To establish an applicable system for advanced quantum information processing between light and atoms, we have demonstrated the quantum non-demolition (QND) measurement with a collective spin of cold ytterbium atoms ( 171 Yb), and observed 1.8 +2.4 −1.5 dB spin squeezing. Since a 171 Yb atom has only a nuclear spin of 1/2 in the ground state, the system is the simplest spin ensemble and robust against decoherence. We used very short pulses with the width of 100 ns, so the interaction time became much shorter than the decoherence time, which is important for multi-step quantum information processing.PACS numbers: 42.50.Ct, 42.50.Dv Quantum non-demolition (QND) measurements are measurements in which the strategy is chosen to evade the undesirable effect of back action [1,2]. They have been developed to manage the quantum noise, and are also useful for a quantum-state preparation device and producing a quantum entanglement [3] as well as a feasible model to capture basic features of a quantum measurement process [1,2]. Previously, the QND measurement of the photon number and the amplitude quadrature of light have been realized [4,5,6,7]. The QND measurement of the collective spin is also considerably interesting, and in fact the QND interaction of collective spin of an atomic ensemble (spin-QND interaction) via the Faraday-rotation interaction with linearly-polarized off-resonant light has been proposed [8,9]. An implication is the spin squeezed state [10], which could improve the measurement precision of the atomic clock transition [11,12] and of the permanent electric dipole moment to test the violation of time reversal symmetry [13]. The spin-QND interaction is also useful for implementing continuous-variable quantum information devices, such as quantum memory and quantum teleportation [3,14,15,16,17]. The variety of the interactions and tunability of their strength are useful characteristics of atoms whereas the property of the interaction is rather fixed by the parameter of the non-linear crystal for the case of the QND measurement in optics [1].Previous experimental approaches for the spin-QND interaction [3,18] used thermal alkali atoms and continuous-wave light or long pulsed light of typically 1 ms width, which is comparable with the decoherence time of the atomic spin [3,18]. Hence, it is essential to implement the interaction with shorter pulses and more controllable cold atoms for composing the quantum interface where more-than-twice interactions between the atoms and light beam are required [17]. In addition, it should be noted that the description of the spin-QND interaction is based on the standard model of the collective spin composed of the spin one-half atoms [19,20]. However, the cesium atoms used in the previous experiments have more complicated multi-level structures, which causes serious difficulties as is pointed out in Ref. [19,20]. Therefore, it is widely valuable to demonstrate the spin-QND interaction with cold spin one-half atoms and short pulses.In this Letter, we report the success...

We report an experimental quantum key distribution that utilizes balanced homodyne detection, instead of photon counting, to detect weak pulses of coherent light. Although our scheme inherently has a finite error rate, it allows high-efficiency detection and quantum state measurement of the transmitted light using only conventional devices at room temperature. When the average photon number was 0.1, an error rate of 0.08 and "effective" quantum efficiency of 0.76 were obtained.PACS numbers: 03.67. Dd, 42.50.Lc According to quantum mechanics, one cannot obtain information about a single quantum system without disturbing its state [1] nor can one clone an unknown state [2]. Quantum cryptography is a technique for realizing secure communications exploiting these principles. The most popular protocol is quantum key distribution (QKD) in which two non-orthogonal states (B92 protocol) [3] or four states (BB84 protocol) [4] are sent via a quantum channel in order to generate random keys owned only by the legitimate sender (usually called Alice) and the receiver (Bob). These keys are then used to encode messages.In practice, a faint laser pulse is usually used as the quantum system, and keys are encoded by its polarization or its phase. Ideally, a single photon is desirable, but it is very difficult to generate it experimentally. Most of the previous experimental and theoretical studies on QKD used or postulated photon counting as a means to detect weak light. However, the usage of photon counting gives rise to two limitations. One is a technical limitation that at present there exists no efficient photon counter for infrared light, especially for 1.55-µm where optical loss in an optical fiber is minimum. State-of-the-art experiments used a specially designed photon-counting system made up of cooled avalanche photo diode operated in a gated Geiger mode [5][6][7]. For example, a quantum efficiency of 7% for 1.55µm with a dark-count probability of 10 −4 per 2.6-nsec time-window was reported [8]. Another limitation is a more fundamental: the quantum state of the transmitted light cannot be directly measured; the state alternation is inferred only from the change of the error rate. For example, when the eavesdropper (usually called Eve) changes the photon number distribution of the transmitted light while keeping the polarization (or the phase) and the mean photon number, Bob cannot notice the presence of Eve. This feature allows Eve to perform many kinds of attacks and leads to security holes (one example is the photon number splitting attack [9]). * Electronic address: hirano@qo.phys.gakushuin.ac.jpIn this paper, we propose using balanced homodyne detection for implementing the BB84 protocol with phase coding [10]. As we will explain, the above limitations associated with photon counting can be resolved by using balanced homodyne detection. In order to demonstrate the experimental feasibility of our scheme, we have performed QKD by sending light pulses at 1.55-µm wavelength through an optical fiber of 20-cm length. Whe...

