New protocol for quantum technology unlocked

Multi-photon entanglement can be used to reveal the multiple parameters that describe optical processes with greater precision than traditional tomography

A new protocol for estimating unknown optical processes, called unitary operations, with precision enhanced by the unique properties of quantum mechanics has been demonstrated by scientists and engineers from the University of Bristol, UK, and the Centre for Quantum Technologies in Singapore.

The work, published in the June issue of Optica, could lead to both dramatically better sensors for medical research and new approaches to benchmark the performance of ultra-powerful quantum computers.

History tells us the ability to measure parameters and sense phenomena with increasing precision leads to dramatic advances in identifying new phenomena in science and improving the performance of technology: famous examples include X-ray imaging, magnetic resonance imaging (MRI), interferometry and the scanning-tunnelling microscope.

Scientists are understanding how to engineer and control quantum systems to vastly expand the limits of measurement and sensing is growing rapidly.  This area, known as quantum metrology, promises to open up radically alternative methods to the current state-of-the-art in sensing.

Members of the experimental team.
From left to right: Jonathan Matthews,
Rebecca Whittaker and Xiao-Qi Zhou

In this new study, the researchers re-directed the sensing power of quantum mechanics back on itself to characterise, with increased precision, unknown quantum processes that can include individual components used to build quantum computers.  This ability is becoming more and more important as quantum technologies move closer to real applications.

Dr Xiao-Qi Zhou of Bristol’s School of Physics said: “A really exciting problem is characterizing unknown quantum processes using a technique called quantum process tomography.  You can think of this as a problem where a quantum object, maybe a photonic circuit of optics or an atomic system, is locked in a box.  We can send quantum states in and we can measure the quantum states that come out.  Our challenge is to correctly identify what is in the box.  This is a difficult problem in quantum mechanics and it is a highly active area of research because its solution is needed to enable us to test quantum computers as they grow in size and complexity.”

One major shortcoming of quantum process tomography is that precision using standard techniques is limited by a type of noise known as ‘shot noise’.  By borrowing techniques from quantum metrology, the researchers were able to demonstrate precision beyond the shot noise limit.  They expect their protocol can also be applied to build more sophisticated sensors that identify molecules and chemicals more precisely by observing how they interact with quantum states of light.

Co-author Rebecca Whittaker, a PhD student in Bristol’s Centre for Quantum Photonics said: “The optical process we measured here can be used to manipulate quantum bits of information in a quantum computer but they can also occur in nature.  For example, our setup could be used to measure how the polarisation of light is rotated by a sample. We could then infer properties of that sample with better precision.

“Increasing measurement precision is particularly important for probing light-sensitive samples where we want to get as much information as we can before our probe light damages or causes alterations to the sample.  We feel this will have a big impact on the tools used in medical research.”

The researchers’ protocol relies on generating multiple photons in an entangled state and this study demonstrates that they can reconstruct rotations which act on the polarisation of light.

Paper

‘Quantum-enhanced tomography of unitary processes’ by Xiao-Qi Zhou, Hugo Cable, Rebecca Whittaker, Peter Shadbolt, Jeremy L O’Brien, Jonathan C. F. Matthews in Optica

Source: http://www.bris.ac.uk/news/2015/june/unitary-operations.html

A SQUID Analog with a Bose-Einstein Condensate

Figure 1: (a) Superposition of horizontal light sheet and a rapidly moving vertical
beam is used to create attractive potentials for BECs. By translating the
vertical beam in the horizontal plane, Ryu et al. can “paint” an arbitrary trapping
configuration. (b) Using this technique, Ryu et al. created a potential in the form
of a loop interrupted by two thin barriers. (c) Absorption image of a BEC in this
potential, resembling a dc-SQUID-like configuration.

Macroscopic quantum effects in Bose-Einstein condensates permit new kinds of ultrasensitive detectors.

