No abstract

A terahertz half-cycle pulse was used to retrieve information stored as quantum phase in an N -state Rydberg atom data register. The register was prepared as a wave packet with one state phase-reversed from the others (the "marked bit"). A half-cycle pulse then drove a significant portion of the electron probability into the flipped state via multimode interference.PACS numbers: 32.80. Rm, 42.30.Rx An atomic Rydberg wave packet is an atom with one of its electrons in a coherent superposition of many highlying orbitals. Wave packets can be viewed as data registers for the information contained in the relative quantum phase of their constituent orbitals. One problem is that phase is difficult to detect. State selective detection on an ensemble of identically prepared registers usually only reveals the component amplitudes; the phases are hidden. Optical techniques have been employed to store and then efficiently retrieve phase information from atomic Rydberg wave packets [1], following a suggestion that binary information stored as phase in quantum registers could be searched more efficiently than the same classical information in classical data registers [2]. This Letter reports a different technique for extracting phase information, based on the interaction between the wave packet and a broadband coherent electric field pulse.In the experiment reported in Ref.[1], a shaped optical pulse carried binary information into the Rydberg atom, by creating an electron wave packet with one or more states phase-flipped (binary 1) with respect to the others (binary 0). A second optical pulse created another wave packet (the decoder) which holographically interfered with the register wave packet [3,4]. This interference converted the phase information to amplitudes by amplifying the flipped states and suppressing the unflipped states. This method of information storage is limited because it depends on mutual coherence between a low-lying launch state and the Rydberg states. Furthermore, the decoding pulse requires that most of the electron's probability amplitude resides in the launch state, and only a small portion goes into Rydberg states. Therefore, the scheme demonstrated in the above experiment cannot be scaled to a very large data register [5].In the new data storage and retrieval experiment presented in this paper, the launch state is no longer part of the data register, and all of the probability resides in the Rydberg states of the atom. The decoding process of amplifying the flipped bits is now performed by a terahertz domain half-cycle pulse(HCP) [6]. This broadband coherent far-infrared radiation directly couples together Rydberg levels. Information stored in the phase domain is converted to amplitude information by coherent redistribution induced by the HCP.HCP interactions with the Rydberg atoms have been studied previously, in HCP-ionization experiments and model calculations [7,8], HCP-redistribution of energy eigenstates [9], and interactions with Rydberg wave packets [10,11]. In the present work, an HCP ...

Abstract. In the perfectly secure message transmission (PSMT) problem, two synchronized non-faulty players (or processors), the Sender S and the Receiver R are connected by n wires (each of which facilitates 2-way communication); S has an -bit message that he wishes to send to R; after exchanging messages in phases 1 R should correctly obtain S's message, while an adversary listening on and actively controlling any set of t (or less) wires should have no information about S's message. We measure the quality of a protocol for securely transmitting an -bit message using the following parameters: the number of wires n, the number of phases r and the total number of bits transmitted b. The optima for n and r are respectively 2t + 1 and 2. We prove that any 2-phase reliable message transmission protocol, and hence any secure protocol, over n wires out of which at most t are faulty is required to transmit at least b = n n−2t bits. While no known protocol is simultaneously optimal in both communication and phase complexity, we present one such optimum protocol for the case n = 2t + 1 when the size of message is large enough, viz., = Ω(t log t) bits; that is, our optimal protocol has n = 2t + 1, r = 2 and b = O(n ) bits. Note that privacy is for free, if the message is large enough. We also demonstrate how randomness can effectively improve the phase complexity. Specifically, while the (worst-case) lower bound on r is 2, we design an efficient optimally tolerant protocol for PSMT that terminates in a single phase with arbitrarily high probability. Finally, we consider the case when the adversary is mobile, that is, he could corrupt a different set of t wires in different phases. Again, the optima for n and r are respectively 2t + 1 and 2; However we show that b ≥ n n−2t bits irrespective of r. We present the first protocol that is (asymptotically) optimum in b for n = 2t + 1. Our protocol has a phase complexity of O(t).

No abstract

We present new results on the quantum control of systems with infinitely large Hilbert spaces.A control-theoretic analysis of the control of trapped ion quantum states via optical pulses is performed. We demonstrate how resonant bichromatic fields can be applied in two contrasting ways -one that makes the system completely uncontrollable, and the other that makes the system controllable. In some interesting cases, the Hilbert space of the qubit-harmonic oscillator can be made finite, and the Schrödinger equation controllable via bichromatic resonant pulses.Extending this analysis to the quantum states of two ions, a new scheme for producing entangled qubits is discovered.

Abstract. We consider perfect verifiable secret sharing (VSS) in a synchronous network of n processors (players) where a designated player called the dealer wishes to distribute a secret s among the players in a way that no t of them obtain any information, but any t + 1 players obtain full information about the secret. The round complexity of a VSS protocol is defined as the number of rounds performed in the sharing phase. Gennaro, Ishai, Kushilevitz and Rabin showed that three rounds are necessary and sufficient when n > 3t. Sufficiency, however, was only demonstrated by means of an inefficient (i.e., exponential-time) protocol, and the construction of an efficient three-round protocol was left as an open problem.In this paper, we present an efficient three-round protocol for VSS. The solution is based on a three-round solution of so-called weak verifiable secret sharing (WSS), for which we also prove that three rounds is a lower bound. Furthermore, we also demonstrate that one round is sufficient for WSS when n > 4t, and that VSS can be achieved in 1 + ε amortized rounds (for any ε > 0) when n > 3t.

No abstract

No abstract