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Articles 181 - 200 of 200
Full-Text Articles in Quantum Physics
Nonclassicality Without Entanglement Enables Bit Commitment, Howard Barnum, Oscar C. O. Dahlsten, Matthew S. Leifer, Ben Toner
Nonclassicality Without Entanglement Enables Bit Commitment, Howard Barnum, Oscar C. O. Dahlsten, Matthew S. Leifer, Ben Toner
Mathematics, Physics, and Computer Science Faculty Articles and Research
We investigate the existence of secure bit commitment protocols in the convex framework for probabilistic theories. The framework makes only minimal assumptions, and can be used to formalize quantum theory, classical probability theory, and a host of other possibilities. We prove that in all such theories that are locally non-classical but do not have entanglement, there exists a bit commitment protocol that is exponentially secure in the number of systems used.
Large-Alphabet Quantum Key Distribution Using Energy-Time Entangled Bipartite States, Irfan Ali-Khan, Curtis J. Broadbent, John C. Howell
Large-Alphabet Quantum Key Distribution Using Energy-Time Entangled Bipartite States, Irfan Ali-Khan, Curtis J. Broadbent, John C. Howell
Mathematics, Physics, and Computer Science Faculty Articles and Research
We present a protocol for large-alphabet quantum key distribution (QKD) using energy-time entangled biphotons. Binned, high-resolution timing measurements are used to generate a large-alphabet key with over 10 bits of information per photon pair, albeit with large noise. QKD with 5% bit error rate is demonstrated with 4 bits of information per photon pair, where the security of the quantum channel is determined by the visibility of Franson interference fringes. The protocol is easily generalizable to even larger alphabets, and utilizes energy-time entanglement which is robust to transmission over large distances in fiber.
Generalized No-Broadcasting Theorem, Howard Barnum, Jonathan Barrett, Matthew S. Leifer, Alex Wilce
Generalized No-Broadcasting Theorem, Howard Barnum, Jonathan Barrett, Matthew S. Leifer, Alex Wilce
Mathematics, Physics, and Computer Science Faculty Articles and Research
We prove a generalized version of the no-broadcasting theorem, applicable to essentially any nonclassical finite-dimensional probabilistic model satisfying a no-signaling criterion, including ones with ‘‘superquantum’’ correlations. A strengthened version of the quantum no-broadcasting theorem follows, and its proof is significantly simpler than existing proofs of the no-broadcasting theorem.
Quantum Dynamics As An Analog Of Conditional Probability, Matthew S. Leifer
Quantum Dynamics As An Analog Of Conditional Probability, Matthew S. Leifer
Mathematics, Physics, and Computer Science Faculty Articles and Research
Quantum theory can be regarded as a noncommutative generalization of classical probability. From this point of view, one expects quantum dynamics to be analogous to classical conditional probabilities. In this paper, a variant of the well-known isomorphism between completely positive maps and bipartite density operators is derived, which makes this connection much more explicit. This isomorphism is given an operational interpretation in terms of statistical correlations between ensemble preparation procedures and outcomes of measurements. Finally, the isomorphism is applied to elucidate the connection between no-cloning and no-broadcasting theorems and the monogamy of entanglement, and a simplified proof of the no-broadcasting …
Conditional Density Operators And The Subjectivity Of Quantum Operations, Matthew S. Leifer
Conditional Density Operators And The Subjectivity Of Quantum Operations, Matthew S. Leifer
Mathematics, Physics, and Computer Science Faculty Articles and Research
Assuming that quantum states, including pure states, represent subjective degrees of belief rather than objective properties of systems, the question of what other elements of the quantum formalism must also be taken as subjective is addressed. In particular, we ask this of the dynamical aspects of the formalism, such as Hamiltonians and unitary operators. Whilst some operations, such as the update maps corresponding to a complete projective measurement, must be subjective, the situation is not so clear in other cases. Here, it is argued that all trace preserving completely positive maps, including unitary operators, should be regarded as subjective, in …
Pre- And Post-Selection Paradoxes And Contextuality In Quantum Mechanics, Matthew S. Leifer, Robert W. Spekkens
Pre- And Post-Selection Paradoxes And Contextuality In Quantum Mechanics, Matthew S. Leifer, Robert W. Spekkens
Mathematics, Physics, and Computer Science Faculty Articles and Research
