Recursos de colección
Caltech Authors (157.532 recursos)
Repository of works by Caltech published authors.
Group = Institute for Quantum Information and Matter
Repository of works by Caltech published authors.
Group = Institute for Quantum Information and Matter
Lai, Yu-Hung; Yang, Ki Youl; Suh, Myoung-Gyun; Vahala, Kerry J.
Fiber tapers provide a way to rapidly measure the spectra of many types of optical microcavities. Proper fabrication of the taper ensures that its width varies sufficiently slowly (adiabatically) along the length of the taper so as to maintain single spatial mode propagation. This is usually accomplished by monitoring the spectral transmission through the taper. In addition to this characterization method it is also helpful to know the taper width versus length. By developing a model of optical backscattering within the fiber taper, it is possible to use backscatter measurements to characterize the taper width versus length. The model uses...
Molignini, Paolo; van Nieuwenburg, Evert; Chitra, R.
Time periodic modulations of the transverse field in the closed XY spin-1/2 chain generate a very rich dynamical phase diagram, with a hierarchy of Z_n topological phases characterized by differing numbers of Floquet-Majorana modes. This rich phase diagram survives when the system is coupled to dissipative end reservoirs. Circumventing the obstacle of preparing and measuring quasienergy configurations endemic to Floquet-Majorana detection schemes, we show that stroboscopic heat transport and spin density are robust observables to detect both the dynamical phase transitions and Majorana modes in dissipative settings. We find that the heat current provides very clear signatures of these Floquet...
Hunter-Jones, Nicholas; Liu, Junyu; Zhou, Yehao
The eigenstate thermalization hypothesis is a compelling conjecture which
strives to explain the apparent thermal behavior of generic observables in
closed quantum systems. Although we are far from a complete analytic
understanding, quantum chaos is often seen as a strong indication that the
ansatz holds true. In this paper, we address the thermalization of energy
eigenstates in the Sachdev-Ye-Kitaev model, a maximally chaotic model of
strongly-interacting Majorana fermions. We numerically investigate eigenstate
thermalization for specific few-body operators in the original SYK model as
well as its $\mathcal{N}=1$ supersymmetric extension and find evidence that
these models satisfy ETH. We discuss the implications of ETH for a
gravitational dual and the quantum...
Bermejo-Vega, Juan; Delfosse, Nicolas; Browne, Dan E.; Okay, Cihan; Raussendorf, Robert
A central question in quantum computation is to identify the resources that are responsible for quantum speed-up. Quantum contextuality has been recently shown to be a resource for quantum computation with magic states for odd-prime dimensional qudits and two-dimensional systems with real wave functions. The phenomenon of state-independent contextuality poses a priori an obstruction to characterizing the case of regular qubits, the fundamental building block of quantum computation. Here, we establish contextuality of magic states as a necessary resource for a large class of quantum computation schemes on qubits. We illustrate our result with a concrete scheme related to measurement-based...
Chao, Rui; Reichardt, Ben W.; Sutherland, Chris; Vidick, Thomas
An ideal system of $n$ qubits has $2^n$ dimensions. This exponential grants
power, but also hinders characterizing the system's state and dynamics. We
study a new problem: the qubits in a physical system might not be independent.
They can "overlap," in the sense that an operation on one qubit slightly
affects the others.
We show that allowing for slight overlaps, $n$ qubits can fit in just
polynomially many dimensions. (Defined in a natural way, all pairwise overlaps
can be $\leq \epsilon$ in $n^{O(1/\epsilon^2)}$ dimensions.) Thus, even before
considering issues like noise, a real system of $n$ qubits might inherently
lack any potential for exponential power.
On the other...
Gosset, David; Mehta, Jenish C.; Vidick, Thomas
In this work we consider the ground space connectivity problem for commuting local Hamiltonians. The ground space connectivity problem asks whether it is possible to go from one (efficiently preparable) state to another by applying a polynomial length sequence of 2-qubit unitaries while remaining at all times in a state with low energy for a given Hamiltonian H. It was shown in [Gharibian and Sikora, ICALP15] that this problem is QCMA-complete for general local Hamiltonians, where QCMA is defined as QMA with a classical witness and BQP verifier. Here we show that the commuting version of the problem is also...
Martin, Ivar; Refael, Gil; Halperin, Bertrand I.
When a physical system is subjected to a strong external multi-frequency
drive, its dynamics can be conveniently represented in the multi-dimensional
Floquet lattice. The number of the Floquet lattice dimensions equals the number
of {\em irrationally}-related drive frequencies, and the evolution occurs in
response to a built-in effective "electric" field, whose components are
proportional to the corresponding drive frequencies. The mapping allows to
engineer and study temporal analogs of many real-space phenomena. Here we focus
on the specific example of a two-level system under two-frequency drive that
induces topologically nontrivial band structure in the 2D Floquet space. The
observable consequence of such construction is quantized pumping of energy
between the...
