@article {3505, title = {Ultrastrong light-matter interaction in a multimode photonic crystal}, year = {2024}, month = {2/21/2024}, abstract = {

Harnessing the interaction between light and matter at the quantum level has been a central theme in atomic physics and quantum optics, with applications from quantum computation to quantum metrology. Combining complex interactions with photonic synthetic materials provides an opportunity to investigate novel quantum phases and phenomena, establishing interesting connections to condensed matter physics. Here we explore many-body phenomena with a single artificial atom coupled to the many discrete modes of a photonic crystal. This experiment reaches the ultrastrong light-matter coupling regime using the circuit quantum electrodynamics paradigm, by galvanically coupling a highly nonlinear fluxonium qubit to a tight-binding lattice of microwave resonators. In this regime, the transport of a single photon becomes a many-body problem, owing to the strong participation of multi-photon bound states arising from interactions that break particle number conservation. Exploiting the effective photon-photon interactions mediated by the qubit, the transport of multiple photons leads to complex multimode dynamics that can be employed for generating a continuous reservoir of strongly-correlated photons, an important resource for quantum networks. This work opens exciting prospects for exploring nonlinear quantum optics at the single-photon level and stabilizing entangled many-body phases of light.

}, url = {https://arxiv.org/abs/2209.14972}, author = {Andrei Vrajitoarea and Ron Belyansky and Rex Lundgren and Seth Whitsitt and Alexey V. Gorshkov and Andrew A. Houck} } @article {3414, title = {An Uncertainty Principle for the Curvelet Transform, and the Infeasibility of Quantum Algorithms for Finding Short Lattice Vectors}, year = {2023}, month = {11/7/2023}, abstract = {

The curvelet transform is a special type of wavelet transform, which is useful for estimating the locations and orientations of waves propagating in Euclidean space. We prove an uncertainty principle that lower-bounds the variance of these estimates, for radial wave functions in n dimensions.\ \ As an application of this uncertainty principle, we show the infeasibility of one approach to constructing quantum algorithms for solving lattice problems, such as the approximate shortest vector problem (approximate-SVP), and bounded distance decoding (BDD). This gives insight into the computational intractability of approximate-SVP, which plays an important role in algorithms for integer programming, and in post-quantum cryptosystems.
In this approach to solving lattice problems, one prepares quantum superpositions of Gaussian-like wave functions centered at lattice points. A key step in this procedure requires finding the center of each Gaussian-like wave function, using the quantum curvelet transform. We show that, for any choice of the Gaussian-like wave function, the error in this step will be above the threshold required to solve BDD and approximate-SVP.\ \ 

}, url = {https://arxiv.org/abs/2310.03735}, author = {Yi-Kai Liu} } @article {3354, title = {An Uncertainty Principle for the Curvelet Transform, and the Infeasibility of Quantum Algorithms for Finding Short Lattice Vectors}, year = {2023}, month = {10/5/2023}, abstract = {

The curvelet transform is a special type of wavelet transform, which is useful for estimating the locations and orientations of waves propagating in Euclidean space. We prove an uncertainty principle that lower-bounds the variance of these estimates, for radial wave functions in n dimensions.
As an application of this uncertainty principle, we show the infeasibility of one approach to constructing quantum algorithms for solving lattice problems, such as the approximate shortest vector problem (approximate-SVP), and bounded distance decoding (BDD). This gives insight into the computational intractability of approximate-SVP, which plays an important role in algorithms for integer programming, and in post-quantum cryptosystems.
In this approach to solving lattice problems, one prepares quantum superpositions of Gaussian-like wave functions centered at lattice points. A key step in this procedure requires finding the center of each Gaussian-like wave function, using the quantum curvelet transform. We show that, for any choice of the Gaussian-like wave function, the error in this step will be above the threshold required to solve BDD and approximate-SVP.

}, url = {https://arxiv.org/abs/2310.03735}, author = {Yi-Kai Liu} } @article {3120, title = {Ultrastrong light-matter interaction in a photonic crystal}, year = {2022}, month = {9/29/2022}, abstract = {

Harnessing the interaction between light and matter at the quantum level has been a central theme in the fields of atomic physics and quantum optics, with applications from quantum computation to quantum metrology. Combining complex interactions with photonic synthetic materials provides an opportunity to investigate novel quantum phases and phenomena, establishing interesting connections to condensed matter physics. Here we explore many-body phenomena with a single artificial atom coupled to the many discrete modes of a photonic crystal. This experiment reaches the ultrastrong light-matter coupling regime using the circuit QED paradigm, by galvanically coupling a highly nonlinear fluxonium qubit to a tight-binding lattice of microwave resonators. In this regime, the transport of a single photon is strongly modified by the presence of multi-photon bound states, owing to interactions that break particle number conservation. Exploiting the effective photon-photon interactions mediated by the qubit, the driven system can be configured as a continuous reservoir of strongly-correlated photons, a resource of interest for quantum networks. This work opens exciting prospects for exploring nonlinear quantum optics at the single-photon level and stabilizing entangled many-body phases of light.

