Showing papers on "Qubit published in 2003"
TL;DR: It is found that in a reasonable time, a qubit can be directly transmitted with better than classical fidelity across the full length of chains of up to 80 spins, and the channel allows distillable entanglement to be shared over arbitrary distances.
Abstract: We propose a scheme for using an unmodulated and unmeasured spin chain as a channel for short distance quantum communications. The state to be transmitted is placed on one spin of the chain and received later on a distant spin with some fidelity. We first obtain simple expressions for the fidelity of quantum state transfer and the amount of entanglement sharable between any two sites of an arbitrary Heisenberg ferromagnet using our scheme. We then apply this to the realizable case of an open ended chain with nearest neighbor interactions. The fidelity of quantum state transfer is obtained as an inverse discrete cosine transform and as a Bessel function series. We find that in a reasonable time, a qubit can be directly transmitted with better than classical fidelity across the full length of chains of up to 80 spins. Moreover, our channel allows distillable entanglement to be shared over arbitrary distances.
1,220 citations
TL;DR: The observed coherent time evolution between two quantum states of a superconducting flux qubit comprising three Josephson junctions in a loop is promising for future solid-state quantum computing.
Abstract: We have observed coherent time evolution between two quantum states of a superconducting flux qubit comprising three Josephson junctions in a loop. The superposition of the two states carrying opposite macroscopic persistent currents is manipulated by resonant microwave pulses. Readout by means of switching-event measurement with an attached superconducting quantum interference device revealed quantum-state oscillations with high fidelity. Under strong microwave driving, it was possible to induce hundreds of coherent oscillations. Pulsed operations on this first sample yielded a relaxation time of 900 nanoseconds and a free-induction dephasing time of 20 nanoseconds. These results are promising for future solid-state quantum computing.
1,109 citations
TL;DR: A universal geometric π-phase gate between two beryllium ion-qubits is demonstrated, based on coherent displacements induced by an optical dipole force, which makes it attractive for a multiplexed trap architecture that would enable scaling to large numbers of ions.
Abstract: Universal logic gates for two quantum bits (qubits) form an essential ingredient of quantum computation. Dynamical gates have been proposed in the context of trapped ions; however, geometric phase gates (which change only the phase of the physical qubits) offer potential practical advantages because they have higher intrinsic resistance to certain small errors and might enable faster gate implementation. Here we demonstrate a universal geometric pi-phase gate between two beryllium ion-qubits, based on coherent displacements induced by an optical dipole force. The displacements depend on the internal atomic states; the motional state of the ions is unimportant provided that they remain in the regime in which the force can be considered constant over the extent of each ion's wave packet. By combining the gate with single-qubit rotations, we have prepared ions in an entangled Bell state with 97% fidelity-about six times better than in a previous experiment demonstrating a universal gate between two ion-qubits. The particular properties of the gate make it attractive for a multiplexed trap architecture that would enable scaling to large numbers of ion-qubits.
838 citations
TL;DR: In this article, the authors proposed an experiment for creating quantum superposition states involving of the order of 10(14) atoms via the interaction of a single photon with a tiny mirror.
Abstract: We propose an experiment for creating quantum superposition states involving of the order of 10(14) atoms via the interaction of a single photon with a tiny mirror. This mirror, mounted on a high-quality mechanical oscillator, is part of a high-finesse optical cavity which forms one arm of a Michelson interferometer. By observing the interference of the photon only, one can study the creation and decoherence of superpositions involving the mirror. A detailed analysis of the requirements shows that the experiment is within reach using a combination of state-of-the-art technologies.
825 citations
TL;DR: In this article, an unambiguous experimental demonstration and comprehensive characterization of quantum controlled-NOT operation in an optical system was presented. But the experimental results were limited to a single operation condition of the gate.
