance of long-range correlations. These correlations are not due to thermal fluctuations but to the intricate structure of a strongly entangled ground state of the system. We present a microscopic computation of the scaling properties of the ground-state entanglement in several 1D spin chain models both near and at the quantum critical regimes. We quantify entanglement by using the entropy of the ground state when the system is traced down to $L$ spins. This entropy is seen to scale logarithmically with $L$, with a coefficient that corresponds to the central charge associated to the conformal theory that describes the universal properties of the quantum phase transition. Thus we show that entanglement, a key concept of quantum information science, obeys universal scaling laws as dictated by the representations of the conformal group and its classification motivated by string theory. This connection unveils a monotonicity law for ground-state entanglement along the renormalization group flow. We also identify a majorization rule possibly associated to conformal invariance and apply the present results to interpret the breakdown of density matrix renormalization group techniques near a critical point.

(Dated: 31st March 2022) Quantum phase transitions occur at zero temperature and involve the appearance of long-range correlations. These correlations are not due to thermal fluctuations but to the intricate structure of a strongly entangled ground state of the system. We present a microscopic computation of the scaling properties of the ground-state entanglement in several 1D spin chain models both near and at the quantum critical regimes. We quantify entanglement by using the entropy of the ground state when the system is traced down to L spins. This entropy is seen to scale logarithmically with L, with a coefficient that corresponds to the central charge associated to the conformal theory that describes the universal properties of the quantum phase transition. Thus we show that entanglement, a key concept of quantum information science, obeys universal scaling laws as dictated by the representations of the conformal group and its classification motivated by string theory. This connection unveils a monotonicity law for ground-state entanglement along the renormalization group flow. We also identify a majorization rule possibly associated to conformal invariance and apply the present results to interpret the breakdown of density matrix renormalization group techniques near a critical point. The study of entanglement in composite systems is one of the major goals of quantum information science [1,2], where entangled states are regarded as a valuable resource for processing information in novel ways. For instance, the entanglement between systems A and B in a joint pure state |Ψ AB can be used, together with a classical channel, to teleport or send quantum information [3]. From this resource-oriented perspective, the entropy of entanglement E(Ψ AB ) measures the entanglement contained in |Ψ AB [4]. It is defined as the von Neumann entropy of the reduced density matrix ρ A (equivalently ρ B ), and directly determines, among other aspects, how much quantum information can be teleported by using |Ψ AB . On the other hand, entanglement is appointed to play a central role in the study of strongly correlated quantum systems [5,6,7], since a highly entangled ground state is at the heart of a large variety of collective quantum phenomena. Milestone examples are the entangled ground states used to explain superconductivity and the fractional quantum Hall effect, namely the BCS ansatz [8] and the Laughlin ansatz [9]. Ground-state entanglement is, most promisingly, also a key factor to understand quantum phase transitions [10,11], where it is directly responsible for the appearance of long-range correlations. Consequently, a gain of insight into phenomena including, among others, Mott insulator-superfluid transitions, quantum magnet-paramagnet transitions and phase transitions in a Fermi liquid is expected by studying the structure of entanglement in the corresponding underlying ground states.
In the following we analyze the ground-state entanglement near and at a quantum critical point in a series of 1D spin-1/2 chain models. In particular, we consider the hamiltonians and which contain both first-neighbor interactions and an external magnetic field, and are used to describe a range of 1D quantum systems [10]. Notice that H XY (γ = 1) corresponds to the Ising chain, whereas H XXZ (∆ = 1) describes spins with isotropic Heisenberg interaction. Both Hamiltonians coincide for γ = ∆ = 0, where they become the XX model.
Osterloh et al [12] and Osborne and Nielsen [11] have recently considered the entanglement in the XY spin model, Eq. (2), in the neighborhood of a quantum phase transition. Their analysis, focused on singlespin entropies [11] and on two-spin quantum correlations [11,12], suggestively shows that these one-and twospin entanglement measures are picked either near or at the critical point. Here, alternatively, we undertake the study of the entanglement between a block of L contiguous spins and the rest of the chain, when the spin chain is in its ground state |Ψ g . Thus, the aim in the following is to compute the entropy of entanglement, Eq.
(1), for the state |Ψ g according to bipartite partitions parameterized by L, where ρ L ≡ trB L |Ψ g Ψ g | is the reduced density matrix for B L , a block of L spins. The XY model, Eq. (2), is an exactly solvable model, in that H XY can be diagonalized by first using a Jordan-Wigner transformation into fermionic modes and by subsequently concatenating a Fourier transformation and a Bogoliubov transformation (see for instance [10]). The calculation of S L , as sketched below, also uses the fact that the ground state |Ψ g of H XY and the corresponding density matrices ρ L are all gaussian states that can be completely characterized by means of certain correlation matrix of second moments.