}, keywords = {FOS: Physical sciences, Quantum Gases (cond-mat.quant-gas), Quantum Physics (quant-ph)}, doi = {10.48550/ARXIV.2209.14972}, url = {https://arxiv.org/abs/2209.14972}, author = {Vrajitoarea, Andrei and Belyansky, Ron and Lundgren, Rex and Whitsitt, Seth and Alexey V. Gorshkov and Houck, Andrew A.} } @article {3030, title = {Universal scattering with general dispersion relations}, journal = {Phys. Rev. Research}, volume = {4}, year = {2022}, month = {4/6/2022}, chapter = {023014}, abstract = {

Many synthetic quantum systems allow particles to have dispersion relations that are neither linear nor quadratic functions. Here, we explore single-particle scattering in general spatial dimension D\≥1 when the density of states diverges at a specific energy. To illustrate the underlying principles in an experimentally relevant setting, we focus on waveguide quantum electrodynamics (QED) problems (i.e. D=1) with dispersion relation ϵ(k)=\±|d|km, where m\≥2 is an integer. For a large class of these problems for any positive integer m, we rigorously prove that when there are no bright zero-energy eigenstates, the S-matrix evaluated at an energy E\→0 converges to a universal limit that is only dependent on m. We also give a generalization of a key index theorem in quantum scattering theory known as Levinson\&$\#$39;s theorem -- which relates the scattering phases to the number of bound states -- to waveguide QED scattering for these more general dispersion relations. We then extend these results to general integer dimensions D\≥1, dispersion relations ϵ(k)=|k|a for a D-dimensional momentum vector k with any real positive a, and separable potential scattering.

}, keywords = {FOS: Physical sciences, Mathematical Physics (math-ph), Quantum Physics (quant-ph)}, doi = {https://doi.org/10.1103/PhysRevResearch.4.023014}, url = {https://arxiv.org/abs/2103.09830}, author = {Wang, Yidan and Michael Gullans and Na, Xuesen and Whitsitt, Seth and Alexey V. Gorshkov} } @article {2760, title = {Universality in one-dimensional scattering with general dispersion relations}, journal = {Phys. Rev. Res.}, volume = {4}, year = {2022}, month = {3/17/2021}, chapter = {023014}, abstract = {

Many synthetic quantum systems allow particles to have dispersion relations that are neither linear nor quadratic functions. Here, we explore single-particle scattering in one dimension when the dispersion relation is ϵ(k)=\±|d|km, where m\≥2 is an integer. We study impurity scattering problems in which a single-particle in a one-dimensional waveguide scatters off of an inhomogeneous, discrete set of sites locally coupled to the waveguide. For a large class of these problems, we rigorously prove that when there are no bright zero-energy eigenstates, the S-matrix evaluated at an energy E\→0 converges to a universal limit that is only dependent on m. We also give a generalization of a key index theorem in quantum scattering theory known as Levinson\&$\#$39;s theorem -- which relates the scattering phases to the number of bound states -- to impurity scattering for these more general dispersion relations.

}, doi = {https://doi.org/10.48550/arXiv.2103.09830}, url = {https://arxiv.org/abs/2103.09830}, author = {Yidan Wang and Michael Gullans and Xuesen Na and Alexey V. Gorshkov} } @article {3020, title = {Unlimited non-causal correlations and their relation to non-locality}, journal = {Quantum}, volume = {6}, year = {2022}, month = {3/22/2022}, pages = {673}, abstract = {

Non-causal correlations certify the lack of a definite causal order among localized space-time regions. In stark contrast to scenarios where a single region influences its own causal past, some processes that distribute non-causal correlations satisfy a series of natural desiderata: logical consistency, linear and reversible dynamics, and computational tameness. Here, we present such processes among arbitrary many regions where each region influences every other but itself, and show that the above desiderata are altogether insufficient to limit the amount of \"acausality\" of non-causal correlations. This leaves open the identification of a principle that forbids non-causal correlations. Our results exhibit qualitative and quantitative parallels with the non-local correlations due to Ardehali and Svetlichny.

}, doi = {https://doi.org/10.22331\%2Fq-2022-03-29-673}, url = {https://arxiv.org/abs/2104.06234}, author = {{\"A}min Baumeler and Amin Shiraz Gilani and Jibran Rashid} } @article {2450, title = {Ultralight dark matter detection with mechanical quantum sensors}, journal = {New Journal of Physics}, volume = {23}, year = {2021}, month = {3/10/2021}, pages = {023041}, abstract = {

We consider the use of quantum-limited mechanical force sensors to detect ultralight (sub-meV) dark matter candidates which are weakly coupled to the standard model. We show that mechanical sensors with masses around or below the milligram scale, operating around the standard quantum limit, would enable novel searches for dark matter with natural frequencies around the kHz scale. This would complement existing strategies based on torsion balances, atom interferometers, and atomic clock systems

}, issn = {1367-2630}, doi = {https://doi.org/10.1088/1367-2630/abd9e7}, url = {https://arxiv.org/abs/1908.04797}, author = {Daniel Carney and Anson Hook and Zhen Liu and J. M. Taylor and Yue Zhao} } @article {2866, title = {Unifying Quantum and Classical Speed Limits on Observables}, year = {2021}, month = {8/9/2021}, abstract = {

The presence of noise or the interaction with an environment can radically change the dynamics of observables of an otherwise isolated quantum system. We derive a bound on the speed with which observables of open quantum systems evolve. This speed limit divides into Mandalestam and Tamm\&$\#$39;s original time-energy uncertainty relation and a time-information uncertainty relation recently derived for classical systems, generalizing both to open quantum systems. By isolating the coherent and incoherent contributions to the system dynamics, we derive both lower and upper bounds to the speed of evolution. We prove that the latter provide tighter limits on the speed of observables than previously known quantum speed limits, and that a preferred basis of \emph{speed operators} serves to completely characterize the observables that saturate the speed limits. We use this construction to bound the effect of incoherent dynamics on the evolution of an observable and to find the Hamiltonian that gives the maximum coherent speedup to the evolution of an observable.