Many seemingly paradoxical effects are known in the predictions for outcomes of intermediate measurements made on pre- and post-selected quantum systems. Despite appearances, these effects do not demonstrate the impossibility of a noncontextual hidden variable theory, since an explanation in terms of measurement disturbance is possible. Nonetheless, we show that for every paradoxical effect wherein all the pre- and post-selected probabilities are 0 or 1 and the pre- and post-selected states are nonorthogonal, there is an associated proof of the impossibility of a noncontextual hidden variable theory. This proof is obtained by considering all the measurements involved in the paradoxical …
Bell's Jump Process In Discrete Time, Jonathan Barrett, Matthew S. Leifer, Roderich Tumulka
Bell's Jump Process In Discrete Time, Jonathan Barrett, Matthew S. Leifer, Roderich Tumulka
Mathematics, Physics, and Computer Science Faculty Articles and Research
The jump process introduced by J. S. Bell in 1986, for defining a quantum field theory without observers, presupposes that space is discrete whereas time is continuous. In this letter, our interest is to find an analogous process in discrete time. We argue that a genuine analog does not exist, but provide examples of processes in discrete time that could be used as a replacement.
Hong-Ou-Mandel Cloning: Quantum Copying Without An Ancilla, Irfan Ali Khan, John C. Howell
Hong-Ou-Mandel Cloning: Quantum Copying Without An Ancilla, Irfan Ali Khan, John C. Howell
Mathematics, Physics, and Computer Science Faculty Articles and Research
In this paper we report an experimental realization of an ancilla-free 1→2 phase-covariant quantum cloner. The cloner is realized by interfering a linearly polarized photon, which we wish to clone with a circularly polarized photon at a beam splitter. The two-photon effect can be understood in light of Hong-Ou-Mandel interference. The fidelity of the cloner was measured as 0.829±0.008 for the 0∕90 basis and 0.835±0.006 for the 45∕135 basis, which is in good agreement with the theoretical prediction of 5∕6 fidelity. The experimental scheme is straightforward and has a high cloning success rate.
Quantum And Classical Coincidence Imaging, Ryan S. Bennink, Sean J. Bentley, Robert W. Boyd, John C. Howell
Quantum And Classical Coincidence Imaging, Ryan S. Bennink, Sean J. Bentley, Robert W. Boyd, John C. Howell
Mathematics, Physics, and Computer Science Faculty Articles and Research
Coincidence, or ghost, imaging is a technique that uses two correlated optical fields to form an image of an object. In this work we identify aspects of coincidence imaging which can be performed with classically correlated light sources and aspects which require quantum entanglement. We find that entangled photons allow high-contrast, high-resolution imaging to be performed at any distance from the light source. We demonstrate this fact by forming ghost images in the near and far fields of an entangled photon source, noting that the product of the resolutions of these images is a factor of 3 better than that …
Measuring Polynomial Invariants Of Multiparty Quantum States, Matthew S. Leifer, N. Linden, A. Winter
Measuring Polynomial Invariants Of Multiparty Quantum States, Matthew S. Leifer, N. Linden, A. Winter
Mathematics, Physics, and Computer Science Faculty Articles and Research
We present networks for directly estimating the polynomial invariants of multiparty quantum states under local transformations. The structure of these networks is closely related to the structure of the invariants themselves and this lends a physical interpretation to these otherwise abstract mathematical quantities. Specifically, our networks estimate the invariants under local unitary (LU) transformations and under stochastic local operations and classical communication (SLOCC). Our networks can estimate the LU invariants for multiparty states, where each party can have a Hilbert space of arbitrary dimension and the SLOCC invariants for multiqubit states. We analyze the statistical efficiency of our networks compared …
Logical Pre- And Post-Selection Paradoxes, Measurement-Disturbance And Contextuality, Matthew S. Leifer, R. W. Spekkens
Logical Pre- And Post-Selection Paradoxes, Measurement-Disturbance And Contextuality, Matthew S. Leifer, R. W. Spekkens
Mathematics, Physics, and Computer Science Faculty Articles and Research
Many seemingly paradoxical effects are known in the predictions for outcomes of measurements made on pre- and post-selected quantum systems. A class of such effects, which we call “logical pre- and post-selection paradoxes”, bear a striking resemblance to proofs of the Bell-Kochen-Specker theorem, which suggests that they demonstrate the contextuality of quantum mechanics. Despite the apparent similarity, we show that such effects can occur in noncontextual hidden variable theories, provided measurements are allowed to disturb the values of the hidden variables.