We generalize the experimental success criterion for quantum teleportation/memory in continuous-variable quantum systems to be suitable for non-unit-gain condition by considering attenuation/amplification of the coherent-state amplitude. The new criterion can be used for a non-ideal quantum memory and long distance quantum communication as well as quantum devices with amplification process. It is also shown that the framework to measure the average fidelity is capable of detecting all Gaussian channels in quantum domain.In quantum information science [1], the manipulation of a quantum system is considered to be a channel that transforms a set of quantum states to another set of quantum states. A fundamental distinction is posed on the channel whether it can be simulated by a measure-and-prepare (M&P) scheme or not. The M&P scheme implies that the output of the channel is produced merely based on the classical data processing from the measurement outcomes and that the channel action breaks quantum entanglement shared between the input of the channel and other systems [2]. Therefore, a natural benchmark for the quantum-domain (QD) operation of a given experimental quantum manipulation is that the channel is outperforming any M&P scheme. The M&P scheme is an intercept-resend attack in the context of the quantum key distribution (QKD), and no secret key can be generated if the input-output relation is explained by an M&P scheme. Hence confirming the QD operation is an important prerequisite for any QKD [3,4]. Hammerer et al. [5] have established the criterion for the QD operation of continuous-variable (CV) channels by proving a limit of the average fidelity achievable by the M&P schemes, Fc, assuming an input set of coherent states: Surpassing Fc ensures the QD operation for transmission and storage of coherent states. This criterion gave a proof for the long-standing conjecture of CV quantum teleportation about Fc [6,7] and provided a firm foundation for the central claims of experimental CV quantum teleportation [7,8,9] and quantum memory [10]. However, the application of this criterion is limited to the unit-gain (UG) channels where the coherent-state amplitudes of the input state and the output state are expected to be the same [9,11].Quantum memory for light (QM) is a challenging protocol [12,13], which requires UG operation and involves not only storage of the states but also transfer of the states between different physical media, such as an optical system and an atomic ensemble. In any implementation [10,14,15,16], it is likely that the effect of linear loss or damping of coherent-state amplitude becomes more significant as the storage period becomes longer. Therefore, some mechanism of gain adjustments seems to be necessary for the complete demonstration of QM. With gain control of the teleportation-based state transfer [12], an above-Fc operation of QM has been reported [10]. Another possible solution for the gain control is to employ an amplifier [17], where it is shown that the quantumlimit phase-insens...