}, url = {https://arxiv.org/abs/2108.04261}, author = {Luis Pedro Garc{\'\i}a-Pintos and Schuyler Nicholson and Jason R. Green and Adolfo del Campo and Alexey V. Gorshkov} } @article {2862, title = {Using an Atom Interferometer to Infer Gravitational Entanglement Generation}, journal = {PRX Quantum}, volume = {2}, year = {2021}, month = {8/20/2021}, abstract = {

If gravitational perturbations are quantized into gravitons in analogy with the electromagnetic field and photons, the resulting graviton interactions should lead to an entangling interaction between massive objects. We suggest a test of this prediction. To do this, we introduce the concept of interactive quantum information sensing. This novel sensing protocol is tailored to provable verification of weak dynamical entanglement generation between a pair of systems. We show that this protocol is highly robust to typical thermal noise sources. The sensitivity can moreover be increased both using an initial thermal state and/or an initial phase of entangling via a non-gravitational interaction. We outline a concrete implementation testing the ability of the gravitational field to generate entanglement between an atomic interferometer and mechanical oscillator. Preliminary numerical estimates suggest that near-term devices could feasibly be used to perform the experiment.

}, issn = {2691-3399}, doi = {10.1103/prxquantum.2.030330}, url = {http://dx.doi.org/10.1103/PRXQuantum.2.030330}, author = {Carney, Daniel and M{\"u}ller, Holger and Taylor, Jacob M.} } @article {2673, title = {Understanding the Frauchiger-Renner Argument}, year = {2020}, month = {9/4/2020}, abstract = {

In 2018, Daniela Frauchiger and Renato Renner published an article in Nature Communications entitled {\textquoteleft}Quantum theory cannot consistently describe the use of itself.\&$\#$39; I clarify the significance of the result and point out a common and persistent misunderstanding of the argument, which has been attacked as flawed from a variety of interpretational perspectives.

}, url = {https://arxiv.org/abs/2008.08538}, author = {Jeffrey Bub} } @article {2494, title = {Unitary Subharmonic Response and Floquet Majorana Modes}, journal = {Phys. Rev. Lett.}, volume = {125}, year = {2020}, month = {10/13/2020}, abstract = {

Detection and manipulation of excitations with non-Abelian statistics, such as Majorana fermions, are essential for creating topological quantum computers. To this end, we show the connection between the existence of such localized particles and the phenomenon of unitary subharmonic response (SR) in periodically driven systems. In particular, starting from highly non-equilibrium initial states, the unpaired Majorana modes exhibit spin oscillations with twice the driving period, are localized, and can have exponentially long lifetimes in clean systems. While the lifetime of SR is limited in translationally invariant systems, we show that disorder can be engineered to stabilize the subharmonic response of Majorana modes. A viable observation of this phenomenon can be achieved using modern multi-qubit hardware, such as superconducting circuits and cold atomic systems

}, doi = {https://doi.org/10.1103/PhysRevLett.125.086804}, url = {https://arxiv.org/abs/1911.05795}, author = {Oles Shtanko and Ramis Movassagh} } @article {2578, title = {Universal one-dimensional discrete-time quantum walks and their implementation on near term quantum hardware}, year = {2020}, month = {1/30/2020}, abstract = {

Quantum walks are a promising framework for developing quantum algorithms and quantum simulations. Quantum walks represent an important test case for the application of quantum computers. Here we present different forms of discrete-time quantum walks and show their equivalence for physical realizations. Using an appropriate digital mapping of the position space on which a walker evolves onto the multi-qubit states in a quantum processor, we present different configurations of quantum circuits for the implementation of discrete-time quantum walks in one-dimensional position space. With example circuits for a five qubit machine we address scalability to higher dimensions and larger quantum processors.

}, url = {https://arxiv.org/abs/2001.11197}, author = {Shivani Singh and Cinthia H. Alderete and Radhakrishnan Balu and Christopher Monroe and Norbert M. Linke and C. M. Chandrashekar} } @article {2534, title = {Universal Constraints on Energy Flow and SYK Thermalization}, year = {2019}, month = {12/10/2019}, url = {https://arxiv.org/abs/1912.04912}, author = {Ahmed Almheiri and Alexey Milekhin and Brian Swingle} } @article {2622, title = {Unforgeable Quantum Encryption}, journal = {In: Nielsen J., Rijmen V. (eds) Advances in Cryptology {\textendash} EUROCRYPT 2018. Lecture Notes in Computer Science, Springer, Cham}, volume = {10822}, year = {2018}, abstract = {