Superconducting Quantum Interference Devices (specifically, dc-SQUIDs) are the world’s most sensitive sensors for magnetic flux. What lies at the heart of this quantum interference technology is the Josephson effect, which is a striking example of a macroscopic quantum phenomenon [1]. When two macroscopically coherent quantum systems such as superconductors are weakly coupled together, through a thin insulating layer for example, a direct current can appear across the junction with no applied voltage. This is known as the dc Josephson effect. As if that were not strange enough, if one tries to put a constant voltage across the junction, the direct current disappears and counterintuitively an alternating current now appears. This is known as the ac Josephson effect. By setting up a loop of a quantum system interrupted by a pair of such junctions, we can exploit these phenomena for interferometry. Magnetic flux acting on the electrons through a superconducting loop causes a phase shift between electric currents that flow across the two junctions and this interference alters the amplitude of the overall oscillation. As a result, dc-SQUIDs find application as ultrasensitive magnetometers [2].

In a neutral system, the role of magnetic field can be played by other effects such as rotational dynamics, and dc-SQUID analogs that work as rotation sensors have been implemented using superfluid helium—an uncharged fluid analog of superconductors [3, 4]. Now, in a paper in Physical Review Letters, Changehyun Ryu and colleagues from Los Alamos National Laboratory, New Mexico, report the creation of a dc-SQUID analog using a Bose-Einstein condensed (BEC) atomic gas [5]. The result should not only encourage studies on BEC properties in complex arrangements but also accelerate the development of an array of cold atom “devices” that may be applied to various other investigations.

How can an ultracold gas act like a dc-SQUID? The Josephson effect relies on the sinusoidal nature of the relation between the current that flows and the relative phase that exists across the junction. In conventional dc-SQUIDs the flow is an electric current, whereas in BECs it is a mass current, and the relative phase is the difference in phase factors of the wave function on either side of the junction. In these macroscopic quantum systems, the relative phase evolves in time in response to the applied potential differences (voltage difference for superconductors, pressure and temperature differences for superfluid helium, and population difference between the two condensates for BECs). A constant potential difference, for example, leads to a relative phase that increases linearly in time, and through the sinusoidal current-phase relation, this then gives rise to an oscillating current. The nonlinear current-phase relation is a key signature of the leakage and the weak coupling of the two macroscopic wave functions.

To achieve this weak coupling, for superconducting systems, researchers employ junctions that are either a tunneling type (where electrons tunnel through a thin nonsuperconducting element) or a constriction type (where the junction is a superconductor, but its physical dimension is made small). For superfluid helium, dc-SQUID analogs are made with a toroidal container interrupted with a pair of constriction-type junctions.

In BEC systems, researchers have observed the Josephson phenomena with tunneling-type junctions made of thin barriers in single-weak-link geometries [6, 7]. Wright et al. recently reported a vividly illuminating experiment that utilized a toroidal BEC interrupted with a single constriction-type junction [8] (see 10 January 2013 Synopsis). In that geometry, no interference takes place, but researchers investigated rich physics in a regime just outside of the Josephson regime where the atomic current as a function of relative phase was hysteretic but not purely sinusoidal.

In paving a way towards the BEC version of the dc-SQUID, Ryu et al. used a technique that they developed several years ago for creating arbitrary potentials for static trapping and dynamical manipulation of BECs [9]. By rapidly moving a laser beam over a static light sheet that provides tight confinement, they effectively “paint” an attractive optical potential in an appropriate geometry (Fig. 1). Here the spatial resolution of the potential is on the order of a few micrometers, which allowed them to create a toroidal potential interrupted by a pair of tunneling-type Josephson junctions. They then moved the junctions circumferentially towards each other, a clever scheme proposed by Giovanazzi et al. [10].

If the junctions are moved slowly enough, atoms tunnel through them and keep the potential difference between the two sectors of the toroidal atomic cloud approximately zero. This is in close analogy to the dc Josephson effect, where a direct current appears across the junction with no driving potential difference. However, as the junction speed is ramped up, the tunneling current cannot respond quickly enough, and a finite potential difference develops at some critical velocity. The atomic current oscillation should then be driven by this energy difference with a frequency proportional to it, which is the ac Josephson effect.

In the absence of dissipation, no net current flows in this ac regime, as the current is only alternating. Hence for speeds any higher than the critical velocity, atoms simply become compressed on one side and expanded on the other side, which appears as a sudden change in the relative population difference in the two sectors of the toroid. Since the Josephson oscillation itself was too small to directly detect, the transition between the dc and ac Josephson effects was sought after to study the Josephson dynamics.