In this letter, first, we investigate the security of a continuous-variable quantum cryptographic scheme with a postselection process against individual beam splitting attack. It is shown that the scheme can be secure in the presence of the transmission loss owing to the postselection. Second, we provide a loss limit for continuous-variable quantum cryptography using coherent states taking into account excess Gaussian noise on quadrature distribution. Since the excess noise is reduced by the loss mechanism, a realistic intercept-resend attack which makes a Gaussian mixture of coherent states gives a loss limit in the presence of any excess Gaussian noise.The security of quantum cryptography is degraded by the presence of realistic experimental imperfections. In particular the transmission loss limits the performance of schemes for a long distance transmission [1].Recently several continuous-variable quantum cryptographic schemes have been proposed [2,3,4,5,6,7,8,9]. Those are sorted into either all-continuous type or hybrid type [5], the all-continuous scheme distributes a continuous key and the hybrid scheme distributes a discrete key. A loss limit, in the sense that the mutual information between Alice and Bob I AB cannot be greater than the Shannon information of an Eavesdropper (Eve) I E , is given for an all-continuous scheme [6] and it is shown that this limitation can be removed by introducing a postselection process for a hybrid scheme [7,8,9]. The existence of loss limit is an open question.The reliable security measure for discrete quantum cryptographic schemes against individual attacks is the secure key gain G which ensures that I E can be arbitrarily small in the long key limit if G is positive [10,11]. The question is how high G can be for a given loss or transmission distance in realistic conditions. The estimations are given for BB84 protocol [11], entangled photon protocol [12], and B92 protocol [13]. The estimation of G for continuous schemes, if possible, is important as a comparison with discrete schemes. At least, the framework [14,15] can be adapted to hybrid schemes.For these discrete schemes, the experimental imperfections are mostly determined by observed bit error rate and dark count rate of single photon detectors [11,12,13]. In continuous-variable schemes, the experimental imperfections appear as the change of quadrature distributions. Experimentally, quadrature measurement is performed slightly above the standard quantum limit and observed quadrature distribution has additional Gaussian noise upon the minimum uncertainty Gaussian wavepacket [8]. Thus, the security analysis including experimental imperfections seems to become qualitatively different from that of the discrete schemes. * Electric address: namiki@qo.phys.gakushuin.ac.jpIn our previous work [9] we estimated G of a hybrid type scheme applying a postselection [8] for a given loss, provided Eve performs quadrature measurement for the lost part of the signal. In this case it is shown that G can be positive if the loss is less...

In this paper we investigate the security of a quantum cryptographic scheme which utilizes balanced homodyne detection and weak coherent pulses (WCP). The performance of the system is mainly characterized by the intensity of the WCP and postselected threshold. Two of the simplest intercept and resend eavesdropping attacks are analyzed. The secure key gain for a given loss is also discussed in terms of the pulse intensity and threshold.

We demonstrate unconditional quantum-noise suppression in a collective spin system via feedback control based on quantum nondemolition measurement. We perform shot-noise limited collective spin measurements on an ensemble of 3.7×10(5) laser-cooled (171)Yb atoms in their spin-1/2 ground states. Correlation between two sequential quantum nondemolition measurements indicates -0.80(-0.12)(+0.11) dB quantum-noise suppression in a conditional manner. Our feedback control successfully converts the conditional quantum-noise suppression into the unconditional one without significant loss of the noise reduction level.

We propose a realistic protocol to generate entanglement between quantum memories at neighboring nodes in hybrid quantum repeaters. Generated entanglement includes only one type of error, which enables efficient entanglement distillation. In contrast to the known protocols with such a property, our protocol with ideal detectors achieves the theoretical limit of the success probability and the fidelity to a Bell state, promising higher efficiencies in the repeaters. We also show that the advantage of our protocol remains even with realistic threshold detectors.Comment: 4 pages, 2 figure

Quantum key distribution (QKD) can offer communication with unconditional security and is a promising technology to protect next generation communication systems. For QKD to see commercial success, several key challenges have to be solved, such as integrating QKD signals into existing fiber optical networks. In this paper, we present experimental verification of QKD co-propagating with a large number of wavelength division multiplexing (WDM) coherent data channels. We show successful secret key generation over 24 h for a continuous-variable QKD channel jointly transmitted with 100 WDM channels of erbium doped fiber amplified polarization multiplexed 16-ary quadrature amplitude modulation signals amounting to a datarate of 18.3 Tbit/s. Compared to previous co-propagation results in the C-band, we demonstrate more than a factor of 10 increase in the number of WDM channels and more than 90 times higher classical bitrate, showing the co-propagation with Tbit/s data-carrying channels.