We study the problem of encrypting and authenticating quantum data in the presence of adversaries making adaptive chosen plaintext and chosen ciphertext queries. Classically, security games use string copying and comparison to detect adversarial cheating in such scenarios. Quantumly, this approach would violate no-cloning. We develop new techniques to overcome this problem: we use entanglement to detect cheating, and rely on recent results for characterizing quantum encryption schemes. We give definitions for (i) ciphertext unforgeability, (ii) indistinguishability under adaptive chosen-ciphertext attack, and (iii) authenticated encryption. The restriction of each definition to the classical setting is at least as strong as the corresponding classical notion: (i) implies\ \  INT-CTXT , (ii) implies\ \  IND-CCA2 , and (iii) implies\ \  AE . All of our new notions also imply\ \  QIND-CPA\  privacy. Combining one-time authentication and classical pseudorandomness, we construct symmetric-key quantum encryption schemes for each of these new security notions, and provide several separation examples. Along the way, we also give a new definition of one-time quantum authentication which, unlike all previous approaches, authenticates ciphertexts rather than plaintexts.

}, doi = {https://doi.org/10.1007/978-3-319-78372-7_16}, author = {Gorjan Alagic and Tommaso Gagliardoni and Christian Majenz} } @article {2215, title = {Unitary Entanglement Construction in Hierarchical Networks}, year = {2018}, abstract = {

The construction of large-scale quantum computers will require modular architectures that allow physical resources to be localized in easy-to-manage packages. In this work, we examine the impact of different graph structures on the preparation of entangled states. We begin by explaining a formal framework, the hierarchical product, in which modular graphs can be easily constructed. This framework naturally leads us to suggest a class of graphs, which we dub hierarchies. We argue that such graphs have favorable properties for quantum information processing, such as a small diameter and small total edge weight, and use the concept of Pareto efficiency to identify promising quantum graph architectures. We present numerical and analytical results on the speed at which large entangled states can be created on nearest-neighbor grids and hierarchy graphs. We also present a scheme for performing circuit placement--the translation from circuit diagrams to machine qubits--on quantum systems whose connectivity is described by hierarchies.

}, url = {https://arxiv.org/abs/1808.07876}, author = {Aniruddha Bapat and Zachary Eldredge and James R. Garrison and Abhinav Desphande and Frederic T. Chong and Alexey V. Gorshkov} } @article {2057, title = {Unforgeable Quantum Encryption}, year = {2017}, month = {2017/09/19}, abstract = {

We study the problem of encrypting and authenticating quantum data in the presence of adversaries making adaptive chosen plaintext and chosen ciphertext queries. Classically, security games use string copying and comparison to detect adversarial cheating in such scenarios. Quantumly, this approach would violate no-cloning. We develop new techniques to overcome this problem: we use entanglement to detect cheating, and rely on recent results for characterizing quantum encryption schemes. We give definitions for (i.) ciphertext unforgeability , (ii.) indistinguishability under adaptive chosen-ciphertext attack, and (iii.) authenticated encryption. The restriction of each definition to the classical setting is at least as strong as the corresponding classical notion: (i) implies INT-CTXT, (ii) implies IND-CCA2, and (iii) implies AE. All of our new notions also imply QIND-CPA privacy. Combining one-time authentication and classical pseudorandomness, we construct schemes for each of these new quantum security notions, and provide several separation examples. Along the way, we also give a new definition of one-time quantum authentication which, unlike all previous approaches, authenticates ciphertexts rather than plaintexts.

}, url = {https://arxiv.org/abs/1709.06539}, author = {Gorjan Alagic and Tommaso Gagliardoni and Christian Majenz} } @article {2036, title = {Universal Security for Randomness Expansion from the Spot-Checking Protocol}, journal = {SIAM Journal on Computing}, volume = {46}, year = {2017}, month = {2017/08/01}, abstract = {

Colbeck (Thesis, 2006) proposed using Bell inequality violations to generate certified random numbers. While full quantum-security proofs have been given, it remains a major open problem to identify the broadest class of Bell inequalities and lowest performance requirements to achieve such security. In this paper, working within the broad class of spot-checking protocols, we prove exactly which Bell inequality violations can be used to achieve full security. Our result greatly improves the known noise tolerance for secure randomness expansion: for the commonly used CHSH game, full security was only known with a noise tolerance of 1.5\%, and we improve this to 10.3\%. We also generalize our results beyond Bell inequalities and give the first security proof for randomness expansion based on Kochen-Specker inequalities. The central technical contribution of the paper is a new uncertainty principle for the Schatten norm, which is based on the uniform convexity inequality of Ball, Carlen, and Lieb (Inventiones mathematicae, 115:463-482, 1994).

}, doi = {10.1137/15M1044333}, url = {http://epubs.siam.org/doi/10.1137/15M1044333}, author = {Carl Miller and Yaoyun Shi} } @article {2069, title = {Use of global interactions in efficient quantum circuit constructions}, journal = {New Journal of Physics}, year = {2017}, month = {2017/12/21}, abstract = {

In this paper we study the ways to use a global entangling operator to efficiently implement circuitry common to a selection of important quantum algorithms. In particular, we focus on the circuits composed with global Ising entangling gates and arbitrary addressable single-qubit gates. We show that under certain circumstances the use of global operations can substantially improve the entangling gate count.

