Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11835011, 12074342) and the Public Welfare Technology Application Research Project of Zhejiang Province, China (Grant No. LY21A040004)
Received Date:24 March 2021
Accepted Date:20 April 2021
Available Online:07 June 2021
Published Online:20 September 2021
Abstract:The Bose-Einstein condensate (BEC) formed by ultracold atomic gases provides an ideal platform for studying various quantum phenomena. In this platform, researchers have explored in depth the important equilibrium and steady phenomena including superfluidity, vortices, and solitons, and recently started to study nonequilibrium problems. In a classical system, nonequilibrium problems, such as explosion, usually occur together with shock waves, which is presented when the explosion speed is larger than the local sound speed. For BEC systems which possess quantum properties, how to produce and understand the shock waves becomes a hot research topic. In this work, we systematically discuss the possibility of quantum shock wave and its essential mechanism in a one-dimensional BEC initially containing dark solitons through quenching interactions. When the system is quenched to the limit of non-interaction, we analytically obtain the post-quench dynamics of initially immobile dark solitons, and find the existence of shock wave, which can be explained through the quantum interference effect. When the system is quenched to finite interaction, we find similar phenomena through numerically solving the Gross-Pitaevskii equation, and analyze different situations. When the system is quenched to a finite weaker interaction, the situation is similar to a non-interaction case; when the system is quenched to a stronger interaction, the shock wave is accompanied by the splitting of the initial soliton, and the two objects can synchronously change; specifically when the quenched ratio of strength is an integer squared, the shock wave disappears, and the soliton is split perfectly. We further explore the properties of the shock wave including its amplitude and speed, and obtain the full scenario as the quenched interaction varies. This work provides theoretical guidance for realizing and measuring shock wave in experiment. Keywords:shock wave/ soliton/ quench/ Bose-Einstein condensate
其中$ n $是背景密度; 声速$s=\sqrt{ng/m}$, 与相互作用强度以及背景密度相关联; 孤子宽度$\xi={\hbar}/{\sqrt{mgn}}$所描述的是灰孤子的尺度; $ u $是孤子速度; $ \mu $为化学势. 当孤子速度$ u $取为0时, 灰孤子变成黑孤子. 本文所淬火的孤子都是静止的暗孤子. 将暗孤子所在的一维BEC系统中的相互作用强度参数进行淬火. 淬火至无相互作用极限时, 可以得到冲击波, 如图2所示. 通过观察数值模拟结果可以发现, 其产生冲击波过程与含背景下凹的高斯波包类似, 因此我们猜测其产生机制也是背景与暗孤子的相干作用. 与高斯波包不同之处在于, 暗孤子在产生冲击波后还可以稳定传输. 在产生冲击波过程中其宽度会发生变化, 但是在产生冲击波后, 暗孤子会依旧保持稳定. 图 2 从$t = -10$至$t= 0$, 暗孤子在凝聚体中稳定演化, 其中背景密度$n = 10$, 相互作用强度$g_1 = 1$. 而在$t = 0$时刻对系统进行了淬火, 将相互作用强度突变至$g_2= 0$, 此后在暗孤子两侧出现对称的激发, 这些激发以恒定速度向两侧运动, 中间孤子宽度变大 Figure2. From $t=-10$ to $t = 0$, the dark soliton evolves stably in the condensate, in which the background density is $n=10$ and the interaction intensity $g_1=1$. At $t=0$, the system is quenched, and the interaction intensity suddenly changes to $g_2=0$. After that, symmetric excitations appear on both sides of the dark soliton, which move to both sides at a constant speed, and the width of the intermediate soliton increases.
其中冲击波的振荡是由后三项叠加造成的. 同样, 从图3(b)可以观察到, 通过解析演化, 消除背景后冲击波不再产生. 进一步证明暗孤子淬火中冲击波的产生机制是背景与暗孤子的相干效应. 图 3 (a)淬火后暗孤子演化至$t = \pi/20$时刻数值与解析对照图, 可以发现两者完全符合; (b) $t = \pi/20$时刻, 除去背景暗孤子淬火后的演化与不除去背景的比较, 在除去背景后冲击波消失 Figure3. (a) When the dark soliton evolves to $t = \pi/20$ after quenching, it can be found that they are completely consistent with the analytical comparison chart; (b) at $t = \pi/20$, the evolution of dark soliton after quenching with background removed is compared with that without background removed, and the shock wave disappears after background removed.