Abstract: The promise of tremendous computational power, coupled with the development of robust error-correcting schemes, has fuelled extensive efforts to build a quantum computer. The requirements for realizing such a device are confounding: scalable quantum bits (two-level quantum systems, or qubits) that can be well isolated from the environment, but also initialized, measured and made to undergo controllable interactions to implement a universal set of quantum logic gates. The usual set consists of single qubit rotations and a controlled-NOT (CNOT) gate, which flips the state of a target qubit conditional on the control qubit being in the state 1. Here we report an unambiguous experimental demonstration and comprehensive characterization of quantum CNOT operation in an optical system. We produce all four entangled Bell states as a function of only the input qubits' logical values, for a single operating condition of the gate. The gate is probabilistic (the qubits are destroyed upon failure), but with the addition of linear optical quantum non-demolition measurements, it is equivalent to the CNOT gate required for scalable all-optical quantum computation.
804 citations
01 May 2003
TL;DR: In this article, the authors demonstrate a universal geometric pi-phase gate between two beryllium ion-qubits, based on coherent displacements induced by an optical dipole force.
Abstract: Universal logic gates for two quantum bits (qubits) form an essential ingredient of quantum computation. Dynamical gates have been proposed in the context of trapped ions; however, geometric phase gates (which change only the phase of the physical qubits) offer potential practical advantages because they have higher intrinsic resistance to certain small errors and might enable faster gate implementation. Here we demonstrate a universal geometric pi-phase gate between two beryllium ion-qubits, based on coherent displacements induced by an optical dipole force. The displacements depend on the internal atomic states; the motional state of the ions is unimportant provided that they remain in the regime in which the force can be considered constant over the extent of each ion's wave packet. By combining the gate with single-qubit rotations, we have prepared ions in an entangled Bell state with 97% fidelity-about six times better than in a previous experiment demonstrating a universal gate between two ion-qubits. The particular properties of the gate make it attractive for a multiplexed trap architecture that would enable scaling to large numbers of ion-qubits.
746 citations
TL;DR: It is shown that coherent time evolution of charge states (pseudospin qubit) in a semiconductor double quantum dot is investigated with a high-speed voltage pulse that controls the energy and decoherence of the system.
Abstract: We investigate coherent time evolution of charge states (pseudospin qubit) in a semiconductor double quantum dot. This fully tunable qubit is manipulated with a high-speed voltage pulse that controls the energy and decoherence of the system. Coherent oscillations of the qubit are observed for several combinations of many-body ground and excited states of the quantum dots. Possible decoherence mechanisms in the present device are also discussed.
696 citations
TL;DR: In this paper, it was shown that quantum computation circuits using coherent states as the logical qubits can be constructed from simple linear networks, conditional photon measurements, and small coherent superposition resource states.
Abstract: We show that quantum computation circuits using coherent states as the logical qubits can be constructed from simple linear networks, conditional photon measurements, and "small" coherent superposition resource states.
623 citations
TL;DR: This work demonstrates conditional gate operation using a pair of coupled superconducting charge qubits using a pulse technique and shows that their amplitude can be transformed by controlled-NOT (C-NOT) gate operation, although the phase evolution during the gate operation remains to be clarified.
Abstract: Following the demonstration of coherent control of the quantum state of a superconducting charge qubit1, a variety of qubits based on Josephson junctions have been implemented2,3,4,5. Although such solid-state devices are not currently as advanced as microscopic qubits based on nuclear magnetic resonance6 and ion trap7 technologies, the potential scalability of the former systems—together with progress in their coherence times and read-out schemes—makes them strong candidates for the building block of a quantum computer8. Recently, coherent oscillations9 and microwave spectroscopy10 of capacitively coupled superconducting qubits have been reported; the next challenging step towards quantum computation is the realization of logic gates11,12. Here we demonstrate conditional gate operation using a pair of coupled superconducting charge qubits. Using a pulse technique, we prepare different input states and show that their amplitude can be transformed by controlled-NOT (C-NOT) gate operation, although the phase evolution during the gate operation remains to be clarified.
523 citations
TL;DR: In this article, the geometric structure of non-local gates is shown to be a 3-torus, and the invariants for local transformations are derived from the coordinates of the 3torus.