}, doi = {10.1088/1367-2630/aaa398}, url = {http://iopscience.iop.org/article/10.1088/1367-2630/aaa398}, author = {Dmitri Maslov and Yunseong Nam} } @article {1517, title = {Upper bounds on quantum query complexity inspired by the Elitzur-Vaidman bomb tester}, journal = {Theory of Computing}, volume = {12}, year = {2016}, month = {2016/11/28}, pages = {1-35}, abstract = {

Inspired by the Elitzur-Vaidman bomb testing problem [arXiv:hep-th/9305002], we introduce a new query complexity model, which we call bomb query complexity $B(f)$. We investigate its relationship with the usual quantum query complexity $Q(f)$, and show that $B(f)=\Theta(Q(f)^2)$. This result gives a new method to upper bound the quantum query complexity: we give a method of finding bomb query algorithms from classical algorithms, which then provide nonconstructive upper bounds on $Q(f)=\Theta(\sqrt{B(f)})$. We subsequently were able to give explicit quantum algorithms matching our upper bound method. We apply this method on the single-source shortest paths problem on unweighted graphs, obtaining an algorithm with $O(n^{1.5})$ quantum query complexity, improving the best known algorithm of $O(n^{1.5}\sqrt{\log n})$ [arXiv:quant-ph/0606127]. Applying this method to the maximum bipartite matching problem gives an $O(n^{1.75})$ algorithm, improving the best known trivial $O(n^2)$ upper bound.