4.不同强度淬火24.1.弱相互作用侧淬火 -->
4.1.弱相互作用侧淬火
上文探究了在无相互作用情况($g_2/g_1= $$ 0$)下生成冲击波的机制. 在淬火比值$ 0 < g_2/g_1 < 1 $ 时, 产生冲击波的机制是否也是一样的呢? 淬火比值已不是无相互作用强度极限, 在淬火后的哈密顿量中存在着相互作用项, 即在演化方程中存在非线性项. 因此不能用解析去求解此类淬火, 只能借助于数值方法进行演化. 选取$ g_2/g_1=0 $, $ g_2/g_1=0.1 $, $ g_2/g_1=0.9 $三个不同的淬火值进行对比. 数值演化结果如图4 所示, 可以看出, 三者形成冲击波的过程类似, 并没有出现密度堆积情况. 在产生冲击波的过程中都先在背景平面上隆起一个波包, 波包不断升高, 与此同时高起的波包与背景相互干涉, 随之产生冲击波. 比较这三者可以发现, 淬火前后比值越大其冲击波振幅越小. 特别关注$ g_2/g_1=0 $与$ g_2/g_1=0.1 $的情况, 从数值模拟上看, 两者形成过程几乎没有差异, 只是在冲击波的振幅上有所差异. 由此可以推断出它们形成冲击波的机制是相同的, 即均为背景与波包的相干. 图 4 淬火强度在$0\leqslant g_2/g_1 < 1$范围时冲击波的形成对比 (a)淬火至无相互作用强度下, 即$g_2/g_1=0$, 可以观察到在背景之上有波包的隆起, 并且伴随着与背景的振荡; (b)相互作用强度淬火前后比值$g_2/g_1 = 0.1$, 除了淬火比值不同外其他都与图(a)相同($n=10,\;g_1=1$); (c)相互作用强度淬火前后比值$g_2/g_1=0.9$, 其他参数与(a), (b)两图相同 Figure4. Comparison of shock wave formation when quenching strength is $0\leqslant g_2/g_1 < 1$: (a) For quenching to the strength without interaction, that is $g_2/g_1 = 0$, it can be observed that there is a bump above the background, accompanied by oscillation with the background; (b) ratio of interaction strength before and after quenching is $g_2/g_1 = 0.1$, values of other parameters are the same as those in panel (a) ($n = 10,\; g_1 = 1,\; m = 1$, $\hbar = 1$); (c) ratio of interaction strength before and after quenching is $g_2/g_1 = 0.9$, and values of other parameters are the same as those in panels (a) and (b).
在前面的讨论中已经知道, 在暗孤子淬火比值在$0\leqslant g_2/g_1 < 1$范围内会有冲击波生成, 这些都是将相互作用强度向小淬火的结果. 同样地, 作为非平衡态演化, 可以将相互作用强度向大淬火. 可以对比$ g_2/g_1 > 1 $与$0\leqslant g_2/g_1 < 1$时的现象, 在此范围内所产生的激发为冲击波, 并且产生的冲击波在左右两侧是对称的. 暗孤子在此范围内淬火时会劈裂出孤子, 并在孤子与背景的交接处产生一个隆起的波包, 随后产生冲击波, 如图5所示. 与之前情形($0\leqslant g_2/g_1 < 1$)比较可以发现, 冲击波产生过程类似, 这两种淬火情形下产生的冲击波应属于同一类型. 在区间$ 1 < g_2/g_1 $内, 冲击波的产生机制也是背景与波包的相干效应. 图 5 淬火强度$ g_2/g_1 > 1 $时冲击波的形成对比 (a)淬火相互作用强度为$ g_2/g_1 = 2 $; (b)淬火相互作用强度为$ g_2/g_1 = 8 $, 其他参数与4.1节相同 Figure5. Comparison of shock wave formation when quenching strength is $ g_2/g_1 > 1 $: (a) Quenching interaction strength is $ g_2/g_1 = 2 $; (b) quenching interaction strength is $ g_2/g_1 = 8 $, and other parameters are the same as those in the section 4.1.