Skolasinski, Rafal; Pikulin, Dmitry I.; Alicea, Jason; Wimmer, Michael
We show that burying of the Dirac point in semiconductor-based
quantum-spin-Hall systems can generate unexpected robustness of edge states to
magnetic fields. A detailed ${\bf k\cdot p}$ band-structure analysis reveals
that InAs/GaSb and HgTe/CdTe quantum wells exhibit such buried Dirac points. By
simulating transport in a disordered system described within an effective
model, we further demonstrate that buried Dirac points yield nearly quantized
edge conduction out to large magnetic fields, consistent with recent
experiments.
Kubica, Aleksander; Beverland, Michael E.; Brandão, Fernando G. S. L.; Preskill, John; Svore, Krysta M.
Three-dimensional (3D) color codes have advantages for fault-tolerant quantum
computing, such as protected quantum gates with relatively low overhead and
robustness against imperfect measurement of error syndromes. Here we
investigate the storage threshold error rates for bit-flip and phase-flip noise
in the 3D color code on the body-centererd cubic lattice, assuming perfect
syndrome measurements. In particular, by exploiting a connection between error
correction and statistical mechanics, we estimate the threshold for 1D
string-like and 2D sheet-like logical operators to be $p^{(1)}_\mathrm{3DCC}
\simeq 1.9\%$ and $p^{(2)}_\mathrm{3DCC} \simeq 27.6\%$. We obtain these
results by using parallel tempering Monte Carlo simulations to study the
disorder-temperature phase diagrams of two new 3D statistical-mechanical
models: the...
Lin, Cheng-Ju; Motrunich, Olexei I.
We numerically construct translationally invariant quasi-conserved operators
with maximum range M which best-commute with a non-integrable quantum spin
chain Hamiltonian, up to M = 12. In the large coupling limit, we find that the
residual norm of the commutator of the quasi-conserved operator decays
exponentially with its maximum range M at small M, and turns into a slower
decay at larger M. This quasi-conserved operator can be understood as a dressed
total "spin-z" operator, by comparing with the perturbative Schrieffer-Wolff
construction developed to high order reaching essentially the same maximum
range. We also examine the operator inverse participation ratio of the
operator, which suggests its localization in the operator...
Savitz, Samuel; Refael, Gil
The technique of continuous unitary transformations has recently been used to provide physical insight into a diverse array of quantum mechanical systems. However, the question of how to best numerically implement the flow equations has received little attention. The most immediately apparent approach, using standard Runge-Kutta numerical integration algorithms, suffers from both severe inefficiency due to stiffness and the loss of unitarity. After reviewing the formalism of continuous unitary transformations and Wegner's original choice for the infinitesimal generator of the flow, we present a number of approaches to resolving these issues including a choice of generator which induces what we...
Yunger Halpern, Nicole; White, Christopher David; Gopalakrishnan, Sarang; Refael, Gil
Many-body-localized (MBL) systems do not thermalize under their intrinsic
dynamics. The athermality of MBL, we propose, can be harnessed for
thermodynamic tasks. We illustrate by formulating an Otto engine cycle for a
quantum many-body system. The system is ramped between a strongly localized MBL
regime and a thermal (or weakly localized) regime. MBL systems' energy-level
correlations differ from thermal systems'. This discrepancy enhances the
engine's reliability, suppresses worst-case trials, and enables mesoscale
engines to run in parallel in the thermodynamic limit. We estimate analytically
and calculate numerically the engine's efficiency and per-cycle power. The
efficiency mirrors the efficiency of the conventional thermodynamic Otto
engine. The per-cycle power scales linearly with...
Baum, Yuval; Refael, Gil
When a d-dimensional quantum system is subjected to a periodic drive, it may
be treated as a (d+1)-dimensional system, where the extra dimension is a
synthetic one. In this work, we take these ideas to the next level by showing
that non-uniform potentials, and particularly edges, in the synthetic dimension
are created whenever the dynamics of system has a memory component. We
demonstrate that topological states appear on the edges of these synthetic
dimensions and can be used as a basis for a wave packet construction. Such
systems may act as an optical isolator which allows transmission of light in a
directional way. We supplement our ideas by...
White, Christopher David; Zaletel, Michael; Mong, Roger S. K.; Refael, Gil
We introduce a method "DMT" for approximating density operators of 1D systems that, when combined with a standard framework for time evolution (TEBD), makes possible simulation of the dynamics of strongly thermalizing systems to arbitrary times. We demonstrate that the method performs well for both near-equilibrium initial states (Gibbs states with spatially varying temperatures) and far-from-equilibrium initial states, including quenches across phase transitions and pure states.