Triplet Superconductors From The Viewpoint Of Basic Elements For Quantum Computers, Armen M. Gulian, Kent S. Wood
Triplet Superconductors From The Viewpoint Of Basic Elements For Quantum Computers, Armen M. Gulian, Kent S. Wood
Mathematics, Physics, and Computer Science Faculty Articles and Research
We discuss possibilities of utilizing superconductors with Cooper condensates in triplet pairing states (where the spin of condensate pairs is S=1) for practical realization of quantum computers. Superconductors with triplet pairing condensates have features that are unique and cannot be found in the usual (singlet pairing, S=0) superconductors. The symmetry of the order parameter in some triplet superconductors (e.g., ruthenates) corresponds to doubly-degenerate chiral states. These states can serve as qubit base states for quantum computing.
Optimal Entanglement Generation From Quantum Operations, Matthew S. Leifer, L. Henderson, N. Linden
Optimal Entanglement Generation From Quantum Operations, Matthew S. Leifer, L. Henderson, N. Linden
Mathematics, Physics, and Computer Science Faculty Articles and Research
We consider how much entanglement can be produced by a nonlocal two-qubit unitary operation, UAB—the entangling capacity of UAB. For a single application of UAB , with no ancillas, we find the entangling capacity and show that it generally helps to act with UAB on an entangled state. Allowing ancillas, we present numerical results from which we can conclude, quite generally, that allowing initial entanglement typically increases the optimal capacity in this case as well. Next, we show that allowing collective processing does not increase the entangling capacity if initial entanglement is allowed.
Experimental Quantum Cloning Of Single Photons, Antia Lamas-Linares, Christoph Simon, John C. Howell, Dik Bouwmeester
Experimental Quantum Cloning Of Single Photons, Antia Lamas-Linares, Christoph Simon, John C. Howell, Dik Bouwmeester
Mathematics, Physics, and Computer Science Faculty Articles and Research
Although perfect copying of unknown quantum systems is forbidden by the laws of quantum mechanics, approximate cloning is possible. A natural way of realizing quantum cloning of photons is by stimulated emission. In this context, the fundamental quantum limit to the quality of the clones is imposed by the unavoidable presence of spontaneous emission. In our experiment, a single input photon stimulates the emission of additional photons from a source on the basis of parametric down-conversion. This leads to the production of quantum clones with near-optimal fidelity. We also demonstrate universality of the copying procedure by showing that the same …
Optimal Simulation Of Two-Qubit Hamiltonians Using General Local Operations, C. H. Bennett, J. I. Cirac, Matthew S. Leifer, D. W. Leung, N. Linden, S. Popescu, G. Vidal
Optimal Simulation Of Two-Qubit Hamiltonians Using General Local Operations, C. H. Bennett, J. I. Cirac, Matthew S. Leifer, D. W. Leung, N. Linden, S. Popescu, G. Vidal
Mathematics, Physics, and Computer Science Faculty Articles and Research
We consider the simulation of the dynamics of one nonlocal Hamiltonian by another, allowing arbitrary local resources but no entanglement or classical communication. We characterize notions of simulation, and proceed to focus on deterministic simulation involving one copy of the system. More specifically, two otherwise isolated systems A and B interact by a nonlocal Hamiltonian H≠HA+HB . We consider the achievable space of Hamiltonians H8 such that the evolution e2iH8t can be simulated by the interaction H interspersed with local operations. For any dimensions of A and B, and any nonlocal Hamiltonians H and H8, there exists a scale factor …
Optically Simulating A Quantum Associative Memory, John C. Howell, John A. Yeazell, Dan Ventura
Optically Simulating A Quantum Associative Memory, John C. Howell, John A. Yeazell, Dan Ventura
Mathematics, Physics, and Computer Science Faculty Articles and Research
This paper discusses the realization of a quantum associative memory using linear integrated optics. An associative memory produces a full pattern of bits when presented with only a partial pattern. Quantum computers have the potential to store large numbers of patterns and hence have the ability to far surpass any classical neural-network realization of an associative memory. In this work two three-qubit associative memories will be discussed using linear integrated optics. In addition, corrupted, invented and degenerate memories are discussed.