}, doi = {10.4086/toc.2016.v012a018}, url = {http://theoryofcomputing.org/articles/v012a018/}, author = {Cedric Yen-Yu Lin and Han-Hsuan Lin} } @article {1450, title = {Universal Subspaces for Local Unitary Groups of Fermionic Systems}, journal = {Communications in Mathematical Physics}, volume = {333}, year = {2015}, month = {2014/10/10}, pages = {541 - 563}, abstract = { Let $\mathcal{V}=\wedge^N V$ be the $N$-fermion Hilbert space with $M$-dimensional single particle space $V$ and $2N\le M$. We refer to the unitary group $G$ of $V$ as the local unitary (LU) group. We fix an orthonormal (o.n.) basis $\ket{v_1},...,\ket{v_M}$ of $V$. Then the Slater determinants $e_{i_1,...,i_N}:= \ket{v_{i_1}\we v_{i_2}\we...\we v_{i_N}}$ with $i_1<...3. If $M$ is even, the well known BCS states are not LU-equivalent to any single occupancy state. Our main result is that for N=3 and $M$ even there is a universal subspace $\cW\subseteq\cS$ spanned by $M(M-1)(M-5)/6$ states $e_{i_1,...,i_N}$. Moreover the number $M(M-1)(M-5)/6$ is minimal. }, doi = {10.1007/s00220-014-2187-6}, url = {http://arxiv.org/abs/1301.3421v2}, author = {Lin Chen and Jianxin Chen and Dragomir Z. Djokovic and Bei Zeng} } @article {1445, title = {Unextendible Product Basis for Fermionic Systems}, journal = {Journal of Mathematical Physics}, volume = {55}, year = {2014}, month = {2014/01/01}, pages = {082207}, abstract = { We discuss the concept of unextendible product basis (UPB) and generalized UPB for fermionic systems, using Slater determinants as an analogue of product states, in the antisymmetric subspace $\wedge^ N \bC^M$. We construct an explicit example of generalized fermionic unextendible product basis (FUPB) of minimum cardinality $N(M-N)+1$ for any $N\ge2,M\ge4$. We also show that any bipartite antisymmetric space $\wedge^ 2 \bC^M$ of codimension two is spanned by Slater determinants, and the spaces of higher codimension may not be spanned by Slater determinants. Furthermore, we construct an example of complex FUPB of $N=2,M=4$ with minimum cardinality $5$. In contrast, we show that a real FUPB does not exist for $N=2,M=4$ . Finally we provide a systematic construction for FUPBs of higher dimensions using FUPBs and UPBs of lower dimensions. }, doi = {10.1063/1.4893358}, url = {http://arxiv.org/abs/1312.4218v1}, author = {Jianxin Chen and Lin Chen and Bei Zeng} } @article {1459, title = {Uniqueness of Quantum States Compatible with Given Measurement Results}, journal = {Physical Review A}, volume = {88}, year = {2013}, month = {2013/7/11}, abstract = { We discuss the uniqueness of quantum states compatible with given results for measuring a set of observables. For a given pure state, we consider two different types of uniqueness: (1) no other pure state is compatible with the same measurement results and (2) no other state, pure or mixed, is compatible with the same measurement results. For case (1), it is known that for a d-dimensional Hilbert space, there exists a set of 4d-5 observables that uniquely determines any pure state. We show that for case (2), 5d-7 observables suffice to uniquely determine any pure state. Thus there is a gap between the results for (1) and (2), and we give some examples to illustrate this. The case of observables corresponding to reduced density matrices (RDMs) of a multipartite system is also discussed, where we improve known bounds on local dimensions for case (2) in which almost all pure states are uniquely determined by their RDMs. We further discuss circumstances where (1) can imply (2). We use convexity of the numerical range of operators to show that when only two observables are measured, (1) always implies (2). More generally, if there is a compact group of symmetries of the state space which has the span of the observables measured as the set of fixed points, then (1) implies (2). We analyze the possible dimensions for the span of such observables. Our results extend naturally to the case of low rank quantum states. }, doi = {10.1103/PhysRevA.88.012109}, url = {http://arxiv.org/abs/1212.3503v2}, author = {Jianxin Chen and Hillary Dawkins and Zhengfeng Ji and Nathaniel Johnston and David Kribs and Frederic Shultz and Bei Zeng} } @article {1221, title = {Universal computation by multi-particle quantum walk}, journal = {Science}, volume = {339}, year = {2013}, month = {2013/02/14}, pages = {791 - 794}, abstract = { A quantum walk is a time-homogeneous quantum-mechanical process on a graph defined by analogy to classical random walk. The quantum walker is a particle that moves from a given vertex to adjacent vertices in quantum superposition. Here we consider a generalization of quantum walk to systems with more than one walker. A continuous-time multi-particle quantum walk is generated by a time-independent Hamiltonian with a term corresponding to a single-particle quantum walk for each particle, along with an interaction term. Multi-particle quantum walk includes a broad class of interacting many-body systems such as the Bose-Hubbard model and systems of fermions or distinguishable particles with nearest-neighbor interactions. We show that multi-particle quantum walk is capable of universal quantum computation. Since it is also possible to efficiently simulate a multi-particle quantum walk of the type we consider using a universal quantum computer, this model exactly captures the power of quantum computation. In principle our construction could be used as an architecture for building a scalable quantum computer with no need for time-dependent control. }, doi = {10.1126/science.1229957}, url = {http://arxiv.org/abs/1205.3782v2}, author = {Andrew M. Childs and David Gosset and Zak Webb} } @article {1444, title = {Universal Entanglers for Bosonic and Fermionic Systems}, year = {2013}, month = {2013/05/31}, abstract = { A universal entangler (UE) is a unitary operation which maps all pure product states to entangled states. It is known that for a bipartite system of particles $1,2$ with a Hilbert space $\mathbb{C}^{d_1}\otimes\mathbb{C}^{d_2}$, a UE exists when $\min{(d_1,d_2)}\geq 3$ and $(d_1,d_2)\neq (3,3)$. It is also known that whenever a UE exists, almost all unitaries are UEs; however to verify whether a given unitary is a UE is very difficult since solving a quadratic system of equations is NP-hard in general. This work examines the existence and construction of UEs of bipartite bosonic/fermionic systems whose wave functions sit in the symmetric/antisymmetric subspace of $\mathbb{C}^{d}\otimes\mathbb{C}^{d}$. The development of a theory of UEs for these types of systems needs considerably different approaches from that used for UEs of distinguishable systems. This is because the general entanglement of identical particle systems cannot be discussed in the usual way due to the effect of (anti)-symmetrization which introduces "pseudo entanglement" that is inaccessible in practice. We show that, unlike the distinguishable particle case, UEs exist for bosonic/fermionic systems with Hilbert spaces which are symmetric (resp. antisymmetric) subspaces of $\mathbb{C}^{d}\otimes\mathbb{C}^{d}$ if and only if $d\geq 3$ (resp. $d\geq 8$). To prove this we employ algebraic geometry to reason about the different algebraic structures of the bosonic/fermionic systems. Additionally, due to the relatively simple coherent state form of unentangled bosonic states, we are able to give the explicit constructions of two bosonic UEs. Our investigation provides insight into the entanglement properties of systems of indisitinguishable particles, and in particular underscores the difference between the entanglement structures of bosonic, fermionic and distinguishable particle systems. }, url = {http://arxiv.org/abs/1305.7489v1}, author = {Joel Klassen and Jianxin Chen and Bei Zeng} } @article {1350, title = {Unified approach to topological quantum computation with anyons: From qubit encoding to Toffoli gate}, journal = {Physical Review A}, volume = {84}, year = {2011}, month = {2011/7/26}, abstract = {Topological quantum computation may provide a robust approach for encoding and manipulating information utilizing the topological properties of anyonic quasi-particle excitations. We develop an efficient means to map between dense and sparse representations of quantum information (qubits) and a simple construction of multi-qubit gates, for all anyon models from Chern-Simons-Witten SU(2)$_k$ theory that support universal quantum computation by braiding ($k\geq 3,\ k \neq 4$). In the process, we show how the constructions of topological quantum memory and gates for $k=2,4$ connect naturally to those for $k\geq 3,\ k \neq 4$, unifying these concepts in a simple framework. Furthermore, we illustrate potential extensions of these ideas to other anyon models outside of Chern-Simons-Witten field theory. }, doi = {10.1103/PhysRevA.84.012332}, url = {http://arxiv.org/abs/1001.4085v2}, author = {Haitan Xu and J. M. Taylor} } @article {1427, title = {Universal low-rank matrix recovery from Pauli measurements}, journal = {Advances in Neural Information Processing Systems (NIPS)}, year = {2011}, month = {2011/03/14}, pages = {1638-1646}, abstract = { We study the problem of reconstructing an unknown matrix M of rank r and dimension d using O(rd poly log d) Pauli measurements. This has applications in quantum state tomography, and is a non-commutative analogue of a well-known problem in compressed sensing: recovering a sparse vector from a few of its Fourier coefficients. We show that almost all sets of O(rd log^6 d) Pauli measurements satisfy the rank-r restricted isometry property (RIP). This implies that M can be recovered from a fixed ("universal") set of Pauli measurements, using nuclear-norm minimization (e.g., the matrix Lasso), with nearly-optimal bounds on the error. A similar result holds for any class of measurements that use an orthonormal operator basis whose elements have small operator norm. Our proof uses Dudley{\textquoteright}s inequality for Gaussian processes, together with bounds on covering numbers obtained via entropy duality. }, url = {http://arxiv.org/abs/1103.2816v2}, author = {Yi-Kai Liu} } @article {1205, title = {Universal computation by quantum walk}, journal = {Physical Review Letters}, volume = {102}, year = {2009}, month = {2009/5/4}, abstract = { In some of the earliest work on quantum mechanical computers, Feynman showed how to implement universal quantum computation by the dynamics of a time-independent Hamiltonian. I show that this remains possible even if the Hamiltonian is restricted to be a sparse matrix with all entries equal to 0 or 1, i.e., the adjacency matrix of a low-degree graph. Thus quantum walk can be regarded as a universal computational primitive, with any desired quantum computation encoded entirely in some underlying graph. The main idea of the construction is to implement quantum gates by scattering processes. }, doi = {10.1103/PhysRevLett.102.180501}, url = {http://arxiv.org/abs/0806.1972v1}, author = {Andrew M. Childs} } @article {2615, title = {Uncertainty principles for compact groups}, journal = {Illinois J. Math. }, volume = {52}, year = {2008}, pages = {1315-1324}, abstract = {