More specifically, let us introduce two Majorana operators, c 2l and c 2l+1 , on each site l = 0, · · · , N − 1 of the spin chain, Operators c m are hermitian and obey the anticommutation relations {c m , c n } = 2δ mn . Hamiltonian H XY can be rewritten as and can be subsequently diagonalized by canonically transforming the operators c m . The expectation value of c m when the system is in the ground state, i.e. c m ≡ Ψ g |c m |Ψ g , vanishes for all m due to the Z 2 symmetry (σ x l , σ y l , σ z l ) → (−σ x l , −σ y l , σ z l ) ∀l of H XY . In turn, the expectation values completely characterize |Ψ g , for any other expectation value can be expressed, by using Wick's theorem, in terms of c m c n . Matrix B reads [13] with real coefficients g l given, when N → ∞, by From Eqs. (7)- (8) we can extract the entropy S L of Eq. (4) as follows. First we compute the correlation matrix of the state ρ L for block B L , namely δ mn + i(B L ) mn , where is constructed by eliminating 2(N − L) contiguous columns and rows from B, those corresponding to the N − L traced-out spins. Let V ∈ SO(2L) denote an orthogonal matrix that brings B L into a block-diagonal form, that is Then the set of 2L Majorana operators d m ≡ Eq. (11) indicates that the above fermionic modes are in a product or uncorrelated state, that is where ̺ m denotes the mixed state of mode m. The entropy of ρ L is a sum over the entropy is the binary entropy] and thus reads For arbitrary values of (a, γ) in H XY and in the thermodynamic limit, N → ∞, one can evaluate Eq. (8) numerically, diagonalize B L in Eq. (9) to obtain ν m and then evaluate Eq. (13). The computational effort grows only polynomically with L and produces reliable values of S L for blocks with up to several tens of spins. However, further analytical characterization is possible in some cases, which speeds the computation significantly. For instance, for the XX model with magnetic field, γ = 0 and 1/a ∈ [−1, 1], one obtains g 0 = φ a /π − 2 and g l = 2 sin(lφ a )/lπ for l = 0, where φ a = arccos(1/a), from which S L can be numerically determined. We obtain, up to L = 100 spins, where k 1 (a) depends only on a. For the Ising model with critical magnetic field, γ = 1, a = 1, one finds g l = −2/l for odd l and g l = 0 for even l and, for up to L = 100 spins, Finally, for the Ising model with magnetic field, γ = 1, and for a close to 1, Kitaev has obtained an analytical expression for the entropy of half of an infinite chain [15]. To use this result in our setting we need to double its value because the entropy resides near the boundary. Thus we get The XXZ model, Eq. (3), cannot be analyzed using the previous method. Instead we have used the Bethe ansatz [16] to exactly determine, through a numerical procedure, the ground state |Ψ The results of the computation of S L for the spin chains (2) and (3) can be summarized as follows.
Non-critical regime. For those values (a, γ) or (∆, λ) for which the models are non-critical, the entropy of entanglement S L either vanishes for all L [e.g. when a sufficiently strong magnetic field aligns all spins into a product, unentangled state] or grows monotonically as a function of L until it reaches a saturation value S max . For instance, in the infinite Ising chain the saturation entropy S max is given by Eq. (16). As shown in Fig. (1), S L often approaches S max already for a small number L of spins.
Critical regime. Instead, critical ground-states are characterized by an entropy S L that diverges logarithmically with L, with a coefficient given by the holomorphic and antiholomorphic central charges c andc of the conformal field theory that describes the universal properties of the phase transition [17], see Fig. (2). This expression was derived by Holzhey, Larsen and Wilczek [18] for the geometric entropy (analogous of Eq. (4) for a conformal field theory), and our calculation confirms it for several critical spin chains. Thus, the critical Ising model corresponds to a free fermionic field theory, with central charges c f =c f = 1/2, whereas the rest of critical regimes in (2) and (3) are described by a free bosonic field theory, c b =c b = 1 (cf. Eqs. (14) and (15)). In particular, the marginal deformation 0 < γ ≤ 1 for a = 1 shows scaling for every γ with universal coefficient c =c = 1/2. It is possible to compare the subleading correction between two different values of γ. The behavior we obtain is described by The singular behavior at γ = 0 is the signature of the fact that that point belongs to the abrupt change of universality class for the XX model. The above characterizations motivate a number of observations, that we move to discuss.