Quantum Computation Through Entangling Single Photons In Multipath Interferometers, John C. Howell, John A. Yeazell
Quantum Computation Through Entangling Single Photons In Multipath Interferometers, John C. Howell, John A. Yeazell
Mathematics, Physics, and Computer Science Faculty Articles and Research
Single-photon interferometry has been used to simulate quantum computations. Its use has been limited to studying few-bit applications due to rapid growth in physical size with numbers of bits. We propose a hybrid approach that employs n photons, each having L degrees of freedom yielding Ln basis states. The photons are entangled via a quantum nondemolition measurement. This approach introduces the essential element of quantum computing, that is, entanglement into the interferometry. Using these techniques, we demonstrate a controlled-NOT gate and a Grover's search circuit. These ideas are also applicable to the study of nonlocal correlations in many dimensions.
Entangling Macroscopic Quantum States, John C. Howell, John A. Yeazell
Entangling Macroscopic Quantum States, John C. Howell, John A. Yeazell
Mathematics, Physics, and Computer Science Faculty Articles and Research
Spatial entanglements of macroscopic quantum systems are proposed. The which-path uncertainty of a single photon passing through a beam splitter is transformed into the which-path uncertainty of two macroscopic fields via two quantum nondemolition measurements. The macroscopic fields are nonlocally correlated.
Reducing The Complexity Of Linear Optics Quantum Circuits, John C. Howell, John A. Yeazell
Reducing The Complexity Of Linear Optics Quantum Circuits, John C. Howell, John A. Yeazell
Mathematics, Physics, and Computer Science Faculty Articles and Research
Integrated optical elements can simplify the linear optics used to simulate quantum circuits. These linear optical simulations of quantum circuits have been developed primarily in terms of the free space optics associated with single-photon interferometry. For an L-bit simulation the number of required free-space optical elements is ∝2L if 50/50 beam splitters are used. The implementation (construction and alignment) of these circuits with these free-space elements is nontrivial. On the other hand, for the cases presented in this paper in which linear integrated optics (e.g., 2L×2L fiber couplers) are used, the number of optical devices does …
Linear Optics Simulations Of The Quantum Baker’S Map, John C. Howell, John A. Yeazell
Linear Optics Simulations Of The Quantum Baker’S Map, John C. Howell, John A. Yeazell
Mathematics, Physics, and Computer Science Faculty Articles and Research
The unitary evolution of linear optics can be used to model quantum computational networks. In this paper, a quantum simulation of a classically chaotic map (the baker’s map) is developed using linear optics. Two different models are presented. The first model employs only 50-50 beam splitters and phase shifters to simulate universal 2-qubit gates of a quantum computer. The second model uses the discrete Fourier transform generated by symmetric N×N fiber couplers. If single photons are used as inputs for these linear optics models, the result is a physical realization of the quantum baker’s map.