We establish an uncertainty principle over arbitrary compact groups, generalizing several previous results. Specifically, we show that if P and R are operators on L2(G) such that P commutes with projection onto every measurable subset of G and R commutes with left-multiplication by elements of G, then ||PR||\≤||P\⋅χG||2||R||2, where χG:g↦1 is the characteristic function of G. As a consequence, we show that every nonzero function f in L2(G) satisfies μ(suppf)\⋅\∑ρ\∈G^dρrankf^(ρ)\≥1.

}, doi = {doi:10.1215/ijm/1258554365}, url = {http://projecteuclid.org/euclid.ijm/1258554365}, author = {Gorjan Alagic and Alexander Russell} } @article {1180, title = {Universal Approach to Optimal Photon Storage in Atomic Media}, journal = {Physical Review Letters}, volume = {98}, year = {2007}, month = {2007/3/19}, abstract = { We present a universal physical picture for describing storage and retrieval of photon wave packets in a Lambda-type atomic medium. This physical picture encompasses a variety of different approaches to pulse storage ranging from adiabatic reduction of the photon group velocity and pulse-propagation control via off-resonant Raman fields to photon-echo based techniques. Furthermore, we derive an optimal control strategy for storage and retrieval of a photon wave packet of any given shape. All these approaches, when optimized, yield identical maximum efficiencies, which only depend on the optical depth of the medium. }, doi = {10.1103/PhysRevLett.98.123601}, url = {http://arxiv.org/abs/quant-ph/0604037v3}, author = {Alexey V. Gorshkov and Axel Andre and Michael Fleischhauer and Anders S. Sorensen and Mikhail D. Lukin} } @article {1409, title = {Ultracold atoms confined in an optical lattice plus parabolic potential: a closed-form approach }, journal = {Physical Review A}, volume = {72}, year = {2005}, month = {2005/9/22}, abstract = { We discuss interacting and non-interacting one dimensional atomic systems trapped in an optical lattice plus a parabolic potential. We show that, in the tight-binding approximation, the non-interacting problem is exactly solvable in terms of Mathieu functions. We use the analytic solutions to study the collective oscillations of ideal bosonic and fermionic ensembles induced by small displacements of the parabolic potential. We treat the interacting boson problem by numerical diagonalization of the Bose-Hubbard Hamiltonian. From analysis of the dependence upon lattice depth of the low-energy excitation spectrum of the interacting system, we consider the problems of "fermionization" of a Bose gas, and the superfluid-Mott insulator transition. The spectrum of the noninteracting system turns out to provide a useful guide to understanding the collective oscillations of the interacting system, throughout a large and experimentally relevant parameter regime. }, doi = {10.1103/PhysRevA.72.033616}, url = {http://arxiv.org/abs/cond-mat/0503477v2}, author = {Ana Maria Rey and Guido Pupillo and Charles W. Clark and Carl J. Williams} } @article {1251, title = {Unified derivations of measurement-based schemes for quantum computation}, journal = {Physical Review A}, volume = {71}, year = {2005}, month = {2005/3/17}, abstract = { We present unified, systematic derivations of schemes in the two known measurement-based models of quantum computation. The first model (introduced by Raussendorf and Briegel [Phys. Rev. Lett., 86, 5188 (2001)]) uses a fixed entangled state, adaptive measurements on single qubits, and feedforward of the measurement results. The second model (proposed by Nielsen [Phys. Lett. A, 308, 96 (2003)] and further simplified by Leung [Int. J. Quant. Inf., 2, 33 (2004)]) uses adaptive two-qubit measurements that can be applied to arbitrary pairs of qubits, and feedforward of the measurement results. The underlying principle of our derivations is a variant of teleportation introduced by Zhou, Leung, and Chuang [Phys. Rev. A, 62, 052316 (2000)]. Our derivations unify these two measurement-based models of quantum computation and provide significantly simpler schemes. }, doi = {10.1103/PhysRevA.71.032318}, url = {http://arxiv.org/abs/quant-ph/0404132v2}, author = {Andrew M. Childs and Debbie W. Leung and Michael A. Nielsen} } @article {1289, title = {Ultracold collision properties of metastable alkaline-earth atoms}, journal = {Physical Review Letters}, volume = {90}, year = {2003}, month = {2003/2/13}, abstract = { Ultra-cold collisions of spin-polarized 24Mg,40Ca, and 88Sr in the metastable 3P2 excited state are investigated. We calculate the long-range interaction potentials and estimate the scattering length and the collisional loss rate as a function of magnetic field. The estimates are based on molecular potentials between 3P2 alkaline-earth atoms obtained from ab initio atomic and molecular structure calculations. The scattering lengths show resonance behavior due to the appearance of a molecular bound state in a purely long-range interaction potential and are positive for magnetic fields below 50 mT. A loss-rate model shows that losses should be smallest near zero magnetic field and for fields slightly larger than the resonance field, where the scattering length is also positive. }, doi = {10.1103/PhysRevLett.90.063002}, url = {http://arxiv.org/abs/physics/0210076v1}, author = {Andrei Derevianko and Sergey G. Porsev and Svetlana Kotochigova and Eite Tiesinga and Paul S. Julienne} } @article {1296, title = {Ultracold Cs$_2$ Feshbach Spectroscopy}, year = {2003}, month = {2003/12/23}, abstract = { We have observed and located more than 60 magnetic field-induced Feshbach resonances in ultracold collisions of ground-state $^{133}$Cs atoms. These resonances are associated with molecular states with up to four units of rotational angular momentum, and are detected through variations in the elastic, inelastic, and radiative collision cross sections. These observations allow us to greatly improve upon the interaction potentials between two cesium atoms and to reproduce the positions of most resonances to accuracies better than 0.5\%. Based on the relevant coupling scheme between the electron spin, nuclear spin, and orbital angular momenta of the nuclei, quantum numbers and energy structure of the molecular states beneath the dissociation continuum are revealed. Finally, we predict the relevant collision properties for cesium Bose-Einstein condensation experiments. }, url = {http://arxiv.org/abs/cond-mat/0312613v2}, author = {Cheng Chin and Vladan Vuletic and Andrew J. Kerman and Steven Chu and Eite Tiesinga and Paul J. Leo and Carl J. Williams} } @article {1261, title = {Universal simulation of Hamiltonian dynamics for qudits}, journal = {Physical Review A}, volume = {66}, year = {2002}, month = {2002/8/30}, abstract = { What interactions are sufficient to simulate arbitrary quantum dynamics in a composite quantum system? Dodd et al. (quant-ph/0106064) provided a partial solution to this problem in the form of an efficient algorithm to simulate any desired two-body Hamiltonian evolution using any fixed two-body entangling N-qubit Hamiltonian, and local unitaries. We extend this result to the case where the component systems have D dimensions. As a consequence we explain how universal quantum computation can be performed with any fixed two-body entangling N-qudit Hamiltonian, and local unitaries. }, doi = {10.1103/PhysRevA.66.022317}, url = {http://arxiv.org/abs/quant-ph/0109064v2}, author = {Michael A. Nielsen and Michael J. Bremner and Jennifer L. Dodd and Andrew M. Childs and Christopher M. Dawson} } @article {1260, title = {Universal simulation of Markovian quantum dynamics}, journal = {Physical Review A}, volume = {64}, year = {2001}, month = {2001/11/9}, abstract = { Although the conditions for performing arbitrary unitary operations to simulate the dynamics of a closed quantum system are well understood, the same is not true of the more general class of quantum operations (also known as superoperators) corresponding to the dynamics of open quantum systems. We propose a framework for the generation of Markovian quantum dynamics and study the resources needed for universality. For the case of a single qubit, we show that a single nonunitary process is necessary and sufficient to generate all unital Markovian quantum dynamics, whereas a set of processes parametrized by one continuous parameter is needed in general. We also obtain preliminary results for the unital case in higher dimensions. }, doi = {10.1103/PhysRevA.64.062302}, url = {http://arxiv.org/abs/quant-ph/0008070v2}, author = {Dave Bacon and Andrew M. Childs and Isaac L. Chuang and Julia Kempe and Debbie W. Leung and Xinlan Zhou} } @article {1207, title = {Universal quantum computation with two-level trapped ions}, journal = {Physical Review A}, volume = {63}, year = {2000}, month = {2000/12/11}, abstract = { Although the initial proposal for ion trap quantum computation made use of an auxiliary internal level to perform logic between ions, this resource is not necessary in principle. Instead, one may perform such operations directly using sideband laser pulses, operating with an arbitrary (sufficiently small) Lamb-Dicke parameter. We explore the potential of this technique, showing how to perform logical operations between the internal state of an ion and the collective motional state and giving explicit constructions for a controlled-not gate between ions. }, doi = {10.1103/PhysRevA.63.012306}, url = {http://arxiv.org/abs/quant-ph/0008065v1}, author = {Andrew M. Childs and Isaac L. Chuang} }