Abstract: We study nonlocal two-qubit operations from a geometric perspective. By applying a Cartan decomposition to su(4), we find that the geometric structure of nonlocal gates is a 3-torus. We derive the invariants for local transformations, and connect these local invariants to the coordinates of the 3-torus. Since different points on the 3-torus may correspond to the same local equivalence class, we use the Weyl group theory to reduce the symmetry. We show that the local equivalence classes of two-qubit gates are in one-to-one correspondence with the points in a tetrahedron except on the base. We then study the properties of perfect entanglers, that is, the two-qubit operations that can generate maximally entangled states from some initially separable states. We provide criteria to determine whether a given two-qubit gate is a perfect entangler and establish a geometric description of perfect entanglers by making use of the tetrahedral representation of nonlocal gates. We find that exactly half the nonlocal gates are perfect entanglers. We also investigate the nonlocal operations generated by a given Hamiltonian. We first study the gates that can be directly generated by a Hamiltonian. Then we explicitly construct a quantum circuit that contains at most three nonlocal gates generated by a two-body interaction Hamiltonian, together with at most four local gates generated by single-qubit terms. We prove that such a quantum circuit can simulate any arbitrary two-qubit gate exactly, and hence it provides an efficient implementation of universal quantum computation and simulation.
379 citations
TL;DR: Microwave spectroscopy in the 4 to 6 gigahertzrange at 20 millikelvin reveals energy levels that agree well with theoretical results for entangled states.
Abstract: We present spectroscopic evidence for the creation of entangled macroscopic quantum states in two current-biased Josephson-junction qubits coupled by a capacitor. The individual junction bias currents are used to control the interaction between the qubits by tuning the energy level spacings of the junctions in and out of resonance with each other. Microwave spectroscopy in the 4 to 6 gigahertzrange at 20 millikelvin reveals energy levels that agree well with theoretical results for entangled states. The single qubits are spatially separate, and the entangled states extend over the 0.7-millimeter distance between the two qubits.
01 Jan 2003
TL;DR: In this paper, the authors present the statistical theory of Mesoscopic noise in 2DEG systems, including shot noise for entangled and spin-polarized Electrons and the role of Andereev reflection.
Abstract: I: Shot Noise. Reversing the Sign of Current-Current Correlations M. Buttiker. Shot Noise and Channel Composition of Atomic-sized Contacts J.M. van Ruitenbeek. Quantum Noise and Multiple Andereev Reflections in Superconducting Contacts A. Martin-Rodero. Shot Noise of Mesoscopic NS Structures: the Role of Andereev Reflection B. Reulet, et al. Current Noise in Diffusive SNS Junctions in the Incoherent MAR Regime E.V. Bezugkyi, et al. Shot Noise in Diffusive Superconductor/Normal Metal Heterostructures C. Strunk, C. Schonenberger. Photo-Assisted Electron-Hole Partition Noise in Quantum Point Contacts D.C. Glattli, et al. Shot Noise of Cotunneling Current E. Sukhorukov, et al. II: Quantum Measurement and Entanglement. Qubits as Spectrometers of Quantum Noise R.L. Schoelkopf, et al. Noisy Quantum Measurement of Solid-State Qubits: Bayesian Approach A.N. Korotkov. Linear Quantum Measurements D.V. Averin. Shot Noise for Entangled and Spin-Polarized Electrons J.C. Egues, et al. The Generation and Detection of Single and Entangled Electrons in Mesoscopic 2DEG Systems W. Oliver, et al. What Quantity is Measured in an Excess Noise Experiment? U. Gavish, et al. Noise Correlations, Entanglement and Bell Inequalities T. Martin, et al. Noise in the Single Electron Transistor and its Back Action during Measurement G. Johansson, et al. Dephasing and Renormalization in Quantum Two-Level Systems A. Schirman, G. Schon. III: Full Counting Statistics. The Statistical Theory of Mesoscopic Noise L.S. Levitov. An Elementary Derivation of Levitov's Formula I. Klich. Full Counting Statistics in Electric Circuits M. Kindermann, Yu.V. Nazarov.Multiterminal Counting Statistics of Superconductor-Normal-Metal Heterostructures W. Belzig. High Cumulants of Current Fluctuations out of Equilibrium D.B. Gutman, et al.