Ryu et al. measured the relative population difference as a function of the total atom number for several fixed rotational speeds of the junctions. Their data from the absorption images of a BEC indeed show dynamic behavior consistent with the system transitioning from dc to ac Josephson regimes, and the overall atom number dependence of atomic current at such transitions is in good agreement with the predictions of the Josephson dynamics. Although it is done indirectly, the current-phase relation is found to be sinusoidal through a simulation on the experiment.

The work of Ryu et al., along with that of Wright et al. [8], offers an exciting opportunity towards sophisticated atom circuits designed and implemented as meaningful tools. In various configurations with or without the Josephson junctions, the cold-atom system has the potential to form an ideal testing ground for phenomena seen in other analogous systems but now with more tunability. That is clear from this result, which advances the fundamental and thought-provoking analogy between the macroscopic quantum physics of superconductivity, superfluids, and BEC gases.

So what remains to be done? A lot, actually. One immediate step is simply more measurements, in particular, obtaining the junctions’ current-phase relation directly. With dynamic variations in the coupling strength, the relation between the atomic current and the relative phase across the junction should exhibit a smooth transition from a linear to sinusoidal shape as the system enters the Josephson regime. This might emerge in a series of destructive interference fringe measurements as the two sections of the BEC toroid are allowed to expand and overlap physically. Another truly revealing measurement would be to directly observe the Josephson oscillation itself in this multijunction system, as done in single-junction experiments, and to demonstrate that the oscillation frequency is proportional to the applied potential difference.

When a beam of light or matter is split and recombined while enclosing a finite area, physical rotation of the instrument leads to a difference in the path lengths in the two directions and gives rise to a rotation-induced phase shift. Through this celebrated Sagnac effect, combined with the quantization condition that the total phase change along a loop must be an exact integer multiple of 2π, a BEC version of the dc-SQUID would function as a rotation sensor with potential applications in navigation, seismology, and geodesy. To explore such possibilities, the demonstration of double-path quantum interference via externally applied rotation may be in order. Studies along those lines should not only reveal intrinsic noises of the system configured as such a device but may also lead to the discovery of various nonlinear phenomena to enhance its sensitivity and utility.

Source: http://physics.aps.org/articles/v6/123?referer=rss

A New Starting Point for Atom Interferometry

Researchers have developed a new atom interferometer that has the potential to be the world’s most sensitive accelerometer.

Interferometers using atoms rather than light can measure acceleration and rotation to high precision. Because atoms are slower than light, atom interferometers have the potential to reach greater inertial sensitivity than their optical counterparts. However, one of the main limitations in atomic interferometry has been discriminating the different contributions to the interference signal. Bright/dark fringes at the output of an atom interferometer result from phase shifts induced by multiple inertial effects as well as interferometer imperfections. In Physical Review Letters, Susannah Dickerson and colleagues from Stanford University, California, now report on a new method that can directly resolve and characterize these phase shifts. They perform their “point source interferometry” on a 10-meter-high atomic fountain, where they can simultaneously measure rotation and acceleration with unprecedented sensitivity [1]. The results open new perspectives in the development of ultraprecise inertial navigation systems as well as future precision tests of general relativity.

The atoms inside an atom interferometer are controlled by beam splitters and mirrors, which in this case are light pulses tuned to particular resonance frequencies in the atoms. The interpretation in terms of matter waves follows from the analogy with optical interferometry. The first beam splitter that an incoming matter wave encounters separates the wave into two different paths. The accumulation of phase along the two paths leads to interference at the last beam splitter, whose two output channels produce complementary probability amplitudes for detecting atoms. The detection probability in each channel is then a sine function of the accumulated phase difference. Most generally, atom interferometers are based on the Mach-Zehnder design, in which two splitting processes are separated by a mirror that folds the paths back together.

These atom interferometers are not only impressive manifestations of quantum physics, but they can also be extremely sensitive inertial sensing devices. The first demonstration of matter-wave inertial sensors occurred about twenty years ago, and since then atom interferometers have evolved into instruments at the leading edge of precision measurements. They measure inertial or gravitational forces affecting the propagation of matter waves with sensitivities comparable to, or even better than, existing classical sensors for rotation as well as for acceleration [2]. With their present performance and technological maturity, these inertial quantum sensors have found a place in a variety of applied and fundamental applications such as navigation [3], geophysics, and gravitation. Current efforts are aimed at either pushing the limits in sensitivity with long baseline experiments [4] or developing compact, commercial devices [5].