Bermejo-Vega, Juan; Delfosse, Nicolas; Browne, Dan E.; Okay, Cihan; Raussendorf, Robert
A central question in quantum computation is to identify the resources that are responsible for quantum speed-up. Quantum contextuality has been recently shown to be a resource for quantum computation with magic states for odd-prime dimensional qudits and two-dimensional systems with real wave functions. The phenomenon of state-independent contextuality poses a priori an obstruction to characterizing the case of regular qubits, the fundamental building block of quantum computation. Here, we establish contextuality of magic states as a necessary resource for a large class of quantum computation schemes on qubits. We illustrate our result with a concrete scheme related to measurement-based...
Geraedts, Scott; Motrunich, Olexei I.
We provide an explicit lattice model of bosons with two separately conserved boson species [U(1)×U(1) global symmetry] realizing a direct transition between an integer quantum Hall effect of bosons and a trivial phase, where any intermediate phase is avoided by an additional symmetry interchanging the two species. If the latter symmetry is absent, we find intermediate superfluid phases where one or the other boson species condenses. We know the precise location of the transition since at this point our model has an exact nonlocal antiunitary particle-hole-like symmetry that resembles particle-hole symmetry in the lowest Landau level of electrons. We exactly...
Bao, Ning; Ooguri, Hirosi
We use the Holevo information to estimate distinguishability of microstates of a black hole in anti-de Sitter space by measurements one can perform on a subregion of a Cauchy surface of the dual conformal field theory. We find that microstates are not distinguishable at all until the subregion reaches a certain size and that perfect distinguishability can be achieved before the subregion covers the entire Cauchy surface. We will compare our results with expectations from the entanglement wedge reconstruction, tensor network models, and the bit threads interpretation of the Ryu-Takayanagi formula.
Pincini, D.; Vale, J. G.; Donnerer, C.; de la Torre, A.; Hunter, E. C.; Perry, R.; Moretti Sala, M.; Baumberger, F.; McMorrow, D. F.
The collective magnetic excitations in the spin-orbit Mott insulator (Sr_(1−x)La_x)_2IrO_4 (x=0, 0.01, 0.04, 0.1) were investigated by means of resonant inelastic x-ray scattering. We report significant magnon energy gaps at both the crystallographic and antiferromagnetic zone centers at all doping levels, along with a remarkably pronounced momentum-dependent lifetime broadening. The spin-wave gap is accounted for by a significant anisotropy in the interactions between J_(eff)=1/2 isospins, thus marking the departure of Sr_2IrO_4 from the essentially isotropic Heisenberg model appropriate for the superconducting cuprates.
Zhong, Tian; Kindem, Jonathan M.; Bartholomew, John G.; Rochman, Jake; Craiciu, Ioana; Miyazono, Evan; Bettinelli, Marco; Cavalli, Enrico; Verma, Varun; Nam, Sae Woo; Marsili, Francesco; Shaw, Matthew D.; Beyer, Andrew D.; Faraon, Andrei
Optical quantum memories are essential elements in quantum networks for long distance distribution of quantum entanglement. Scalable development of quantum network nodes requires on-chip qubit storage functionality with control of its readout time. We demonstrate a high-fidelity nanophotonic quantum memory based on a mesoscopic neodymium ensemble coupled to a photonic crystal cavity. The nanocavity enables >95% spin polarization for efficient initialization of the atomic frequency comb memory, and time-bin-selective readout via enhanced optical Stark shift of the comb frequencies. Our solid-state memory is integrable with other chip-scale photon source and detector devices for multiplexed quantum and classical information processing at...
Zhong, Tian; Kindem, Jonathan M.; Bartholomew, John G.; Rochman, Jake; Craiciu, Ioana; Miyazono, Evan; Bettinelli, Marco; Cavalli, Enrico; Verma, Varun; Nam, Sae Woo; Marsili, Francesco; Shaw, Matthew D.; Beyer, Andrew D.; Faraon, Andrei
Optical quantum memories are essential elements in quantum networks for long distance distribution of quantum entanglement. Scalable development of quantum network nodes requires on-chip qubit storage functionality with control of its readout time. We demonstrate a high-fidelity nanophotonic quantum memory based on a mesoscopic neodymium ensemble coupled to a photonic crystal cavity. The nanocavity enables >95% spin polarization for efficient initialization of the atomic frequency comb memory, and time-bin-selective readout via enhanced optical Stark shift of the comb frequencies. Our solid-state memory is integrable with other chip-scale photon source and detector devices for multiplexed quantum and classical information processing at...