TL;DR: This work uses electron counting to measure directly the quantum dot's tunnelling rate and the occupational probabilities of its charge state and provides evidence in favour of long (10 µs or more) inelastic scattering times in nearly isolated dots.
Abstract: Nanostructures in which strong (Coulomb) interactions exist between electrons are predicted to exhibit temporal electronic correlations1. Although there is ample experimental evidence that such correlations exist2, electron dynamics in engineered nanostructures have been observed directly only on long timescales3. The faster dynamics associated with electrical currents or charge fluctuations4 are usually inferred from direct (or quasi-direct) current measurements. Recently, interest in electron dynamics has risen, in part owing to the realization that additional information about electronic interactions can be found in the shot noise5 or higher statistical moments6,7 of a direct current. Furthermore, interest in quantum computation has stimulated investigation of quantum bit (qubit) readout techniques8,9, which for many condensed-matter systems ultimately reduces to single-shot measurements of individual electronic charges. Here we report real-time observation of individual electron tunnelling events in a quantum dot using an integrated radio-frequency single-electron transistor10,11. We use electron counting to measure directly the quantum dot's tunnelling rate and the occupational probabilities of its charge state. Our results provide evidence in favour of long (10 µs or more) inelastic scattering times in nearly isolated dots.
TL;DR: An idea to directly encode the qubit of quantum key distributions, and then present a quantum secret sharing scheme where only product states are employed, where the theoretic efficiency is doubled to approach 100%.
TL;DR: For the simplest and most important case of local projective measurements on an entangled Bell pair state, it is shown that exact simulation is possible using local hidden variables augmented by just one bit of classical communication.
Abstract: What classical resources are required to simulate quantum correlations? For the simplest and most important case of local projective measurements on an entangled Bell pair state, we show that exact simulation is possible using local hidden variables augmented by just one bit of classical communication. Certain quantum teleportation experiments, which teleport a single qubit, therefore admit a local hidden variables model.
TL;DR: In this paper, the decoherence of the qubit state from noise and dissipation was analyzed for the current-biased Josephson junction and the effect of dissipation can be entirely accounted for through a semiclassical noise model that appropriately includes the effects of zero-point and thermal fluctuations from dissipation.
Abstract: We calculate for the current-biased Josephson junction the decoherence of the qubit state from noise and dissipation. The effect of dissipation can be entirely accounted for through a semiclassical noise model that appropriately includes the effect of zero-point and thermal fluctuations from dissipation. The magnitude and frequency dependence of this dissipation can be fully evaluated with this model to obtain design constraints for small decoherence. We also calculate decoherence from spin echo and Rabi control sequences and show they are much less sensitive to low-frequency noise than for a Ramsey sequence. We predict small decoherence rates from 1/f noise of charge, critical current, and flux based on noise measurements in prior experiments. Our results indicate this system is a good candidate for a solid-state quantum computer.
TL;DR: A new concept for a two-qubit gate operating on a pair of trapped ions based on laser coherent control techniques is proposed, which is insensitive to the temperature of the ions, works also outside the Lamb-Dicke regime, and can be orders of magnitude faster than the trap period.
Abstract: We propose a new concept for a two-qubit gate operating on a pair of trapped ions based on laser coherent control techniques. The gate is insensitive to the temperature of the ions, works also outside the Lamb-Dicke regime, requires no individual addressing by lasers, and can be orders of magnitude faster than the trap period, which is presently the speed limit of all two-qubit proposals.
TL;DR: In this article, a scheme to achieve maximally entangled states, controlled phase shift gate, and SWAP gate for two superconducting-quantum-interference-device (SQUID) qubits, by placing SQUIDs in a microwave cavity is presented.
Abstract: We present a scheme to achieve maximally entangled states, controlled phase-shift gate, and SWAP gate for two superconducting-quantum-interference-device (SQUID) qubits, by placing SQUIDs in a microwave cavity. We also show how to transfer quantum information from one SQUID qubit to another. In this scheme, no transfer of quantum information between the SQUIDs and the cavity is required, the cavity field is only virtually excited and thus the requirement on the quality factor of the cavity is greatly relaxed.