Critical and non-critical ground states contain structurally different forms of quantum correlations. Noncritical ground-state entanglement corresponds to a weak, semi-local form of entanglement driven by the appearance of a length scale, e.g. a mass gap. Indeed, for any L, the reduced density matrix ρ L is supported on a small, bounded subspace of the Hilbert space of the L spins, and can be obtained by diagonalizing the hamiltonian corresponding to the block B L and only a few extra neighboring spins, as skillfully exploited in White's density matrix renormalization group (DMRG) techniques [19]. We note here that a bounded rank for ρ L (relatedly, a saturation value for S L ) is instrumental for the success of DMRG schemes, where only of a finite number ). Finally, curve ( * ) comes from a XXX chain of 20 spins without magnetic field, HXXZ(∆ = 1, λ = 0) in Eq. (3). These finite-chain results, obtained using the Bethe ansatz, combine the logarithmic behavior (L = 1, · · · , 5) of a free boson field theory with a finite-size saturation effect (L = 6, · · · , 10). We have added the lines c+c 6 [log 2 (L) + π] both for bosons and fermions to highlight their remarkable agreement with the numerical diagonalization.
of eigenvectors of ρ L can be kept. Critical bound-state entanglement corresponds, on the contrary, to a stronger form of entanglement, one that embraces the system at all length scales simultaneously. DMRG techniques have reportedly failed to reproduce quantum critical behavior [20] and we may, in view of Eq. (17), be in a position to understand why. Indeed, the divergent character of S L is just one particular manifestation of the fact that the number of relevant eigenvectors of ρ L unboundedly grows with L. If, as is the case in DMRG schemes, only a finite number of levels can be considered, then a sufficiently large L will always make the computation of ρ L impossible (arguably, even in an approximate sense) by using such schemes. This strongly suggests that overcoming the above difficulties necessarily requires techniques that do not attempt to reproduce the critical behavior of the ground state through a local, real space construction.
Another remarkable, far-reaching fact is that, as mentioned below Eq. (17), our results coincides with entropy computations performed in conformal field theory. There, a geometric or fine-grained entropy analogous to Eq. (4) but for a continuous field theory has been considered by several authors, including Srednicki [21], Callan and Wilczek [22], Holzhey et al [18] and Fiola et al [23]. Thus, starting from non-relativistic spin chain models, and by performing a microscopic analysis of a relevant quantity in quantum information, we have obtained a universal scaling law for entanglement that is in full agreement with previous findings in the context of, say, black-hole thermodynamics in 1+1 dimensions [18,23].
The above connection has a number of implications to be exploited. For instance, Srednicki [21] has obtained the behavior of entropy in 2+1 and 3+1 dimensional conformal field theories. For a region R in 2 or 3 spatial dimensions, the entropy of R is proportional to the size σ(R) of its boundary, That is, the entropy per unit of boundary area, κ, is independent of the size of R. [This is in sharp contrast with the same quantity in 1D, where the boundary consists of two points and S R /σ(R) diverges logarithmically with the length L of R]. Accordingly, Eq. (19) also describes the critical ground-state entanglement of 2D and 3D spin lattices. Also the fact that the entropy of entanglement for 1D critical spin chains, Eq. (17), matches well-known conformal field theory parameters carries an extra bonus. The coefficient in control of the divergent behavior of S L at critical points is the central charge, which is subject to Zamolodchikov's c-theorem [24]. The c-theorem states that the central charges associated to the ultraviolet and infrared end points of renormalization group flows, labeled by C UV and C IR , obey the inequality C UV > C IR for unitary theories. This powerful result establishes an irreversible arrow as renormalization group transformations are performed. The translation of this idea to the quantum information setting is that entanglement decreases along renormalization group flows. An infrared theory carries less global entanglement than the ultraviolet theory where it flowed from. The c-theorem seems natural as renormalization group transformations integrate out short distance degrees of freedom, accompanied with their quantum correlations. Yet, it is not at all trivial due to, first, the infinite degrees of freedom (needing regularization) existing in a quantum field theory and, second, the rescaling step in the renormalization group transformation. It is noteworthy, then, that entanglement decreases both (i) under the local operations and classical communication and (ii) along renormalization group trajectories. The former case corresponds to local manipulation of an entangled system while the second is made out of a block-spin transformation followed by a rescaling of the system. Both actions do reduce quantum correlations and become irreversible [24,25].
One more remark. From Eqs. This allows us to look in more detail to the reshuffling of the ground state as more sites are incorporated in the block B L . Every time a new spin is added, the amount of local surprise due to quantum correlations with the rest of the chain increases, and so does the entropy. But critical quantum correlations entangle every single subset of the system, and the way they are reordered is far more subtle than the relation hinted by entropy arguments. We have numerically verified that also a majorization relation [26] holds for the ground-state reduced density matrices of the infinite XY spin chain, namely where the jump in steps of two is forced by the subtleties of the microscopic model. Thus, a critical ground state orderly redistributes weights, so as to accommodate for the new correlations, according to a detailed, exponentially large set of inequalities as contained in Eq. (21). In this sense, majorization may be a signature -admittedly a very refined one-of conformal invariance. But the majorization counterpart in the continuum conformal field theory is not yet known. Perhaps, then, the correspondence between concepts of quantum information science and conformal field theory, between critical ground-state entanglement and geometrical entropy, can also be exploited in the reverse direction.
This work was supported by the by the Spanish grants GC2001SGR-00065 and MCYT FPA2001-3598, by the National Science Foundation of USA under grant EIA-0086038, and by the European Union under grant ISF1999-11053.