Why can we build such sensitive inertial sensors with atom interferometers? “Classical” arguments can provide a basic understanding. When atoms are subject to acceleration or rotation along their trajectory, their speed along this trajectory is modified. This variation of atomic speed results in a variation of the atomic de Broglie wavelength, which itself leads to a dephasing between the two interferometer arms that changes the detection probability at each output channel. In general, atoms enter an interferometer with a wide range of velocities, which means the incoming matter wave has a large spread in de Broglie wavelengths. The outgoing matter wave blends together the effects of rotation and acceleration, as well as unwanted contributions from wave-front distortion and mirror vibrations. For navigation applications, the rotation response needs to be isolated from the acceleration response, and for precision measurements, the noise sources need to be fully understood and characterized. In practice, the different inputs are separated using multiple interferometers aligned along different directions [6].

The experiment reported by Dickerson et al. is admirable in many respects. It employs the longest baseline in atom interferometry, with a 10-meter-tall vacuum enclosure. It also manages to resolve the velocity-dependent phase shifts, allowing multiaxis inertial sensing with a single device. To achieve this, the Stanford team generated a small ultracold atom cloud just 200 micrometers in diameter as their device input. This “point source” contained a few million rubidium atoms at a temperature of less than 50nanokelvin. Using lasers, the team launched their cloud from the bottom of the vacuum enclosure and applied a three-pulse accelerometer sequence, which separates the cloud into two ballistic trajectories—one going slightly higher than the other [see Fig.1(a)]. With this apparatus, Dickerson et al. succeeded in realizing an experimental idea from 60 years ago [7].

Because the atoms are so cold, the cloud is still relatively small when the two paths recombine 3 seconds after the initial split [see Fig.1(b)]. The final cloud—with its interference fringes—is just a few millimeters across, allowing it to be fully imaged on two CCD cameras at the bottom of the enclosure. Nevertheless, the cloud is a factor of 30 larger than its initial size, and this expansion means that each pixel of the CCD cameras corresponds to the probability amplitude for a specific parabolic trajectory through the interferometer. It is as if the point source spreads out into multiple atom “lasers,” i.e., particularly well-collimated atomic wave packets with specific de Broglie wavelengths, and each pixel records the output of the interferometer from each of these lasers. As for classical optics, the resolution set by the pointlike source makes it possible to record directly on the CCD the full interference pattern, not just the probability amplitude. The methods developed by Dickerson and co-workers allow them to extract, at will, information about rotation, acceleration, and even interferometer imperfections such as optical wave-front distortion. They show that the potential acceleration sensitivity of their device is 6.7×10−12g, which is two orders of magnitude better than the previous limit.

This long baseline experiment brings atom interferometry into a new regime of quantum manipulation. To demonstrate this, the researchers imaged the atoms about 1 second after the first beam splitting pulse, at the peak of the fountain trajectory, where a CCD camera captured the atoms in a quantum superposition of 2-millimeter-wide clouds separated by 1.4 centimeters. Even though this wide separation is a natural consequence of coherent wave-packet manipulation (which can also be achieved with large momentum beam splitting), it is still fascinating to have a direct image of this quantum object. Such a long-lived (seconds) macroscopically separated wave packet raises new challenges for studying relativistic effects, or even decoherence induced by spacetime fluctuations [8].

What can happen next? Very long baseline atom interferometers are actively being studied in several ground-based experiments as well as for certain proposed space missions. The hope is that these precise quantum devices may eventually test the equivalence principle or detect gravitational waves [9]. In this ongoing development, one question concerns what type of atom source will achieve the greatest precision: incoherent “light bulbs” or coherent “lasers.” The achievement by Dickerson et al. argues that “light bulbs” can power extremely sensitive devices. However, other ongoing experiments with atom lasers [10] will certainly give other answers—or raise new questions.http://physics.aps.org/articles/v6/92?referer=rss