09 Jun 2003
TL;DR: Recently, Goldreich et al. as mentioned in this paper showed that a 2-query LDC can be decoded with only 1 quantum query, and then proved an exponential lower bound for such 1-query locally quantum-decodable codes.
Abstract: A locally decodable code encodes n-bit strings x in m-bit codewords C(x), in such a way that one can recover any bit xi from a corrupted codeword by querying only a few bits of that word. We use a quantum argument to prove that LDCs with 2 classical queries need exponential length: m=2Ω(n). Previously this was known only for linear codes (Goldreich et al. 02). Our proof shows that a 2-query LDC can be decoded with only 1 quantum query, and then proves an exponential lower bound for such 1-query locally quantum-decodable codes. We also show that q quantum queries allow more succinct LDCs than the best known LDCs with q classical queries. Finally, we give new classical lower bounds and quantum upper bounds for the setting of private information retrieval. In particular, we exhibit a quantum 2 server PIR scheme with O(n3/10) qubits of communication, improving upon the O(n1/3) bits of communication of the best known classical 2-server PIR.
TL;DR: In this article, the authors consider whether quantum coherence in the form of mutual entanglement between a pair of qubits is susceptible to decay that may be more rapid than the decay of the coherence of either qubit individually.
Abstract: We consider whether quantum coherence in the form of mutual entanglement between a pair of qubits is susceptible to decay that may be more rapid than the decay of the coherence of either qubit individually. An instance of potential importance for solid-state quantum computing arises if embedded qubits (spins, quantum dots, Cooper pair boxes, etc.) are exposed to global and local noise at the same time. Here we allow separate phase-noisy channels to affect local and nonlocal measures of system coherence. We find that the time for decay of the qubit entanglement can be significantly shorter than the time for local dephasing of the individual qubits.
TL;DR: In this paper, a scalable design for silicon-germanium quantum-dot qubits is presented, which incorporates vertical and lateral tunneling, and simulations of a four-qubit array suggest that the design will enable single electron occupation of each dot of a many-dot array.
Abstract: Spins based in silicon provide one of the most promising architectures for quantum computing. A scalable design for silicon-germanium quantum-dot qubits is presented. The design incorporates vertical and lateral tunneling. Simulations of a four-qubit array suggest that the design will enable single electron occupation of each dot of a many-dot array. Performing two-qubit operations has negligible effect on other qubits in the array. Simulation results are used to translate error correction requirements into specifications for gate-voltage control electronics. This translation is a necessary link between error correction theory and device physics.
TL;DR: In this article, a two-qubit gate for the $\mathrm{XY}$ interaction which combines the CNOT and SWAP operations was proposed, which can be implemented efficiently even if only nearest-neighbor coupling between the qubits is available.
Abstract: The two-qubit interaction Hamiltonian of a given physical implementation determines whether or not a two-qubit gate such as the controlled-NOT (CNOT) gate can be realized easily. It can be shown that, e.g., with the $\mathrm{XY}$ interaction more than one two-qubit operation is required in order to realize the CNOT operation. Here we propose a two-qubit gate for the $\mathrm{XY}$ interaction which combines the CNOT and SWAP operations. By using this gate quantum circuits can be implemented efficiently, even if only nearest-neighbor coupling between the qubits is available.
TL;DR: In this article, a deterministic secure direct communication protocol using single qubit in mixed state was proposed, the security of this protocol is based on the security proof of BB84 protocol.
Abstract: We show a deterministic secure direct communication protocol using single qubit in mixed state. The security of this protocol is based on the security proof of BB84 protocol. It can be realized with current technologies.
TL;DR: It is shown that for an odd number of sites a spin cluster qubit can be defined in terms of the ground state doublet, and this qubit is remarkably insensitive to the placement and coupling anisotropy of spins within the cluster.