在淬火比值在$1\leqslant g_2/g_1 < 4$范围内, 分别在原孤子左右两边劈裂出1个孤子, 并伴随着冲击波, 而在$4\leqslant g_2/g_1 < 9$范围内, 除了冲击波产生, 左右两侧劈裂出孤子变为2个. 冲击波产生处都在最外侧孤子与背景相交处. 可以发现, 在整个范围内存在着两个特殊值: $g_2/g_1= 4$和$g_2/g_1 = 9$, 见图6. 当淬火至此二值时, 冲击波消失只存在孤子的劈裂, 并且淬火比值越接近这两个值时, 所产生的冲击波振幅越小. 此结果与Gamayun等[53]对BEC中的灰孤子进行淬火的探究不谋而合, 他们提出了在相互作用强度为整数的平方倍$g_2/g_1 = n^2, \;n = 2, 3, 4\cdots$时可以完美地劈裂出孤子. 所谓完美劈裂是指一个暗孤子在淬火后在原暗孤子两侧各产生一个带速度的灰孤子, 而没有伴随着其他的激发. 孤子的劈裂并非只在这些特殊的值上, 在非整数的平方倍时也可以劈裂出孤子但是会伴随着其他激发, 称为不完美劈裂. 图 6 淬火强度$g_2/g_1=4$与$ g_2/g_1=9 $时孤子完美劈裂 (a)淬火相互作用强度为$ g_2/g_1=4 $时在原孤子两侧各完美劈裂出1个灰孤子; (b)淬火相互作用强度为$ g_2/g_1=9 $时在原孤子两侧各完美劈裂出两个灰孤子. 可以观察到完美劈裂情况下除了孤子并没有其他激发 Figure6. When the quenching strength is $ g_2/g_1=4 $ and $ g_2/g_1=9 $, the soliton splits perfectly: (a) When the quenching interaction intensity is $ g_2/g_1=4 $, a gray soliton is perfectly split on both sides of the original soliton; (b) when the quenching interaction intensity is $ g_2/g_1 =9 $, two gray solitons are split perfectly on both sides of the original soliton. It can be seen that in the case of perfect splitting, there is no excitation except soliton.
24.3.淬火暗孤子全景图 -->
4.3.淬火暗孤子全景图
我们发现在不同的淬火参数下冲击波的速度与振幅是不同的, 为此进行了探究. 如图7所示, 在$0\leqslant g_2/g_1 < 1$范围内冲击波振幅有剧烈的变化, $ g_2/g_1 $的比值越接近0, 冲击波的振幅越大, 比值越大振幅越小, 在$g_2/g_1 = 1$时不会有冲击波产生; 而在此范围内所产生的冲击波速度变化情况与振幅相反. 在$g_2/g_1 = 0$时速度最小, 比值越接近1, 速度越大, 当比值到达1时刻突变为0. 图 7 淬火后孤子与冲击波的振幅、速度随淬火强度的变化 (a)冲击波最高点振幅以及劈裂出的孤子深度与相互作用强度淬火比值关系, 虚线为左侧, 实线为右侧, 两者完全重合; (b) 速度与相互作用强度淬火比值关系, 红色线所描述的是冲击波, 绿色和粉丝的线是劈裂出的孤子. 在原孤子的左侧为负, 右侧为正 Figure7. Changes of amplitude and velocity of soliton and shock wave after quenching: (a) Quenching ratio relationship between the peak amplitude of shock wave, the depth of split soliton and the interaction strength. The dashed line is on the left side and the solid line is on the right side, which are completely coincident; (b) quenching ratio relationship between velocity and interaction strength. The red line describes shock wave, and the green and vermicelli lines are split solitons. It is negative on the left side and positive on the right side of the original soliton.