Abstract: We study the low energy states of finite spin chains with isotropic (Heisenberg) and anisotropic (XY and Ising-like) antiferromagnetic exchange interaction with uniform and nonuniform coupling constants. We show that for an odd number of sites a spin cluster qubit can be defined in terms of the ground state doublet. This qubit is remarkably insensitive to the placement and coupling anisotropy of spins within the cluster. One- and two-qubit quantum gates can be generated by magnetic fields and intercluster exchange, and leakage during quantum gate operation is small. Spin cluster qubits inherit the long decoherence times and short gate operation times of single spins. Control of single spins is hence not necessary for the realization of universal quantum gates.
TL;DR: In this paper, the authors implemented remote state preparation of a qubit from a hydrogen to a carbon nucleus in molecules of carbon-13 labeled chloroform 13 CHCl 3 over interatomic distances using liquid-state nuclear magnetic resonance techniques.
TL;DR: In this paper, it was shown that even states generated from the initial state with all qubits being spin down, via the one-axis twisting Hamiltonian, are spin squeezed if and only if they are pairwise entangled.
Abstract: We show that spin squeezing implies pairwise entanglement for arbitrary symmetric multiqubit states. If the squeezing parameter is less than or equal to 1, we demonstrate a quantitative relation between the squeezing parameter and the concurrence for the even and odd states. We prove that the even states generated from the initial state with all qubits being spin down, via the one-axis twisting Hamiltonian, are spin squeezed if and only if they are pairwise entangled. For the states generated via the one-axis twisting Hamiltonian with an external transverse field for any number of qubits greater than 1 or via the two-axis countertwisting Hamiltonian for any even number of qubits, the numerical results suggest that such states are spin squeezed if and only if they are pairwise entangled.
TL;DR: An architecture for quantum computing with spin-pair encoded qubits in silicon that is scalable to a highly parallel operation and insensitive to tuning errors and easy to model is suggested.
Abstract: We suggest an architecture for quantum computing with spin-pair encoded qubits in silicon. Electron-nuclear spin-pairs are controlled by a dc magnetic field and electrode-switched on and off hyperfine interaction. This digital processing is insensitive to tuning errors and easy to model. Electron shuttling between donors enables multiqubit logic. These hydrogenic spin qubits are transferable to nuclear spin-pairs, which have long coherence times, and electron spin-pairs, which are ideally suited for measurement and initialization. The architecture is scalable to a highly parallel operation.
TL;DR: An all-optical scheme for the experimental realization of a quantum phase gate based on the polarization degree of freedom of two traveling single-photon wave packets and exploits giant Kerr nonlinearities that can be attained in coherently driven ultracold atomic media.
Abstract: We present here an all-optical scheme for the experimental realization of a quantum phase gate. It is based on the polarization degree of freedom of two traveling single-photon wave packets and exploits giant Kerr nonlinearities that can be attained in coherently driven ultracold atomic media.
TL;DR: In this paper, the authors investigated the thermal entanglement in the two-qubit Heisenberg XY model with a non-uniform magnetic field and found that the critical temperature T{sub C} may be enhanced under a non uniform magnetic field.
Abstract: We investigate the thermal entanglement in the two-qubit Heisenberg XY model with a nonuniform magnetic field. Concurrence, the measurement of entanglement, is calculated. The behavior of concurrence is present at three different cases. Contrary to the uniform magnetic field case, we find that the entanglement and the critical temperature T{sub C} may be enhanced under a nonuniform magnetic field.
TL;DR: An LC circuit implemented using a current-biased Josephson junction (CBJJ) as a tunable coupler for superconducting qubits is studied and a simple recoupling scheme leads to a generalization to arbitrary qubit designs.
Abstract: We study an LC circuit implemented using a current-biased Josephson junction (CBJJ) as a tunable coupler for superconducting qubits. By modulating the bias current, the junction can be tuned in and out of resonance and entangled with the qubits coupled to it. One can thus implement two-qubit operations by mediating entanglement. We consider the examples of CBJJ and charge-phase qubits. A simple recoupling scheme leads to a generalization to arbitrary qubit designs.